Mechanisms and Kinetics of Gel Formation in Geopolymers

Mechanisms and kinetics of gel formation in geopolymers

By Catherine Anne Rees

Supervisors: Professor Jannie S.J. van Deventer, Dr John Provis and Dr Grant C. Lukey

A thesis submitted in total fulfilment of the requirements of the degree of Doctor of Philosophy

Department of Chemical and Biomolecular Engineering The University of Melbourne March 2007

There is no duty we so much underestimate as the duty of being happy. Being happy we sow anonymous benefits upon the world…

RoRoRobert Ro bert Louis Stevenson (1850(1850--1894)--1894)

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Abstract

Geopolymer chemistry governs the formation of an X-ray amorphous aluminosilicate cement material. Binders form at ambient temperatures from a variety of different raw material sources, including industrial wastes. Early research in this field was based around investigating binder material properties; however, more recently, geopolymer formation chemistry has been intensively studied. Better understanding of the chemical processes governing geopolymer curing reactions will allow a wider variety of waste materials to be utilised and also the tailoring of binder properties for specific applications.

Two different gel phases have been found previously to develop consecutively in a fly ash geopolymer system. However, an understanding of the factors which control the phase development and the process of transformation into the final binder are not well understood. This necessitates the use of a variety of analytical techniques, and has led in this thesis to the application of a novel in situ method, capable of analysing the high pH gels without destructive sample preparation.

Attenuated total reflectance Fourier Transform infrared spectroscopy (ATR-FTIR) is used to analyse partially reacted geopolymers and hardened pastes both in and ex situ. In situ analysis is performed at 1 minute intervals over 3 days, in what is believed to be the first set of true in situ experiments involving fly ash geopolymers. The kinetics of geopolymer formation in systems of different composition are quantified and directly compared. Geopolymer network formation occurs after a lag period, the length of which is dependent on activator concentration; this is followed by a linear growth period. Linear kinetics are observed for all geopolymers investigated.

Ex situ analysis is also performed on geopolymers of up to 6 months of age, investigating structural changes occurring in samples of different composition. X-ray diffraction is also used to compare the formation of crystalline phases in the different samples, giving insight into the early gel chemistry and Si/Al ratio. Microstructural differences are observed between samples with equal silica concentrations but different solution

speciation. The solution speciation significantly alters the early gel formation reactions and binder microstructure.

A conceptual model is developed for fly ash geopolymer synthesis. The model is based around the formation of different gels resulting from early fly ash dissolution. Al release from the fly ash initially is followed by the dissolution of the remaining Al-depleted, Si- rich layer on the ash surface. Early gel development involves a nucleation event and particle growth, followed by a slow network rearrangement, catalysed by the presence of hydroxide and water. The nucleation hypothesis is tested by the addition of potential nucleating sites in the form of nano-particles. Reaction rates are very similar with and without the particles; however, the lag at the start of the reaction is eliminated when the nano-particles are present, supporting the nucleation model.

Various different raw materials are tested as potential additives to fly ash geopolymers and as a primary binder material. A 1-part mix (“just add water”) geopolymer cement is also developed, involving the addition of solid sodium aluminate to geothermal silica waste. This is shown to produce a binder with similar network structure to the aged fly ash geopolymers. Further work investigating the material properties of this new binder is required. It is hoped that the methods and ideas presented in this thesis can be further developed and used to apply geopolymer chemistry to a wider variety of materials, increasing the industrial applicability.

Declaration

This is to certify that:

(i) The thesis comprises only my original work towards the PhD except where indicated in the Preface,

(ii) Due acknowledgement has been made in the text to all other material used,

(iii) The thesis is less than 100,000 words in length, exclusive of tables, maps, bibliographies and appendices.

______

Catherine Anne Rees

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Acknowledgements

Many people and organisations from both within and outside of the University have contributed time, knowledge and funding towards this project. I am extremely fortunate and very grateful to have met so many wonderful people in the course of my studies who have helped me in different ways.

I would first like to thank Louise Keyte, the person who initially introduced me to research. Without her early encouragement and guidance I would most certainly not be completing this thesis right now.

I would like to thank my initial supervisors Professor Jannie van Deventer and Dr Grant Lukey for allowing me to take on this project and giving me the independence and freedom to fully explore the topics and use my creativity. Guidance and support was much appreciated. Thank you to my other supervisor Dr John Provis, for his tireless efforts in reading and correcting my thesis and also for our many discussions around the office. His assistance was imperative to the timely completion of this thesis.

Thank you also to the members (past and present) of the Geopolymer and Minerals Processing Research Group for their friendship and help.

I would also like to thank many other people who have generously contributed their time and resources to this project, in no particular order... Department of Chemical and Biomolecular Engineering staff, Angus Johnston, Simon Crawford, John Marsden (PQ chemicals), Simon Spiers (HullTech), Finlays Stonemasonry, Geoff Shields (Queensland Magnesia), Bob Laughlin (Torftech, Canada), Keith Hopkins (Genesis Power, NZ), Lauren Gomez, Professor Cesar Diaz Trujillo, John Blaik, Peter Manson and Ray Jones (Bundaberg Sugar).

This work was made possible by funding received from the Australian Research Council, as well as additional funding from the Particulate Fluids Processing Centre. The University of Melbourne also made significant contributions in the form of a Special Postgraduate Studentship and Melbourne Abroad Travel Scholarship. I would also like to thank my parents and family for their ongoing support of my rather lengthy education. And of course, a big thank you to Xavier for his support and love and for putting up with me in what has been an interesting time in both our lives.

Table of Contents

Abstract…………………………………………………….. v

Declaration……………………………………………….... vii

Acknowledgements…………………………………………ix

Table of contents…………………………………………...xi

List of Figures and Tables…………………………………xiv List of Figures…………………………………………………………... xiv List of Tables………………………………………...... xxii Chapter 1 Introduction………………………………….. ...1

Chapter 2 Critical Literature Review……………………... 5 2.1 A History of Geopolymer Technology……...... 5 2.2 Fundamental Geopolymer Research………….………………... 7 2.3 Recent Work in Fly Ash Geopolymer Research...... 11 2.4 Fourier Transform Infrared Spectroscopy……………………... 23 for use in Geopolymer Research 2.5 Conclusions……………………………………………………... 34 Chapter 3 Experimental Methods…………………………37 3.1 Materials…………………………………………………………37 3.1.1 Primary Binder Materials………………………………………... 37 3.1.2 Secondary Binder Materials………………………………..……. 40 3.1.3 Activating solutions………………………………………… ...... 41 3.2 Synthesis of Geopolymer Samples………………………….……45 3.3 Characterisation Techniques………………………………….…47 3.3.1 X-ray Diffraction…………………………………………..……….47 3.3.2 Scanning Electron Microscopy………………………..…………47 3.3.3 Attenuated Total Reflectance - Fourier Transform Infrared Spectroscopy……………………………………………………….. 48

3.4 Conclusions…...... ………………………………………...51 Chapter 4 FTIR Analysis of Geopolymer Gel Ageing ……. .53 4.1 Introduction…………………………………………………….. 53 4.2 Materials and Experimental Methods…………………………. 54 4.3 Results and Discussion…………………………………………55 4.3.1 FTIR Spectra of Fly Ash Geopolymers…………………… 55 4.3.2 Structural Changes in Geopolymer Gel over Long Timescales – Low NaOH………………………….... 59 4.3.3 Structural Changes in Geopolymer Gel over Long Timescales – Increased NaOH Concentration…… 64 4.3.4 Effect of Activator Concentration on Microstructure….. 73 4.3.5 Formation of Crystalline Phases in the Geopolymer Gel…………………………………………………………….. 76 4.4 Conclusions……………………………………………………. 81 Chapter 5 An In Situ Study of Geopolymer Gel Formation ………………………………………………………………..…85 5.1 Introduction……………………………………………………. 85 5.2 Materials and Experimental Methods…………………………86 5.3 Results and Discussion…………………………………………86 5.3.1 In situ FTIR Spectra of Fly Ash Geopolymers…………… 86 5.3.2 Functional Group Analysis……………………….………... 91 5.3.3 Effect of Sodium Hydroxide Concentration on Geopolymer Kinetics……………………………………………………….. 94 5.3.4 The Role of Sodium Hydroxide in Geopolymer Formation …………………………………………………………………. 98 5.4 Conclusions……………………………………………………. 101

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Chapter 6 Alternative Raw Materials for Geopolymer Synthesis: Effect of Si/Al ratio on

Reaction Rate and Chemical Structure……..... 103 6.1 Introduction……………………………………………………. 103 6.2 Materials and Experimental Methods…………………………105 6.3 Results and Discussion………………………………………. ... 106 6.3.1 Gel Formation in Fly Ash Geopolymers Containing Solid Silica Powder…………………………………………. 106 6.3.2 Use of a Metakaolin as a Secondary Silicate…… 111 6.3.3 Implications of heterogeneous solids addition in geopolymer gel formation………………………………….. 114 6.3.4 Effect of High Early Aluminium Concentration………....117 6.4 Investigation of a 1-Part Mix Geopolymer……………………. 120 6.5 Conclusions………………………………………………….…. 123 Chapter 7 Geopolymer Gel Formation with Seeded

Nucleation…………………………………….. 125 7.1 Introduction…………………………………………………… 125 7.2 Materials and Methods……………………………………….... 126 7.3 Results and Discussion……………………………………….... 127 7.3.1 Zeolite Formation…………………………………………….127 7.3.2 In situ FTIR Spectra of Geopolymers Seeded with Nano-particles………………………………………………... 128 7.3.3 Geopolymer Gel Structure with Seeded Nucleation……... 136 7.3.4 Nucleation and Geopolymer Gel Growth………………….141 7.4 Conclusions……………………………………………………... 147 Chapter 8 Conclusions and Recommendations…………... 151

Chapter 9 References………………………………………157

Publications ...... 177

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List of Figures and Tables

List of Figures

Figure 2.1. Deconvolution of FTIR spectrum of an alkali silicate activated fly ash geopolymer. From reference [36]...... 9

Figure 2.2. Correlation between the extent of activation and position of the asymmetric T- O-Si band in alkali and alkali silicate activated fly ash geopolymers. From reference [36]...... 10

Figure 2.3. (a) 29 Si MAS NMR and (b) 27 Al MAS NMR spectra of the initial ashes (L and M) and 1% HF-insoluble resides (LHF and MHF). From reference [46]. ……...... 13

Figure 2.4. Effect of hydrochloric acid concentration on primary and secondary fly ash leaching. From reference [50]...... 15

Figure 2.5 Shift in the position of the T–O asymmetric stretching band over time in alkali activation of different fly ashes. From reference [58]...... 20

Figure 2.6. (a) Reaction degree versus time for activation of three different Spanish fly ashes with 8M NaOH; (b) unreacted Al 2O3 in ash versus time (horizontal lines represent the maximum quantity of aluminium that can react in each ash). From reference [60]...... 22

Figure 2.7 Relationship between the bond length and position of the asymmetric stretch of Si-O-T bonds in the FTIR spectra of silicates. From reference [37]...... 27

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Figure 2.8. Shift in position of main Si-O-T asymmetric stretch in the FTIR spectra of aluminosilicates with different Al contents. (a) asymmetric stretch T-O bonds, (b1) and (b2) symmetric stretch of Si-O-T bonds, (c) Double 6-ring vibrations, (d) T-O bending, (e) pore opening. From reference [71]...... 28

Figure 2.9 Schematic representation of intensity differences between transmission FTIR spectra and ATR-FTIR due to penetration depth...... 34

Figure 3.1 SEM micrographs of purified geothermal silica. ……………….………….40

Figure 3.2 ATR-FTIR spectra for activating solutions with low (approximately 3M) [NaOH]. Numbers refer to the silicate concentration in moles per litre. ………………42

Figure 3.3 ATR-FTIR spectra for activating solutions with intermediate (approximately 6M) [NaOH]. Numbers refer to the silicate concentration in moles per litre. …………………………………………………………………………………………..43

Figure 3.4 ATR-FTIR spectra for activating solutions with high (approximately 9M) [NaOH]. Numbers refer to the silicate concentration in moles per litre. ……………….44

Figure 3.5 A. A picture of the MKII Golden Gate diamond ATR, B. Schematic diagram showing the beam path through the ATR (1) torque head screw with limiter screw; (2) ATR crystal, (3) clamp bridge, (4) ZnSe lens barrel, (5) mirrors. From reference [102]...... 50

Figure 3.6 Schematic diagram of in situ ATR-FTIR experimental setup (diagram not to scale)...... 50

Figure 4.1. FTIR spectra of geopolymer development for a sample with Na/Al = 0.5 and [SiO2]=0 A. Spectra of unreacted fly ash (bottom) and geopolymers after different reaction times, B. Enlarged spectra of separated solids after 1 and 2 days of reacting; bottom spectrum is unreacted fly ash. The line at approximately 1055cm-1 shows the position of the broad peak of the unreacted fly ash which appears in the reacted geopolymer spectra. Numbers refer to the age of the sample in days. ……..…...... 57

Figure 4.2. FTIR spectra of geopolymer development for a sample with Na/Al = 0.5 and [SiO2]=2.5M. Numbers refer to the age of the samples. The vertical line shows the position of the main Si-O-T stretching band for the geopolymer network. ..…...... 59

Figure 4.3 Shift over time of main asymmetric Si-O-T stretching band for geopolymers activated with different concentrations of soluble silicate. Na/Al = 0.25, numbers refer to molar silicate concentration in activating solution. A. low silicate, B. medium silicate, C. high silicate. …………………………………………………………………...... 60

Figure 4.4. Shift of main asymmetric Si-O-T stretching band over time for geopolymers activated with different molar concentrations of soluble silicate and Na/Al = 0.5, numbers refer to silicate concentrations in activating solution. A. low silicate, B. intermediate silicate, C. high silicate. ……………………………………………………...... 65

Figure 4.5. Shift of main asymmetric Si-O-T stretching band over time for geopolymers activated with different molar concentrations of soluble silicate. Na/Al = 0.75, numbers refer to silicate concentrations in activating solution: A. low silicate, B. intermediate silicate, C. high silicate. ……………………………………………….……...... 68

Figure 4.6 Variation in the final position of the main Si-O-T asymmetric stretching band in the FTIR spectra of geopolymer samples. ………………………………...... 70

Figure 4.7. Concentration of silicate monomer (Q0) in the activating solutions used for geopolymer synthesis [120]. …………………………………………….…...... 72

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Figure 4.8 SEM micrographs of the geopolymers with Na/Al = 0.5 activated with 0.6M SiO2. ………………………………………………………………………...... 73

Figure 4.9 SEM micrographs of samples activated with sodium silicate solutions, 3.5M SiO2 with overall mix ratio of A. Na/Al = 0.25, B. Na/Al = 0.5, C. Na/Al = 0.75 ……………………………………………………………………………...... 75

Figure 4.10 XRD diffractograms for all geopolymers samples with Na/Al = 0.5. Numbers refer to molar concentration of silicate in the activating solution, F = Faujasite zeolites, M = mullite, Q = quartz. ……………………………………………………...... 77

Figure 4.11 Geopolymers with Na/Al = 0.75 and various concentrations of silicate in the activating solutions (values shown on graph are molar [SiO2]) F = faujasite, S = hydroxysodalite. …………………………………………………………...... 78

Figure 4.12 FTIR spectra for geopolymers, numbers refer to molar concentration of silicate in the activating solution; A. Na/Al = 0.5, B. Na/Al = 0.75. S = hydroxysodalite, F = Faujasite type zeolite. …………………………………………………...... 80

Figure 5.1. FTIR spectra of geopolymer development for samples with Na/Al = 0.5, A [SiO2] = 0 M and B. [SiO2] = 2.5M. Numbers refer to age of sample in hours. ……………………………………………………………………………...... 89

Figure 5.2 FTIR spectra of geopolymer development for a deuterium exchanged sample with Na/Al = 0.5, [SiO2] = 0 M. Numbers refer to age of sample in hours. ……………………………………………………………………………...... 90

Figure 5.3 Functional group analysis for geopolymers activated with: A. sodium hydroxide solution (Na/Al = 0.5), and B. sodium hydroxide solution (Na/Al = 0.5) with 2.5M SiO2...... 93

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Figure 5.4 functional group chromatograms for geopolymers activated with sodium hydroxide solution, A. Na/Al = 1.0, and B. Na/Al = 0.75...... 95

Figure 5.5 Functional group chromatograms for geopolymer peak in samples activated with different concentrations of sodium hydroxide...... 96

Figure 5.6 Rate of change of geopolymer peak intensity (IGP) with time for samples activated with varying concentrations of sodium hydroxide...... 97

Figure 5.7 Functional group chromatograms of unreacted fly ash in geopolymers activated with different concentrations of sodium hydroxide...... 100

Figure 6.1 In situ ATR-FTIR spectra of geopolymer development for a sample with Na/Al = 0.5 and silica fume as secondary silica source, equivalent [SiO2] = 1.7 M. Numbers refer to age of sample in hours...... 107

Figure 6.2 Peak intensity change with time for a fly ash geopolymer containing silica fume as a solid secondary silica source with Na/Al = 0.5 and equivalent [SiO2] = 1.7M...... 108

Figure 6.3 Peak intensity change with time for samples containing silica fume as a solid secondary silica source. Nomenclature: preceding number = Na/Al ratio, SF[#]= silica fume effective molar concentration...... 109

Figure 6.4. In situ FTIR spectra of geopolymer formation for samples with Na/Al = 0.5 and metakaolin as secondary silica source, A. equivalent [SiO 2] = 1.7 M and B. equivalent [SiO 2] = 4.9M. Numbers refer to the age of the samples in hours...... 112

Figure 6.5 Peak intensity change with time for fly ash geopolymers containing metakaolin, with A. equivalent [SiO 2] = 1.7 M and Na/Al = 0.44, B. equivalent [SiO 2] = 4.9M and Na/Al = 0.36...... 113

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Figure 6.6. FTIR spectra of fly ash geopolymer samples containing: MK = metakaolin,

SF = silica fume, both with equivalent [SiO 2] = 4.9M. Sample labelling: preceding number = Na/Al ratio, “Reference” = alkali activated fly ash sample with Na/Al = 0.5...... 115

Figure 6.7. X-ray diffractograms of geopolymer samples containing solid secondary silicates: Mk = metakaolin, SF = silica fume, both with equivalent [SiO 2] = 4.9M. Sample labelling: preceding number = Na/Al ratio, “Reference” = alkali activated fly ash sample with Na/Al = 0.5. F = faujasite zeolite...... 116

Figure 6.8 In situ FTIR spectra of geopolymer development for samples with solid sodium aluminate with A. Na/Al = 0.49 and equivalent [Al] = 0.1M, B. Na/Al = 0.43 and equivalent [Al] = 1.0M. Numbers refer to age of sample in hours...... 118

Figure 6.9 Peak intensity change with time for samples with solid sodium aluminate with A. Na/Al = 0.49 and equivalent [Al] = 0.1M, B. Na/Al = 0.43 and equivalent [Al] = 1.0M...... 119

Figure 6.10 FTIR spectra for a geopolymer synthesised from purified geothermal silica and sodium aluminate with Na/Al = 1.3 and Si/Al = 2.0. A. In situ spectra, numbers refer to age of sample in hours. B. FTIR spectrum after 100 days, Dotted line is an alkali activated fly ash geopolymer with Na/Al = 0.75, also after 100 days. ……...... 121

Figure 6.11 Functional group analysis of a sodium aluminate activated geothermal silica geopolymer with Na/Al = 1.3 and Si/Al = 2.0...... 123

Figure 7.1 FTIR spectra of geopolymer development for sample with Na/Al = 0.5 and

[SiO 2] = 0 M, A. reference sample without nano-particles, B. with 0.01g Al 2O3 nano- particles. Numbers refer to activation time in hours...... 129

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Figure 7.2 Changes in intensity at 960cm -1 with time for geopolymers with Na/Al = 0.5 and [SiO 2] = 0 M. Straight lines have been graphically fitted (grey), m = gradient of linear fit...... 130

Figure 7.3 FTIR spectra of seeded and un-seeded geopolymers after 70 hours at 30ºC with Na/Al = 0.5 and [SiO 2] = 0 M. Water and fly ash have been spectrally subtracted...... 133

Figure 7.4 FTIR spectra of seeded geopolymers after 70 hours at 30ºC with Na/Al = 0.5 and [SiO 2] = 0 M. Black line = 0.1g nano-particles, Grey line = 0.01g nano-particles...... 135

Figure 7.5 FTIR spectra after 100 days at 30ºC for A. Black = seeded and grey = un- seeded geopolymers, GFA has been spectrally subtracted. B. Spectral subtraction of the un-seeded geopolymer from the seeded geopolymer. Arrows show the new bands...... 138

Figure 7.6 X-ray diffractograms for geopolymers with Na/Al = 0.5 and [SiO 2] = 0 M. F = faujasite type zeolite, M = mullite, Q = quartz, SF = zeolite species F...... 139

Figure 7.7C. Phase separation results from hindered gel diffusion. Bulk gel forms as congruent dissolution occurs (relative sizes of objects not to scale)...... 143

Figure 7.7B. Si rich species dissolve from the Al depleted layer, creating a siliceous gel...... 143

Figure 7.7A. Initial early release of Al growth of nuclei with Al rich gel...... 143

Figure 7.8A. Initial early release of Al and formation of Al rich primary gel...... 146

Figure 7.8B. Congruent dissolution allows the gel to become enriched with Si...... 146

Figure 7.8C. Si rich gel comes to pseudo-equilibrium with the surrounding solution/solids and after some time, nucleation occurs...... 146

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List of Tables

Table 2.1 FTIR band assignments for sodium silicate solutions. Data from reference [81]. ……………………………………………………………………………………...... 32

Table 3.1. Composition of raw materials as determined by XRF (mass %). ……………………………………………………………………………………...... 41

Table 4.1. Composition of samples ……………………………………………………………………………………...... 54

Table 5.1. Composition of samples studied ……………………………………………………………………………………...... 86

Table 6.1 Composition of samples …………………………………………………………….………………………...... 105

xxii ~ Chapter 1 ~

Chapter 1 Introduction

Cementitious materials are widely used in modern society. Applications for cement can be quite varied due to the flexibility of on site casting or prefabrication, all at ambient temperatures. Due to these properties, Ordinary Portland Cement (OPC) has become one of the most widely used building materials in the world. A geopolymer is a particular class of binder broadly described as an alkali aluminosilicate cement. Geopolymers are generally produced from the mixing of appropriate alkali, silica and aluminium sources at high pH and ambient temperatures. Furthermore, these binders are often produced from waste materials such as coal fly ash and blast furnace slags. Geopolymers have an extensive range of applications due to their unique properties, such as the ability to withstand elevated temperatures and fire, along with resistance to acid corrosion. The use of various additives such as alkali silicate solutions also gives control over setting times and material properties.

Geopolymers have often been referred to as a “green cement”; not only can industrial waste be utilised as binder materials, but there is also a significantly lower CO 2 emission per tonne than that of OPC [1]. OPC production requires high temperatures and therefore has a high energy demand. This coupled with the stoichiometry of the reaction means that production of 1 tonne of OPC by conventional methods produces over 0.8 tonnes of CO 2 Greenhouse gas [2]. Production of the hydroxide or silicate activator for geopolymer cements does still produce CO 2. However, per tonne of geopolymer cement, the amount of CO 2 produced can be up to 10 times lower than for OPC, representing significant environmental benefits [2]. The importance of stemming global warming and climate change is receiving increasing attention along with the need to lower CO 2 emissions [3]. The use of geopolymers as an OPC replacement represents the potential for a substantial

1 ~ Chapter 1 ~

reduction in world CO 2 emissions, as the cement industry is responsible for an estimated

5% of global CO 2 emissions [2].

Geopolymer chemistry has much wider implications than simply a cement replacement or specialised building material. Other uses include: toxic waste encapsulation, fireproofing and mining waste stabilisation. Within the geopolymer binder, the chemical network of a zeolite is combined with the amorphous structure of a glass, giving unique waste confinement properties. Hazardous substances can be rendered harmless, trapped within the aluminosilicate network, and remaining bound inside the glassy matrix.

While geopolymer technology is of significant commercial interest due to the intrinsic properties, much remains unknown about the chemistry and mechanisms of binder formation. One major difficulty is the presence of several reactive phases, including solids, gels and solutions, the compositions and structures of which are changing with time. Presently, the understanding of gel formation and transformation mechanisms is limited. Recent work has identified the development of different gel phases which form throughout the hardening process. However, no true in situ method has successfully been applied to waste based geopolymers to elucidate the various stages involved in this complicated chemical process. The thesis addresses this problem, and a novel in situ analytical technique, based on attenuated total reflectance Fourier Transform infrared spectroscopy (ATR-FTIR) is developed for application to geopolymer systems. The technique has been successfully applied to investigate and compare the reaction kinetics of different fly ash geopolymer compositions, and also to monitor the changes in gel structure over time. It should be noted that the method is innovative, and thus has been applied to the best of the current knowledge. Further development is likely to be needed to fully unlock the entire potential of this novel analytical procedure.

The first experimental chapter of this thesis (Chapter 4) assesses the changes in geopolymer gel structure over 200 days for a wide range of fly ash geopolymer compositions. This study is designed to map system behaviour in different concentration ranges. Three critical silicate concentration ranges were identified to have similar gel

2 ~ Chapter 1 ~ formation and ageing behaviour. Chapter 5 develops and applies the novel in situ ATR- FTIR technique to a simple geopolymer system of sodium hydroxide and fly ash. Different hydroxide concentrations are tested and the rates of network formation in the fly ash geopolymers are, for the first time, quantified and directly compared.

A series of solid silicate materials are tested in Chapter 6 for suitability in fly ash geopolymers as a secondary silica source to replace the silicate solutions. Material selection is performed on the basis of industrial applicability. The effect on the kinetics of a high early dissolved aluminate concentration is then investigated. The knowledge gained about the performance of these materials in the alkali activated geopolymer system is then used in the development of a 1-part mix geopolymer cement. The “just add water” geopolymer, believed to be the first of its kind, is compared to the conventional fly ash geopolymers, and demonstrates a similar chemical structure.

Finally, Chapter 7 tests the hypothesis of a nucleation mechanism in geopolymer formation. This is done by adding potential nucleating sites to the activating solutions of a geopolymer prior to mixing. The rates of reaction are monitored and the final gel structure is characterised. A detailed reaction model is presented, encompassing theory developed using the most current results from both the present study and the literature. This is a significant step forward in understanding the wide ranging chemical processes occurring in a complex system with many transient states, which will continue to be studied for some time before a complete picture of the chemistry is achieved. In summary, this thesis contributes both a new experimental technique for analysis of geopolymer systems and advancements in the understanding of reactions and processes occurring during geopolymer binder formation.

4 ~ Chapter 2 ~

Chapter 2 Critical Literature Review

2.1 A History of Geopolymer Technology

A geopolymer can be described as a low calcium alkali activated aluminosilicate cement. The structure is comprised mainly of Si-O-Al and Si-O-Si tetrahedral bonds arranged in a solid x-ray amorphous aluminosilicate network. The Al³ + is in IV-fold coordination, charge stabilised by an alkali cation. Unreacted materials and small amounts of other newly formed phases are typically present in the material, trapped in the network. Geopolymer synthesis typically requires an aluminosilicate and an aqueous alkali hydroxide source, which is often combined with an alkali silicate solution. Traditional aluminosilicate sources include kaolin, metakaolin and coal fly ash.

The further development and understanding of geopolymer technology is of significant commercial interest because these materials can be cost-competitive with OPC and furthermore can exhibit superior chemical and mechanical properties [4]. Geopolymers can also be used for toxic waste encapsulation [5]. Geopolymers can display high early strengths, and be chemical and fire resistant [6]. This gives rise to numerous applications for the technology, especially in the building products and construction industry [4]. Furthermore, the utilisation of industrial waste products such as fly ashes and slags presents an opportunity for an environmentally friendly replacement for OPC. Benefits include reductions in landfill waste and Greenhouse gas emissions such as carbon dioxide.

Geopolymer technology was first applied by Joseph Davidovits more than 25 years ago [6, 7]. Since then, various research groups throughout the world have adopted this field of study and hundreds of publications now exist [8]. Much of the very early geopolymer research was application driven and as such, published in the patent literature [9]. This

5 ~ Chapter 2 ~

work was primarily concerned with setting and material properties of different mixtures, focusing on binder compressive strength and other properties, such as fire response, which would delineate the potential uses [10-12]. Some composite materials were also developed and investigated with a similar approach [13].

In the 1990’s a number of publications on geopolymers began to emerge in the scientific literature; the effects on the synthesised material of variables such as mix design were investigated [14]. Interest in geopolymers for different applications grew, in particular for waste encapsulation; a number of studies investigating binder suitability were published [15-17]. Work on determining the effects of metal contaminants such as copper and lead was also carried out [5]. The importance of material structure was recognised and geopolymer microstructural evolution and the effect of additives on sample microstructure and chemistry were also studied [18-20]. Several different reaction models emerged, all with a common theme envisaging a dissolution reaction followed by a precipitation or polymerisation reaction, referred to as “geopolymerisation” [16]. The use of new terminology proposed by Davidovits, specific to geopolymer chemistry was widespread [16]. The terminology referred to the three known chemical units in the geopolymer structure, Si-O-Al-O was called polysialate, Si-O-Al-O-Si-O was called poly(sialate-siloxo), and Si-O-Al-O-Si-O-Si-O was called a poly(sialate-disiloxo) group [6]. These chemical “groups” were thought to be the only possible combinations to exist in the geopolymer, and so it was implied that the Si/Al ratio could only take on integer values between 1 and 3 [6].

Towards the end of last century there was a gradual shift in the way geopolymer research was conducted. There was a move away from the geopolymer specific terminology and scientific segregation, towards uniting this field of research with other related areas such as sol-gel science and zeolite chemistry. Many more analytical techniques were utilised in carefully designed experimental systems to shed light on the effects of various components such as the alkali metal activator [21, 22]. Several new raw materials were also tested for the first time, such as synthetic aluminosilicate powders, albite [23], stilbite [24], basalt fibers [25], alkali-feldspar [26] and blast furnace slag [27, 28]. With

6 ~ Chapter 2 ~

the use of a wider range of aluminosilicate sources came the need to understand the effect of different contaminants on binder formation, spurring the study of various additives on geopolymer material properties and formation chemistry [29, 30].

2.2 Fundamental Geopolymer Research

The turn of the century saw the emergence of a more fundamental geopolymer research methodology. Model systems were used to investigate particular facets of geopolymer chemistry and more detailed models were developed to explain phenomena such as microstructural evolution [31-34]. One such study was carried out by Lee and van Deventer. It was well known at the time that the addition of silicate solutions to geopolymer mixtures could reduce setting times and improve material properties; however, there was little understanding of why this occurred and of the chemistry involved. Several papers published by Lee and van Deventer assessed the effects of silicate solution concentration on the destruction of fly ash.

One of the papers of Lee and van Deventer [35] involved leaching a class F fly ash in alkali and alkali silicate solutions. A wide range of silicate concentrations was investigated from 0-0.57M SiO 2 with 0.6M alkali hydroxide, using Na or K alkali cations. Although the model system chosen for the leaching analysis employed a liquid to solids ratio much higher than that generally used in geopolymer synthesis, information was obtained on the relative dissolution rates and re-precipitation processes occurring during alkali and alkali silicate activation. The studies found a minimum value in the model system of 0.2M SiO 2, above which the fly ash destruction was enhanced. At low soluble silicate concentrations, the dissolution was found to be inhibited by secondary precipitates on the surface of the fly ash; this effect was reduced with increasing soluble silicate concentration.

Fourier Transform Infrared (FTIR) spectroscopy confirmed the results of the leaching tests, whereby samples activated in higher concentration silicate solutions demonstrated a greater shift of the main T-O-Si asymmetric stretching vibration. The FTIR spectrum of

7 ~ Chapter 2 ~

the dried leaching solution in a high silicate sample was similar to the spectra of a geopolymer. It was thus hypothesised that the geopolymer structure and composition were similar to the leach solution. A similar spectrum was also obtained by drying a 5.3M potassium silicate solution (SiO 2/Na 2O = 3.5). However, the absence of a band at 778cm -1, present in the dried solution but not in the geopolymer or the dried leaching solution, was thought to indicate that there was little contribution in the geopolymer gel from a phase similar to the structures present in the dried potassium silicate solution. The differences in structure were thought to be caused by variation in solution speciation, without further discussion.

The pioneering work of Lee and van Deventer also involved an in depth sequential FTIR study using Fourier self deconvolution to assess structural changes during alkali (or alkali silicate) activation from leaching and during geopolymer formation, as illustrated by Figure 2.1 [36]. The study investigated K, Na and mixed alkali systems at the same silicate concentrations discussed above. Four geopolymer samples were also examined, utilising a high solids to liquids ratio with high alkali (5 and 10M) and silicate concentrations (0 and 2.5M), as normally used in geopolymer synthesis. They found that, over time, there was a shift of the main T-O-Si asymmetric stretching band with increased “alkali activation” of the fly ash, where alkali activation was defined as the depolymerisation of the ash. This band shift to lower wavenumbers was assigned to the increase in non bridging oxygen (NBO) content arising from depolymerisation in the ash matrix. Explanation of this band assignment was based on the reduced molecular vibrational force constant of the Si-O- Na + bond compared to the Si-O-T bond (T = Si or Al). Other factors which could cause the peak shift were not mentioned.

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Figure 2.1. Deconvolution of FTIR spectrum of an alkali silicate activated fly ash geopolymer. From reference [36].

The higher silicate concentration in the activating solution was thought to lead to a greater inclusion of alkali metal in the ash matrix and/or greater extent of glass depolymerisation, as the solids leached in more highly concentrated silicate solutions demonstrated the greatest peak shift. The geopolymer spectra also demonstrated a peak shift to lower wavenumbers over time. However, the peak shifted back to higher wavenumbers after some time for geopolymer samples activated without soluble silicate. This shift was thought to be due to “repolymerisation” of the newly formed phase.

However, using this theory, geopolymer samples activated with 2.5M SiO 2 failed to repolymerise, even though the samples gained strength over time. No explanation was presented for this occurrence except that further work was required.

The position of the T-O-Si asymmetric stretching band was then correlated with the “extent of alkali activation”, shown in Figure 2.2. The extent of activation (%) was calculated from the leaching experiments as [100 x (maximum apparent Si dissolution) / (theoretical maximum Si dissolution)]. The theoretical maximum Si dissolution was calculated from the overall X-ray amorphous Si content of the fly ash. The relationship between extent of activation and peak position was then applied to the changes in the geopolymer spectra over time, assuming that the phases responsible for the spectral bands

9 ~ Chapter 2 ~

under consideration were the same in both the leaching system and the geopolymer system. While it is known that depolymerisation or dissolution of silica or aluminosilicate species does increase the NBO concentration in the system (largely in the solution), the actual structure and composition of the precipitates known to form in this system [35] were not considered in detail.

Figure 2.2. Correlation between the extent of activation and position of the asymmetric T- O-Si band in alkali and alkali silicate activated fly ash geopolymers. From reference [36].

The FTIR literature review presented in the papers by Lee and van Deventer was well researched and the band assignments presented in the summary table were accurate. However, there was one factor which was overlooked, and which is fundamental to the arguments presented in the work. The shift of the band due to the T-O-Si asymmetric stretch was assigned entirely to the existence of NBOs. It is true that in a pure silicate, the addition of alkali to the glass structure caused a shift of this band to lower wavenumbers, and the arguments presented in the aforementioned papers are accurate to this point. However, other factors which can increase the Si-O-T bond length and angle will also have the effect of decreasing the molecular vibrational force constant and thus shift the asymmetric stretch to lower wavenumbers [37, 38]. Such factors include increases in: porosity volume fraction (increases in pore size), compressive stress and hydrostatic pressure [39]. An additional factor influencing the position of this band, and thought to be

10 ~ Chapter 2 ~

the most relevant, is the amount of Al 3+ substituted for Si 4+ in the structure. As the substitution of Al into a tetrahedral silicate network is increased, there is an increase in bond length and angle and therefore also a shift of the asymmetric Si-O-T band to lower wavenumbers [40, 41]. It was recognised by Lee and van Deventer that the composition of the precipitates in the system could change with time. However, the effect of a changing Si/Al ratio was not considered.

While the experimental work itself was very well planned and executed, many of the arguments and explanations presented were invalidated by the interpretation of the FTIR data. For example, it is very unlikely that there will be 100% activation (complete silica dissolution) of a fly ash geopolymer sample in 48 hours at ambient temperatures [42]. However, much can still be taken from the work, which was in fact the starting point for this thesis.

2.3 Recent Work in Fly Ash Geopolymer Research

In the past two to three years, more fundamental geopolymer research has been published than ever before. Particularly noteworthy is the extensive work by Fernández-Jiménez, Palomo and co-workers, with research into the structure and composition of both raw materials and geopolymer products. Emphasis was placed on the intermediate phases formed. Several analytical techniques, in particular selective chemical attack, have been used to shed new light on the reaction products.

One study highlighted some of the similarities and differences between the traditional sol-gel chemistry and fly ash geopolymers, particularly the acid versus alkali catalysed sol-gel processes [43]. A model system was employed in the study, using potassium silicate, potassium hydroxide and aluminium nitrate to synthesise gels, then investigating their chemical composition, structure and thermal stability. Gels synthesised under alkaline conditions demonstrated a much higher thermal stability than the acid catalysed equivalent, remaining amorphous up to 1200ºC.

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The purity of the system and the absence of residual unreacted solid materials in the high pH gels allowed a more accurate determination of the gel composition. Giving insight into how the composition (in particular the Si/Al ratio) affects the thermal stability of the gel, this information can also be applied to the production of geopolymer gels.

An interesting extension of the work on gels would be to include FTIR spectra for the already characterised gels of different compositions. This would assist in band assignments and understanding peak variation with composition and structural changes in geopolymer systems. A greater understanding of the FTIR spectra would enable this convenient technique to be more widely used in geopolymer research.

An extensive characterisation of two different class F fly ashes was carried out in a separate publication by Fernández-Jiménez and co-workers as shown in Figure 2.3 [44]. Selective chemical attack, X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) were used to quantify the glassy and crystalline phases in the ashes. The selective chemical attack involved a 1% HF treatment; the procedure has been developed elsewhere and used in the past to dissolve the vitreous component of the fly ash, without affecting the crystalline components [45]. This allows the chemical composition of the glassy and crystalline phases to be analysed separately, allowing more information to be gained about the reactive (glassy) phase of the fly ash. Both the original and HF modified ash were subjected to 27 Al and 29 Si MAS (Magic Angle Spinning) NMR and XRD using the Rietveld method for quantitative data analysis.

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Figure 2.3. (a) 29 Si MAS NMR and (b) 27 Al MAS NMR spectra of the initial ashes (L and M) and 1% HF-insoluble resides (LHF and MHF). From reference [46].

Results of the quantitative XRD gave a higher glassy content for both ashes than the chemical attack method. The XRD results indicated that the HF treatment had dissolved less than 80% of the X-ray amorphous component of each fly ash. This was thought to be due to the existence of non-reactive X-ray amorphous phases, which may be poorly crystallised mullite or quartz phases. The vitreous SiO2 and Al 2O3 contents of each ash were calculated, with emphasis on the vitreous component being the reactive part of the ash. Qualitative information from NMR coincided well with the findings of the other two analytical methods. While the overall glassy Si and Al oxides were quantified, no data were given for different glass compositions within the one fly ash [47, 48]; this is of interest as some glasses may not be as reactive in the alkaline solutions used in geopolymer synthesis. The type of glass forming in fly ash is a function of the coal composition and the burning conditions, if the composition is similar, the glasses are more likely to also be similar [49].

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The importance of the Si/Al ratio in the vitreous phase of the fly ash was highlighted in a later publication by the same authors [46]. A procedure was used in this paper to quantify the degree of reaction from the alkali activation of the two ashes characterised in the previous paper. The degree of reaction was calculated by subjecting the partially reacted geopolymers to a 1:20 HCl solution and determining the amount of material dissolved during the acid attack as shown in Figure 2.4. The use of chemical attack was said to remove only the reaction products from the alkali activation. This allowed quantification of the degree of ash reacted and the Si/Al ratio of the newly formed material, without contribution from the unreacted ash. The effect of the HCl solution on the ash itself was not shown. This is particularly relevant given that HCl solutions at similar concentrations are known to dissolve components of fly ash (Figure 2.4) including Al, and can cause significant changes in the surface area and microstructure [50-53].

The chemical attack method, used to quantify the degree of the reaction in the alkali activated fly ash [46], had been developed from a technique used previously by other authors to dissolve the reaction products of the alkali activation of metakaolin [54]. The original method used a 1:9 HCl solution followed by a 5% solution of Na 2CO 3. The effect of the acid attack on the unreacted metakaolin was also not shown. It was stated that previous tests were conducted to check the effect of the HCl on metakaolin and that it does in fact remain insoluble [54]. No details of this test were given. This method for calculating the degree of reaction was applied to fly ash geopolymers and developed for this purpose in a separate publication [55].

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Figure 2.4. Effect of hydrochloric acid concentration on primary and secondary fly ash leaching. From reference [50].

The same method of acid attack has been used to quantify the degree of reaction in alkali activated fly ashes in a series of publications by Fernández-Jiménez and co-workers, and is central to the work. A valuable extension of this would be to validate the method for use on the fly ash system using FTIR to show the before and after spectra of the geopolymers, along with before and after spectrum of the fly ash itself. A spectral subtraction could be used to demonstrate that the HCl insoluble residue is in fact unreacted ash, or part thereof. This would show explicitly that the ash is not affected by the HCl and that all the newly formed geopolymer phase is completely removed during the treatment.

15 ~ Chapter 2 ~

After the fly ash was reacted with 8M NaOH solution, the product was acid treated to calculate the degree of reaction [46]. The HCl insoluble residue was then alkali activated with an 8M NaOH solution for a slightly shorter time and the process repeated. The second acid attack showed an overall increase in the degree of reaction by 5%, such that the total amount of original ash reacted was over 70%. This ash was previously calculated to be 80% vitreous material. However, the amount of vitreous SiO 2 and Al 2O3, which undergo alkali activation, was found to be less than 65% in total. This indicates that some other material in the fly ash has reacted during alkali activation or acid attack. The alkali attack of quartz and mullite in the fly ash was also observed using XRD. Another possibility is that the HCl acid attack has leached the heavy metals or other components from the fly ash. Since the degree of reaction is determined from the mass loss after alkali activation and then acid leaching, any other material lost in the process will inflate these values. As discussed earlier, HCl is known to leach various components from fly ash; the actual amounts will depend on the original composition of the ashes [50, 53].

In the same study [46], the Si/Al ratio of the newly formed gel was calculated from the NMR data. The ash with the lower Si/Al (vitreous) ratio formed a gel with a higher Si/Al ratio; the significance of this finding was not discussed. It is possible that the ash with the greater vitreous Al 2O3 content reacted faster and thus the gel is enriched with Si at 7 days, after initially forming the Al rich gel, previously observed by the same research group [55]. The study showed that the ash with the highest vitreous Al content had a greater degree of reaction, concluding that a minimum amount of Al must be present in the raw material for the reactions to occur. It may also be true that the reactions occur faster with increased Al content, and this is why the lower Al fly ash demonstrated a lower degree of reaction at 7 days. It may be worthwhile to show the degree of reaction of an aged sample for comparison. Further strength could be added to this work by demonstrating the trend between vitreous Al content and reactivity using a large number of different fly ashes. Correlations between reactivity and glassy Al content could then be quantified.

In a previous study by the Palomo group, a single class F fly ash was characterised using NMR and the HF chemical attack procedure described earlier. The fly ash was activated

16 ~ Chapter 2 ~

with 8M NaOH and reacted at 65ºC or 85ºC and analysis was carried out at various time intervals up to 30 days [55]. One sample was also reacted at 45ºC for 7 days and analysed only at this time. At 85ºC there was a much faster reaction, with the degree of reaction greater than 50% after just 24 hours, compared to 46.6% after 30 days for the sample activated at 65ºC. This may be due to the condensation of silicate species (i.e. reduction in NBO) at the higher temperature. This behaviour has been observed by de Jong and co-workers for sodium silicate solutions, which were found only to condense and form siloxane bonds at temperatures over 80ºC, even when dehydrated at the lower temperatures [56]. The rapid gain in mechanical strength observed for this system may also be attributed to silica phases formed from the condensation of silicates liberated from the fly ash.

The NMR spectra of the alkali activated ash indicated the formation of an Al rich gel initially. However, results of the same sample reacted for longer times indicated that the amount of Si in the gel had increased. It was hypothesised that the initial Al rich gel dissolves to form a more stable gel, richer in Si, with reference to Ostwald’s law. It was also found that higher temperatures led to the incorporation of less Al in the gel in total. This was correlated with compressive strength development, where the Al rich geopolymer gels displayed relatively low strengths compared to the Si rich gels.

This work identified a major step in the process of alkali activation of fly ash; identifiying the formation of an initially Al rich gel which subsequently undergoes Si enrichment before forming the final geopolymer gel. This change was in fact observed in the FTIR results of Lee and van Deventer discussed earlier [36], however, no such conclusions were drawn due to the inadequate interpretation of the FTIR data. More recently, the finding has been supported by the work of other authors using model systems [57].

The high curing temperatures and widely spaced time intervals in the study by Palomo and co-workers did not reveal further details of the two different gels forming during the alkali activation [55]. The time and rate of change of the gel, and the mechanism by which this change occurs, were not investigated, nor were the effects of other parameters.

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A lower temperature and the analysis of many more time intervals would allow a more in depth understanding of these changes, and a more detailed reaction model could then be developed. However, this is difficult with a technique such as 29 Si NMR where the data acquisition is slow and true in situ measurements are complex. Energy Dispersive XRD has been used previously for in situ work on geopolymers; however the study was based on metakaolin [57]. It would be difficult to use this method on fly ash geopolymers and furthermore, an appropriate synchrotron beam line is required, which is often not readily available. FTIR is a method allowing data to be rapidly acquired, ideally, an in situ method would best suit the further investigation of this work.

The two gel concept was further investigated in a series of publications from the Palomo group. One such paper used FTIR to investigate changes in the gel structure and composition over time [58]. The paper aimed to correlate the changes in the vibrational spectra with that in the reaction products formed from the alkali activation. Selective chemical attacks as described earlier were again used in this publication, combined with spectral subtraction, to separate the FTIR spectral features and identify contributions from the newly formed gels and the raw material. Three different fly ashes were tested in the study, all activated with 8M NaOH solution. Again, a relatively high temperature (85ºC) was used, with relatively widely spaced time intervals, 2h, 5h, 8h, 20h and 7 days.

Band assignments in the FTIR study were based on those commonly used in silicate and zeolite systems. Initial band assignments for the system were made using the 1% HF chemical attack to remove the vitreous phase, and with it the broad spectral contribution of the glass. The remaining FTIR bands were then assigned to the various crystalline phases identified from the XRD results. The FTIR spectra of the HF insoluble material (the crystalline component) of the fly ashes were then subtracted from that of the untreated ash, giving the spectra of the reactive or glassy component of the ash. The same method was applied to the alkali activated ashes.

The position of the asymmetric T-O stretching vibration was correlated with the Al content of the unreacted ashes. A higher amount of Al shifts the T-O asymmetric stretch

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to lower wavenumbers, a fact well documented in the FTIR literature [59]. This band was monitored in the alkali activated ashes over time, and found to shift (Figure 2.5), with a similar trend to that observed by Lee and van Deventer [36]. Advances were made in the use of FTIR in the study by Fernández-Jiménez and Palomo, with the assignment of this band shift to changes in the Al content of the gel. As shown in Figure 2.5, the asymmetric T-O stretch was observed to shift to lower wavenumbers in the first two hours for all three ashes, then back to slightly higher wavenumbers over time. This demonstrated the formation of the Al rich gel followed by the formation of the Si rich gel as observed previously [55].

The presence of various ring structures was also investigated in this study. Small relatively sharp bands, visible in the FTIR spectra of the geopolymer between 800- 500cm -1 were assigned to different types of ring structures. A band at 720-730cm -1, observed to form early in the reaction in all samples, was assigned to four membered ring structures. However, this band was previously assigned to the symmetric stretch of Si-O- Si and Si-O-Al bonds in geopolymer structures [36]. It is difficult to assign bands in this region as there is likely to be some degree of overlap between these species. Some spectral features were correlated with the presence of newly formed zeolite and hydroxysodalite crystals, also identified using XRD. In particular, bands at 630-640cm -1 were assigned to single and double six membered ring structures, and correlated with XRD results indicating the presence of a zeolite with this type of structure. These bands were observed to form later in the reaction.

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Figure 2.5 Shift in the position of the T–O asymmetric stretching band over time in alkali activation of different fly ashes. From reference [58].

The alkali activated ash sample with a low vitreous Al content did not demonstrate the bands associated with the double six membered (D6R) rings in the FTIR spectrum, nor the presence of a zeolite with the D6R structure in XRD. This was thought to indicate insufficient available Al for the structural evolution of the alkali activation to continue. However, it is possible that the high Si ash had simply not formed zeolites, and thus no rings due to the zeolites are present in the FTIR spectra, thus the bands observed in the other two activated ashes in the region 800-500cm -1 did not form. Further work needs to be completed, combining both XRD and FTIR for a composition series of geopolymers to confirm or disprove this alternate explanation.

The reaction was hypothesised to proceed by the formation of 4-rings from the available Al, which then formed 6-rings, requiring additional Al. The ring formation hypothesis was based solely on the results of the FTIR. While the presence of the zeolite phases was correlated with the FTIR structural features of these zeolite types, these bands in the FTIR spectra were assigned to the geopolymer gel network as well as to the zeolites. Crystalline materials exhibit sharp, well-defined vibrations in FTIR. Even a small amount of zeolite will demonstrate these bands, in conjunction with overlapping vibrations of the other species present. It is therefore possible that these bands are attributable only to the zeolite phases and not to the remaining geopolymer network. This would also explain

20 ~ Chapter 2 ~

why the ash high in amorphous Si did not demonstrate these bands. The role of the zeolite formation in the structural evolution model was not discussed in the context of geopolymer network formation. An in depth discussion of the role of zeolites can be found in a recent review paper by Provis and co-workers [8].

The importance of the reactive Al content on the formation of alkali activated fly ash binders was further investigated in a subsequent publication [60]. The three fly ashes selected for analysis varied in the amounts of glassy material, with different amounts of reactive (vitreous) silica and alumina. NMR was used to assess changes in the amount of Al per tetrahedra in the geopolymer network over time. The formation of an Al rich gel initially, before the subsequent formation of a more Si rich gel was again found in all cases. Furthermore, the final, and thus more thermodynamically stable gel formed from alkali activation of each fly ash was found from the 29 Si MAS-NMR spectra to be similar in all cases.

The degree of reaction was found by chemical attack with HCl to be similar for all three ashes in the first few hours (Figure 2.6). However, the sample with the lower vitreous Al content demonstrated a reduced rate after this time and a lower overall degree of reaction after 7 days. These results further supported the hypothesis that a certain amount of reactive Al was required for the reactions to continue and the system to continue forming aluminosilicate gel.

A three stage reaction model was proposed based on the results. Stage one involved the dissolution of the fly ash. The formation of an Al rich gel, “Gel 1”, then occurred coating some of the fly ash particles, and this was called the “induction period”. Stage three, termed the “silicon incorporation stage”, involved the transformation of Gel 1 into Gel 2, a more Si rich gel. This process was hypothesised to occur due to the relatively slow dissolution of Si from the fly ash. Gel 2 was thought to be primarily responsible for the mechanical strength development in the system.

21 ~ Chapter 2 ~

This series of publications has made a valuable contribution to the understanding of geopolymer formation. Not only has this work linked geopolymer formation chemistry more strongly with that of zeolite chemistry, but a metastable intermediate phase was discovered and characterised by several different analytical techniques. The actual method of gel development and the transformation of Gel 1 into Gel 2 are not yet well understood. An appropriate in situ analytical technique is needed to give information on the structural and chemical development of the geopolymer gel. Further work is required to better understand this transition and elucidate the mechanism of geopolymer formation from fly ash.

To date there is no comprehensive study assessing the effect of different concentrations of both soluble silicate and alkali on the synthesis chemistry of fly ash geopolymers at ambient temperatures. Furthermore, no simple method is available for examining the reaction kinetics in a geopolymer system, or of even directly comparing the rate of reaction of two samples. The present thesis aims to bridge this gap in the current literature, giving insight into the role of soluble silicates and silicate speciation in the formation and ageing of fly ash geopolymers. Furthermore, new information on the process of gel formation, transformation and ageing is also required. An in depth

22 ~ Chapter 2 ~

understanding of this chemistry would allow greater control over geopolymer properties through mix design, potentially increasing the commercial viability of this material.

2.4 Fourier Transform Infrared Spectroscopy for use in Geopolymer Research

Many different techniques are now available to obtain FTIR spectra, including in situ methods, capable of obtaining spectra at very short intervals over long periods of time. Another advantage of modern FTIR analysis techniques is that sample preparation can be non-destructive. Furthermore, sampling accessories such as Attenuated Total Reflectance (ATR) can be designed for use on highly corrosive systems, allowing the spectra of early age geopolymers to be obtained. ATR has the added advantage of allowing the water and hydroxyl related bands in the spectrum to be analysed. This method has not before been used for fly ash geopolymers.

A great deal of information is embedded in the FTIR spectra of a material; the main problem lies in the interpretation of the spectra. A common misconception is that the FTIR spectra are ambiguous, particularly with glassy materials, as bands can be broad and overlapping due to the presence of a whole range of sites [61]. If not performed with care, incorrect band assignments can be made; these are then cited by other authors and propagated through the literature. Other issues arise from the variable results obtained from the use of different measurement conditions [62]. The accurate use of FTIR as an analytical technique involves a careful review of the literature and a sequence of well designed experiments. If these conditions are satisfied, ambiguity can be removed and much information regarding the structure and bonding in a material can be revealed, using this simple, non-destructive analytical technique.

There is a plethora of infrared spectroscopy literature which lends itself to a critical literature review. The inclusion of poor quality material in this section does not serve the purpose of increasing the understanding of FTIR for use in geopolymer chemistry, so this review is selective rather than exhaustive. Emphasis in this literature review has been

23 ~ Chapter 2 ~

placed on citing the original work which identified and assigned particular FTIR bands associated with various bonds. Only the region from 1300-400cm -1 has been included in this review, as this region is the most significant for silicate networks. Although it is not strictly correct to assign bands in the spectra to particular vibrations in a glass, it is commonplace in the literature to do so. Association of various motions with particular vibrational frequencies will be used for FTIR analysis in this thesis [63].

To understand and interpret the FTIR spectrum of a material, some information about the structure must be known. A geopolymer is a hydrous alkali aluminosilicate, a tetrahedral silicate network with a number of the tetrahedral positions occupied by Al 3+ in four fold coordination, charge stabilised by an alkali cation. Other possible functional groups existing in the system are Al-O-Al groups (in high Al systems) and NBO of the form Si- OH, Si-O- Na + or Al-OH. In addition, there may be small amounts of other less stable compounds, such as 5-coordinated Si species, however these are likely to have a negligible spectral contribution. This review assesses literature regarding the spectra of a wide variety of materials related to these chemical groups.

The normal modes of vibration for vitreous silica were studied by Bell and Dean using computational methods [64]. A hand built model of the silica network containing up to

600 SiO 4 tetrahedra was used; the structure was randomised while considering the limitations imposed by bond angle constraints. Vibrational spectra were computed for the model, and three normal modes of vibration were observed at approximately 1050, 750 and 400cm -1, described as the stretching, bending and rocking motion of the Si-O-Si bonds in the network. The results agreed well with those observed experimentally which occurred at approximately 1100, 800 and 500cm -1. This model forms the basis for the FTIR band assignments in the silicate literature and also the present thesis.

The vibrational modes of silicate structures are closely related to those of vitreous silica. The three normal modes of vibration are preserved [40], although degeneracy can occur and new modes are introduced when more complex structures arise from network substitution. Changes to the length and angle of the Si-O-T (T= Si or Al) bonds in the

24 ~ Chapter 2 ~

silicate network will change the position of the band due to the asymmetric stretch of this bond. Factors include network substitution such as Al or the presence of NBO in the network. Changes can also be caused by increased compressive stress, fictive temperature or porosity volume fraction, even in pure silica systems [38, 39]. Any factor which strains, lengthens or alters the Si-O-Si bond will affect the FTIR spectra.

Stubican and Roy investigated the effect on the silicate network of the substitution of ions with a different charge, including the substitution of Al 3+ into the silicate network, replacing the Si 4+ [65, 66]. This was done by obtaining FTIR spectra for a large number of synthetic clays, comparing trends in peak position and intensity with composition. Spectra were recorded in transmittance mode using the standard KBr disk technique.

Stubican and Roy found that as the amount of Al 3+ substitution in the silicate network increased, there was an increase in the absorption of a weak band at 877cm -1, and that new bands appeared between 800-850cm -1. Slight changes in the position of other bands at lower wavenumbers were also noted. The authors concluded that the FTIR spectra of silicates were very sensitive to the substitution of ions of different charge, but much less sensitive to substitution of ions of the same charge. The substitution of trivalent ions into the tetrahedral positions in a silicate network caused the main Si-O stretching band (1100-900cm -1) to shift to lower wavenumbers due to the average increase in the (Si, Al)- O bond length and the increased ionic character of the bond. Interestingly, no band relating to Al was assigned between 1200 and 950cm -1, with the bands in this region all found to be related to the Si-O bonds. Some publications refer to this region as representing Si-O and/or Al-O vibrations, however it has been shown through studies of both silicate and aluminate systems that there are no pure Al-O vibrations in this region [67].

A discussion by Tarte clarified the band positions in the infrared spectra relating to tetrahedral Al [67]. The work identified the need to distinguish between “isolated” and “condensed” tetrahedra, as these would have different spectral features. In the case of a geopolymer, the tetrahedra form a network and are therefore mostly condensed, and as

25 ~ Chapter 2 ~

such the vibrations are affected by the surrounding network. Tarte proposed that the presence of network bound tetrahedral Al 3+ was indicated by a strong and complex absorption in the region 900-750cm -1, and that a more detailed discussion of individual bands in the spectra was useless due to the complexity of the structure and the spectra. -1 Thus the broad band in the region 900-750cm was assigned to AlO 4 lattice vibrations.

Several other authors have also recognised that pure Al-O vibrations do not occur above 950cm -1. One recent study was conducted using substituted mullite type compounds, comparing FTIR spectra for Al-Si type mullite to those of a Al-Ge and Ga-Ge mullites to assign various spectral features to the different bond vibrations [68]. The KBr pellet technique was again used. Bands in the region 1200-950cm -1 were assigned only to Si-O vibrations. The Al-O in plane and out of plane stretch were assigned to bands at 828 and 909cm -1 respectively.

Henderson and Taylor examined 19 different sodalite minerals, correlating the cell edge parameters with the positions of the vibrational bands [37]. Again, no pure Al-O vibrations were identified between 1200-950cm -1. In this work, the relationship between T-O bond distance and position of the main asymmetric stretch was also investigated. The study concluded that larger T-O bond lengths tend to be associated with smaller T-O- T bond angles. A linear inverse relationship was found to exist between the position of the T-O asymmetric stretch and bond length, for compounds with bond angles close to 145º and 125º, as shown in Figure 2.7.

The correlation observed by Stubican and Roy, relating tetrahedral Al substitution and the shift of the main Si-O asymmetric stretch in the FTIR spectra of silicates, has also been observed by other authors. Tuddenham and Lyon developed a quantitative relationship allowing the extent of Al substitution for Si to be calculated from FTIR data alone [69]. The work used 21 different chlorite mineral samples with Si/Al ratios from 0-1. It was found that the number of major absorption bands between 1100-950cm -1 in the FTIR spectra varied with Al substitution, from one band to three bands in some cases. A distinct change in the FTIR spectra was found to occur when the Si/Al ratio was around

26 ~ Chapter 2 ~

2.3. Overall, the position of the band near 1000cm -1 was found to correlate linearly with the Al substitution. Samples fit into two series, depending on the Fe content. The relationship was also confirmed for other naturally occurring minerals in subsequent studies [70].

Figure 2.7 Relationship between the bond length and position of the asymmetric stretch of Si-O-T bonds in the FTIR spectra of silicates. From reference [37].

Chao and co-workers also investigated this relationship by comparing the FTIR spectra of 16 faujasite zeolites with Si/Al = 1.7 to 3.7 [41]. The study once again found that the external asymmetric stretching vibration (1100-940cm -1) and also the internal symmetric stretch (840-740 cm -1) of the O-T-O bond shifted to lower wavenumbers with increased Al content. The relationship was again found to be linear for both bands. The correlation also described literature data for other zeolite samples well. A linear relationship was also observed between the Al content and the wavenumber of the main absorption in the region 1100-950cm -1 and 800-750cm -1, by Shigemoto and co-workers as shown in Figure 2.8 [71].

27 ~ Chapter 2 ~

Another factor which causes a significant shift in the main asymmetric stretch of the Si- O-T (T: Si, Al or Na +) network is addition of Na +. Sweet and White investigated the FTIR spectra of sodium silicate glasses with varying amounts of Na [72]. The main Si-O- Si asymmetric stretch shifted to lower wavenumbers with increasing Na content. However glasses containing Na also demonstrated a second peak, not present in pure silica glass, at around 950cm -1.

The band at 950cm -1 in alkali silicates has been assigned by many authors to the Si-O asymmetric stretch of NBOs of the form Si-O-M+ where M + is an alkali cation [36, 63]. However, Sweet and White [72] found that the frequency of the band assigned to BOs (~1050cm -1) increases at a faster rate than the band due to NBOs (~950cm -1), when the

28 ~ Chapter 2 ~

number of NBOs in the glass decreases. Furthermore, if the correlation between Na content and position of the band at around 950cm -1 is extrapolated to zero Na, the value becomes 1050cm -1, which is the position of the only peak in the region found in the pure silica glass. This was explained as a strong coupling of this vibrational mode with the remainder of the glass structure.

Uchino and Yoko furthered the work on the vibrational bands in sodium silicate glass using ab initio molecular orbital calculations [73]. Calculated vibrational spectra fitted well with those observed experimentally. The band at 950cm -1 was assigned to the asymmetric stretch (Si-O-Si) of bridging oxygens in the network. This band assignment was justified by explaining the changes occurring in the glass structure; the BO (Si-O-Si) bonds in the network are substantially weakened and elongated with the addition of Na + ions. Increased bond length leads to a reduction in the molecular vibrational force constants and thus a shift of the band to lower wavenumbers. This assignment is also in agreement with other computational studies [74].

Two other types of NBO also exist in the geopolymer system, Al-O-Na + and Al-OH [22]. Although it is preferable for the NBOs to be located on the Si [75], both species will be present in the system in the early stages during the dissolution and gel formation. Due to the very high pH in the geopolymer system, many of the NBOs in the system will be deprotonated [22]. However, it has been shown that even at high pH, aluminate species - contain Al-OH groups, the dominant species being Al(OH) 4 [76]. The vibrational assignments used here will be based on those made for sodium aluminate solutions, as it is mainly the smaller Al species in solution which will contain NBOs. Due to the relatively high reactivity of these groups, they are unlikely to exist in large quantities in the gel [77].

The FTIR and Raman spectra of sodium aluminate solutions (0.5-6M) in both H 2O and

D2O were investigated by Moolenaar and co-workers [76]. The FTIR spectra of sodium aluminate solutions was found to be concentration dependent. The presence of a single aluminate species was detected up to 1M aluminate, and the formation of new species

29 ~ Chapter 2 ~

occurred as the concentration increased. A low intensity band at 950cm -1 was observed in the spectra, however a large amount of uncertainty was associated with this due to the broad bands located at slightly lower wavenumbers, causing significant overlap. The tentative assignment of this band was to O-H bending modes in Al-OH groups. This band is of less importance due to the relatively low intensity compared with others in the aluminate spectra; however it does overlap with the region of interest in the silicate network and silicate solution species. Strong bands in the infrared spectra were identified -1 -1 at 725 and 900cm in H 2O (745 and 895cm in D 2O), with only the former bands present for concentrations of the order 1M Al and 1M NaOH. The band at 725cm -1 was assigned -1 to the AlO 4 asymmetric stretch. The band at 900cm , present only in highly concentrated solutions, was assigned to the Al-O stretching vibration.

Other lower intensity vibrations were observed in the highly concentrated solutions at 705 and 540cm -1. Two possible band assignments were made for these. The first proposed assignment was an Al(OH) 2 asymmetric and Al(OH) 2 symmetric stretch respectively. This was deemed to be a feasible interpretation; however a different explanation was favoured by the authors. This alternative involved the formation of a 2- dimer species in solution of the form (OH) 3Al-O-Al(OH) 3 . The band assignments -1 assuming dimer formation involved two AlO 3 modes at approximately 900cm , four -1 -1 AlO 3 modes at around 700 cm and one Al-O-Al mode at 545 cm . The NMR data were also consistent with the proposed dimer formation which has also been supported by other authors [78, 79].

Another vibrational spectroscopic study was conducted on aluminate solutions more recently by Wattling using ATR-FTIR and Raman spectroscopy [80]. Sodium aluminate solutions in the concentration range 1.25-9.38M NaOH and 0-6.85M Al were analysed, along with their deuterated analogues. Spectral subtraction of solvent bands was performed by minimising the water bending vibration at 1640cm -1. This ensured that strong H 2O and D 2O vibrational bands were not overshadowing the aqueous species of interest, as occurred in previous studies. The most intense vibration occurred at approximately 700cm -1and this was attributed to the asymmetric Al-O stretch. A very

30 ~ Chapter 2 ~

broad and low intensity band was also observed at approximately 940cm -1, corresponding to the observation of Moolenaar and co-workers [76]. This band was assigned to Al-OH bending modes. The assignment was confirmed when the deuterated analogue was examined, as the 940cm -1 band was observed to shift significantly. In more highly concentrated solutions, additional, less intense bands occur at 880, 625 and 550cm -1. The assignments made for these bands are in agreement with Moolenaar and co-workers [76], suggesting the formation of a dimer species in solution at higher concentrations.

The ATR technique has also been used to investigate the vibrational spectra of sodium silicate solutions; again the strong contribution of the water can be removed using spectral subtraction. For this analysis it is not possible to use the standard KBr technique most frequently used in the literature, as KBr dissolves in silicate solutions [81]. This also renders the KBr pellet technique inappropriate for use on partially reacted geopolymers, as these systems can still contain silicate species in solution. The method can be used if the geopolymer gels are dried, however, this causes structural changes and precipitation, thus giving less accurate spectra of the system components. The ATR method allows the solvent to remain in the system, but the contribution to be removed from the spectra using a subtraction method. Therefore the overlap of the solvent peaks is removed, without altering the sample.

A study by Bass and Turner used the ATR technique along with 29 Si NMR to study sodium silicate solutions [81]. A wide range of Si/Na ratios was investigated with concentrations of up to 6M Si. By correlating changes in FTIR band position and intensity at different concentrations with the NMR results, band assignments were made and are summarised in Table 2.1 below.

31 ~ Chapter 2 ~

Table 2.1 FTIR band assignments for sodium silicate solutions. Data from reference [81]. Anion Type * Component band region Polymer 1300-1100 cm -1 Q3 4 membered rings 1020-1050 cm -1 Q3 3 membered rings 1070-1030 cm -1 Q2 3 membered rings 1050-1020 cm -1 SiO - cyclic anions 1020-1010 cm -1 Linear Q 1 (dimer, trimer, tetramer) and monomer 1005-995cm -1, 985-965 cm -1 Monomer and dimer 950-910 cm -1 SiO - small anions 900-850 cm-1

These band assignments are in good agreement with those discussed earlier for sodium silicate glasses, in which the degree of connectivity in the silicate was proportional to shifts in the position of the asymmetric Si-O stretch. The assignments will be applied to the FTIR spectra of the silicate solutions used for activation in the present thesis.

ATR-FTIR is an extremely valuable analytical tool, particularly for aqueous systems. The technique was developed independently by both Fahrenfort [82] and Harrick [83] in the early 1960’s and has become a popular technique in materials science. A study by Kline and Mullins used ATR-FTIR to monitor kinetics during a sol-gel synthesis [84]. A solution of tetraethylorthosilicate (TEOS) was mixed with aluminium nitrate and the resulting solution pH was adjusted using a hydroxide solution. Immediately after mixing, the solutions were placed in the cylindrical liquid ATR cell allowing rapid analysis. The first 15 minutes of this reaction had not before been monitored, as techniques such as NMR did not allow such fast data collection. The solutions in the ATR remained static for the 8 hour experiment and data were collected every one minute for the first fifteen minutes and longer intervals thereafter. A functional group analysis was conducted, in which the intensity of the strongest band assigned to the functional group of interest was monitored over time. The kinetics of various reaction mixtures with different Si/Al ratios were compared in the study. The bands due to the main reactant, solvent and the product of the hydrolysis reaction were monitored over time. A clear inverse relationship was

32 ~ Chapter 2 ~

evident between the band intensity of the vibrations assigned to TEOS and that of the hydrolysed TEOS, demonstrating the effectiveness of the in situ ATR-FTIR to monitor the reaction.

Kline and Mullins also used ATR-FTIR to develop kinetic equations for the sol-gel reactions [85]. Initially, the chemistry of the system was defined, accounting for the major reactions occurring during hydrolysis and gel formation. Computer software was then used to fit rate constants to the ATR-FTIR data. Kinetic rate constants from each chemical equation were then discussed and compared in the context of the sol-gel theory. This paper demonstrates the efficacy of the in situ ATR technique for investigating the kinetics of reacting systems. The technique has also been used by other authors to probe reaction kinetics in different sol-gel systems, including organic-inorganic hybrid materials [86, 87] and will be used in this thesis to probe the solid-liquid and gel reactions occurring within the reacting geopolymer.

While ATR-FTIR is a convenient and useful technique for aqueous systems, the spectra obtained can differ from those obtained by the KBr pellet technique. Unlike conventional transmission FTIR, the infrared beam penetrates the sample to a depth of up to approximately 10 microns when using the ATR technique. The actual depth of sample penetration depends on the refractive index of the sample, the angle of incidence and the wavelength of the radiation [83]. A schematic representation of the dependence of sample penetration depth on wavenumbers for transmission versus ATR spectra is illustrated by Figure 2.9 below.

33 ~ Chapter 2 ~

ATR

Transmission Relative absorbance Relative

4000 3000 2000 1000

Wavenumber (cm -1)

Figure 2.9 Schematic representation of intensity differences between transmission FTIR spectra and ATR-FTIR due to penetration depth

There is generally a slightly higher sensitivity to peaks at lower wavenumbers, which is actually beneficial for geopolymer research since the peak associated with the geopolymer network occurs at lower wavenumbers than that of the main raw material, thus its formation can be monitored closely at lower concentrations, this will be further investigated in Chapter 4.

2.5 Conclusion

Recent advances in fly ash geopolymer research have identified the existence of a gel transformation during geopolymer formation. Initially an Al rich gel forms, which later becomes enriched with Si. One technique which has been used to investigate these gels is FTIR, a valuable technique for use on geopolymer systems. Changes in reacting geopolymers have been monitored in the literature using FTIR. However, incorrect band assignments in some cases led to conflicting results and the development of incorrect geopolymer formation models.

The FTIR literature contains much information regarding the spectra of silicate, aluminate and aluminosilicate systems, including the effects of alkali addition. FTIR allows the identification and monitoring of these groups over time. The basic FTIR

34 ~ Chapter 2 ~

spectrum of amorphous silica contains three main bands due to the stretching, bending and rocking of the Si-O-Si bonds which occur at approximately 1100, 800 and 475cm -1 respectively. A geopolymer network can be envisaged as a tetrahedral silica network, with some of the Si replaced with Al 3+ charge balanced by an alkali cation. In addition, non-bridging oxygens can also exist. An increase in the amount of Al 3+ in the silica network elongates and weakens the bonds to neighbouring Si-O groups, and causes a shift of the main Si-O stretch to lower wavenumbers. Monitoring the position of this band during the reaction can allow the time of gel formation and the relative changes in the Al content of the gel to be observed.

The ATR-FTIR technique is particularly suited for use on geopolymer systems as destructive sample preparation is not required. Furthermore, this type of spectroscopy allows sample analysis in situ.

The time at which the geopolymer gel forms and the mechanism by which the gel transformations occur are not yet fully understood, nor are the factors which affect the process. An in situ analytical technique is required to further investigate these changes. In this thesis the gel formation and ageing in geopolymer systems of different compositions is studied by a novel in situ ATR-FTIR technique, applied to geopolymers for the first time. Various additives are used to further probe the system and gain insight into the chemical processes bringing about the changes in the gel composition and structure. The comprehensive review of fundamental FTIR literature presented in this Chapter will be used as a basis for understanding and interpreting the FTIR spectra.

36 ~ Chapter 3 ~

Chapter 3 Experimental Methods

To make a geopolymer, reactive sources of silica, alumina and alkali are mixed in water at high pH. The majority of studies use a single solid as a source of both silica and alumina, which is added to a high pH solution containing the alkali, and often additional silica as well (i.e. an alkali silicate solution). In some cases, the solution contains a secondary source of alumina; however the alumina and silica are not normally included in the same solution, as this leads to rapid precipitation. This chapter describes the materials and methods employed in the research presented in this thesis. A chemical analysis of the raw materials used for geopolymer synthesis is presented and the significance of the raw material selection will be explained.

The experimental methods used to make the geopolymer samples are also explained in detail in this chapter along with a description of the analytical equipment employed. The main technique used in this study is Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). A novel in situ analytical procedure using ATR- FTIR is developed and applied to the geopolymer system for the first time. Other techniques used include Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD).

3.1 Materials

3.1.1 Primary Binder Materials

Coal fly ash is the by-product of the pulverised coal combustion process, generally removed from the flu gases by electrostatic precipitators. This material has often been used as an aluminosilicate raw material for geopolymer synthesis [88]. One reason this material is so widely used to make geopolymers is that it is in good supply and presents a hazardous waste disposal problem [89].

37 ~ Chapter 3 ~

The coal used in power stations contains mostly carbon, however there is a fraction of material which is non-combustible. The non-combustible matter is often high in silica and alumina due to the presence of clays; however, it can also contain other oxides of calcium, iron and various other elements, some of which are hazardous. Very high temperatures occur during the burning process, and mineral species in the coal particles can become molten or even vapourise. These species often coalesce and condense to form various phases on the surface of residual solids, or form very small discrete particles [90]. Various factors influence the formation of residue during the coal burning process, primarily the mineralogical content of the coal, the combustion environment and the fineness of grinding [90]. The complexity of the starting materials and transport processes operating in the furnace lead to highly heterogeneous particles, in which one particle can contain several different phases [45]. Particles are mostly spherical with both crystalline and glassy components often existing in single spheres which can be solid or hollow and can even contain other particles [91, 92]. The heterogeneity of fly ashes must be understood in order to design experimental procedures which will provide reproducible results.

The principal binder material used for geopolymer synthesis in the present thesis was a class F coal fly ash, originating from Gladstone Power Station in Queensland, Australia. The oxide composition of the Gladstone fly ash (GFA) is determined using a Siemens SRS3000 X-Ray Fluorescence (XRF) spectrometer and is shown in Table 3.1 and the 2 BET surface area is 1.2m /g. The GFA contains 28.7% Al 2O3 and 46.8% SiO 2, however, a significant proportion of this material is crystalline. The crystalline fraction of the fly ash is largely unreactive in the alkaline solutions used for geopolymer synthesis, and only the X-ray amorphous material reacts to a significant extent [44]. It is calculated from a quantitative XRD analysis using SiroQuant software that only 37% of the SiO 2 and 14% of the Al 2O3 in the GFA is X-ray amorphous. It is for this reason that mix formulations are based on the amount of X-ray amorphous SiO 2 and Al 2O3 in the GFA.

38 ~ Chapter 3 ~

To ensure experimental reproducibility and accuracy, a single 20kg bag of fly ash was used throughout. This bag was well mixed and then batched such that each geopolymer would contain a similar particle size distribution and overall composition.

Geopolymers in this thesis are also synthesised from geothermal silica waste and sodium aluminate. The sodium aluminate powder, supplied by Aldrich was produced by Riedel- de Haën (lot #41180) and contained 55.2 wt% Al 2O3 and 42.7 wt% Na2O. Geothermal silica was sourced from the pipes of the Cerro Prieto Geothermal Power Plant in Baja California, Mexico where the total geothermal silica waste production is estimated to be 50,000 tpa [93]. This material has no present commercial use and is discarded to a waste pond with the threat of overflow and damage of nearby agricultural lands [94].

The raw silica scale for use in this work is milled to reduce the particle size, then stirred in polypropylene beakers with distilled water at 80ºC (liquid to solids ratio = 10) for 30 minutes to remove salts. The resultant slurry is filtered and washed with more distilled water, then dried at 40ºC and milled again before use. This method has been used previously to purify geothermal silica [95]. The purified geothermal silica is 96% SiO 2 and contains a small amount of various salts; the oxide composition is determined by XRF and shown in Table 3.1. This material has a very small particle size, however the particles are aggregated, as can be seen in the SEM micrograph in Figure 3.1 below. This gives a high surface area for reaction (approximately 30m 2/g [96]), while the geopolymer paste maintains good workability at liquid to solids ratios of around 0.28.

39 ~ Chapter 3 ~

100 µm 2µm

Figure 3.1 SEM micrographs of purified geothermal silica

3.1.2 Secondary Binder Materials

The secondary solids are materials containing Si which alter the overall Si/Al ratio of the fly ash geopolymer mixtures and can potentially replace the solution silicate frequently used in the activating solutions. The most important selection criterion for these materials is the dissolution rate, which must be faster than that of fly ash, so that the solution will rapidly become rich in Si similar to a silicate activating solution. Therefore, both structure and surface area of the solids are important. Other important factors include cost and availability.

A solid silica waste material, silica fume (grade SF-98), is sourced from the Australian Fused Materials plant at Rockingham, Western Australia. The silica fume is a byproduct of the zircon desilication process, performed in electric arc furnaces [97], and the oxide composition of the waste is shown in Table 3.1. The approximate surface area of the silica fume is 15m 2/g, which is around ten times that of fly ash; a relatively high surface area is necessary for rapid dissolution.

Metakaolin was also used as a secondary solid in this thesis, to investigate the effect of pre-dissolved species on the reaction of fly ash. Metakaolin is formed from the calcination of kaolin clay at temperatures between 500-750ºC. During heating, water of hydration is removed and the crystalline structure is destroyed. Metakaolin is an obvious choice for this study as it is known to have a much faster dissolution rate than fly ash in

40 ~ Chapter 3 ~

alkaline solutions and is a widely used source of aluminosilicate for geopolymer synthesis [98]. The metakaolin, sold as Metastar 402, is sourced from Imerys Minerals, UK. The chemical composition, determined by XRF is shown in Table 3.1, and its empirical formula is determined to be 2.3SiO 2.Al 2O3.

Table 3.1. Composition of raw materials as determined by XRF (mass %).

Sample GFA Geothermal SiO 2 SF Metakaolin Na 2O 0.3 0.5 t 0.1 MgO 1.4 0.1 - 0.4 Al 2O3 28.7 0.3 0.2 40.3 SiO 2 46.8 95.6 93.0 54.5 P2O5 0.5 t 0.3 0.1 SO 3 0.2 t - 0.3 K2O 0.5 0.4 t 2.7 CaO 5.8 0.4 t 0.1 TiO 2 1.4 t t t MnO 0.2 0.1 - t Fe 2O3 11.5 0.3 0.4 0.9 CuO t t - - ZnO t t - - SrO 0.1 t - - ZrO 2 0.1 t 4.2 - LOI 2.4 1.3 1.0 0.6 Total 100 99 99.1 100 t = trace amounts detected

3.1.3 Activating solutions

All activating solutions are prepared with NaOH pellets (Merck, 99.5%) and Milli-Q water in polyethylene vessels. Containers are kept sealed when not in use, to avoid atmospheric carbonation. Alkali silicate solutions are prepared using a standard commercial silicate solution obtained from PQ Australia, class N, with composition:

8.9% Na 2O, 28.7% SiO 2 and 62.4% H 2O. The sodium hydroxide is initially added to the water and agitated until dissolved. This solution is cooled before the alkali silicate solution is added. All alkali silicate solutions are stored for 24 hours prior to use. The speciation in the activating solutions was investigated using ATR-FTIR, the spectra are shown in Figures 3.2, 3.3 and 3.4 below.

41 ~ Chapter 3 ~

PQ silicate solution

4.9

3.5

2.5 1.2 1.0 0.8 Absorbance 0.6 0.4

1200 1100 1000 900 800 Wavenumber (cm -1)

Figure 3.2 ATR-FTIR spectra for activating solutions with low (approximately 3M) [NaOH]. Numbers refer to the silicate concentration in moles per litre.

In Chapter 2, a detailed review of the FTIR literature was conducted, and Chapter 2, Table 2.1 outlines the band positions for the various silicate solution species. This table is used in the following discussion of the different silicate solution species and the FTIR bands thereof. Figure 3.2 shows the FTIR spectrum of the original silicate stock solution . Three overlapping bands are present in the original silicate solution at 1120, 1000 and 880cm -1; these regions are assigned to vibrations of silicate polymers, smaller molecules (monomer to tetramer), and small anionic species respectively [81]. Altering the concentration of silica in the solutions or the concentration of hydroxide changes the speciation in the solutions [77].

42 ~ Chapter 3 ~

In Figure 3.2, the more concentrated silicate solutions, 2.5-4.9M SiO 2, demonstrate similar bands in the FTIR spectra to those of the original PQ silicate solution. However, when the silicate solution concentration is decreased to 1.2M, a distinct change occurs. Bands in the region 1000-1100cm -1 disappear completely and the main bands in the system occur at approximately 930 and 980cm -1. Bands in the former region are attributed to vibrations of the monomer and dimer species, while the latter region is attributed to these and some slightly larger molecules, up to the tetramer [81]. It is clear that at relatively low sodium hydroxide concentrations (around 3M) larger polymeric species dominate at high silicate concentrations, while the smaller species dominate at lower silicate concentrations, consistent with known behaviour observed by NMR and mass spectrometry [77].

4.9

3.5 2.5 1.2 1.0 0.8 0.6 0.4 0.2 Absorbance Absorbance

1200 1100 1000 900 800 Wavenumber (cm -1)

Figure 3.3 ATR-FTIR spectra for activating solutions with intermediate (approximately 6M) [NaOH]. Numbers refer to the silicate concentration in moles per liter.

43 ~ Chapter 3 ~

Similar trends are also observed at intermediate NaOH concentrations (around 6M); Figure 3.3 shows the FTIR spectra for these silicate solutions. The highest silica concentration is the only solution containing a significant contribution from polymeric species (1100cm -1). The remaining solutions demonstrate two main bands at 920 and 975cm -1, indicating the presence of only small species, the largest of which is the tetramer. A similar trend is also observed for the solutions with high NaOH (approximately 9M, Figure 3.4). Two main bands are again observed at 920 and 975cm -1. However, in this case there is a change in the relative intensity of the bands, such that the band at 920cm -1 is of higher intensity, indicating an increase in the amount of monomer and dimer relative to the larger trimer and tetramer species. Furthermore, at the high silicate concentrations, up to 4.9M, there is still a significant contribution from monomer and dimer species when the NaOH concentration is also high.

4.9 3.5 2.5 1.2 1.0 0.8 0.6

0.4 0.2 Absorbance

1200 1100 1000 900 800

Wavenumber (cm -1)

Figure 3.4 ATR-FTIR spectra for activating solutions with high (approximately 9M) [NaOH]. Numbers refer to the silicate concentration in moles per liter.

44 ~ Chapter 3 ~

3.2 Synthesis of Geopolymer Samples

Geopolymer samples are prepared by mixing the solid materials with the activating solution (solution preparation described above). In the case of fly ash geopolymers with secondary solids, the solid materials are dry mixed initially, before the activating solutions are added. A similar technique is also used for the 1-part mix geopolymer, synthesised from geothermal silica and sodium aluminate, whereby the solids were dry mixed before the solution was added. Immediately after the addition of the solution to the solids, the preparations are stirred mechanically for no more than two minutes, before transfer to a sealed polyethylene reaction vessel which is stored at 30ºC. The compositions of samples will be presented at the start of each Chapter. In this section, a general description of mix formulation is given along with reasoning for the sample design.

All fly ash geopolymers are prepared from 60g batches of fly ash. It was calculated that 60g of fly ash contained approximately 0.2 moles of Al in an X-ray amorphous state. This represents the amount of potentially reactive Al in the system. Mix design was based on three different Na/Al ratios. It is know that one alkali metal atom is required to charge balance each 4-coordinated Al 3+ , however not all of the X-ray amorphous Al in the fly ash may be reactive. As such, three different Na/Al ratios were tested (referring only to the amorphous Al): 0.25, 0.5 and 0.75. The NaOH activating solution provided the Na + ions. To each 60g batch of fly ash, 2g of NaOH was added in solution to give Na/Al = 0.25, 4g of NaOH was added to give Na/Al = 0.5 and so on.

The effect of silicate concentration on binder formation was a key element of this work. To ensure that changes in behaviour at different silicate concentrations were observed, eleven different silica concentrations were tested at each Na/Al ratio. The silicate concentrations used in this thesis started from 0.2M, the minimum amount found in previous work to represent the onset of silicate activation [35]. This theory is to be tested here, as the previous work was based on a leaching system with a much higher liquid to solid ratio than that employed in geopolymer synthesis. Silica concentrations used

45 ~ Chapter 3 ~

increase in 0.2M increments up to 1.2M, then three higher levels are tested at 2.5, 3.5 and 4.9M. The silicate concentrations selected represent the range of possible concentrations achievable using the standard commercial sodium silicate solution used in the work with the imposed constraints on NaOH content of the batch and water content. The three NaOH concentrations were approximately 3, 6 and 9M (corresponding to the Na/Al = 0.25, 0.5 and 0.75). However, the exact concentration of NaOH varied slightly between the samples because the water content was calculated as 28wt% of solids and aqueous silica, such that additional water was added to geopolymers with high concentrations of aqueous silica. This was done to allow direct comparison between different geopolymers containing aqueous or solid silicates, as water contents are generally calculated from a water to binder ratio. The content of NaOH will henceforth be referred to by the Na/Al ratio, rather than a concentration, as the Na/Al ratio is constant with silicate concentration.

The effect of alkali activation was also examined, using sodium hydroxide solution activation of fly ash. Several different sodium hydroxide contents are tested giving ratios of Na/Al = 0.25, 0.38, 0.5, 0.63, 0.75 and 1. These samples were compared to the other geopolymers to determine the minimum amount of silica required for silicate activation.

To assist with band assignments in the FTIR spectra, one alkali activated sample was produced with Na/Al = 0.5 using NaOD solution (40wt % NaOD in D 2O, 99.5 atom % D, from Aldrich) diluted with D 2O (99.9% D, from Cambridge Isotope Laboratories). The heavier isotope causes a significant shift of all hydroxyl related bands, meaning that the bands relating to Al(OH) 4 and Si-OH groups can be isolated from the other vibrations and identified. This is important as some bands for these terminal groups overlap with other network groups; using the NaOD solution, the contributions from these groups can be differentiated to some extent.

Secondary solids were used to adjust the Si/Al ratio in the mix with the view of achieving silicate activation with a solid silica source rather than a solution source. To enable direct comparison of the results for the solid silicate geopolymers and the solution silicate

46 ~ Chapter 3 ~

activated geopolymers, the quantity of solid silica added is calculated on the basis of an effective solution concentration. The mass of solids added to the fly ash contains the quantity of silica which would be added to the solution to give the silica concentration quoted. If the solid is pure silica then the mass of solid is easy to calculate, but if the solid is not pure silica, then the quantity of solid added will be greater to achieve the same effective silicate concentration.

3.3 Characterisation Techniques

3.3.1 X-ray Diffraction

X-ray diffraction is a technique allowing the identification of crystalline phases in a solid material. The technique relies on the constructive interference of X-rays diffracted from planes of atoms within the solid. XRD powder diffractograms of geopolymer specimens are collected on a Philips PW 1800 diffractometer with CuK α radiation generated at 20 mA and 40kV. Samples are ground in air with a mortar and pestle, and then mounted in aluminium sample cups. Step scans are performed from 5-70º 2 θ at 0.02º 2 θ steps, integrated at a rate of 2.0 second per step.

3.3.2 Scanning Electron Microscopy

Scanning electron microscopy (SEM) allows the surface of a sample to be imaged on very short length scales, below 1 µm in some cases. The technique relies on surface bombardment with high energy electrons in an evacuated chamber. Topological information is yielded by the method. The surfaces of the solid being analysed must be conductive, and are thus coated with monolayers of a conductive substance such as gold or carbon. SEM is performed in the present thesis using a Philips XL30 Field Emission Gun Scanning Electron Microscope (FEG-SEM). Fractured specimens are mounted on stubs using adhesive carbon pads and gold coated prior to analysis. Unfortunately, this method dehydrates the samples, which causes precipitation of pore solutions and can affect the structure. This must be taken into account when analysing the SEM micrographs.

47 ~ Chapter 3 ~

3.3.3 Attenuated Total Reflectance - Fourier Transform Infrared Spectroscopy

FTIR is an analytical technique which exploits both classical and quantum vibrational theory, allowing the identification of different chemical bonds in a molecule from the absorption of infrared radiation at various wavelengths [99]. Materials are characterised by the normal modes of vibration of the bonds which result in a change in dipole moment of the molecule. Vibrations can involve stretching, which is the change in one of the interatomic distances, as well as bending and other deformations [100].

The FTIR spectrum of a molecule or network can be quite complex, as some bands are due to fundamental vibrations, while others are due to combination or overtone vibrations [100]. As such, analysis commonly utilises empirical correlations from the literature, associating the chemical structure of compounds with certain spectral features. The FTIR technique has been used for over 70 years and is therefore very well established. A review of the FTIR literature relating to silicates, aluminates and aluminosilicates was presented in Chapter 2. Here, the fundamental theory of the ATR technique is discussed and the experimental apparatus and procedures for data collection are outlined.

The ATR technique is quite different to conventional transmission infrared spectroscopy as it allows absorption spectra to be calculated for opaque samples, without dispersing in a transmitting matrix such as KBr. In traditional transmission FTIR, a very small amount (0.5mg) of the sample is milled with approximately 5mg of KBr and pressed into a disk. Fly ash is highly heterogeneous, so a larger sample volume is desirable. The milling process can also change the structure of a material and affect results [101]. Furthermore, the pore solutions of geopolymers contain dissolved species, particularly at short reaction times before the geopolymer gel has formed. This is an issue because KBr dissolves in silicate solutions [81]. This can be overcome by drying the samples, however, drying causes precipitation and thus a change in structure, the nature of which will depend on the progress of the reaction. Thus, to obtain a series of spectra giving an accurate

48 ~ Chapter 3 ~

representation of the chemical structure of a geopolymer at different reaction stages, another technique is required.

ATR-FTIR allows the calculation of transmission spectra without destructive sample preparation. The method allows solids, gels and solutions of the geopolymer mixture to be analysed simultaneously. Spectra are obtained from the absorptions of an evanescent wave which is transmitted through an internal reflection element (IRE) of high refractive index and penetrates a short distance into the sample, in contact with the IRE [82]. The IRE used is diamond, selected because of its resistance to high pH and abrasion from sample removal and cleaning.

For ex-situ experiments, FTIR spectra of the samples are collected using a Varian FTS 7000 FT-IR spectrometer, with a Specac MKII Golden Gate single reflectance diamond ATR attachment with KRS-5 optics and heater top plate. Absorbance spectra are collected from 4000-400cm -1 at a resolution of 2cm -1 and a scanning speed of 5kHz with 64 scans. Geopolymer samples are removed from sealed containers and a freshly fractured surface is immediately mounted onto the ATR crystal and clamped to obtain good contact. A picture of the ATR attachment along with a schematic diagram of the beam path through the apparatus is shown in Figure 3.5. This procedure was designed to minimise atmospheric exposure which can cause changes through carbonation and evaporation of water from pore solutions or partially reacted samples.

The same parameters are used for the data collection in the in situ experiments, except that in this case, 32 scans are collected and the ATR top plate is maintained at 30ºC. A schematic diagram of the in situ ATR-FTIR setup is shown in Figure 3.6. A simple polyethylene reaction cell, with Teflon seal, is clamped onto the diamond ATR crystal. Masking tape was also used to minimise any water loss. The reaction cell is filled with geopolymer slurry, prepared according to the procedure in section 3.2 and scans are started within 2 minutes.

49 ~ Chapter 3 ~

A B

All spectra are processed using Varian Resolutions Pro software, version 4.0.5.009. Baselines are corrected and the spectrum of water is subtracted, such that the intensity contributions at 3240 and 1635cm -1 are minimised. This is done to remove the strong spectral contribution of the water, which overlaps with bands of interest at wavenumbers below 900cm -1. In addition, the geopolymer spectra can then be compared to the sample made with NaOD after similar spectral subtraction of D 2O.

Co ver

Sample Heated top plate

Diamond Incident radiation Reflected radiation

Figure 3.6 Schematic diagram of in situ ATR-FTIR experimental setup (diagram not to scale).

50 ~ Chapter 3 ~

3.4 Conclusions

This section has described the material selection and experimental techniques applied in the current work. Several analytical techniques have been proposed to analyse the samples. ATR-FTIR has been selected as the main analytical technique as it can be used to analyse samples at any time after mixing. A novel in situ technique has also been described which will allow kinetics of different systems to be directly compared. The sample set has been designed to cover a wide range of silicate and sodium hydroxide concentrations, to investigate the different system behaviours brought about by changes in concentration.

52 ~ Chapter 4 ~

Chapter 4 FTIR Analysis of Geopolymer Gel Ageing

4.1 Introduction

The first step in geopolymer formation is the liberation of aluminosilicate species from a solid, typically resulting from alkali hydroxide attack on an aluminosilicate particle. Initially, the surface of the solid particle contacts the activating solution and hydrolysis reactions begin to occur, depolymerising the particle and liberating the network species into the solution. The presence of soluble silicates has been found to significantly affect this process of alkali attack and accelerate the dissolution of the aluminosilicate glass in fly ash [35]. Changes in the rate of solid dissolution will in turn alter the rate of geopolymer formation. Furthermore, the presence of hydrolysed silicate in the early stages of the reaction affects the subsequent formation of the aluminosilicate geopolymer gel. Much research has been carried out on the effects of activating solution silicate concentration on geopolymer compressive strength and microstructure [33, 34, 36, 103- 106]. However, conflicting information still exists in the literature about the effect of soluble silicates in the activating solutions used for geopolymer formation. The presence of silicates in activating solutions has been reported to have both an accelerating and a retarding effect on geopolymer binder formation and strength development [36, 106, 107].

The use of silicates in the activating solutions for geopolymer formation can potentially enable more rapid setting in slowly reacting systems, such as those based on low reactivity class F fly ash. As such, an understanding of the effect of different silicate activating solution concentrations is essential to allow tailoring of geopolymer properties to specific applications.

53 ~ Chapter 4 ~

The present Chapter presents a comprehensive study on the effect of different concentrations of both soluble silicate and alkali on the synthesis chemistry of fly ash geopolymers at ambient temperatures. This provides insight into the role of soluble silicates and silicate speciation in the formation and ageing of fly ash geopolymers. An in depth understanding of this chemistry would allow greater control over geopolymer properties through mix design, potentially increasing the commercial viability of this material.

4.2 Materials and Experimental Methods

GFA was used as the primary binder material to make geopolymers as described in Section 3.2. The compositions of all prepared samples are shown in Table 4.1. Sodium silicate and alkali solutions prepared 24 hours in advance as described in Section 3.1.3.

The water to (GFA+SiO 2) mass ratio used was 0.28 throughout. ATR-FTIR spectroscopy was performed on all samples at various time intervals over 200 days as described in Section 3.3.3.

Table 4.1. Composition of samples

[SiO 2] M 0 0.2 0.4 0.6 0.8 1.0 1.2 2.5 3.5 4.9 Na/Al 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 Na/Al 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Na/Al 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

H2O (g)* 16.8 16.9 16.9 17.0 17.1 17.2 17.2 17.7 18.1 18.6 * Total water in activating solution

During the experiment, two samples, [SiO 2] = 0 and 2.5 both with Na/Al=0.5, were removed from the oven at various time intervals (before the paste had hardened) and the solid phase was separated from the solution via centrifugation. The solids were washed with distilled water and centrifuged 4 times, at which point the residual water registered pH=7. The solids were then dried at 60ºC on the ATR crystal and analysed using the same ATR-FTIR method described earlier (Section 3.3.3).

54 ~ Chapter 4 ~

To test for the formation of geopolymers, a 3-5g sample of the reacted solid was placed in 10mL of MilliQ water and left for 12 hours at ambient temperature. The vessels were vibrated for 30 seconds and changes to the solid and solution were observed.

X-ray diffraction (XRD) was used to identify the crystalline phases present in the raw materials and reacted geopolymers after 100 days reacting at 30ºC (Section 3.3.1). Scanning electron microscopy was also performed after 100 days (Section 3.3.2)

4.3 Results and Discussion

4.3.1 FTIR Spectra of Fly Ash Geopolymers

An example of the typical FTIR spectra observed during geopolymer formation over 200 days is shown in Figure 4.1A. The FTIR spectrum of GFA is also shown for comparison. The spectrum of GFA is broad and relatively featureless, owing to the predominantly glassy nature and heterogeneity of this material; with a variety of different bonding environments for the main elemental constituents [61]. The many crystalline inclusions, identified in Chapter 3, also have different peaks in the FTIR spectrum which fall under the broad hump. As such, it is difficult to identify the position of the infrared bands in the spectra of fly ash by conventional methods such as deconvolution. In order to include the FTIR spectral vibrations of all the crystalline and amorphous phases present in the fly ash and the newly formed products, the number of component bands becomes quite large.

Taking a relatively straightforward approach to this issue, the identification of the position of the main Si-O-T asymmetric stretching vibration (the strongest band) was estimated to be represented by the point of maximum absorbance in the region 1200- 900cm -1 [65]. This “peak” will henceforth be referred to as the “main band” in the spectra of fly ash and the synthesised geopolymers. The main band in the FTIR spectra of GFA is centred at approximately 1055cm -1.

55 ~ Chapter 4 ~

The spectrum of GFA closely resembles the spectrum of the geopolymers shortly after mixing (see Chapter 5). This is due to the relatively small spectral contribution from the activating solution which contains only sodium hydroxide and water. In the spectra shown in Figure 4.1, the contribution of water has been spectrally subtracted. As the paste reacts, changes in the FTIR spectra are visible due to the destruction of certain phases and the development of new ones with different structure and bonding. From the time of mixing up to 2 days, an increase in intensity in the region 850-1000cm -1 is evident in the FTIR spectra of the reacting mixture. This region corresponds to dissolved silicate species (demonstrated in Chapter 3), which may also contain aluminium, indicating that there is dissolution taking place. This was further confirmed by the centrifugation experiments, as will be discussed in detail below.

The centrifugation experiments involved the removal of the partially reacted solids from the reaction mixture. After one day reacting at 30ºC, the geopolymer paste was mixed with water and centrifuged to remove water soluble gel and solution species from the paste. Both the separated solids and the original spectra of the unreacted GFA are shown in Figure 4.1B. There has been little appreciable change in the FTIR spectra of the solid fly ash in the geopolymer mixture after one day. Only a slight shift and intensity reduction of the main band is evident. Thus the contribution to the intensity growth in the region 850-1000cm -1, observed in Figure 4.1A, is due to the liberation of solution species or formation of a new phase easily removed from the unreacted fly ash particles by water. The slight band shift observed in the fly ash is most likely due to the presence of non- bridging oxygens (NBOs), created when silicate species are liberated from the solid network [63, 72]. However, while this shift is anticipated, the change is too small to rule out experimental error.

56 ~ Chapter 4 ~

A B 2 1 200 100

44 29 15 Unreacted GFA 9 5 Absorbance Absorbance 2 Unreacted GFA 1

1200 1000 800 600 1200 1000 800 600 Wavenumber (cm -1) Wavenumber (cm -1)

Figure 4.1. FTIR spectra of geopolymer development for a sample with Na/Al = 0.5 and

[SiO 2] = 0 A. Spectra of unreacted fly ash (bottom) and geopolymers after different reaction times, B. Enlarged spectra of separated solids after 1 and 2 days of reacting; bottom spectrum is unreacted fly ash. The line at approximately 1055cm -1 shows the position of the broad peak of the unreacted fly ash which appears in the reacted geopolymer spectra. Numbers refer to the age of the sample in days.

Between two and five days, there is a significant change in the FTIR spectra of the reacting geopolymer paste (Figure 4.1A). A new band with a significantly narrower peak becomes visible at 960cm -1. Over time, the position of this band shifts and increases in intensity. There are two identified factors contributing to this intensity increase. The depth of sample penetration and therefore the absorbance of infrared radiation are proportional to the wavelength when using an ATR attachment on an FTIR instrument [108]. Thus, greater absorption occurs when a peak is shifted to lower wavenumbers. However, after 9 days (Figure 4.1A), the band position remains stable yet the intensity increases further, indicating that this relationship between band position and intensity cannon be solely responsible for the intensity changes. Alternatively, this intensity increase could be due to a greater amount of the particular functional group which absorbs infrared radiation at that frequency.

57 ~ Chapter 4 ~

Another important feature of Figure 4.1A is the presence of a number of peaks in the region 630-760cm -1. This region corresponds to the secondary building units in a material such as ring structures [109]. The sharp nature of the peaks in this region indicates the presence of crystalline phases [110]. The aforementioned peaks first became visible at 9 days and persisted beyond 200 days. The position of these bands indicates the presence of faujasite type zeolite, found previously to exhibit three overlapping bands of similar intensity at 745, 655 and 555cm -1 [111]. The timescale of formation observed in the present study also corresponds well with the work of Valtchev and Bozhilov who found that faujasite type zeolite begins to form after 11 days in mixed solution of sodium silicate and aluminate (composition 4Na 2O : 0.2Al 2O3 : 1.0SiO 2 : 200H 2O) reacted at 25ºC [111]. The present reaction conditions employ a slightly higher temperature, 30ºC, (although much less water) therefore it would be expected that the reactions would proceed at a slightly faster rate. Further identification of the type of crystalline phases suggested from FTIR spectra was performed using X-ray diffraction; this will be further discussed in the present chapter.

The FTIR spectra of silicate activated geopolymers appear quite different in the early stages to those without added soluble silicate. Silicate solution activated geopolymers contain a main band of high intensity in the early stages of reaction due to the spectral contribution of the silicate species in the added solution (Figure 4.2). Only small changes in the FTIR spectra are visible over the 200 days; the primary spectral changes are complete after just one day. It should also be noted that the position of the main band after 200 days is also at slightly higher wavenumbers than the sample activated without soluble silicates.

58 ~ Chapter 4 ~

200

100 44 29 15 9 Absorbance 5 2 1

1200 1000 800 600 Wavenumber (cm -1)

Figure 4.2. FTIR spectra of geopolymer development for a sample with Na/Al = 0.5 and

[SiO 2] = 2.5M. Numbers refer to the age of the samples. The vertical line shows the position of the main Si-O-T stretching band for the geopolymer network.

The geopolymer structure after 200 days also shows significant differences from that of the hydroxide activated geopolymer. The region relating to secondary building units and ring structures contains smooth broad overlapped bands in the present sample, indicating that the structure is predominantly amorphous [61]. The FTIR spectra of the reacting geopolymer paste furnish much information about the structure and composition of the gel.

4.3.2 Structural Changes in Geopolymer Gel over Long Timescales – Low NaOH

The position of the main Si-O-T stretching band gives an indication of the length and angle of the Si-O bonds in the silicate network. For amorphous silica, this peak occurs at approximately 1100cm -1 [112]. A shift of the main Si-O-T asymmetric stretching band to lower wavenumbers indicates a lengthening of the Si-O-T bond, a reduction in bond angle and therefore a decease of the molecular vibrational force constant [38]. As discussed at length in Chapter 2, this can be caused by many factors in a silicate network, including an increase in the proportion of silicon sites with non-bridging oxygen (NBO)

59 ~ Chapter 4 ~ charge stabilised by sodium cations [72] or an increase in the substitution of tetrahedral Al in the silicate network [40]. The position of the main band can be used to monitor change in geopolymer gels over time. The effect of varying activating solution silicate concentration (at fixed alkali content) on the position of the main band in geopolymers over 200 days is shown in Figure 4.3.

1020 A 1010 0 0.2 1000 990 980 970 960 950 0 20 40 60 80 100 120 140 160 180 200 B ) ) 1020 1 - 1010 1000 990 0.4 980 0.6 0.8 970 1 960 1.2

Wavenumber (cm Wavenumber 950 0 20 40 60 80 100 120 140 160 180 200 C 1020 1010 1000 990 980 2.5 970 3.5 960 4.9 950 0 20 40 60 80 100 120 140 160 180 200

Time (days)

60 ~ Chapter 4 ~

Figure 4.3A demonstrates the shift of the main band for samples with the lowest silicate content, 0 and 0.2M SiO 2. The position of the main band remained unchanged for the first 2 days, and then shifted to lower wavenumbers between days 2 and 15. After 15 days a -1 plateau occurred at approximately 965cm . The sample with 0.4M SiO 2 demonstrated a similar trend in the initial stages. However, the minimum wavenumber was 990cm -1, reached after 35 days and then increasing slightly before a plateau at approximately 995cm -1. Conversely, samples with higher silicate content and higher water content did not exhibit any significant peak shift. The sample with 3.5M SiO 2 is an exception to this and it is unknown why this is the only high silica sample which demonstrated a significant peak shift.

Pastes from the samples with the lowest silicate content (therefore lowest water content)

0 and 0.2M SiO 2 hardened after curing at 30ºC. This was not the case for all geopolymers. The sample with 4.9M SiO 2 was malleable and did not form a solid product over 200 days. The remaining samples with Na/Al=0.25 demonstrated a weak solid product. This finding comes back to the much debated question “what is a geopolymer?” To resolve this problem, a more practical working definition of a geopolymer has been adopted. For the purposes of this work, a geopolymer is defined as a solid material formed from the mixing of an alkali source, silica source and aluminium source in water at high pH which transforms into a final hardened solid, capable of maintaining rigidity in water. Essentially, enough of the aluminosilicate product (network of Al-O-Si tetrahedral bonds, Al 3+ charge balanced by an alkali cation) must form to hold the unreacted particles together, even in water with physical agitation. This differs from the working definitions used by other authors, which include parameters such as ambient temperature strength development, lack of long range order, high temperature thermal stability and a structure comprised of Al-O and Si-O tetrahedral units only [104, 113]. The problem is that some samples which fit these specifications are very low in Al and actually break down in water, and would therefore not be useful cementitious materials.

When exposed to water for 12 hours and then brief physical agitation, samples with 0,

0.2, 0.4, 2.5 and 3.5M SiO 2 showed no change in the turbidity of the water or shape of the

61 ~ Chapter 4 ~

solid. However, samples with 0.6-1.2 and 4.9M SiO 2 changed shape in the water and with a strong visible increase in turbidity. A significant amount of the fly ash was not bound to the solid, and was released into the water. The samples which formed geopolymers demonstrated a final peak position below 995cm -1, while the samples which did not form a hardened product capable of withstanding water demonstrated final band positions greater than 995cm -1. One drawback of the macroscopic approach to peak position is that when the sample is poorly reacted, the original fly ash peak dominates the spectra and will therefore skew the overall position of the main band. Using spectral subtraction of the original fly ash is one way to overcome this problem giving a more clear separation between the samples.

Competing mechanisms are responsible for the varied behaviour in peak shift at different silicate concentrations in the low alkali system (Na/Al = 0.25). There are two activators which assist in breaking down the original fly ash network, dissolved silicate and dissolved sodium hydroxide. Since the water content was calculated on the total mass of silica in the system, there is an increase in the water content with increasing silicate content and therefore a reduction in the concentration of sodium hydroxide. Furthermore, a reduction in the concentration of hydroxide ions in the solution occurs with increased silicate concentration, as silica is an acid (Equation 4.1). The hydroxide ion concentration is critical in the dissolution process.

OH - OH - - 2- 3- 4.1 SiO 2 + 2H 2O ↔ Si(OH) 4 ↔ Si(OH) 3O ↔ Si(OH) 2O2 ↔ Si(OH)O 3

The exact form of the silicate monomer in the solution is dependent on the concentration of sodium hydroxide in the solution. As such, for every mole of silica there is the potential for three less moles of free hydroxide to participate in the dissolution process (depending on pH). At these low sodium hydroxide concentrations (Na/Al = 0.25) the effect of decreasing pH outweighs the benefits of the catalytic effect of the silicate on the dissolution process at some concentrations. Thus geopolymers are not formed in most cases with medium and high silicate concentrations and low Na/Al ratios.

62 ~ Chapter 4 ~

Another reason why increases in silicate concentration may not lead to faster GFA dissolution and geopolymer formation is the absence of monomeric species in the solution. Increased silicate concentration will only be effective catalysing GFA network hydrolysis if there are a large number of silanol groups, found in silicate monomer and small species. In the case of the 4.9M silicate solution, there will be very little monomer present due to the high SiO 2/Na 2O mole ratio and a significant amount of the silicate will be Q 3 and Q 4, and it is likely that colloids will be present [81, 114].

Silicate colloids are inhibitory to the dissolution process as the surface adsorbs layers of Na + ions, which act to stabilise the colloid in solution [115]. In a system where there is an excess of NaOH, this will assist in the dissolution reactions by buffering the amount of Na + able to stabilise the corroding glassy GFA surface. Layers of Na + stabilise the surface and hinder the corrosion process [115]. However, in the current system, there is a deficiency of NaOH and therefore of Na +; these species are required to facilitate reactions and stabilise the Al 3+ in IV-fold coordination. It is possible that the colloids are competing with other reactions for the available Na+, therefore hampering the dissolution and geopolymer formation. Furthermore, the reduction in sodium hydroxide concentration with increasing silica further inhibits the dissolution process. Without significant depolymerisation of the fly ash network, there is insufficient reactive aluminosilicate material to form geopolymer gel.

Although silicate can act to reduce the dissolution and gel formation by sequestering sodium and hydroxide ions, the presence of silicate at low alkali can still be beneficial. Activating solutions high in soluble silicate are preloaded with solution species such that less GFA dissolution is required before the solution becomes saturated. The many different actions of silicates in leaching and gel forming systems are most likely responsible for the wide variety of results reported in the geopolymer literature. Many factors other than silicate concentration must be assessed in order to understand the effect of different silicate concentrations on geopolymer formation.

63 ~ Chapter 4 ~

The geopolymers with 0-0.4M SiO 2 and Na/Al = 0.25 still contained a high enough sodium hydroxide concentration such that even in the absence of silicate, there was ample GFA degradation and supply of reactive species to form the geopolymer gel. Samples with 2.5-3.5M SiO 2 also formed geopolymers. The silicate monomer concentration in these samples was high enough to overcome the low sodium hydroxide concentration. Sufficient monomer was present to enhance the dissolution without sequestering excessive amounts of additional vital sodium and hydroxide ions, which are essential to the reaction.

Samples with 0.6-1.2M SiO 2 and Na/Al = 0.25 did not form geopolymers at 30ºC. There was a slightly higher sodium hydroxide concentration in the medium silicate concentrations than the samples with 3.5M SiO 2. However, the slight increase in hydroxide concentration was not enough to offset the effect of lower silicate monomer concentration in these samples or the reduction in pH. There appears to be two factors which allow geopolymer formation; the concentration of the sodium hydroxide activator must be above 2.8M or there must be sufficient active silicate in the system, not in excess, maintaining an adequate concentration of free hydroxide in the solution.

In geopolymers with Na/Al = 0.25 the sodium hydroxide concentration was extremely low. Given that geopolymers did not form in most cases, a comparison of the effect of varied silicate concentration on the rates of gel ageing at this Na/Al ratio was unable to be made.

4.3.3 Structural Changes in Geopolymer Gel over Long Timescales – Increased NaOH

By increasing the sodium hydroxide concentration in the activating solution, geopolymers form and the effect of varied silicate activating solution concentration can be investigated. Band shifts in the FTIR spectra of samples with Na/Al = 0.5 and silicate concentrations varying from 0 to 4.9M are shown in Figure 4.4. The samples have been grouped according to the time at which the lowest wavenumber was recorded for the

64 ~ Chapter 4 ~ main band; this appears to be related to the silicate concentration in the activating solution. Three regions of silicate activating solution concentrations have been identified, low, medium and high silicate. Geopolymer samples in each category demonstrate similar peak shift behaviour; the three groups are outlined in Figure 4.4.

A 1010 0 1000 0.2 990 0.4 980 0.6 970 0.8 960 950 0 20 40 60 80 100 120 140 160 180 200

) ) B 1 1010 - 1000 1 990 1.2 980 970 960 950 Wavenumber (cm Wavenumber 0 20 40 60 80 100 120 140 160 180 200

1010 C 2.5 1000 3.5 990 4.9 980 970 960 950 0 20 40 60 80 100 120 140 160 180 200

Time (days)

Silicate concentrations [SiO 2] = 0-0.8M have been defined as low silicate for Na/Al = 0.5. Initially, there is a shift of the main band to lower wavenumbers. A minimum value

65 ~ Chapter 4 ~ of approximately 952cm -1 occurs after 15 days, followed by an increase in wavenumber until a final value of 958cm -1 is reached. The rate of peak shift in the early stages, up to five days, for samples with Na/Al = 0.5 is much greater than that of the previous samples with the same silicate content and Na/Al = 0.25. A significant peak shift in the first 48 hours is observed for samples with Na/Al = 0.5. This rate increase is not surprising, since these samples have double the amount of sodium hydroxide activator compared to the samples discussed in Section 4.3.2, allowing for more rapid GFA dissolution.

Intermediate silicate activation was defined as samples with [SiO 2] = 1.0-1.2M. These geopolymers demonstrated a similar peak shift profile to the low silicate geopolymers; however the minimum wavenumber was reached after 9 days. This band shift, initially to lower and then back to higher wavenumbers, did not occur for samples with a high concentration of silicate in the activating solution (2.5M or above). Samples containing concentrations of [SiO 2] = 2.5-4.9M demonstrated no peak shift over the timeframe investigated, with the final position of the main band reached within 24 hours of mixing. The rate of peak shift gives an indication of the rate of reaction, defined broadly as the time taken for the geopolymer to reach its pseudo-equilibrium structure. Structural changes were indicated by the FTIR spectra, which remained unchanged between 50 and 200 days.

In the present study, the rate of reaction was found to increase with increasing silicate activating solution concentration. This finding contrasts that of Kovalchuk and co- workers who used XRD to investigate geopolymer structure and found that increased silicate concentrations retarded the reaction kinetics [107]. This was concluded from the formation of less hydroxysodalite in the higher silicate geopolymer compared to the lower silicate geopolymer, which was believed to indicate slower zeolite formation. The present work has found that the formation of hydroxysodalite or detectable quantities of other zeolites does not occur in high silicate systems, even after 6 months of curing at 30ºC (see section 4.3.5). The finding of Kovalchuk and co-workers may be indicative of different mechanisms of reaction or gel formation, rather than a reduced rate of the same reactions occurring in other systems. This will be further discussed in Chapter 7. In

66 ~ Chapter 4 ~ addition, the varied results may be a product of the different reactivities of the ashes studied.

The final position of the main band for samples with Na/Al = 0.5 was dependent on the silicate concentration in the initial activating solution. The geopolymer with the highest wavenumber after 200 days was the sample with the greatest amount of silicate in the activating solution. Conversely, the samples grouped with the least amount of silica in the activating solution have bands at the lowest wavenumber after 200 days. The final peak position is dependent on many factors affecting the bond length and angle of the Si-O bonds in the tetrahedral network. The greatest of these factors is thought to be the overall amount of Al substituted into the network. The addition of secondary silicate, from the activating solution, lowers the overall Al/Si ratio in the system. However, this does not prove that the Al/Si ratio in the gel has changed. Determination of gel composition could potentially be made using SEM/EDX, however, this was not undertaken in the present study as many of the geopolymers were too soft to undergo polishing for sample preparation.

To further investigate the system, the same silicate activating solution concentrations were employed at yet higher alkalinity, with Na/Al = 0.75. Unlike the lower alkali systems, a similar peak shift trend is observed for all samples, including the geopolymers with highly concentrated silicate activating solutions. This result is in good agreement with the work of Lee and van Deventer which first demonstrated that this trend for geopolymers with 2.5M SiO 2 only occurs at high-alkali content [36]. In the present high alkali sample set, the main band in all samples shifted to lower wavenumbers and then back to higher wavenumbers before a reaching a final steady value.

The rate of band shift for geopolymers with Na/Al = 0.75 was again affected by the silicate activating solution concentration (Figure 4.5). Samples containing very low silicate concentrations, [SiO 2] = 0-0.2, demonstrated the slowest band shift out of all geopolymers with Na/Al = 0.75. For other samples, with higher silicate activating solution concentrations, the main band position reaches a minimum after 2 days.

67 ~ Chapter 4 ~

A 980 970

960 0 950 0.2 0.4 940 0 20 40 60 80 100 120 140 160 180 200

) 1 - 980 B 0.6 970 0.8 1 960 1.2 950 940 0 20 40 60 80 100 120 140 160 180 200 Wavenumber (cm Wavenumber 980 C 970 960 2.5 950 3.5 4.9 940 0 20 40 60 80 100 120 140 160 180 200

Time (days)

The peak shift to lower wavenumbers and then back to higher wavenumbers indicates a change in the length and angle of the Si-O-T bonds in the system over time. When sodium hydroxide alone is used to activate class F fly ash, an aluminium rich gel is formed in the first instance, followed by a silica rich gel in the later stages [60]. In a system activated with a high concentration of soluble silicate, much more aluminium would need to be liberated before aluminium rich gel can form. Therefore samples high in soluble silicate (Na/Al = 0.5) did not demonstrate a peak shift back to higher wavenumbers; the gel composition remained much more stable than geopolymers activated with low or no soluble silicate. The shift back to higher wavenumbers in the

68 ~ Chapter 4 ~ high silicate samples with Na/Al = 0.75 indicates that significantly more Al was liberated from the GFA into the gel within the first 2 days, relative to geopolymers with equivalent silica and Na/Al = 0.5.

The minimum wavenumber observed for the main band is the point of maximum NBO and/or Al substitution in the silicate network of the geopolymer gel. With Na/Al = 0.5, the minimum wavenumber of the geopolymers increased with increasing silicate concentration (Figure 4.4), indicating that there was a lower substitution of Al in the geopolymer network. Additional silicate in the activating solution was not participating in liberating more Al rich species from the fly ash in the early stages. Thus a more siliceous gel was formed, indicated by the higher wavenumbers for the main band. However, when 50% more sodium hydroxide was used (Na/Al = 0.75), the minimum wavenumber of the main band decreased with increasing silicate concentration until the lowest value was reached at 947cm -1 (Figure 4.5). The silicate activating solutions with Na/Al = 0.75 had significantly more monomer than the equivalent silicate concentration with Na/Al = 0.5 [114]. This further supports the suggestion that the higher silicate monomer concentrations assists in the liberation of additional Al from the fly ash, increasing the Al/Si ratio in the early geopolymer gel.

The final band position for a geopolymer furnishes information about the Si/Al ratio in the hardened binder, and for all geopolymers this was in the range 950-995cm -1. The final band position for all geopolymers with Na/Al = 0.5-0.75, compared in Figure 4.6, were dependent on both Na/Al ratio and activating solution silicate concentrations. For geopolymers activated with [SiO 2] = 0-0.6M, the final band position of samples with Na/Al = 0.5 was slightly below that for geopolymers with equivalent silicate and Na/Al =

0.75. However, samples with [SiO 2] = 0.8-4.9M, the final band position for the geopolymer with Na/Al = 0.75 was at higher wavenumbers than that for samples with Na/Al = 0.5. This trend confirms the theory that band positions are not simply due to the NBO content, as was previously reported [36], since geopolymers with higher sodium hydroxide content did not have consistently lower wavenumbers.

69 ~ Chapter 4 ~

970

Na/Al = 0.75 968 Na/Al = 0.5 966

964

962

960

958

956

954 Final bandFinal position 100. after days

952

950 0 0.2 0.4 0.6 0.8 1 1.2 2.5 3.5 4.9 Activating solution silicate concentration (M)

Figure 4.6 Variation in the final position of the main Si-O-T asymmetric stretching band in the FTIR spectra of geopolymer samples.

The difference between the final band positions for the two samples sets, with [SiO 2] ≥ 0.8M (Na/Al=0.5 and 0.75), decreased with increasing silicate concentration. As such, both geopolymers with activating solution concentrations with 3.5-4.9M SiO 2 were of similar values after 100 days, differing by just 2cm -1. The intermediate silicate concentration, [SiO 2] = 0.8-1.2, demonstrated the greatest deviation; the final wavenumber for samples with Na/Al = 0.75 was 9cm -1 lower than geopolymers activated with the same silicate concentration and Na/Al = 0.5.

There are competing affects altering the position of the main Si-O-T asymmetric stretching band, forcing it in opposite directions. One effect involves the increased GFA dissolution experienced when silicate activation is used [35]. This would allow higher early dissolution and therefore more Al to be liberated from the glass network in the initial stages and thus the formation of an Al rich gel. Greater substitution of Al into a

70 ~ Chapter 4 ~ silicate network shifts the main band to lower wavenumbers. However, more silicate in the starting solution means a higher overall silicate content and lower pH in the system. Since the silicate is already in the solution, this is likely to be incorporated into the geopolymer gel. Therefore a relatively low Al substitution occurs and the main band appears at higher wavenumbers. Thus, when an excess of silicate is used above the amount required to liberate the maximum amount of Al at that concentration of sodium hydroxide, the main band is expected to be positioned at higher wavenumbers. This is evident in the trends seen in Figures 4.4 and 4.5.

The varying behaviour of silicate concentrations at different sodium hydroxide contents relates to the silicate speciation in the activating solution (discussed at length in chapter

3). With increasing SiO 2/Na 2O mole ratio, the amount of higher order species increases at the expense of the monomer [114]. Therefore activating solutions with Na/Al = 0.75 can have higher silica concentrations while maintaining high percentages of monomeric species, whereas solutions with very high silica and Na/Al = 0.5 can contain fewer monomer than some lower in silica with equal sodium hydroxide. It is the silicate monomers which assist the dissolution process. The larger, higher order species are bulky and could potentially cause steric hindrance in the reaction mixture or simply remain unreacted. Thus the additional silica in the high silicate samples with Na/Al = 0.5 acted as a matrix filler. On the contrary, activating solutions with high silica concentrations and Na/Al = 0.75 contained a greater quantity of monomeric species, which assisted in breaking down the fly ash network.

71 ~ Chapter 4 ~

1.80 Na/Al = 0.5 . . 1.60 Na/Al = 0.25 Na/Al = 0.75 1.40

1.20

1.00

0.80

0.60

0.40

0.20 Activatingsolution monomer concentration (M) 0.00 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Overall silicate concentration in activating solution (M)

Figure 4.7. Concentration of silicate monomer (Q 0) in the activating solutions used for geopolymer synthesis [114].

A strong correlation exists between monomer concentration and silicate reactivity. Figure 4.7 shows the monomer concentration in the activating solution for a given silica concentration at each Na/Al ratio. By equating the results from the FTIR study with the monomer concentration diagram, a trend can be observed. There appears to be a minimum monomer concentration below which the silicate in the activating solution has little effect on the changes in geopolymer structure, indicated by the FTIR spectra of the reacting samples. Above this value however, the silicate concentration alters the reactions occurring in the geopolymer, indicated by the FTIR spectra. This apparent value for minimum silicate monomer is approximately 0.6M SiO 2, which corresponds to solutions with 0.8M SiO2 for Na/Al =0.5 and 0.4M for Na/Al = 0.75. It can be seen in Figure 4.7, that this is the value for which the monomer concentration in systems with Na/Al = 0.5 and 0.75 systems first diverge. The system higher in NaOH has more monomer for a given silicate concentration.

72 ~ Chapter 4 ~

4.3.4 Effect of Activator Concentration on Microstructure

While the amount of sodium hydroxide and silicate in the activating solutions affects the chemical structure of the geopolymer gel, there is also a large effect on the microstructure. It has been observed previously that increasing the silicate concentration in the activating solution for metakaolin based geopolymers changes the microstructure, reducing the number of large interconnected pores as well as the size and density of the gel particulates [34]. A similar trend has been found for fly ash geopolymers, where low silicate geopolymer microstructures appear highly porous when compared with high silicate samples [116], and this was also confirmed in the present work. Figure 4.8 shows a scanning electron micrograph for a geopolymer with Na/Al = 0.5 containing 0.6M SiO 2 in the activating solution. Many unreacted spherical fly ash particles are visible. The geopolymer gel appears to be coating the unreacted particles and binding them together. The microstructure is coarse and large pores are visible between the gel coated particles. Overall there appears to be quite a number of unreacted particles compared to a relatively small amount of gel.

10 µm 0.5LIQ600

Figure 4.8 SEM micrographs of the geopolymers with Na/Al = 0.5 activated with 0.6M

SiO 2.

73 ~ Chapter 4 ~

By increasing the amount of silica in the activating solution at fixed sodium hydroxide concentration (Na/Al = 0.5) there is a distinct microstructural change. Figure 4.9B shows an electron micrograph of a geopolymer with the same amount of sodium hydroxide but with 3.5M SiO 2 used in the activating solution. The binder coverage is more continuous and appears coarser in texture than the sample with 0.6M SiO 2. Small pores in the binder itself are visible, as opposed to the larger pores in the low silica geopolymer resulting from the voids between the gel covered particles. This was explained by Duxson and co- workers using a gel densification mechanism, in which the varying microstructure is a product of the lability of silicate species in the activating solution and early gel [34].

The gel densification mechanism has been further investigated in the present work by varying the amount of sodium hydroxide used at a fixed silica concentration. Figure 4.9C shows an SEM micrograph of a geopolymer activated with the same silicate as that shown in Figure 4.9B, but with 50% more sodium hydroxide (Na/Al = 0.75). The continuity of the binder has increased dramatically and there are fewer small pores visible in this sample. This is most likely to be due to the increased GFA dissolution experienced at high alkali hydroxide and high silicate monomer concentrations. Relatively few unreacted particles are visible and the sample appears quite well reacted.

In contrast to the increased continuity, apparent skeletal density and reduced voids seen when the sodium hydroxide concentration was increased, lower sodium hydroxide concentrations have the reverse effect. Figure 4.9A shows an SEM micrograph for a geopolymer with Na/Al = 0.25 and 3.5M SiO 2, the gel texture is again different to the other high silicate activated geopolymer, and appears coarsest. The small pores appear larger than in both other high silicate samples. Few unreacted particles are visible, most likely because the highly porous gel was so weak that fracture caused failure within the gel, instead of at the interface with the unreacted particles [24]. The poor dissolution experienced with such a low sodium hydroxide concentration would mean that there is less gel forming aluminosilicate available. Therefore the skeletal density of the gel would be lower, since all samples with the same silicate concentration had the same bulk density.

74 ~ Chapter 4 ~

10 µm

Figure 4.9 SEM micrographs of samples activated with sodium silicate solutions, 3.5M

SiO 2 with overall mix ratio of A. Na/Al = 0.25, B. Na/Al = 0.5, C. Na/Al = 0.75.

75 ~ Chapter 4 ~

4.3.5 Formation of Crystalline Phases in the Geopolymer Gel

Features in the FTIR spectra of geopolymers with no silicate in the activating solutions, shown in Figure 4.1A, indicated that there may be crystalline phases present. The formation of secondary phases in the reacting mixture gives an indication of the amounts of Si, Al and Na in the gel for a given sample. Different zeolites will form at different reactant quantities [117], depending on the local chemistry. However, since fly ash is a heterogeneous material, it is possible that there will be “pockets” rich in certain reactants and other regions which are deficient in those species. Therefore it is not a definitive method for quantifying the overall Si/Al ratio in the gel.

The XRD diffractograms of all samples and the GFA raw material are shown in Figure 4.10. The main crystalline constituents of GFA are quartz and mullite. These phases have been found in the past to be unreactive in the geopolymer system [36, 118]. This is in good agreement with the present study where no change in the intensity of the quartz and mullite peaks was observed after geopolymer formation. It is generally the glassy material in the fly ash which reacts.

76 ~ Chapter 4 ~

Q M F F F M F Q F F F 0.0 0.2

0.4 0.6 0.8

1.0 1.2 2.5

3.5 4.9 Unreacted GFA ash

5 10 15 20 25 30 35 40 45 50 º2 θ

While the geopolymer gel is X-ray amorphous, there can be new crystalline phases formed within the detection limits for conventional XRD. Crystalline phases were found in the geopolymers activated with low or no soluble silicate. For the geopolymers with Na/Al = 0.5, the only crystalline phase identified from the XRD (Figure 4.10) was a

Faujasite type zeolite (thought to be Na 2Al 2Si 2.4 O8.8 .6.7H 2O, PDF#00-012-0246). The amount of zeolite formed was more than 3% by weight (the detection limit of this technique [111]). The quantity of zeolite formed, estimated from the relative intensity of the zeolite bands in the diffractograms, was roughly inversely proportional to the silicate activating solution concentration. The relatively broad nature of the peaks indicates that the zeolite particles are quite small, which has been found previously for low temperature zeolite synthesis [8, 111]. The absence of peaks in the samples activated with 2.5M SiO 2

77 ~ Chapter 4 ~ or higher can mean two things; there are less than 3% by weight of the crystals present, or that crystal size is smaller than 8nm [119].

Q M

Q S F F S F S M F F 0.0 F F 0.2 0.4 0.6 0.8 1.0 1.2 2.5 3.5 4.9 Unreacted GFA

5 10 15 20 25 30 35 40 45 50

º2 θ

Figure 4.11 Geopolymers with Na/Al = 0.75 and various concentrations of silicate in the activating solutions (values shown on graph are molar [SiO 2]) F = faujasite, S = hydroxysodalite.

The formation of hydroxysodalite (general formula Na6+x [SiAlO 4]6(OH)x.nH 2O [120]), was detected in geopolymers with Na/Al = 0.75 at very low silicate concentrations, 0- 0.6M (Figure 4.11). Hydroxysodalite formation is favoured by high sodium hydroxide and Al concentrations in the reacting medium [121]. In zeolite synthesis from gel systems, the composition of the secondary gel from which zeolites crystallise is similar to the zeolite itself [122]. Thus, it is expected that the reacting gel in this case would have a relatively low Si/Al ratio, at least locally where the zeolite was forming (hydoxysodalite Si/Al = 1). As the silicate activating solution concentration increased, the Si/Al ratio in

78 ~ Chapter 4 ~ the newly formed gel was too high. The formation of zeolites from sources with a similar Si/Al ratio to the final crystalline structure has a lower activation energy for the transformation [123].

The moderate silicate samples (0.8-1.2M) with Na/Al = 0.75 formed faujasite zeolites (Figure 4.11), similar to the geopolymers with Na/Al = 0.5. The only difference here between the different sample sets was that the 2.5M silicate sample with Na/Al = 0.75 showed very small peaks in the XRD which were not present in the equivalent silicate content at lower [NaOH]. It is thought that the additional aluminium liberated by the higher alkali content allowed the Si/Al ratio in the gel to stay high enough, even at the high silicate concentration of 2.5M, to allow faujasite zeolite formation. This further supports the conclusions from the FTIR analysis, that the additional sodium hydroxide liberated more Al species, forming a gel relatively rich in Al even at moderately high silicate concentrations.

No zeolites were identified in any samples with [SiO 2] ≥ 3.5M. One possible reason for this is that any zeolite crystals present in these samples were too small for XRD detection. FTIR has been used in the past to identify zeolite crystals smaller than can be identified by XRD [119, 124]. The study by Jacobs, Derouane and Weitkamp identified X-ray amorphous zeolites with catalytic properties similar to that of the crystalline counterparts [119]. These X-ray amorphous zeolites demonstrated the same structure sensitive peaks in the FTIR spectra as the X-ray crystalline zeolites, even though the crystals were as small as 4 unit cells. As such, the FTIR spectra of the geopolymer samples were analysed in the regions sensitive to zeolite structure to determine if the increase in silicate concentration was inhibiting zeolite formation or merely causing a reduction in the crystal size. Figure 4.12 shows the FTIR spectra for the geopolymers with Na/Al = 0.5-0.75 in the region sensitive to ring vibrations [125] and thus the fingerprint for different zeolite structures.

79 ~ Chapter 4 ~

A B

4. 9 4. 9 3.5 3.5 2.5 2.5

1.2 1.2

1. 0 1. 0 0. 8 0.6 0. 8 Absorbance 0.6 F 0. 4

Absorbance 0. 4 0.2 0.2 0 0 F S 800 700 600 800 700 600 Wavenumber (cm -1) Wavenumber (cm -1)

The spectra in Figure 4.12A show three overlapping peaks at approximately 745, 655 and 555cm -1, the vibrations of faujasite type zeolite identified earlier. The peaks become indistinguishable when the silicate solution activating concentration is 0.8M silica or higher. Samples with silicate concentrations of 0.8-1.2M show very weak peaks in the X- ray diffractograms in Figure 4.11; however these and other geopolymers do not show clear zeolite peaks in the FTIR spectra. Figure 4.12B shows the subtle differences in the FTIR spectra for samples with different zeolites. The low silica geopolymers contain hydroxysodalite, exhibiting vibrations at approximately 735 and 665cm -1 [126]. Medium silicate geopolymers have peaks in the regions 745, 655 and 555cm -1, indicating faujasite type zeolites, in good agreement with the XRD results. However, the high silicate geopolymers demonstrate no distinct peaks in the region indicative of zeolites. This indicates that the diminishing peaks in the X-ray diffractograms for the geopolymers with increasing silicate activating solution concentration is due to a reduced quantity of zeolite

80 ~ Chapter 4 ~ formation rather than a decreased crystal size. Although there is a broad hump in the FTIR spectra in the region indicative of zeolites, the absence of definite peaks in the appropriate areas means that it is unlikely that the system contains significant quantities of nano-crystals. Zeolite nano-crystals contain the same structural units as the larger crystals, and so would exhibit similar vibrational characteristics [119]. Thus the amount of crystalline material, not the crystal size, is decreasing with increasing silicate concentration.

4.4 Conclusions

A study of geopolymer gel ageing over a wide composition range was conducted using Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy (ATR-FTIR). This method allowed the direct analysis of the different phases in partially reacted geopolymer slurries and gels. The position of the asymmetric stretch of the Si-O-T bonds (main band) was compared for all samples at different time intervals over a period of 200 days at 30ºC. Changes in the position of the main band were used to compare the relative Si/Al ratio in the gel at any point in time, giving insight into the selective dissolution/gellation processes with varying silicate and sodium hydroxide concentrations.

Shifts of the main band in the FTIR spectra of geopolymer samples with activating solution concentrations from 0-1.2M SiO 2 indicated that an Al rich gel formed before the final gel composition was reached, regardless of the NaOH concentration. The time required for the system to reach a steady gel composition was dependent on silicate activating solution concentration and speciation. Increasing the amount of monomeric silicates in the activating solution led to a more rapid attainment of steady gel composition.

- The speciation of the silicate solution is a product of the SiO 2/OH ratio. It was found that increasing the concentration of sodium hydroxide in the activating solution controlled the extent of Al liberation from the fly ash network in the early stages of the reaction. In

81 ~ Chapter 4 ~ samples with very high silicate concentrations, an Al rich gel only formed when the sodium hydroxide concentration was high, thus the speciation in the activating solution consisted mainly of monomers. Only monomers and smaller silicate species were found to accelerate early fly ash dissolution in this system.

Increasing the concentration of silicate in the activating solution of geopolymers can have an accelerating or retarding effect on gel formation. An accelerating effect is observed if the speciation of the activating solution is comprised mainly of monomer and smaller silicate entities. A retarding effect is observed in geopolymers activated with solutions deficient in sodium hydroxide, where the speciation consists mainly of high order oligomers, particularly when silicate colloids were present. This was explained in terms of a competition between the colloid surface and the dissolution/gelation reactions for the limited available sodium hydroxide. Lowered NaOH activity decreased the rate of OH - attack and dissolution rate of the fly ash, thereby hampering the gel growth by starving the reaction of (alumino)silicate components. Furthermore, there is a reduction in the mobility of other solution species due to the presence of bulky silicate groups in the solution. Sodium hydroxide deficiency was found to occur in geopolymers with Na/Al=0.25 (approximately 3M NaOH). It was hypothesised that the formation of silicate colloids would be beneficial if there was an excess of NaOH in the system as the surface of the fly ash particles would not be saturated with Na + ions, which are known to have a stabilising effect on the glassy (alumino)silicate surface.

The minimum amount of silicate in the activating solution required to alter the reactions was dependent on the sodium hydroxide concentration. This is because at higher NaOH there will be more monomer for a given silicate concentration. The present study has shown that 0.2M SiO 2, the amount previously assigned as the minimum concentration required to give silicate activation [35], is too low in the real geopolymer system to have any effect. It was found that a minimum silicate monomer concentration of approximately

0.6M SiO 2 was required to alter the reaction rates significantly.

82 ~ Chapter 4 ~

The formation of crystalline phases was found in samples activated without very high concentrations of silicate. The concentration of both sodium hydroxide and silicate in the activating solution, therefore the silicate speciation, controlled the type of zeolites formed. No zeolites were detected using XRD in samples with 3.5-4.9M SiO 2 in the activating solution, regardless of the concentration of NaOH. FTIR spectra of these samples also indicated the absence of large quantities of nano-crystalline zeolites, which would be undetectable by XRD. Therefore the geopolymers gel in systems with very high silicate concentrations are thought to be predominantly amorphous. Zeolites with a lower Si/Al ratio were formed in geopolymer gels which were indicated to be richer in Al from the FTIR results.

The silicate monomer concentration has been found to affect the rate of development as well as the chemical composition and microstructure of fly ash geopolymer gels at ambient temperatures over 200 days. The next chapter will investigate the kinetics of early geopolymer gel development and the effects of sodium hydroxide concentration.

84 ~ Chapter 5 ~

Chapter 5 An In Situ Study of Geopolymer Gel Formation

5.1 Introduction

It has been known for some time that the properties of a geopolymer depend on the type and ratio of reactants used for synthesis. The effect of certain reagents on the mechanical properties of the resultant geopolymers is well established in the literature [6, 30, 104- 106]. Many studies have assessed the changes in microstructural characteristics and leachability of cured geopolymers of varied composition [18, 34, 106, 118, 127, 128]. However, an in-depth understanding of how the chemical interaction of the starting materials alters the rates of reaction in the system is still lacking. To date, there is no true in situ study of the formation of fly ash geopolymers. Changes over time have been assessed by taking samples of a reacting geopolymer at various time intervals [36, 60, 129], however the intervals and reaction temperatures vary between studies and valuable information on intermediate phases may be overlooked depending on the time intervals used.

Geopolymer formation from solid materials is a complicated process involving the formation of transient phases, which exist in pseudo-equilibrium in the early stages [60]. The early chemistry determines the composition and ratio of the initial phases in the system, which ultimately forms the final binder. With an order of magnitude increase in viscosity, observed when a system gels, transport of chemical species is dramatically slowed and thus the rate of reactions decreases by several orders of magnitude. Thus, an understanding of the early reactions occurring in the system is critical for developing the technology which will allow tailoring of material properties for specialised applications.

85 ~ Chapter 5 ~

In this chapter, an in situ study was performed using ATR-FTIR. The present work used a time interval between data collections of just one minute with ambient reaction temperatures for up to 3 days. Since the reaction kinetics are temperature dependent, short time intervals between measurements and ambient temperatures were used to ensure that all changes occurring during early binder formation would be discernible. This method allows continuous FTIR analysis of the outer surface of a reacting sample to be performed over long periods of time.

5.2 Materials and Experimental Methods

GFA was used as the primary binder material to make geopolymers as described in Section 3.2. The compositions of all prepared samples are shown in Table 5.1. Sodium silicate and alkali solutions prepared 24 hours in advance as described in Section 3.1.3.

The water to (GFA+SiO 2) mass ratio used was 0.28 throughout. In situ ATR-FTIR spectroscopy was performed on all samples for up to 3 days as described in Section 3.3.3.

Table 5.1. Composition of samples studied

Na/Al 0.25 0.375 0.5 0.625 0.75 1 H2O (g)* ☺ [SiO 2] M 0 0 0, 2.5 0 0 0 16.8 * Total mass of water in activating solution ☺ 17.7g H 2O in activating solution

5.3 Results and Discussion

5.3.1 In situ FTIR Spectra of Fly Ash Geopolymers

Examples of the FTIR reaction profiles for geopolymers with hydroxide and silicate activation are shown in Figure 5.1. Although spectra were collected at 1 minute intervals, the times shown in this figure were selected to reflect only major spectral changes. Figure 5.1A shows the development of a geopolymer activated without secondary silicates.

86 ~ Chapter 5 ~

Small changes are evident in the first 6 hours of reaction with a slight intensity increase in the region from 860-920cm -1. This has been previously assigned to dissolved species liberated from the fly ash by dissolution into the alkaline activating solution (Chapter 3).

The shoulder in the geopolymer spectra at approximately 1055cm -1, due to the main Si- O-T asymmetric stretching band for unreacted GFA, reduced in intensity only slightly between 6 and 36 hours. A more significant reduction in this band was observed between 36 and 48 hours. Simultaneously, the main band for the system, previously located at 1055cm -1 for unreacted GFA, shifted to lower wavenumbers while increasing in intensity. This intensity increase is due to formation of a new gel phase [60]. A small contribution from silicate solution species is also possible, however, this has been ruled out as the major contributor. This is because the silicate species with vibrations in this region are the monomer and dimer [81]. It is unlikely that such small species will reach concentrations high enough to give a strong infrared absorption in a system with such low water and high solids content and relatively low sodium hydroxide concentration.

The shift and intensity increase of the main T-O-Si asymmetric stretching band in the early stages of geopolymer formation was investigated in Chapter 4, Section 4.3.1. In this Chapter, the solid material was separated from the reacting paste by water washing and centrifugation. The FTIR spectra of the separated solids showed no appreciable change in the structure of the solid material. The solid phase present in the early age geopolymer was simply unreacted fly ash. This indicates that the new phase, solution or gel, formed in the first 48 hours was easily removed from the system with water and probably not covalently bound to the fly ash particles. This suggests that early peak shifts are attributable to solution species or a newly formed (not yet solidified) gel phase. This evidence implies a solution mediated process in the early stages of geopolymer formation for samples activated without secondary silicates and Na/Al = 0.5; whereby species are dissolved from the glassy fly ash particles and move to the bulk solution, where the new phase (geopolymer) begins to form, which ultimately transforms into the geopolymer binder.

87 ~ Chapter 5 ~

After the first 48 hours, significant changes were seen in the FTIR spectra of the geopolymer activated only with sodium hydroxide solution (Figure 5.1A). A new intense peak develops at approximately 965cm -1. Between 48 hours and 70 hours, this peak increases in intensity, reduces in width and shifts to slightly lower wavenumbers. In the same time period, a reduction in the shoulder due to unreacted fly ash is also observed. This intense peak in the FTIR spectra, centred at around 960cm -1, was found to exist in all geopolymer samples investigated in Chapter 4, having a stable peak position between 30 and 200 days after activation. This particular vibrational band was identified as the main asymmetric stretch of the Si-O-T bonds in the newly formed material (Chapter 4); growth of this band was indicative of geopolymer formation in the alkali activated fly ash. The development of a geopolymer could therefore be monitored by the formation of this peak, in the sample activated only with sodium hydroxide, this process is evident in the period between 36 and 70 hours. While visual spectral observations give some information, a quantitative analysis is required to determine the exact lag time before geopolymer formation such that different samples can be directly compared.

In contrast to the long activation period observed in the sample synthesised in the absence of secondary silicate, the sample prepared with a 2.5M SiO 2 activating solution (Figure 5.1B) demonstrates a much faster reaction. At time zero, there are two overlapping peaks observed at approximately 910 and 990cm -1 due to the vibrations of the silicate species in the activating solution (Chapter 3). Between 0 and 3 hours, the peak at 910cm -1 grows in intensity and further overlaps the band at 990cm -1. After just 24 hours, there is only one band visible in the geopolymer spectrum which is positioned in the middle of the two original silicate solution bands at 957cm -1. Little change in the position of the main band was observed between 24 and 48 hours, in good agreement with the results of the 200 day study of geopolymer gel ageing (Section 4.3.3, Chapter 4). The shoulder due to unreacted GFA (1055cm -1) also reduces in intensity throughout the reaction.

88 ~ Chapter 5 ~

A B

70 60 48 48 36 36 30 24 24 12

12 6 6 3 0 0 GFAA Absorbance Absorbance Unreacted fly ash GFA

1200 1100 1000 900 800 700 600 1200 1100 1000 900 800 700 600 - 1 - 1 Wavenumber (cm ) Wavenumber (cm )

Figure 5.1. FTIR spectra of geopolymer development for samples with Na/Al = 0.5, A

[SiO 2] = 0 M and B. [SiO 2] = 2.5M. Numbers refer to age of sample in hours.

In order to make more definitive band assignments, a geopolymer was synthesised using NaOD solution in place of the NaOH solution. The exchange of hydrogen for deuterium causes the vibrations involving the hydroxyl groups to shift significantly [130]. Therefore, by using deuterium exchange, the vibrations due to OH groups can be isolated by comparing the FTIR spectra of the both systems. Figure 5.2 shows the in situ FTIR spectra of a geopolymer activated with NaOD. This geopolymer has the same concentration of NaOD in D 2O as that of the sample activated with NaOH in H 2O, shown in Figure 5.1A.

89 ~ Chapter 5 ~

70 70

60 6048 48 3636 2424 1212 55 0.50.5 00 Absorbance Absorbance

1100 1000 900 800 700 600

Wavenumber (cm -1)

Figure 5.2 FTIR spectra of geopolymer development for a deuterium exchanged sample with Na/Al = 0.5, [SiO 2] = 0 M. Numbers refer to age of sample in hours.

The FTIR spectra of the NaOD activated geopolymer shown in Figure 5.2 look very similar to those of the standard NaOH activated geopolymer in Figure 5.1A. Small differences are visible, including a change in the relative intensity of the bands at around -1 550cm . This change is an artefact created by the spectral subtraction of H 2O/D 2O as -1 H2O has a much greater absorbance than D 2O at around 550cm . The main Si-O-T asymmetric stretch, (at approximately 970cm -1) has also shifted to slightly higher wavenumbers in the sample made with NaOD. According to Davis and Tomozawa, this region has been assigned to both the OH bend and Si-O stretch of O 3Si-OH groups [131]. However, this shift is too small to for the band to be predominantly due to hydroxyl -1 stretching. The Si-O stretch of O 3Si-OD groups occurs at 944cm [131], thus if the dominant vibration involved Si-OH groups, the band in the geopolymer spectra would shift to lower wavenumbers, which it does not. This confirms the band assignments used

90 ~ Chapter 5 ~

in this thesis in which the region 950-1100cm -1 is assigned to Si-O-Si and Si-O-Al stretching.

Another small difference is observed in the sharpness of the bands due to dissolved species at approximately 940 and 1005cm -1. While the position of the bands is the same, indicating that the original assignments are correct, the bands appear sharper in the NaOD sample, as the effect of the strong water absorption is removed completely. This was partially removed using spectral subtraction in the NaOH activated samples, however the entire effect cannot be accounted for by this method, as the FTIR spectrum of water is slightly altered by the presence of NaOH and other solution species. These changes are caused by factors such as hydrogen bonding in the solution, orientation of the water molecules and ion hydration. Although not perfect, spectral subtraction is still a very effective tool in removing sufficient water band intensity to allow the solute bands to be identified.

5.3.2 Functional Group Analysis

Several changes have been observed in the FTIR spectra of the reacting geopolymers. In order to quantify the time dependence of these changes, and thereby analyse the reaction rate, a functional group analysis was conducted. The intensity change of a particular peak was monitored over time. The peaks selected for monitoring were due to the asymmetric stretch of the Si-O-T group in the geopolymer network and in the original GFA. Band positions for this vibration were determined from the FTIR spectra of the unreacted GFA and the final band position for the reacted geopolymers, identified in the 200 day study (Section 4.3.2, Chapter 4). An increase in absorbance at the wavenumber of a functional group generally means an increase in the amount of this functional group in the sample [132]. Likewise, a decrease in intensity generally represents a reduction in these species. A similar functional group analysis has been used previously in a cylindrical ATR-FTIR cell to monitor changes and sol-gel kinetics in aluminosilicate gelation, in a TEOS (tetra ethyl orthosilicate) system [84]. In the TEOS system the functional group of interest was the silanol group, and changes in intensity of the band due to Si-OH stretching were

91 ~ Chapter 5 ~

correlated with changes in the number of silanol groups in the system. This is the first time the method has been applied to monitor kinetics in a geopolymer system.

A functional group analysis for geopolymers activated with and without soluble silicate is shown in Figure 5.3. The sample activated with sodium hydroxide alone demonstrates little change in the intensity of the peak due to the asymmetric stretch of the Si-O-T bonds in the geopolymer network (960cm -1) from 0 to 42.5 hours. At 42.5 hours there is an increase in intensity of the 960cm -1 band which continues linearly until the end of the 70 hour experiment. The shoulder due to unreacted GFA (1055cm -1) reduces slightly in intensity between 0 and 42.5 hours and appears to be approximately linear. Between 42.5 and 70 hours, the gradient becomes steeper, indicating that the rate of fly ash breakdown has increased during this period. As predicted from Figure 5.1, an intensity increase of the band at 960cm -1 is accompanied by a reduction in intensity of the shoulder at 1055cm -1. The changes in intensity with time are linear in both identified regions for the unreacted GFA band and the peak due to the asymmetric stretch of the Si-O-T bonds in the geopolymer network.

The initial period where there was little change in intensity of the 1055cm -1 band and no change in the intensity of the 960cm -1 peak indicates that there is a lag in this system. This time will be referred to as the “induction period”, as it represents a time in which the reagents are in contact, however no product is formed. The cause of this phenomenon was further investigated using lower and higher sodium hydroxide solution concentrations in the activating solution and will be discussed further in this chapter.

92 ~ Chapter 5 ~

0.5 0.5 0.45 0.45 0.4 0.4 Geopolymer Peak -1 0.35 Geopolymer Peak 0.35 963cm 958cm -1 0.3 0.3 0.25 0.25 Peak Intensity Peak intensity Unreacted GFA 0.2 0.2 -1 Unreacted GFA 1055cm 0.15 0.15 1055cm -1 0.1 0.1 0 10 20 30 40 50 60 70 0 5 10 15 20 25 30 35 40 45 50 Time (hours) Time (hours)

Figure 5.3 Functional group analysis for geopolymers activated with: A. sodium hydroxide solution (Na/Al = 0.5), and B. sodium hydroxide solution (Na/Al = 0.5) with

2.5M SiO 2.

Geopolymers activated with high concentrations of soluble silicate do not demonstrate the lag at the beginning of the reaction. Figure 5.3B shows that the destruction of the fly ash occurred without delay in a linear fashion for the geopolymer activated with 2.5M

SiO 2. Similarly, the increase in intensity of the peak in the region relating to the geopolymer matrix was also linear, and again no lag was observed. In good agreement with the work of Lee and van Deventer, the presence of aqueous silicate was found to speed the destruction of the fly ash [35]. Higher early dissolution increases the availability of hydrolysed aluminosilicate, aluminate and silicate species. Once in contact, such entities can be highly reactive in forming aluminosilicate oligomers and gels [77]. In addition, the amount of silicate already in solution initially is high, due to the soluble silicate in the activating solution. Silicate species in solution can more readily undergo silicate exchange and rearrangement, compared to the solid fly ash, providing the reactive precursors necessary to form the geopolymer network.

The presence of these silicates in the activating solution also makes it more difficult to use the in situ FTIR data for kinetic analysis. Dissolved silicate species exhibit strong stretching vibrations around the same region as the final geopolymer peak (as seen in Figure 5.1B, time=0), making it difficult to distinguish between the two chemical groups. This violates the initial assumption that the peak intensity being quantified relates

93 ~ Chapter 5 ~

specifically to the vibrations of the of geopolymer gel network. Monomeric and dimeric silicate species, which exhibit vibrational bands in this region, exist in high quantities in this activating solution (Chapter 4, Section 4.3.3, Figure 4.7). For this reason, the functional group analysis will not be further applied to geopolymers activated with concentrated silicate solutions in this thesis.

Figure 5.3A also demonstrates small but sharp reduction in peak intensity at 23 hours, which affects both the 1055cm -1 and 960cm -1 bands. A small increase in intensity is also seen in Figure 5.3B from 0-2 hours, which affects both bands. This type of occurrence is possibly due to a physical disturbance which alters the contact between the ATR crystal and the sample. Reduced contact between the crystal and sample causes a drop in the absorption of infrared (IR) radiation and therefore a reduction in the intensity of all peaks. The reverse is also true; an increase in contact causes an increase in the intensity of all peaks. Anything which changes the depth of penetration of the IR radiation into the sample changes the absorbance of the overall spectra. For this reason it is important to view more than one peak in the spectra with functional group analysis.

5.3.3 Effect of Sodium Hydroxide Concentration on Geopolymer Kinetics

To further investigate the effect of sodium hydroxide on the rate of geopolymer formation, higher sodium hydroxide concentrations were employed. A functional group analysis for samples with Na/Al = 1.0 and Na/Al = 0.75 is shown in Figures 5.4A and B respectively. Other than a small physical disturbance in the first 10 hours, there appears to be little change in the intensity of the band due to the Si-O-T stretch in the geopolymer network and only a small reduction in the band due to unreacted GFA for both samples up until 27 hours of reaction. After 27 hours there is a linear increase in the intensity of the band due to the geopolymer network and a linear reduction in the intensity of the unreacted GFA peak with time. The activation period observed in both samples is significantly shorter than in the sample with Na/Al = 0.5, which demonstrated no significant intensity change for the geopolymer peak in the first 42 hours. A 25% increase in sodium hydroxide activator led to a 36% reduction in the induction period. However,

94 ~ Chapter 5 ~

further increases in sodium hydroxide did not achieve further rate increases, indicating that the system was already saturated with the activator at Na/Al = 0.75 (approximately 9M NaOH).

-1 Geopolymer Peak (962cm ) -1 0.5 Geopolymer 962cm -1 0.5 GeopolymerGeopolymer 955cmPeak (955cm-1 ) 0.4 0.4

0.3 0.3 -1 -1-1 UnreactedUnreacted GFA GFA (1055cm (1055cm -1 ) UnreactedUnreacted GFA GFA (1055cm (1055cm )) Peak intensity 0.2 0.2 Peak intensity

0.1 0.1 0 20 40 60 0 20 40 60 TimeTime (hours) (hours) Time (hours)

Figure 5.4 functional group chromatograms for geopolymers activated with sodium hydroxide solution, A. Na/Al = 0.75, and B. Na/Al = 1.0.

As discussed earlier, the rate of intensity increase of the geopolymer peak is proportional to the rate of growth of the geopolymer gel network. A direct comparison of reaction rates for geopolymers activated with different concentrations of sodium hydroxide, from Na/Al = 0.25, 0.5, 0.75 and 1.0, is shown in Figure 5.5. The intensities were normalised to the same starting point at t = 25 hours, for the purpose of comparing gradients. The sample with Na/Al = 0.25 shows no change in intensity for the geopolymer peak over the entire 72 hours; the induction period was longer than the experiment. All geopolymer samples activated with higher sodium hydroxide concentrations exhibit a linear rate immediately following the induction period until the end of the experiment. Linear functions were fitted to the data from the end of the activation period. Microsoft Excel was used for curve fitting and R 2 values of 0.99 or greater were obtained. The calculated gradients are summarised in Figure 5.6.

95 ~ Chapter 5 ~

0.550.55

0.50.5 Na/AlNa/Al = =0.75 0.75

0.450.45 Na/AlNa/Al = 1.0= 1.0

0.40.4 Na/Al = 0. 5 0.350.35 Na/Al = 0.50

0.3 Peak intensity intensity Peak 0.3

Intensitygeopolymerpeak of 0.25 0.25 Na/AlNa/Al = =0. 0.2525

0.20.2

0.150.15 2525 30 30 35 35 40 40 45 45 50 55 55 60 60 65 65 70 75 75 Time (hours) Time (hours)

Figure 5.5 Functional group analysis for samples activated with different concentrations of sodium hydroxide.

The Na/Al ratio of the geopolymer was found to have a profound effect on the rate of network formation. Figure 5.6 illustrates the relationship between Na/Al ratio and rate of geopolymer gel growth. There is an increase in the rate of geopolymer formation for increasing sodium hydroxide concentrations, until a maximum is reached, after which the rate is reduced by additional sodium hydroxide. Sodium hydroxide is known to speed the dissolution of silica (and thus aluminosilicate) in water [133]. Faster dissolution may appear to imply faster geopolymer formation due to rapid fly ash destruction; however, this is not necessarily the case. Although the increase in sodium hydroxide did reduce the length of the induction period, the rate of network formation after this period was not always increased. The detrimental effect of too much excess sodium hydroxide on geopolymer formation has also been observed previously. Geopolymer compressive strength is known to increase with increasing M +/Al mole ratio, nevertheless there is a

96 ~ Chapter 5 ~

maximum value [105, 134]. Alkali concentrations above the maximum lead to reductions in geopolymer compressive strength [105, 134]. Generally, an ideal value for the system is Na/Al = 1, as this corresponds to the amount of alkali cation required to charge balance the Al 3+ in tetrahedral coordination. However, the amount of Al in the present work is calculated for the amorphous Al in the system, as it is the glassy material which is known to be reactive [35]. Some of this X-ray amorphous Al may be bound in low solubility glasses, quasicrystalline or cryptocrystalline phases, unobservable by XRD [135]. Due to the heterogeneity of fly ash, it is difficult to say what proportion of readily available glassy Al exists in the system. For this reason, the Na/Al ratio should be regarded as relative, rather than an absolute value. Furthermore, this analysis is giving a comparison of the relative rates of geopolymer formation and the fastest rate of formation may not always give the highest compressive strength.

0.009

p 0.008 g 0.007 0.006 0.005 0.004 0.003 Rate ofRate I change 0.002 0.001 0 0 0.25 0.5 0.75 1 1.25 1.5

Na/Al mole ratio

Figure 5.6 Rate of change of geopolymer peak intensity (I GP ) with time for samples activated with varying concentrations of sodium hydroxide.

To understand the effect of sodium hydroxide on the rate of geopolymer gel formation, the role of the Na + and OH - must be clarified. Understanding the dominant process limiting the reaction at a particular sodium hydroxide concentration is crucial, since any reaction is only as fast as its slowest step.

97 ~ Chapter 5 ~

5.3.4 The Role of Sodium Hydroxide in Geopolymer Formation

Sodium hydroxide is involved in many processes occurring in the reacting geopolymer paste. These include: accelerating the dissolution, stabilising solution species and colloids, increasing the solubility limits of silica and alumina and reducing electrostatic repulsion between the anions, catalysing gel formation and rearrangement [122, 133, 136, 137]. The dissolution rate of silica is increased due to the catalytic effect of the hydroxide ion. The Si-O network bonds undergo nucleophilic attack from the OH - ions, forming a 5- coordinated reaction intermediate, which breaks down to form terminal Si-O bonds [138]. After the rupture of the first bond, the silanol groups created will deprotonate, generally charge stabilised by an alkali cation. The presence of the Si-O- M + complex lengthens and weakens adjacent Si-O-Si bonds, allowing easier cleavage [133]. The rate of dissolution becomes proportional to the number of deprotonated silanols on the surface. The protonation state of the silanol groups is controlled by the pH, which is affected by the sodium hydroxide concentration and levels of dissolved silica. A similar mechanism can be applied to aluminate groups in an aluminosilicate [139].

The solubility of silicate is also greatly increased at high alkali hydroxide concentrations. This is due to the formation of ionic species which have a different equilibrium with the solid phase [137]. Highly charged species forming at elevated hydroxide concentrations also repel each other and resist condensation reactions. Altering the solid-liquid equilibrium allows the stability of many more reactive fragments in the limited solution available in the geopolymer system.

While OH - is responsible for creating the reactive precursors, the alkali cation catalyses the gel formation, potentially acting as a structure directing agent [21]. The association of alkali cations with the deprotonated silanol anions is called ion pairing [140]. This interaction reduces the electrostatic repulsion between the silicate/aluminate anions in the solution making the condensation reaction more energetically favourable [136]. The structure directing effects arise when the silicate/aluminate anions replace the waters of hydration of the alkali cation long enough for a bond to form [136]. This is more likely in

98 ~ Chapter 5 ~

water deficient systems such as a geopolymer. In addition to assisting the gelation reactions, the alkali cation balances the negative charge on the tetrahedral Al 3+ , eventually forming part of the geopolymer network [141].

While the alkali cation accelerates gelation by reducing anion electrostatic repulsion, this can also limit the gelation. Strongly paired cations resist the formation of a siloxane bond, stabilising the anions in solution and significantly reducing the reactivity. Ion pairing is enhanced by high alkalinities, lower temperatures and with smaller alkali cations. This effect is demonstrated in Figure 5.6 Initially, increases in alkali hydroxide give increased rate, until a maximum is reached at Na/Al = 0.75, beyond this point, further sodium hydroxide increases give a reduced rate of geopolymer formation. The lag time is still inversely related to the sodium hydroxide content (Figure 5.5). Dissolution rates are increased at higher alkalinities. Faster dissolution leads to more gel forming species in solution, earlier in the reaction allowing more rapid gelation onset. However, the rate of geopolymer gel formation, once started, may be slowed by strong ion pairing.

Another mechanism causing a reduction in the rate of geopolymer gel formation is the increased dissolution rate and higher overall solubility at high sodium hydroxide concentrations. Newly formed gel phases break down at a faster rate when very high sodium hydroxide concentrations are employed. Thus the geopolymer gel growth is hampered by an increased rate of reverse reaction [122]. This effect is compounded by the reduced rate of network formation from the strong ion pairing effects discussed earlier.

Based on these arguments it would appear that the rate of fly ash destruction in the geopolymer paste would increase with increasing sodium hydroxide concentration. This was observed in leaching experiments, conducted with a much higher liquid to solid ratio [35]. However, the functional group analysis for the shoulder in the spectra due to unreacted fly ash (Figure 5.7), demonstrated that this relationship was not always observed.

99 ~ Chapter 5 ~

0.2

0.19 -1

0.18 Na/Al = 0.25

0.17

0.16

0.15 Na/Al = 0.75 0.14 Na/Al = 1.0 Intensity of shoulder at at 1055cm shoulder of Intensity 0.13 Na/Al = 0.5

0.12 40 45 50 55 60 65 70 75 80 Time (hours)

Figure 5.7 Functional group chromatograms of unreacted fly ash in geopolymers activated with different concentrations of sodium hydroxide.

The samples with Na/Al ≥ 0.5 demonstrated a slope double the magnitude of the geopolymer with Na/Al = 0.25. This follows with the original model from the leaching data. Conversely, for the samples with higher sodium hydroxide concentrations, the fly ash destruction rates were proportional to the rate of geopolymer network formation, although the difference between the gradients was small. Thus the sample with the highest sodium hydroxide concentration did not demonstrate the maximum dissolution rate, as would be expected from dissolution theory. This indicates that the dissolution reaction is not the rate limiting factor in geopolymer gel growth. Due to the extremely low water concentration in the geopolymer system, the leaching characteristics have been altered. In order to liberate additional species from the fly ash, hydrolysed species already in the solution must be consumed and incorporated into the gel network. Thus at higher sodium hydroxide concentrations, the network formation reaction becomes limiting, rather than the dissolution reaction, which limits the rate at lower sodium hydroxide concentrations (Na/Al = 0.25). Furthermore, it can be seen from Figures 5.3A, 5.4A and 5.4B that the rate of fly ash destruction in all samples increases after the onset of network

100 ~ Chapter 5 ~

formation. This indicates that the network formation reaction is rapidly consuming solution species, allowing faster liberation of the additional molecules from the fly ash.

The in situ spectroscopic technique employed to analyse geopolymer kinetics in an alkali activated geopolymer can also be used to compare kinetic effects of various additives or different raw materials in the system. This is the first time that the dissolution reaction has been monitored in situ in the real system at the high liquid to solids ratios normally employed in geopolymer synthesis. The method gives an accurate picture of the real dissolution rate without altering the concentrations of reactants, as was done in previous work [35].

5.4 Conclusions

An in situ spectroscopic technique has been used for the first time to monitor the kinetics of geopolymer formation in the real system. Using a functional group analysis, changes in intensity of the band due to the asymmetric stretch of the Si-O-T bonds in the geopolymer network were used to quantify rates of reaction. Rates of formation of geopolymers activated with sodium hydroxide solutions of varied concentration were compared. It was found that when sodium hydroxide alone was used as the activator, there is a lag. This lag was termed the “induction period” during which there was dissolution of the fly ash taking place, but no geopolymer network formation. The length of the induction period depended on the sodium hydroxide concentration. Increased sodium hydroxide concentration reduced the induction time. The rate of geopolymer network formation (from the end of the lag period) increased with increasing NaOH, with a maximum at Na/Al = 0.63. At higher Na/Al ratios, the rate then decreased with increasing NaOH. The reduced geopolymer formation rate at high Na/Al ratios was explained using ion pairing theory and thought to be further compounded by an increased rate of gel dissolution relative to gel formation at higher sodium hydroxide concentrations. The in situ ATR-FTIR technique employed for the first time in this work can be potentially used to directly compare rates of geopolymer formation in an alkali activated system with various additives.

101

102 ~ Chapter 6 ~

Chapter 6 Alternative Raw Materials for Geopolymer Synthesis: Effect of Si/Al ratio on Reaction Rate and Chemical Structure

6.1 Introduction

Geopolymers have typically been synthesised from solid X-ray amorphous aluminosilicates such as metakaolin or fly ash activated with alkali hydroxide solutions, often containing high concentrations of soluble silicate [8]. A great deal of work has been published regarding the changes in microstructure and mechanical strength with Si/Al ratio. Some studies have used solid raw materials with varying Si/Al ratio [19, 60, 118]. However the majority of studies have used a single solid raw material and varied the Si/Al ratio by changing the proportion of silica in the activating solution [32-34, 36, 103- 106, 134, 142]. It has been recognised that the Si/Al ratio has a very significant effect on geopolymer characteristics, particularly the microstructural evolution and final geopolymer pore structure [32, 34, 103, 106, 134]. Microstructure has been linked in many of these studies to the compressive strength and is also an important parameter in determining the leachability and chemical stability of the binder.

In Chapter 4, it was found that the amount of silicate monomer in the activating solutions affected both the rates of reaction and the final gel structure. In Chapter 5, the effect of the Na/Al ratio was observed to have a profound effect on the early kinetics of geopolymer formation. In the present chapter, the addition of various additives to alter the Na/Al ratio and Si/Al ratio of the geopolymer mixes is investigated, including: geothermal silica, silica fume, metakaolin and sodium aluminate. The rate of network formation of the geopolymers was monitored using the novel in situ ATR-FTIR technique developed in Chapter 5.

103 ~ Chapter 6 ~

Silicate activation in geopolymer synthesis can be highly beneficial. Increasing the amount of silicate in the binder leads to improved chemical resistance due to lower permeability from the reduction in total pore volume [106]. However, the addition of viscous silicate solutions for commercial geopolymer cement production poses handling difficulties and can also lead to rapid setting. The incorporation of additional silica as a solid material could potentially increase the overall silicate content without these issues. Furthermore, this is a necessary step in the formulation of a 1-part mix geopolymer cement, similar to the readily available Ordinary Portland Cement products. The development of a 1-part mix geopolymer cement is expected to significantly increase the commercial viability of these materials.

The prerequisites for successful silicate activation were found in Chapter 4; monomeric silicate species were the most beneficial in the system under investigation. Therefore to enable silicate activation, a solid material which dissolves faster than the fly ash and will form monomeric species is necessary. Due to the industrial motivation of this work, secondary silica sources which were inexpensive or waste materials were selected for analysis.

One such waste material was purified geothermal silica. The production of geothermal power results in large amounts of precipitated silica [95]. At the Cerro Prieto geothermal plant in Baja California, Mexico, 5 tonnes per month of silica are removed from the pipes alone and discarded into an evaporation lake [94]. This material has no present use and is stored in a waste dam with the threat of overflow into nearby agricultural lands [94]. These residues can be purified with water to produce an almost pure amorphous silica, with a specific surface area over ten times greater than fly ash [96]. This material is a potentially valuable source of amorphous silica, which can be used as a replacement for the soluble silicate in geopolymer synthesis. Other materials selected for this analysis included metakaolin and silica fume. While metakaolin is not a waste material, it is a relatively inexpensive feedstock, particularly as only small quantities are used compared to fly ash. Previous work has shown that metakaolin incorporation into fly ash geopolymers enhances the chemical resistance, but can also be detrimental to the

104 ~ Chapter 6 ~

compressive strength of the binder [143]. However, detailed analysis of the effect of the metakaolin on the alkali activation of the fly ash has not been done.

6.2 Materials and Experimental Methods

The sample compositions are shown in Table 6.1. Fly ash (60g) was mixed with the required mass of solid silicate powder to achieve the equivalent solution concentrations to that shown Table 6.1, such that direct comparison could be made with the solution silicate samples investigated in Chapter 4. The water to binder (GFA + SiO 2) mass ratio used was 0.28 throughout. Sodium hydroxide solutions were added to the dry mix and geopolymers were prepared in accordance with Section 3.2 (Chapter 3).

In situ ATR-FTIR spectroscopy was performed on all samples for up to 3 days at 30ºC and ex situ ATR-FTIR was also used to analyse samples after 100 days at 30ºC, as described in Section 3.3.3. X-ray diffraction (XRD) was used to identify the crystalline phases present in the raw materials and reacted geopolymers after 100 days reacting at 30ºC (as described in Section 3.3.1).

Table 6.1 Composition of samples

Equivalent 0.1 1 1.7 4.9 concentration M ♥ Silica fume mass - - 1.7g (0.50) 4.9g (0.50) Silica fume mass - - - 4.9g (0.75) Sodium aluminate mass 0.29 (0.49) 2.9 (0.43) - - Metakaolin mass - - 3.2g (0.44) 9.3g (0.36)

H2O (g)* 16.8 16.8 17.3 18.2 * Total mass of water in activating solution ♥ Solid silicate mass equivalent to activating solution with specified concentration # Values in brackets are Na/Al ratio

105 ~ Chapter 6 ~

A 1-part mix geopolymer was also synthesised: 10g of geothermal silica was dry mixed with 7.6 g of sodium aluminate, and then 7.9g of water was added before mechanical stirring for no more than 2 minutes. This sample had an approximate Si/Al = 2.0 and Na/Al = 1.2

6.3 Results and Discussion

6.3.1 Gel Formation in Fly Ash Geopolymers Containing Solid Silica Powder

The in situ ATR-FTIR spectra of the formation of a fly ash geopolymer with Na/Al = 0.5 containing silica fume are shown in Figure 6.1. The composition of the paste was equivalent to a geopolymer activated with a solution of 1.7M [SiO 2], where the solution silica has been replaced by the solid silica fume. The most distinct feature of these spectra is the relatively sharp peak at 1100cm -1, due to the asymmetric stretch of the Si-O-Si bonds [64] in the silica fume. The intensity of the band at 1100cm -1 decreases with time, indicating that the silica is being consumed in the reaction. While the peak due to the asymmetric Si-O-Si stretch in the silica fume is clearly visible and sharper than the GFA peak, the two peaks overlap to some extent and since the infrared intensity is additive. A change in intensity of one of these peaks could be due to a change in the other peak. This effect can be separated by monitoring the shoulder at 1055cm -1 and comparing this with the changes at 1100cm -1.

At the beginning of the reaction (t = 0) the relatively sharp Si-O-Si stretch dominates the spectrum of the geopolymer mix in Figure 6.1. Between 0.5 and 24 hours, an increase in intensity at approximately 1005 and 910 cm -1 indicates new bands forming in this region. These bands are attributed to the Si-O- Na + bonds forming on the surface of the silica as it is hydrolysed, and to the silicate species being liberated into solution from the particles [81]. Between 24 and 36 hours, there is a small increase in intensity at around 980cm -1, however it is difficult to distinguish any bands in this region due to the high degree of overlap. At 42 hours a large peak centred at approximately 970 cm -1 becomes visible and then shifts to lower wavenumbers. This band, which increases in intensity over time, is

106 ~ Chapter 6 ~

attributed to the newly formed geopolymer network. The band at 1005 cm -1 is still visible as a shoulder in the spectra at 70 hours, indicating the presence of an Si rich phase, possibly a sodium silicate gel in the vicinity of the silica particles. The small shoulder at approximately 1055cm -1, more clearly visible in the 48 and 70 hour spectra, is attributed to unreacted fly ash.

70 48 42 36

24 12 5 Absorbance Absorbance 0.5

1200 1000 800 600 Wavenumber (cm -1)

Figure 6.1 In situ ATR-FTIR spectra of geopolymer development for a sample with

Na/Al = 0.5 and silica fume as secondary silica source, equivalent [SiO 2] = 1.7 M. Numbers refer to age of sample in hours.

To quantify the relative changes of the FTIR bands of interest, peak intensity was monitored over time for the unreacted GFA (1055cm -1), newly formed geopolymer (960cm -1) and unreacted solid silica (1100cm -1). Figure 6.2 shows the peak intensity changes for these bands in the two geopolymers containing silica fume. Apart from a small physical disturbance (discussion of which can be found in Chapter 5), the intensity of the peak assigned to the geopolymer network increases slightly in the first 42 hours.

107 ~ Chapter 6 ~

The slight increase in intensity during this time is attributed to the liberated solution species, which also demonstrate vibrational bands in this region. A full discussion of these bands can be found in Chapters 2 and 3; a justification of the band assignments used here can be found in Chapter 5. After 42 hours, the intensity of the peak attributed to the geopolymer network increases more rapidly, with a linear trend. The intensity of the band related to unreacted GFA decreases linearly over time until 42 hours, at which time the gradient becomes steeper, indicating an increased rate of solid material consumption. This coincides with the intensity increase of the peak associated with the geopolymer network.

0.5 Geopolymer Peak 0.4 (960cm -1 )

0.3 Unreacted GFA 0.2 (1055cm -1 ) Peak intensity 0.1 Silica Fume (1100cm -1 ) 0 0 20 40 60 80 Time (hours)

Figure 6.2 Peak intensity change with time for a fly ash geopolymer containing silica fume as a solid secondary silica source with Na/Al = 0.5 and equivalent [SiO 2] = 1.7 M

The rapid increase in intensity of the peak relating to the geopolymer network at 42 hours represents the onset of geopolymer gel formation. As aluminosilicate species are incorporated into the geopolymer gel, the water and hydroxyl groups previously bound to the aluminosilicates are released. The free water and activator are then able to participate in further hydrolysis reactions, to liberate more species from the solid. As the gel network is already formed, newly liberated solution species can add immediately to the growing gel interface. Mass transfer is assisted by concentration gradients between the hydrolysed solid surface, producing the solution species and the gel interface which is consuming similar species.

108 ~ Chapter 6 ~

In Chapter 5 it was found that the concentration of sodium hydroxide activator altered the rate of geopolymer gel growth. The effect of solid silica on this process is illustrated by Figure 6.3 below, showing the change in rate for both high and low silica content geopolymers with different Na/Al ratios. Initially, there are small increases in intensity in the region associated with the geopolymer network for all samples; the gradient of a straight line fitted to the data of all three samples in this region is 0.0014. After a certain time, there is an increase in the rate for all samples. The greatest rate of intensity increase

(0.0079) was recorded for the sample with Na/Al = 0.75 and equivalent [SiO 2] = 4.9M.

The sample with lower NaOH and lower SiO 2 had a very similar rate, 0.0076. However, the sample with Na/Al = 0.5 and [SiO 2] = 4.9M demonstrates a much slower rate of network growth than the other two samples. One factor likely to have caused the slower growth rate is the additional water added to the sample. The water content was calculated as a percentage of the combined solid silica and fly ash mass. Thus, the overall concentration of sodium hydroxide in this sample was lower than the other two. These data correspond well with those obtained in Chapter 5, where the concentration of sodium hydroxide was found to control the rate of network formation.

0.45 0.75SF4.9 Slope = 0.0 079 0.35 0.5SF1.7 Slope = 0.00 76

0.25

Peak intensity 0.5SF4.9 Slope = 0.0014 Slop e = 0. 0027

0.15 20 30 40 50 60 70 80 Time (hours)

109 ~ Chapter 6 ~

During the hydrolysis reaction, hydroxide is consumed according to Equation 4.1 (Chapter 4). One mole of silica can consume up to four moles of hydroxide during dissolution. This dissolution process lowers the pH of the system and will ultimately slow the reactions which are dependent on the high hydroxide concentration. At the onset of gelation, much of the hydroxide should be released back into the system according to equation 6.1. This equation represents the reaction between the dominant species in solution at high pH [144].

Na + Na + - - 2- - - Si(OH) 3O + Al(OH) 4 ↔ (OH)O 2Si-O-Al(OH) 3 + OH 6.1

However, if phase separation results from the high silica solids content, silicate phases can form in the regions close to the silica particles. The silica begins to dissolve, initiating hydroxide uptake and forms a sodium silicate phase. If this silicate phase does not form a condensed gel, the hydroxide remains captive and cannot participate in further reactions.

Aluminium has been identified as a critical element in geopolymer formation. Insufficient Al has been found to lower the degree of reaction in fly ash geopolymers [60], and aluminate promotes the condensation reactions [144]. When sufficient aluminate is present, the sodium silicate formed during the dissolution process reacts with the aluminate and surrounding silicate groups, releasing the hydroxide and forming the aluminosilicate network. However, if a relatively pure sodium silicate phase forms in the absence of sufficient aluminate, the network formation reaction may not take place even if the phase is dehydrated at temperatures up to 80ºC [56, 104].

In a system with high solids content such as a geopolymer, hindered gel diffusion can lead to phase separation and thus the formation of Si rich phases, as will be further discussed in Chapter 7. It is postulated that the overall effective alkali hydroxide concentration in the system has been lowered by the formation of localised silicate phases which resist network formation due to low local Al concentrations. Thus, a large amount of secondary solid silica can potentially hinder the geopolymer formation reactions in the early stages, reducing the overall rate of network formation. Reaction rates could be

110 ~ Chapter 6 ~

increased by increasing the sodium hydroxide concentration or lowering the overall amount of solid silica in the system.

6.3.2 Use of Metakaolin as a Secondary Silicate

In situ ATR-FTIR spectra for fly ash geopolymers containing metakaolin as a source of secondary silica are shown in Figure 6.4A and B below. The primary difference between this system and the silica fume system is that the metakaolin also contains reactive Al. The early spectra demonstrate a broad spectral contribution at approximately 1035cm -1 due to the solid metakaolin. This broad band appears to shift to lower wavenumbers with time, increasing in intensity with peak shift. Between 60 and 70 hours, the sample shown in Figure 6.4A demonstrates a change in band shape, with a reduction in band width accompanying the increase in intensity. This change was not observed for a sample containing a higher amount of metakaolin, shown in Figure 6.4B. The strong band forming at approximately 960cm -1 has been attributed to the geopolymer network (see Chapter 4). Over time, this band is found to increase in intensity and reduce in width.

The dissolution of metakaolin in alkaline solutions is an order of magnitude faster than that of fly ash [98]. Fly ash has both a larger average particle size and a glass structure compared to metakaolin, which is a calcined clay with a disordered layered structure [145]. As such, it was expected that the samples containing metakaolin would react faster than the alkali activated fly ash geopolymers. Furthermore, the additional Al in the system, due to the metakaolin (with Si/Al =1), maintained the high Al content required for geopolymer formation. To investigate the rates of network formation in the metakaolin containing system, a functional group analysis was conducted. The intensity of bands in the spectral region associated with the geopolymer network and the aluminosilicate raw material were monitored over time. It should be noted that the metakaolin spectra will contribute to the region associated with unreacted GFA such that intensity changes could be due to either a variation in the metakaolin or GFA content in the system.

111 ~ Chapter 6 ~

A B

70 70 60 60 48 42 48 42 36 24 36 12 24 12 5

Absorbance 5

Absorbance Absorbance 0 0

1200 1000 800 600 1200 1000 800 600

Wavenumber (cm -1) Wavenumber (cm -1)

Figure 6.5A shows a small increase in intensity of the bands in the region associated with the geopolymer network up until approximately 62 hours, where there is a sudden change in the rate of growth. Both regions are linear, as observed for all previous geopolymer samples (Chapter 5). The rate of change in the initial region is quite small, 0.001. This increases by nearly an order of magnitude to 0.0093 after 62 hours. The initial rate is quite similar to that observed in the silica fume-containing geopolymers (0.0014).

The sample with a greater amount of metakaolin, equivalent [SiO 2] = 4.9M (Figure 6.5B), demonstrated only one linear region of intensity growth with a low gradient (0.0013). This value was also similar to the initial intensity increase of the silica fume- containing samples, which was attributed to the liberation of solution species. It is thought that the induction period was longer than 70 hours and therefore the growth period was not able to be observed in this experiment.

112 ~ Chapter 6 ~

0.5 0.5 A Geopolymer Peak B 0.4 -1 (958 cm ) 0.4 Geopolymer Peak (958 cm -1 ) 0.3 0.3

0.2 0.2 Peak intensity Peak intensity Peak intensity

0.1 Unreacted GFA 0.1 Unreacted GFA -1 (1055 cm ) (1055 cm -1 ) 0 0 0 20 40 60 80 0 20 40 60 80 Time (hours) Time (hours)

Figure 6.5 Peak intensity change with time for fly ash geopolymers containing metakaolin, with A. equivalent [SiO 2] = 1.7 M and Na/Al = 0.44, B. equivalent [SiO 2] = 4.9M and Na/Al = 0.36.

The initial peak intensity increase in the region around 960cm -1 was assigned to solution species from the rapid dissolution of metakaolin. Interestingly, the onset of geopolymer network formation, indicated by the sharp change in intensity growth of the band associated with the geopolymer network, occurs at a much longer time in the system containing metakaolin solids. This is not completely unexpected, as the increase in Al content has lowered the Na/Al ratio of the system, which was found in Chapter 5 to affect the rate of network formation and the length of the lag period. The additional water added to the system due to the increased solids also lowers the overall sodium hydroxide concentration. For the sample with higher metakaolin content (Na/Al = 0.36), this lag period is longer than the period for which the in situ experiment was able to be conducted. The solution species liberated during the dissolution dominate the spectra. Eventually, a geopolymer is formed with a similar structure to that of the fly ash geopolymer without metakaolin, inidcated by the FTIR spectra at 200 days, Figure 6.6.

While the lag period is extended for samples containing metakaolin, the rate of network formation is increased after the onset when compared to the equivalent samples without metakaolin. The rate of intensity increase in the second linear region is 0.0093 for the metakaolin contaning geopolymer (Figure 6.5A), compared to 0.0041 for a similar

113 ~ Chapter 6 ~

sample without metakaolin (Chapter 5). The rate has more than doubled. In zeolite chemistry, more lengthy induction periods are generally followed by higher growth rates [122]. In fact, synthesis mixtures are often aged at lower temperatures to extend the induction period and thus speed subsequent development. A lengthy induction period allows the formation of a greater number of stable nuclei, which develop independently and thus give a greater overall growth rate.

It is also possible that a different formation mechanism is operating in the alkali activation of metakaolin. Reactions involving the metakaolin alone (or with limited participation from the fly ash) may be forming an initial gel phase, with a different structure to that of the fly ash geopolymer. Further discussion on geopolymer formation from metakaolin is outside the scope of this thesis, as the discussion here focusses on the effect of metakaolin on the activation of the fly ash. This appears to be an inhibitory effect initially, whereby the dissolution of metakaolin dominates early, prolonging the onset of more rapid fly ash dissolution. These findings are in good agreement with the work of van Jaarsveld and co-workers which found that metakaolin adversely affected the compressive strength of fly ash geopolymers [143]. These results indicate that metakaolin reacts very differently to fly ash during geopolymer formation; the variation is much more complex than first thought. Further work is needed to fully understand the different behaviour of metakaolin and fly ash in alkaline media.

6.3.3 Implications of heterogeneous solids addition in geopolymer gel formation

The effect of secondary solids addition on geopolymer binder structure was investigated using FTIR. The FTIR spectra of geopolymer samples containing metakaolin and silica fume are shown in Figure 6.6 below. The positions of the bands due to the main Si-O-T asymmetric stretch in all samples shown were very similar to that of the reference sample, containing fly ash as the only solid. However, the position of this band for geopolymers containing silica fume was slightly higher than for the metakaolin- containing sample or the plain fly ash sample. This was to be expected, as the position of

114 ~ Chapter 6 ~

the main asymmetric Si-O-T stretch in the FTIR spectra was found in Chapter 4 to be dependent on the Al content of the network.

Residual SF

0.5SF 0.75SF

0.36MK Reference Absorbance

1200 1000 800 600

Wavenumber (cm -1)

Figure 6.6. FTIR spectra of fly ash geopolymer samples containing: MK = metakaolin,

SF = silica fume, both with equivalent [SiO 2] = 4.9M. Sample labelling: preceding number = Na/Al ratio, “Reference” = alkali activated fly ash sample with Na/Al = 0.5.

Another interesting feature of Figure 6.6 is the accentuated shoulder at 1100cm -1 (marked with an arrow) for the silica fume-containing geopolymer activated with Na/Al = 0.5, not seen in the silica fume-containing sample with Na/Al = 0.75. This region is attributed to the asymmetric stretch of the Si-O-Si bonds in the unreacted silica fume. It is thought that the lower sodium hydroxide concentration in the sample with Na/Al = 0.5 was insufficient to completely hydrolyse the silica fume. Some of this material remains unreacted, maintaining the original silica structure but trapped inside a hardened geopolymer matrix. This work agrees well with that of Singh and co-workers in which unreacted colloidal silica was observed using NMR in metakaolin/Ludox™ blend geopolymers [129].

115 ~ Chapter 6 ~

An indication of the amount of silica fume reacted in the early stages of gel development can be obtained from assessing the crystalline phases formed in the geopolymers. In Chapter 4, the relative amount and type of crystalline phase formed was found to be dependent on the soluble silicate concentration and the Na/Al ratio. For geopolymers containing concentrations of SiO 2 above 2.5M, no crystalline phases were detected. Figure 6.7 shows the XRD diffractograms for the solid silicate containing geopolymers after 100 days along with a reference geopolymer containing only fly ash and sodium hydroxide solution. The most noticeable difference between the samples is the smaller relative size of the zeolite peaks for the samples containing silica fume, compared to that of metakaolin and the reference sample.

F F F 0.5SF F F 0.75SF

0.36 Mk

Reference

5 10 15 20 25 30 35 40 45 50 º2 θ

Faujasite type zeolite of the form Na 2Al 2Si 2.4 O8.8 (PDF#00-012-0246) was the only new crystalline phase detected in the solid silicate geopolymers. These zeolites were found in Chapter 4 to occur in geopolymers activated with Na/Al = 0.5 for silicate concentrations

116 ~ Chapter 6 ~

up to 2.5M, and samples with Na/Al = 0.75 for silicate concentrations between 0.6M and 2.5M. The XRD results in Figure 6.7 show that faujasite zeolite formed in the silica fume-containing sample with Na/Al = 0.75, which would not have occurred in the absence of at least 0.6M hydrolysed silica, or unless the effective sodium hydroxide concentration was lowered to around Na/Al = 0.5 by the formation of silicate phases. This confirms the in situ FTIR findings which indicate that the silica fume began to dissolve in the first few days. These results indicate that the addition of silica in the form of a solid can enable silicate activation to take place. Further work is needed to optimise the physical parameters of the solid silica source to give the most favourable binder properties.

6.3.4 Effect of High Early Aluminium Concentration

To further investigate the effect of early Al addition to the gel, sodium aluminate was added to the geopolymer synthesis mixtures. Two aluminate contents were investigated; 0.1M Al was dissolved into the activating solution of the first sample. In the second sample, 1M Al was used; however the concentration of aluminate was too high to dissolve in the solution, so it was added as a solid, NaAlO 2. The rate of aluminate dissolution is likely to be much higher than that of the fly ash, so the aluminate will dissolve first. No additional alkali hydroxide was added to the high Al sample (with 1M sodium aluminate), as such, alkali activation of the fly ash could only begin when sufficient sodium aluminate had dissolved to increase the solution pH. The in situ FTIR spectra of the two samples are shown in Figure 6.8 below.

The spectra of the sample containing 0.1M aluminate looks very similar to geopolymers activated with sodium hydroxide only, with one difference. Between 36 and 60 hours, a Si rich gel dominates the spectra. However, the main band position shifts to lower wavenumbers between 60 and 70 hours indicating that a more Al rich gel has formed. The appearance of a shoulder in the spectra at 990cm -1 at 70 hours indicates that the Si rich phase is still present. The mechanism of reaction leading to this phase formation is unknown at this time.

117 ~ Chapter 6 ~

A B 70

60 70 48 60 42 48

42 36 24 36

24 12

Absorbance Absorbance 12 0

1200 1000 800 600 1200 1000 800 600 - 1 Wavenumber (cm ) -1 Wavenumber (cm )

Figure 6.8 In situ FTIR spectra of geopolymer development for samples containing sodium aluminate with A. Na/Al = 0.49 and equivalent [Al] = 0.1M, B. Na/Al = 0.43 and equivalent [Al] = 1M. Numbers refer to age of sample in hours.

The sample containing a greater amount of aluminate demonstrated a different FTIR spectral profile. A large band at around 710cm -1 is attributed to the Al-O vibrations in the sodium aluminate; over the 70 hour period, the intensity of this band decreases indicating that, as expected, the amount of free aluminate is decreasing. The rate of geopolymer network formation was monitored using a similar functional group analysis to that employed earlier. Figure 6.9A shows the rate of intensity increase of the band associated with the geopolymer network for the sample with 0.1M Al as aluminate. Two linear regions have been fitted to the data. Initially, there is a very small slope, 0.0008, followed by a more rapid increase in intensity from around 30 hours, with a gradient of 0.0051. This increase is slightly higher than that of a geopolymer with a similar activating solution, without aluminate, investigated in Chapter 5 (0.0043). The sample with high aluminate (Figure 6.9B) demonstrates only a single linear rate of 0.0015, similar to that

118 ~ Chapter 6 ~

observed during the induction period of the silica fume containing geopolymers examined earlier (0.0014).

0.5 0.5 Geopolymer Peak A -1 B (958 cm ) 0.4 0.4 Geopolymer Peak (958 cm -1 ) 0.3 0.3

0.2 0.2 Peak intensity Peak intensity

Unreacted GFA Unreacted GFA 0.1 -1 0.1 (1055 cm ) -1 (1055 cm ) 0 0 0 20 40 60 80 0 20 40 60 80 Time (hours) Time (hours)

Figure 6.9 Peak intensity change with time for samples with solid sodium aluminate with A. Na/Al = 0.49 and equivalent [Al] = 0.1M, B. Na/Al = 0.43 and equivalent [Al] = 1.0M.

- High Al(OH) 4 activity in solution has been found previously to inhibit glass dissolution [146], and this may explain the extended lag period observed with high early Al concentrations. This may seem unlikely, since an Al rich phase has been found to form initially in most fly ash geopolymers (Chapter 4). However, when the Al rich gel forms during the alkali activation of fly ash, the removal of the Al species from the glassy fly ash leaves a partially detached Si rich layer. This silicate will dissolve more rapidly after the Al removal, as fewer bonds require hydrolysis before each Si atom can be released. Thus if the driving force for the Al removal from the fly ash is reduced, the overall fly ash dissolution rate will be lower and thus the lag period extended. An understanding of the amount of Al in the system, the form it is in and the time at which it will be released are all essential to controlling the process of geopolymer formation.

119 ~ Chapter 6 ~

6.4 Investigation of a 1-Part Mix Geopolymer

Work in this chapter thus far has examined the effects of solid secondary silica, aluminosilicate and sodium aluminate on fly ash geopolymer gel formation. It was found that solid silica, metakaolin and the sodium aluminate dissolve and participate in the reactions, altering the early chemistry of the system and subsequent rate of geopolymer network formation.

It is known that when concentrated sodium silicate and sodium aluminate solutions are mixed, aluminosilicate precipitates are rapidly formed [147]. However, it has been shown that either sodium silicate or sodium aluminate can be added to a solid aluminosilicate source to produce a slowly forming, relatively stable geopolymer network [148]. What has not been tested before in the academic literature is the addition of sodium aluminate to amorphous silica to produce a geopolymer. It is necessary to use an amorphous silica with a high enough surface area to allow rapid dissolution, but low enough surface area to produce a workable binder with a reasonable viscosity at these water to binder ratios. For this reason, geothermal silica was selected. This material has a small particle size, however the particles in the solid state are aggregated, such that the mixture is still workable at reasonable water to binder ratios.

Geopolymer pastes produced using this method have a slightly more viscous consistency than that of the fly ash geopolymers. The samples were caramel in colour, during mixing with the final hardened product taking on a white to honey colour. While the mechanical properties of these geopolymers were not tested, a longer setting time was observed than for a fly ash geopolymer activated with an equivalent Na/Al ratio. However, when tested at 100 days the samples maintained rigidity in water and when examined manually after several days in water, still appeared strong.

Figure 6.10A and B demonstrate the changes in FTIR spectra over time of sodium aluminate activated geothermal silica. The spectra appear somewhat different to that of the fly ash geopolymers examined thus far in the thesis. Bands at 1100, 800 and 475cm -1

120 ~ Chapter 6 ~

relate to the stretching bending and rocking of the Si-O-Si bonds respectively in the network of the unreacted geothermal silica [64]. The band at 625cm -1 is related to the Al- O vibrations in the unreacted solid aluminate (Chapter 3). Bands at approximately 900 -1 -1 and 700cm were assigned to AlO 3 vibrations and a band at 545cm was assigned to a single Al-O-Al mode in the aluminate [76]. Another band at 940cm -1 was assigned to Al- OH vibrations [76].

A B 70 60

48 42 36 24 Absorbance Absorbance Absorbance 12 5 0

1200 1000 800 600 1200 1000 800 600 -1 Wavenumber (cm ) -1 Wavenumber (cm )

Over time, there was a reduction in intensity of the band at 1100cm -1, indicating that the solid silica was dissolving or changing in molecular structure. The absence of a strong band at 780cm -1 in the initial spectra, due to Al-O vibrations in solid unreacted aluminate [37, 67, 149], indicates that much of the sodium aluminate has already dissolved during the mixing process. Although not quantified, in this work a large amount of heat was generated during the mixing process prior to mounting on the ATR, providing further

121 ~ Chapter 6 ~

evidence of the rapid sodium release during the NaAlO 2 dissolution. The small band at 625cm -1 also reduces in intensity over time and is barely visible after 70 hours, indicating that the remaining aluminate is reacting. At the same time, a new band appears to be forming at approximately 950cm -1, with increasing intensity over time. This region is associated with the stretching vibrations of the Si-O-T bonds of the geopolymer network; however, it is also associated with the Al-OH bonds in the dissolved aluminate species. The intensity in this region continues to increase until there is only one clear band at 955cm -1.

Figure 6.10B shows the spectrum of the sample after 100 days. Some geothermal silica remains unreacted, indicated by the shoulder extending to 1100cm -1. However, very little residual aluminate is thought to be present, due to the lack of intensity in the associated regions discussed earlier. The bands at 745, 655 and 555cm -1 are attributed to the presence of faujasite type zeolites [111], similar to those observed in the fly ash geopolymers investigated in Chapter 4. The spectrum of a fly ash geopolymer of similar overall composition to the geothermal silica geopolymer is shown for comparison in Figure 6.10B. The main difference between the spectra of the two samples lies in the contribution of the unreactive fly ash components such as mullite and quartz. There is also a slight shift in the main band between the two geopolymers, indicating a different Si/Al ratio in the gels. The similarity in FTIR spectra of these geopolymers indicates that there is also likely to be a similar structure in both systems.

The rate of change in intensity in the regions associated with the geothermal silica and the geopolymer network was monitored over time (Figure 6.11). Initially, there is a rapid increase in intensity in the region around 960cm -1, with a gradient of 0.0066, followed by a lesser slope (0.0018). The band attributed to unreacted silica decreased in intensity linearly over time, with a slope of -0.0011. It is thought that the initial sharper intensity increase at 960cm -1 was due to the rapid dissolution of the sodium aluminate, since the intensity change for the Si-O-Si asymmetric stretch (1100cm -1) was constant throughout the reaction, indicating a constant rate of silica dissolution. The rate of intensity growth in the region around 960cm -1 was also relatively low, similar to that of the induction period

122 ~ Chapter 6 ~

observed in the silica fume containing fly ash geopolymers (0.0014). It is thought that there may also be a lengthy induction period in this sample, longer than 70 hours.

0.45 Geopolymer peak 0.4 -1 Slope = -0.0011 960 cm

0.35 Slope 2 = 0.0018

0.3

Peak intensity Peak 0.25 Unreacted silica 1100cm -1 0.2 Slope 1 = 0.0066

0.15 0 10 20 30 40 50 60 70 80 Time (hours)

Figure 6.11 Functional group analysis of a sodium aluminate activated geothermal silica geopolymer with Na/Al = 1.3 and Si/Al ≈ 2.0.

Although experimentally very interesting, this type of mix may be impractical as a cement replacement, due to the large amount of heat generated during the early stages. Perhaps there is some application for this material in very cold climates, where the system must produce sufficient internal heat to allow dissolution to take place. Although the structure of the geopolymer appears to be similar to that of fly ash geopolymers, further research is required in order to investigate the material properties of these new geopolymers to determine industrial applicability.

6.5 Conclusions

In this Chapter, the effects of additives such as silica fume, metakaolin and sodium aluminate on fly ash geopolymers were investigated. It was found that the presence of dissolved species from each additive had an effect on the dissolution of the fly ash and

123 ~ Chapter 6 ~

the subsequent rate of geopolymer network formation. The critical factor found to affect the geopolymers containing a secondary solid silica or aluminosilicate source was the sodium hydroxide concentration or Na/Al ratio. The additional solid materials led to lower values, in some cases so low that the reactions were slowed. This effect can be compensated by using more highly concentrated sodium hydroxide solutions.

Silica fume was found to dissolve preferentially to fly ash in the early stages of reaction and alter the Si/Al ratio in the gel. This is most likely due to the higher surface area of the silica fume compared to the fly ash. The use of small amounts of sodium aluminate in solution had a slightly accelerating effect, while a large amount of sodium aluminate was found to retard the onset of geopolymer formation, prolonging the induction period. A similar effect was observed when metakaolin was used, however after the lengthy induction period, the rate of network formation was expedited. It is hypothesised that the lengthy induction period could allow the formation of a greater number of stable nuclei, resulting in the faster network formation after onset.

A one part mix geopolymer system was also investigated, consisting of solid sodium aluminate, geothermal silica and water. This system demonstrated a lengthy induction period similar to that of the aluminate containing fly ash geopolymer. The ultimate structure of the material was also found to be similar to that of an alkali activated fly ash geopolymer of similar composition. Further work should be conducted on the aluminate activation of silicate materials to investigate material properties of the resultant geopolymers for specialised applications.

124 ~ Chapter 7 ~

Chapter 7 Geopolymer Gel Formation with Seeded Nucleation

7.1 Introduction

The structural similarity between zeolites and geopolymers was noted some time ago by Joseph Davidovits [150]. Several authors have since recognised the presence of various types of zeolite phases in reacted geopolymers [33, 104, 151-154]. More recently, a review has been published examining existing experimental data on the subject, suggesting that geopolymer structure is, in many cases, made up of nano-crystalline zeolites in an amorphous gel [8]. Many authors in geopolymer research have equated zeolite chemistry with geopolymer chemistry due to the similarities between the two systems [19, 42, 155], with some referring to a geopolymer as a “zeolitic precursor” [42]. Others have taken the parallel even further, suggesting nucleation as a mechanism for gel growth and using this analogy to explain microstructural observations [31] and strength development [156] in metakaolin geopolymers. Furthermore, a nucleation step has been included in a recently developed mathematical model proposed by Provis and co-workers to describe geopolymer solid-gel transformations [8]. Despite these analogies the exact nature of the geopolymer gel and the mechanism by which the binder forms are still not fully understood.

Zeolite synthesis generally involves the rearrangement of aluminosilicate species in a highly alkaline activating solution to produce crystalline materials with uniform pore sizes. Fly ash and metakaolin have often been used as aluminosilicate sources for these reactions [117, 157-159] and the synthesis is generally performed under hydrothermal conditions [122]. However, it is possible to produce zeolites at ambient temperatures, and synthesis mixtures are often aged at room temperature prior to hydrothermal treatment to promote nucleation [160, 161]. Structure directing agents are often employed to

125 ~ Chapter 7 ~

encourage the formation of certain crystalline structures, and alkali cations often perform this task [122].

The main difference between the reaction mixtures used for zeolite and geopolymer synthesis is the amount of water and alkali hydroxide. Zeolite forming systems generally - employ mole ratios of the order: H 2O/SiO 2 = 10-100, and OH /SiO 2 = 2-20, in - comparison to the geopolymer system where commonly H 2O/SiO 2 = 1-2, and OH /SiO 2 = 0.1-0.5. Despite these differences, the chemistry of the two systems has been compared many times in the literature, with the knowledge from zeolite chemistry being used to explain experimental observations in geopolymer formation.

In the present chapter, the possibility of a nucleation mechanism occurring during geopolymer gel formation will be directly explored through experiment and results will be discussed in the context of well developed theory on zeolite chemistry. The in situ ATR-FTIR method and interpretation developed in previous chapters is utilised and a new model for geopolymer gel development presented.

7.2 Materials and Methods

A 6M NaOH solution was mixed with 60g of fly ash (Sections 3.1.3 and 3.2). Two additional samples were prepared (with the same composition as above) in which 0.01g and 0.1g of Al 2O3 nano-particles (NanoScale Materials, Manhattan) respectively was dispersed in the activating solution immediately before mixing with the fly ash.

In situ ATR-FTIR spectroscopy was performed on all samples for up to 3 days as described in Section 3.3.3. Ex situ ATR-FTIR and XRD was also performed after 100 days at 30ºC (Section 3.3).

126 ~ Chapter 7 ~

7.3 Results and Discussion

7.3.1 Zeolite Formation

The process of zeolite formation has been widely investigated over the past 50 years. A recent review by Cundy and Cox [122] presented a unified model for zeolite synthesis from precursor gels, encompassing many of the experimental observations to date. The model proposed the formation of two different gels in the system prior to the appearance of zeolite crystals, drawing on the Ostwald Law of Successive Transformations to explain the phenomenon. This law states that a phase will not always transform directly into the most stable state, but will often change into a transient state of greater likeness to itself.

The first gel formed in a zeolite synthesis is an inhomogeneous, amorphous gel, which is not in equilibrium with the surrounding solution. This gel approaches equilibrium with the surrounding solution through a series of depolymerisation-repolymerisation reactions, catalysed by the presence of the hydroxide ion. The result of this process is designated as the “second gel”. Ordering in the gel increases throughout this process, and small regions of more ordered material develop, referred to as “proto nuclei” [122]. The stoichiometry of the second gel closely resembles the zeolites which are subsequently produced. Reactions involving gel reorganisation continue throughout the whole process, however when regions of higher order develop, the depolymerisation of these areas is slowed relative to the repolymerisation. This change in the relative dissolution rates is due to the greater stability of the more highly crystalline structure. The increase in structural order reduces the relative rate of depolymerisation compared with the surrounding amorphous gel [122]. When sufficient order is present, these regions become nuclei and crystal growth begins. A hydrated cation was proposed by Cundy and Cox [122] to mediate the propagation, sequestering monomer from solution to a site on the growing centre, and then guiding the species to a more favourable co-ordination being that of the crystal. Thus, zeolite crystals are formed.

To speed this crystal formation and promote the formation of specific crystal structures, reaction mixtures are often seeded with small crystals. These crystals act as nucleation

127 ~ Chapter 7 ~

centres, eliminating the time taken for the system to produce its own stable nuclei. Supposing that the reactions occurring in geopolymer formation are analogous to the zeolite system, a nucleation event would then be expected. The lag observed initially in geopolymer systems activated with sodium hydroxide, (Chapter 5) could be called the “induction period”, potentially representing the time taken to form stable nuclei. Similarly, the initial introduction of potential nucleation sites into the geopolymer system would then eliminate the lag period at the start of the reaction. Therefore faster formation of the geopolymer network would be expected.

7.3.2 In situ FTIR Spectra of Geopolymers Seeded with Nano-particles

Figure 7.1B shows the in situ FTIR spectra of a geopolymer sample at various time intervals over 3 days at 30ºC. The geopolymer was activated with sodium hydroxide solution (Na/Al = 0.5) in which 0.01g Al 2O3 nano-particles with a mean particle size of 200nm, and a specific surface area of 275m 2/g, were dispersed immediately before mixing with 60g of fly ash. For comparison, the in situ FTIR spectra of the equivalent geopolymer (identical activating solution) synthesised without the nano-particles is shown in Figure 7.1A. The initial spectra of both samples are the same, because the seeds are present in such a small quantity that they cannot be detected by FTIR. The sample shown in Figure 7.1B, seeded with nano-particles, shows an increase in intensity of the main Si-O-T asymmetric stretching band at 960cm -1, the region related to the geopolymer network, (Chapter 4), after just 12 hours. The equivalent un-seeded sample shows little change in the FTIR spectra after 12 hours. This system was found to have a lag in geopolymer gel growth lasting up to 42.5 hours (Chapter 5).

128 ~ Chapter 7 ~

B A

70 60 70 60 48 48 42 42 36 36

24 24 12 12 5 5

Absorbance 0.5 0. 5 Absorbance 0 0

1200 1000 800 600 1200 1000 800 600 - 1 Wavenumber (cm ) Wavenumber (cm -1)

Figure 7.1 FTIR spectra of geopolymer development for sample with Na/Al = 0.5 and

[SiO 2] = 0 M, A. reference sample without nano-particles, B. with 0.01g Al 2O3 nano- particles. Numbers refer to activation time in hours.

The rate of increase in intensity of the main band in the spectra was investigated using a functional group analysis similar to that conducted in Chapters 5 and 6. Figure 7.2 shows the intensity change for the band at 960cm -1 over time. For comparison, data for both the seeded and un-seeded geopolymer samples are shown. A lag period was observed in the geopolymer activated with sodium hydroxide solution (discussed in detail in Chapter 5). This lag is absent in the geopolymer seeded with nano-particles. The rate of intensity increase of the main band in both samples is very similar. This is interesting as it indicates that after the gel begins to grow, the rate of growth is comparable in both samples, with and without seeding. It is likely that the same mechanism of gel growth is rate limiting in both systems.

The most noticeable difference between the results obtained for the two systems (Figure 7.2) is that the geopolymer gel growth occurs from time zero in the seeded sample, compared to after 40 hours in the geopolymer with no seeds. This experiment

129 ~ Chapter 7 ~

demonstrates the change in rate observed in sodium hydroxide activated geopolymers when potential nucleation sites or gel growth sites are added in very tiny amounts (less than 0.01 w/w%) but with a very high BET surface area (275m 2/g). This change could not have come about from the chemical contribution of the added solids if they dissolved. This was investigated in Chapter 6 with the addition of sodium aluminate. It was found that the aluminate needed to be added in very high concentrations to significantly change the reaction profile observed using in situ FTIR.

0.35 Al 2O3 Nano-particles 0.3 Slope = 0.0043

0.25 0.2 Un-seeded Slope = 0.0039 0.15 0.1

0.05

Intensityof geopolymer peak . 0 0 20 40 60 80 Time (hours)

Figure 7.2 Changes in intensity at 960cm -1 with time for geopolymers with Na/Al = 0.5 and [SiO 2] = 0 M. Straight lines have been graphically fitted (grey).

Given that the action of the alumina was not a chemical one in the aqueous phase, the particles must have had a physical effect to accelerate the gel development and remove the induction (lag) period. It is possible that the surface of the nano-particle seeds has acted as a nucleating site for the gel; suggesting a nucleation mechanism for geopolymer gel growth in the fly ash/sodium hydroxide system, similar to that which occurs in zeolite formation. Furthermore, the rates of formation are linear in the geopolymer system (Figure 7.2), as is the rate of growth of zeolite crystals [122]. It is proposed that the seed particles are playing a catalytic role in the nucleation of geopolymer gel formation, enabling each seed particle to interact with the reacting system on multiple occasions and thus explaining the strong effects seen at such a low seed dosage.

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In a zeolite system, seeding has several effects, such as reducing synthesis time by eliminating the need for lengthy self nucleation. This also gives control over other system parameters, such as crystal size. Adding a greater number of nucleating sites leads to smaller crystals as the nuclei compete for species in the solution to grow in size. Furthermore, seeding with a specific crystal can promote the formation of a single type of zeolite before other zeolite phases have nucleated, therefore allowing higher yields of the desired product. While allowing better control over the synthesis, the seed particles do not change the mechanism by which the zeolite develop, widely believed to be monomer addition to the growing crystal [122]. The seeds simply provide a surface on which the nuclei growth can begin. In the un-seeded system, the primary and secondary gel (discussed in section 7.3.1) must develop first then nuclei form within the gel before growth can begin.

A similar mechanism is proposed for geopolymer synthesis, in which the seed particles act in the same way. The seed does not alter the mechanism of network growth, only the time at which the growth starts. This was observed in Figure 7.2, where a lag period occurs for the un-seeded geopolymer but not for the seeded sample. However, the rate of geopolymer network growth in both systems is similar after the nuclei are present.

Another way in which the nano-particles could be affecting the reactions is by acting to remove aqueous Al as it dissolves from the fly ash, preventing potential inhibition of further dissolution. As discussed earlier, Al preferentially dissolves from the fly ash in the initial stages of alkali activation [162]. Previous research has shown that even small amounts of aqueous Al can inhibit silica and other mineral dissolution, and can reduce the dissolution rate by an order of magnitude, [137, 163-166]. Conversely, other studies have shown that a higher Al content in aluminosilicates can increase the dissolution rate. Hydrolysis of more relatively weaker Al-O bonds leaves a partially detached silicate network, and an increase in the overall dissolution rate [167]. The characteristics of aluminosilicate dissolution depend strongly on the structure of the material and the conditions used, particularly the pH [168]. One study on fly ash leaching found that Al was not preferentially dissolved, and in fact no Al was detected in the leach solution in the initial sampling, only Si and Ca [35]. In contrast, other work on alkali activation of fly

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ash found that Al rich phases do form initially, supporting the theory of preferential Al dissolution [60]. Further research is required before the effect of Al inhibition in fly ash dissolution can be confirmed or refuted. The mechanism postulated below applies to a scenario where Al inhibition is significant.

Initially, there is a preferential dissolution of Al from the fly ash network, creating a siliceous layer. The aqueous Al species then adsorb to the surface sites of the silicate rich layer, passivating the surface by preventing the approach of hydroxyl ions [163]. This drastically slows the dissolution process. Over time the inhibited silicate dissolution process slowly releases enough Si to react with the aqueous Al and cease the inhibition. Dissolution of the fly ash then becomes more rapid leading to the formation of a gel. Aqueous Al could thus be responsible for the lag seen in geopolymer samples activated with sodium hydroxide alone. This explains the enhanced dissolution observed when silicate activation is used [35], as silicate monomers quickly react with the liberated Al in solution, reducing the surface passivation. The action of silica would therefore be to complex or react with the aqueous Al and remove the impact of the surface adsorption to allow more rapid fly ash dissolution. In this model, nucleation is still proposed; however, the nuclei will form in time within the gel, which begins growing on the nano-particle. The alternative model of nano-particles acting as nuclei will be discussed in more detail later in this Chapter.

Although the rate of growth is similar for both seeded and un-seeded geopolymers, there are differences in the network structures, observed as variations in the FTIR spectra shown in Figure 7.1. The differences can be observed as spectral shoulders occurring at wavenumbers higher than 960cm -1. The seeded geopolymer (Figure 7.1B) has a distinct shoulder at 1100cm -1 and a slightly weaker shoulder at 1040 cm -1 after 70 hours. The non-seeded geopolymer (Figure 7.1A) contains only one shoulder at approximately 1055cm -1, the region due to unreacted fly ash (Chapter 4). This difference is further highlighted by Figure 7.3; the FTIR spectra of both seeded and un-seeded geopolymers (70 hours) are shown after water and fly ash spectral subtraction. The shoulder due to unreacted fly ash in the un-seeded geopolymer has been completely removed. In contrast, the bands in the spectra of the seeded geopolymer at 1100 and 1040cm -1 are more clearly

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visible when the broad overlapping fly ash peaks are removed. Results in Figure 7.3 indicate that the 1100cm -1 and 1040cm -1 bands are due to a new phase, present in the geopolymer early in the reaction, assignment of which is discussed below.

Un-seeded Absorbance Seeded 1100 1000 900 800 700 600

Wavenumber (cm -1)

Figure 7.3 FTIR spectra of seeded and un-seeded geopolymers after 70 hours at 30ºC with Na/Al = 0.5 and [SiO 2] = 0 M. Water and fly ash have been spectrally subtracted.

Bands due to the asymmetric stretch of the Si-O-T bonds occur in the region 1100- 950cm -1 [169], and the position of this vibration is sensitive to both the length and angle of the Si-O-T bond [39]. In a silicate material, the length and angle of the Si-O-Si bond is also affected by the local network and thus the next nearest neighbour [170]. Therefore -1 while the asymmetric stretch of a pure SiO 2 phase is found at 1100cm , a small amount of network modifier (Na or Al) will reduce the intensity of this band and new bands, at lower wavenumbers, will be seen for the new Si-O-T bonds and also for the neighbouring Si-O-Si bonds which are affected. The number of well resolved features attributed to the newly formed gel in the area due to asymmetric stretch of the Si-O bond in Figure 7.3 indicates that isolated regions exist in which silicon is in different, specific environments. It therefore is proposed that these new bands in the FTIR spectra at 1100 and 1040cm -1 are due to isolated gel pockets of different composition to the bulk gel, representative of a phase separation.

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The presence of the band at 1100cm -1 (assigned to the Si-O-Si bond in amorphous silica [64]) in the seeded geopolymer indicates that there are condensed, highly siliceous regions forming in the gel. This band was not observed in any other geopolymers in the present study, activated with either sodium hydroxide or sodium silicate activating solutions (Chapter 4). The shoulder at 1040cm -1 is due to the asymmetric stretch of the Si-O-Si bond with an increased bond length and angle [38]. This may be attributed either to an increase in the Al substitution in the silicate network or an increase in the number of NBOs (as discussed in Chapter 4). Since the alkalinities of the systems are the same, it is unlikely that there has been a significant increase in the amount of NBO. Therefore, the new phase is thought to be a silicate network with a lower Al substitution than the bulk of the geopolymer gel, possibly at the interface of the highly siliceous region and the bulk geopolymer gel. While there are new bands visible in the spectra of the seeded geopolymer, the position of the “main band” (the strongest peak in the spectrum) is the same for both samples, indicating that the bulk geopolymer gel structure and extent of Al substitution are similar.

To further investigate the different phases forming with seeded geopolymer synthesis, the amount of nano-particles being used was increased by a factor of 10. The FTIR spectrum (after 70 hours at 30ºC) for both high and low seeded geopolymers are shown in Figure 7.4 below. The shoulder at 1040cm -1 is greatly enhanced in the spectra of the geopolymer with more nano-particles. Furthermore, the main band has broadened at the top and changed shape slightly, indicating that there may be an additional band adding to this peak. The additional band is at higher wavenumbers than the main band in the original sample, and thus it represents a more silica rich phase.

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0.1g nano - particles

0.01g nano-particles Absorbance Absorbance 1200 1000 800 600 Wavenumber (cm -1)

Figure 7.4 FTIR spectra of seeded geopolymers after 70 hours at 30ºC with Na/Al = 0.5 and [SiO 2] = 0 M. Black line = 0.1g nano-particles, Blue line = 0.01g nano-particles.

Although not shown, the line fitted to the functional group analysis of the geopolymer gel development with 0.1g nano-particles had a gradient of 0.043, which was exactly the same as that of the straight line fitted to the intensity versus time data for geopolymer samples seeded with 0.01g of nano-particles. This indicates that the rate limiting factor for gel growth was not the availability of nucleating sites. Other potential rate limiting factors include: 1) dissolution, thus availability of species for gel growth, 2) hindered gel diffusion, starving growing surfaces for potential nutrients and gel depolymerisation and 3) slow rate of hydroxide catalysed gel rearrangement providing the required monomeric/small species for addition to the growing nuclei. All these mechanisms have one thing in common, i.e. the supply of small species to the growing surface. One other possible mechanism is the rate of addition of species to the nuclei, however this has been ruled out due to similarities in the gradients of the seeded and un-seeded systems. The rate at which addition occurs is likely to change throughout the reaction, as the viscosity of the gel changes and so does the adding species mobility and ability to rearrange once bound to an active site on the growing nuclei. Thus linear kinetics would not be expected

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in this system. It is difficult to say which mechanism is rate limiting in the present system; this is an area of research which deserves further investigation.

While the seeding does not change the mechanism of network growth, the local composition at the nuclei surface will not necessarily be the same for seeded and un- seeded systems. The relative amounts of Al and Si in the aqueous phase will change with time during the dissolution/growth process. Since the growth of the seeded geopolymer starts from time zero, the phase in contact with the nuclei will be rich in Al. By contrast, the phase in contact with the nuclei in the un-seeded sample will have a different composition, likely to have a higher relative amount of Si. This is because the Al is selectively leached from the fly ash in the first instance [60, 167, 171]. Thus, the phase which grows in the initial stages in the seeded system will be different to that which develops on the self generated nuclei in the more Si rich gel of the un-seeded geopolymer. Evidence to support this hypothesis along with an explanation of the proposed mechanisms will be presented in the next sections.

7.3.3 Geopolymer Gel Structure with Seeded Nucleation

Seeded geopolymer synthesis induced both a change in reaction profile and a phase separation in the geopolymer structure. New phases were observed to form in the geopolymer with seeding, not before observed for this system. These differences in gel structure were further investigated by characterising the cured geopolymers after 100 days at 30ºC. Figure 7.5A demonstrates the FTIR spectra of the seeded and un-seeded geopolymers. The position of the main Si-O-T asymmetric stretching vibration is 960cm -1, the same in both samples. The shoulder at 1100cm -1 in the seeded geopolymer is again visible, indicating that the highly siliceous gel observed after 70 hours is still present. Furthermore, the shoulders at 1040, 990 and 930cm -1 are clearly visible when the spectra of the seeded and un-seeded geopolymers are superimposed (Figure 7.5A).

Figure 7.5B shows the result of the subtraction of the un-seeded geopolymer from the seeded geopolymer spectra shown in Figure 7.5A. The result clearly shows the position of the new bands present in the seeded system, not observed in the un-seeded sample.

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This subtraction removed the spectral contribution of the bulk gel, formed after the onset of congruent dissolution, leaving only the bands for the new phases, which formed prior to the onset of congruent dissolution. The assignment of these new bands was made based on the literature review presented in Chapter 2. The band at 1100cm -1 is assigned to asymmetric stretch of Si-O-Si bonds in a relatively pure silica network [64], as the amount of network substitution of Al increases, this band shifts to lower wavenumbers [65, 71, 172]. As such, the band at 930cm -1 was assigned to a phase rich in Al, forming on the nuclei during the preferential dissolution of Al from the fly ash in the initial stages. The relatively pure silica phase is therefore attributed to the dissolution of the silica rich layer, formed during the initial dissolution stage when the fly ash surface became depleted of Al. The silica rich phase also added to the outer surface of the growing nuclei, coating the Al rich core. The band at 990cm -1 is due to a gel with a composition in between that of the Al rich gel and the silica rich gel, and thus assigned to the interface between these two gels. The band at 1040cm -1 is due to a silica rich phase with some Al substitution. This band was assigned to the boundary between the silica rich gel and the bulk gel formed after the onset of congruent dissolution.

The relative increase in the intensity of the band at 1040cm -1 is much greater than that of the band at 1100cm -1 when the amount of nano-particles is increased from 0.01g to 0.1g (Figure 7.4), indicating that the amount of silica gel is constant. This is because the amount of silica being released from the Si enriched layer (formed by the selective leaching of Al initially) is controlled by the dissolution mechanism which in unaffected by the change in nano-particle content. The amount of silica in the Al depleted layer on the fly ash surface will be similar in both systems, thus the amount of silica rich gel will be the similar in both systems. The band at 1040cm -1 was assigned to an interfacial region between the silica gel and the bulk. If there is an increase in the number of nuclei, there will be an increase in surface area of the silica gel in contact with the bulk gel. Therefore there will be more interface and thus more interfacial gel, giving a greater absorbance in the FTIR spectra at around 1040cm -1. This finding further supports the proposed band assignments.

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A B

Absorbance Absorbance

1 2 00 1000 800 600 1100 1000 900 - 1 -1 Wavenumber (cm ) Wavenumber (cm )

Figure 7.5 FTIR spectra after 100 days at 30ºC for A. Black = seeded and pink = un- seeded geopolymers, GFA has been spectrally subtracted. B. Spectral subtraction of the un-seeded geopolymer from the seeded geopolymer. Arrows show the new bands.

Significant differences between the two geopolymer structures in Figure 7.5A are also observed in the region relating to ring structures, below 760cm -1 [173]. Both samples demonstrate a band at 540-555cm -1; this region has previously been assigned to the vibration of Al-O-T bonds in tetrahedral coordination [67, 174]. It is likely that this band encompasses both ring and tetrahedral Al-O-T vibrations. Network bound Al is in tetrahedral coordination in geopolymer structures [175]. Differences in the region relating to ring vibrations (below 760cm -1) indicate that the two samples contain different zeolites, which demonstrate bands in this region as discussed in Chapter 4.

To further investigate the different gels forming, new crystalline phases were analysed using X-ray diffraction. X-ray diffractograms for the seeded and un-seeded geopolymers are shown in Figure 7.6. The main crystalline components of the fly ash are quartz and mullite; the intensity of these peaks does not change from the original ash (Chapter 4). It was found in Chapter 4 that the geopolymer with Na/Al = 0.5 and [SiO 2] = 0 M, without

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seeding, formed faujasite type zeolites, Na 2Al 2Si 2.4 O8.8 .6.7H 2O (PDF # 00-012-0246). It is clear from the diffraction pattern (Figure 7.6) that the crystalline phase formed in the seeded sample is quite different to this. The new crystalline phase has been identified as zeolite Species F, Na 2Al 2Si 2O8.xH 2O (PDF # 00-025-0777), a synthetic sodium zeolite with an edingtonite-like (EDI) framework [176]. Zeolite F has a lower Si/Al ratio than the faujasite type zeolite found in the un-seeded sample.

Q M non-seeded F Unseeded Q F F M F F

ZF ZF Al2O3 Seeded nano-crystals ZF

5 15 25 35 45 55 65

Degrees 2 θ

Figure 7.6 X-ray diffractograms for geopolymers with Na/Al = 0.5 and [SiO 2] = 0 M. F = faujasite type zeolite, M = mullite, Q = quartz, ZF = zeolite species F.

The structure of the zeolite F unit cell is tetragonal, primitive [177] containing units similar to the double four membered ring. Zeolite F has not before been synthesised in a system using only sodium as an alkali source. Generally, this zeolite is found in potassium or mixed alkali systems [117] and can be exchanged to produce the sodium zeolite F [178]. It is possible that the small amount of potassium and other alkali metals present in the fly ash (Chapter 3) have leached out of the fly ash early in the reaction and participated in the zeolite formation. Alternatively, the unique conditions in the early stages of geopolymer formation may have created an environment where Zeolite F (Na) can be directly synthesised. Zeolite F forms at relatively low temperatures (80ºC) using metakaolin or kaolin as an aluminosilicate source, in synthesis mixtures with an approximate Si/Al = 1 [117]. This is not surprising, as the Zeolite F stoichiometry has Si/Al = 1. In the present system, it is thought that Zeolite F is forming in the initial Al rich gel, close to the original nuclei which explains the formation of the relatively high Al

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zeolite. Potentially, the added nano-particles also acted as the nucleating sites for the zeolite formation. The initially-formed gel is thus envisaged also to have an approximate Si/Al = 1. This is in agreement with the work of Palomo and co-workers, who performed NMR on geopolymers activated with only sodium hydroxide solution and found that initially, the system forms a gel with Si/Al = 1, which changes to Si/Al = 1.86 over time [55].

It is not surprising that different zeolites have formed in the gels of the two systems. Often in zeolite synthesis, the choice of seed for these reactions is selected specifically to promote the formation of one zeolite over another. In this case, the seed has formed separate Al rich and Si rich phases in the gel; a change in gel stoichiometry will affect the types of zeolites formed. The zeolites formed in the seeded geopolymer have Si/Al = 1, compared to Si/Al ~ 1.2 for the faujasite zeolite in the un-seeded geopolymer. This indicates that zeolite formation in the un-seeded system took place in a more Si rich gel, which was expected to form with the onset of congruent dissolution. The bulk geopolymer gel in the seeded and un-seeded geopolymers was of similar Si/Al ratio at 70 hours, as indicated by the position of the main band in the FTIR spectra (Figure 7.1). Thus, if the zeolites had formed within this gel phase (away from the nuclei in the seeded system) similar zeolites would be expected for both systems.

Aqueous and gel species have more mobility in the early stages and are therefore able to rearrange more readily into the appropriate co-ordination geometry required for crystal propagation. As such, it is proposed that the nucleation mechanism occurring in geopolymer formation for systems activated with sodium hydroxide begins with the formation of zeolite nuclei in the secondary gel. These nuclei grow rapidly until a critical point is reached at which the surrounding gel density becomes so great, that diffusion of species to the growing nuclei slows crystal growth significantly. Even when the appropriate species are available at the crystal gel border, the steric hindrance of surrounding gel and the lack of water prevents the species from attaining the appropriate conformation to maintain the order of the crystal structure. Thus there will be a reduction in crystallinity or order of the new phase from the initially formed zeolite material, leading to the formation of an X-ray amorphous gel, the “bulk geopolymer gel”.

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Over time, the depolymerisation, re-polymerisation reactions (catalysed by the hydroxide ion) continue to occur in the bulk gel, and slowly the order in the system increases. Thus a geopolymer is a zeolite precursor with very slow reactions caused by severe water deficiency, forcing crystal growth thorough gel rearrangement, rather than the traditional zeolite synthesis occurring through monomer addition from a much more dilute solution.

Although the geopolymer structure may have started its life with the view of becoming a zeolite, this does not necessarily mean that the ultimate thermodynamically stable product will be a crystalline material. If the gel structure is made up of very small particles, such as those observed by Duxson and co-workers [34] then the amorphous phase in the system may be both kinetically and thermodynamically more stable than the crystalline analogue [179]. This is because the free energy of each phase is dependent on the size of the particles, and for both nano-particles and nano-clusters, the free energy of the amorphous polymorph is lower than that of the crystal. The long term stability of the geopolymer structure is vital to the industrial applicability of these materials and thoroughly deserves further investigation.

7.3.4 Nucleation and Geopolymer Gel Growth

The wealth of knowledge in zeolite chemistry has provided a great deal of insight into the reactions occurring between aluminosilicate species in a highly alkaline environment. As described earlier, the two systems, zeolites and geopolymers, share much in common in terms of input components. In this section, diagrams are used to illustrate the adaptation of zeolite theory proposed to explain the experimental results in this thesis.

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The in situ FTIR spectra of geopolymers containing low and high quantities of nano- particles have revealed the formation of new phases not present in the spectra of the reference sample (un-seeded) and not attributed to the nano-particles themselves. An increase in the amount of nano-particles enhanced the spectral features indicating an increase in the relative amount of some new phases. Furthermore, the lag present at the start of the reaction in the un-seeded reference sample, (where no geopolymer gel formed) was not observed when seeding was used. However, the rate of geopolymer gel growth, as estimated by the functional group analysis, was the same for all systems after the onset of geopolymer gel formation.

Figure 7.7A depicts the preliminary stages of geopolymer formation in a seeded system. Initially, there is a release of Al from the fly ash, due to the lower strength of the Al–O bonds compared to the Si-O bonds [171]. Species rich in Al dissolve from the fly ash and are immediately sequestered by the nuclei; network growth begins. This Al rich phase is observed in the FTIR spectra of the seeded geopolymers even when well reacted. The addition of Al species to the growing nano-particle is a driving force for dissolution, since the concentration of species in solution will be kept low as the monomer adds rapidly to the growing nuclei, forming the Al rich phase. The uptake of Al species by the nuclei also prevents the aqueous Al species from retarding further glass dissolution [163, 180], thereby accelerating the onset of congruent fly ash dissolution.

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Release of Al rich A Al rich species through gel dissolution

Fly ash particle

NaOH solution Overall Concentration gradient

Figure 7.7A. Initial early release of Al and growth of nuclei with Al rich gel.

B Replenishing Al rich aluminosilicate gel species

Fly ash Si rich gel particle

Si Concentration gradient

Figure 7.7B. Si rich species dissolve from the Al depleted layer, creating a siliceous gel.

C Al rich gel Si rich gel Fly ash particle Si rich and bulk gel interface

Al rich and Si rich gel interface Bulk geopolymer gel

Figure 7.7C. Phase separation results from hindered gel diffusion. Bulk gel forms as congruent dissolution occurs (relative sizes of objects not to scale).

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The early release of Al creates an Al depleted layer on the fly ash which is partially depolymerised, due to Al removal and is therefore rich in silica. This layer is more highly soluble than the remaining fly ash due to its close proximity to the surface and its partially depolymerised structure, requiring the hydrolysis of fewer bonds for each molecule released. As the Si rich layer dissolves, the species again add to the surface of the growing nuclei, but hindered gel diffusion limits the mixing of the two phases. This phase separation was evident in the new bands in the FTIR spectra, observed with seeded nucleation (Figure 7.3). Furthermore, there will also be a boundary between each phase, where the composition will be somewhere in between the two phases bordering one another.

Stoichiometric release of the aluminosilicate from the fly ash begins after the dissolution of any siliceous layer. Simultaneously, the newly formed phase surrounding the added nuclei grows and consumes species from the surrounding solution or gel. In this way, the composition of the bulk geopolymer gel remains the same as that of the un-seeded geopolymer, as the bulk gel is formed in the later stages after the onset of congruent dissolution.

In the seeded system, where nucleating sites are present close to the start of the reaction, the first gel forming does not come to equilibrium with the solution initially, and the solution composition is changing with time. This leads to the phase separation, occurring due to the swing in composition caused by the early release of Al-rich species and later release of Si-rich species, before congruent dissolution. This phase separation was observed in the FTIR spectra, where the new bands were present in the seeded geopolymer (Figure 7.5).

The phase separation identified in the seeded sample does not occur in an un-seeded geopolymer. Figure 7.8 illustrates the phenomena occurring in geopolymer formation in an un-seeded system. Initially, there is the release of Al rich species, similar to the seeded system. However, in the un-seeded geopolymer, the Al-rich species are not immediately removed from the solution. Dissolved species, initially rich in Al, migrate from the fly ash particle into the “bulk” (solution between the particles), driven by the overall

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concentration gradient. A gel only forms after some time, most likely when the super saturation limit is reached. When gelation occurs, via condensation reactions, water is released. This water is then available for further fly ash dissolution. The congruent dissolution begins after the formation of an Al depleted layer in the fly ash. Due to the high Si/Al ratio in the amorphous material in the fly ash, (Si/Al ~ 2, Chapter 3) further dissolution enriches the newly formed Al rich gel with silica (Chapter 4). This new silica rich gel slowly comes to equilibrium with the surrounding solution phase, through a series of depolymerisation, re-polymerisation reactions, catalysed by the hydroxide ion. This equilibrated gel is then referred to as the “secondary gel” [46] analogous to that formed in zeolite synthesis [122].

In a system where no nucleating sites are provided, the initial gel formed was observed to be rich in Al before subsequent Si augmentation, as was found in Chapter 4, and by other authors [46]. The Si/Al ratio of the gel has stabilised and congruent dissolution is occurring before the nucleation occurs. Thus the formation of phase separated gels, such as the silica rich phase seen in the seeded geopolymer, would not be expected. The nucleating sites form in a secondary gel, which is in pseudo-equilibrium with the aqueous phase. Due to the long time taken for nucleation (indicated by the lag period, Figure 7.2), it is likely that the composition of the aqueous phase had stabilised somewhat by the time the first nuclei had formed. These nuclei are of similar composition to the gel from which they developed and therefore form a phase which is of similar composition to the surrounding gel. Species being added to the growing nuclei need not migrate far from the gel border.

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Release of Al rich A Al rich species through gel dissolution Fly ash particle

NaOH solution Overall Concentration gradient

Figure 7.8A. Initial early release of Al and formation of Al rich primary gel.

B Replenishing aluminosilicate species Gel enriched with Si

Fly ash particle

Si Concentration gradient

Figure 7.8B. Dissolution of Si-rich layer allows the gel to become enriched with Si prior to the onset of congruent dissolution.

C Nuclei form Residual Fly ash

“Secondary gel”

Figure 7.8C. Si rich gel comes to pseudo-equilibrium with the surrounding solution/solids and after some time, nucleation occurs.

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7.4 Conclusions

A novel in situ method has been used to investigate the early gel formation chemistry in a geopolymer system. High surface area Al 2O3 nano-particles were added in small quantities to a geopolymer activating solution prior to mixing. It was found that the 42 hour reaction lag occurring in regular sodium hydroxide activation of fly ash (Chapter 5) does not occur when the synthesis mixture contained the Al 2O3 nano-particles. Furthermore, there was a phase separation in the gel in the system seeded with nano- particles, where a gel very high in silica was formed, indicated by the appearance of a band at 1100cm -1 in the FTIR spectra. Several other silicate phases of varying Si/Al ratio were also present, indicated by new features in the FTIR spectra, not seen in the un- seeded samples.

A reaction model for gel formation in alkali activated fly ash geopolymer systems was presented. The model proposed a reaction mechanism similar to that observed in zeolite synthesis. The selective leaching of Al from the fly ash produces an Al rich gel, referred to as the “primary gel”. The Al deficient layer remains on the fly ash surface, with a partially detached silicate network. This dissolves, followed by the stoichiometric release of Al and Si species from the fly ash. Over time, diffusion in the gel, driven by the concentration gradients, allows the aqueous phase to become more homogeneous. The gel slowly comes to pseudo-equilibrium with the surrounding solution, through a series of depolymerisation, re-polymerisation reactions, catalysed by the hydroxide ion. This new equilibrated gel is referred to as the “secondary gel”.

Through further gel reorganisation, local areas with increased structural order begin to form referred to as “proto nuclei”. These are regions with sufficient order such that the rate of re-polymerisation can exceed that of depolymerisation; if this occurs, the region begins to grow and nucleation has occurred. Monomers, supplied by local gel dissolution, begin to add to the nuclei and form zeolite nano-crystals. The crystals grow until the gel density is so great that diffusion and gel rearrangement become hindered. The gel structure rapidly reduces in crystallinity outwardly from the original nuclei, and the

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structure becomes X-ray amorphous; this material is referred to as the bulk geopolymer gel.

When nano-particles are added to the geopolymer system, no lag occurs in the reaction. Two possible mechanisms were developed to explain this observation. One model was proposed in which the nano-particles catalysed the formation of nuclei or acted as the nuclei, thus the time taken for the system to form stable nuclei was eliminated. In the initial stages, when Al-rich species are released into the solution, immediate addition to the nuclei occurs, forming a dense Al-rich gel. Structural reorganisation of this gel around the nuclei leads to the formation of the zeolite crystals. Crystal growth occurs until the mobility of chemical species is reduced by the high concentration and further addition becomes disordered.

The dissolution of the siliceous layer on the fly ash then releases Si rich species into the solution, which also add rapidly to the growing nuclei, forming a silica rich gel layer. The congruent dissolution then releases Al and Si species which also add to the growing nuclei in a similar way to the un-seeded system, creating the bulk geopolymer gel, with a similar composition and structure to that of the un-seeded system. The similarity in the bulk gel of the seeded and un-seeded systems was observed in the identical positions of the main Si-O-T asymmetric stretching band in the FTIR spectra. The phase separation in the seeded geopolymer was hypothesised to result from the different gel layers and the interfacial regions of the gels, which will have a composition in between the adjacent phases.

An alternative model was also proposed in which the initially dissolved Al species act to passivate the fly ash surface by adsorption, limiting the early dissolution. The added nano-particles act only to remove Al species from the solution initially, and provide a surface for gel growth. Within the gel, a structural reorganisation process (equivalent to that described in the un-seeded system) forms the stable nuclei for network growth. This alternative model also explained why only a small amount of nano-particles was required (less than 0.1% by weight) and why the rates of formation in the seeded and un-seeded

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systems were the same at the end of the lag period. At this stage, further research is required to fully understand the dominant mechanism.

Although it was determined that the rate for both seeded and un-seeded geopolymer gel growth was similar, the rate limiting factor in each system was not able to be determined. Several possibilities were proposed, including hindered gel diffusion and dissolution of the precursor gel. This area should also be further investigated in future research.

149

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Chapter 8 Conclusions and Recommendations

The principal outcome of this thesis is the development and application of a novel in situ analytical technique, allowing the kinetics of gel development and transformation in geopolymer systems to be monitored over time. The use of this technique allows the development of a new conceptual model. Changes in gel structure are investigated over 200 days at ambient temperatures for a large sample set, mapping the composition ranges used for geopolymer synthesis. This work provides a better understanding of the gel ageing process at different activator concentrations. Other solid materials are also tested for suitability both as an additive to fly ash geopolymers and as the main binder material. This work led to the development of a 1-part mix geopolymer cement, structurally similar to fly ash analogue and believed to be the first of its kind.

Fly ash geopolymer synthesis begins with the solid particles contacting the activating solution. Predominantly Al-containing species are liberated from the surface, leaving behind a partially detached Si-rich layer. When the diffusion of the Al species through the siliceous layer into the solution becomes rate limiting, the Si-rich layer begins to dissolve more rapidly than the Al species. The rate and extent of Al liberation are found in Chapter 4 to be a function of the NaOH and Si concentration in the initial activating solution. Silicate monomers and smaller species have an accelerating effect on early fly ash dissolution; these species are only present in concentrated silicate solutions if the sodium hydroxide concentration is also high. The enhanced dissolution may be caused by the direct participation of the silicate monomers in the dissolution reaction; the silicate monomer may act to destabilise the T-O bonds (T = Al or Si) allowing easier cleavage. Alternatively, the silicate may sequester any dissolution inhibiting species in the solution, such as alkaline earth ions. Inhibition can be caused by co-adsorption of ions to the fly

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ash surface or even film formation, preventing the approach of hydroxide and therefore reducing the rate of bond cleavage in the aluminosilicate.

At some point during the dissolution, as species are released from the fly ash into the Al containing solution medium, a gel is formed. Geopolymers with activating solution concentrations from 0-1.2M SiO 2 or with higher silicate and high hydroxide concentrations are found in Chapter 4 to form an Al-rich gel before the final composition is reached. Thus the entire Si-rich layer has not completely dissolved prior to gel formation. The preliminary gel, rich in Al, transforms into a more Si enriched gel due to the dissolution of the residual Si-rich layer from the fly ash.

The local order in the gel continues to increase over time due to the constant breaking and reforming of bonds, catalysed by the hydroxide in solution. At some stage, within localised regions, the order becomes so great that the rate of bond formation exceeds that of bond destruction. The network grows from these regions of local order, or nuclei. A nucleation mechanism in geopolymer synthesis has previously been proposed by other authors by assimilation of zeolite chemistry with the geopolymer system; however little other evidence has been provided to support the hypothesis. Similar to zeolite synthesis, the rate of network growth is found in Chapter 5 to be linear, after a lag period in which there is dissolution but no significant geopolymer network formation.

Both the early fly ash dissolution mechanism and the nucleated network growth are supported by the results of Chapter 7, in which nano-particles are added to the activating solution as potential nucleation sites. Species liberated from the fly ash added immediately to the nuclei and geopolymer network formation started from time zero. The lag period, observed for all other geopolymers (Chapters 5 and 6), is eliminated by the addition of the nano-particles. This lag period is identified as the time taken for the system to produce its own stable nuclei. Different gel phases, not present in the reference samples, are also observed to form in the geopolymers seeded with nano-particles. The initial dissolution of Al from the fly ash led to the formation of an Al-rich gel phase close to the nuclei, followed by a Si-rich phase closer to the bulk of the geopolymer. Interfacial

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boundary regions are also observed with compositions between those of the different phases. These relatively stable phases of different compositions are still present in the cured geopolymers after 100 days. Unlike the early gel development in the un-seeded geopolymer, the initially formed gel in this case did not change in composition before the final geopolymer network is formed.

The same network growth mechanism is not observed in Chapter 4 for geopolymers with very high silicate activating solution concentrations. An Al-rich gel only formed when the sodium hydroxide concentration is also high, meaning that the speciation in the activating solution consisted mainly of monomers. In the systems containing larger silicate species, the silicate is found to simply add to the geopolymer network forming a Si-rich gel initially, rather than a dissolution accelerant liberating more Al. An Al-rich gel is not detected; instead, the final gel composition is rapidly reached, often within 24 hours. It is hypothesised that the amount of dissolved species in these activating solutions is so high, that without additional hydroxide to stabilise the aqueous phase, a gel forms prematurely during the dissolution of predominantly Al species from the fly ash. At this stage, the dissolution of the Si-rich layer on the fly ash has not yet begun. The permeability of this gel is hypothesised to be sufficiently low that no further diffusion of dissolved species into the network occurs and thus the initial Si/Al ratio of the gel remains unchanged. This low permeability is thought to be due to the large number of polymeric silicate species present in the activating solutions.

The rate of fly ash dissolution is found in Chapter 5 to be dependent on the free hydroxide concentration in the solution; the rate of network formation and the length of the lag period are also found to be dependent on the sodium hydroxide concentration for alkali activated fly ash geopolymers. A wide range of hydroxide concentrations are tested and the maximum rate is observed at approximately 9M NaOH. Higher concentrations reduce the rate of network formation, thought to be due to the effects of strong ion pairing.

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A similar trend is also observed in Chapter 6 for the solid silicate-containing geopolymers; the addition of a large amount of additional solid silicate material, at a constant hydroxide concentration slowed the initial reactions and elongated the lag period. In these systems, the free hydroxide concentration is lowered by the liberation of

SiO 2, from the solid silicate into the solution in the early stages. High silicate concentrations require an increased hydroxide concentration to maintain an adequate level of free hydroxide in the solution. Similar observations are also made for the silicate solution activated geopolymers in Chapter 4. It is clear from these results that the early reactions are highly dependent on the free sodium hydroxide concentration, also affected by the presence of soluble silica.

Early gel formation in geopolymers is caused by the presence of sufficient dissolved species. An appropriate Al/Si ratio and a high pH solution are also needed. Furthermore, the accumulation of Al and Si species in the solution must be slow enough to avoid sudden precipitation, thus allowing a dense network to form. The silicate activating solutions used in Chapter 4 are stable for several weeks at room temperature and did not form a gel without the addition of an Al source (fly ash in Chapters 4 and 5). Furthermore, it is known that the geopolymer network cannot be formed from a source of Al alone; the Si species are needed. Thus, to form a geopolymer, a Si-rich phase can be slowly dissolved into an Al-rich solution, or an Al-rich phase can be slowly dissolved into a solution containing Si species. This hypothesis is tested in Chapter 6, whereby a rapidly dissolving sodium aluminate powder is dry mixed with a relatively pure source of silica, and water is added, making a 1-part mix geopolymer cement. The sodium aluminate dissolved immediately, raising the pH and catalysing dissolution of the silica. The lag period for this system is quite long. However, the molecular structure of the geopolymer at 100 days is found to be comparable to that of a similar composition fly ash geopolymer examined in Chapter 4. Thus, sodium aluminate activation of amorphous silicates can also be used to produce geopolymer materials.

This thesis has provided new insight into the processes governing geopolymer formation, and a novel experimental tool was developed by which the process can be analysed,

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allowing a better understanding of how different materials behave during geopolymer formation. Ultimately, this will lead to the development of superior binder materials and the wider application of geopolymer technology.

Further work is required to study the various geopolymer systems using the novel analytical technique developed in this thesis. Kinetic equations could then be formulated, leading to a semi-empirical model which will allow the kinetics of network growth in geopolymer systems of different compositions to be estimated. Such equations would allow specific tailoring of geopolymer cements for various applications. This method also has the potential to give a better understanding of the differences between metakaolin and fly ash geopolymer systems. This will lead to more accurate use of the recent advances in the understanding of metakaolin geopolymer systems, when applied to fly ash geopolymer formation.

A more detailed model of the chemical reactions occurring in geopolymer synthesis could also be developed using results of the in situ ATR-FTIR. In particular, peak deconvolution and the use of deuterium exchange in the region relating to silanol groups would give insight into the hydrolysis reactions and gel formation. Such chemical equations could be incorporated into the kinetic model for geopolymer synthesis. Further work should also be done using different nano-particles for seeding the mixtures. Different nano-particles may induce varied behaviour if the surface has different properties which affect the attraction and bonding of aluminate or silicate species. Of particular interest are the type and extent of zeolite formation, as these are known to significantly affect binder properties. This method represents another potential mechanism for controlling binder structure on a molecular level and therefore tailoring the system for specific applications.

The physical properties of the 1-part mix, aluminate activated silica geopolymer also need to be tested. This system requires further development for use as a novel cement material and also for waste stabilisation applications. The suitability of other silica waste sources must also be investigated, such as rice hull ash, slags, other agricultural ashes,

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different fly ashes and mine tailings. In particular, the effect of various contaminants such as Ca and Fe could be investigated for high Fe or Ca wastes. The ATR-FTIR technique is suitable to use on all geopolymer systems and so could be applied to other pure systems such as the ethoxide derived aluminosilicate sol gel process used recently.

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Publications

C.A. Rees, J.L. Provis, G.C. Lukey, J.S.J. van Deventer, ATR-FTIR analysis of fly ash geopolymer gel ageing, Langmuir In press, DOI 10.1021/la700713g (2007).

C.A. Rees, J.L. Provis, G.C. Lukey, J.S.J. van Deventer, In situ ATR-FTIR study of the early stages of fly ash geopolymer gel formation, Langmuir Accepted for publication

(2007).

C.A. Rees, J.L. Provis, G.C. Lukey, J.S.J. van Deventer, Geopolymer gel formation with seeded nucleation, and mechanistic implications, submitted to Colloids and Surfaces A:

Physicochemical and Engineering Aspects (2007).

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176

Minerva Access is the Institutional Repository of The University of Melbourne

Author/s: REES, CATHERINE ANNE

Title: Mechanisms and kinetics of gel formation in geopolymers

Date: 2007

Citation: Rees, C. A. (2007). Mechanisms and kinetics of gel formation in geopolymers. PhD thesis, Faculty of Engineering, Chemical and Biomolecular Engineering, The University of Melbourne.

Publication Status: Unpublished

Persistent Link: http://hdl.handle.net/11343/39579

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