Mechanisms and Kinetics of Gel Formation in Geopolymers
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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 ii There is no duty we so much underestimate as the duty of being happy. Being happy we sow anonymous benefits upon the world… RoRoRobertRo bert Louis Stevenson (1850(1850----1894)1894) iii iv 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 v 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. vi 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 vii viii 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). ix 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. x 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 xi 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 xii Chapter 6 Alternative Raw Materials for Geopolymer Synthesis: Effect of Si/Al ratio on Reaction Rate and Chemical Structure……....