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The Hydrothermal Formation of Calcium Silicate Hydrates

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The Hydrothermal Formation of Calcium Silicate Hydrates THE HYDROTHERMAL FORMATION OF CALCIUM SILICATE HYDRATES. BY D. R. MOOREHEAD THESIS SUBMITTED AS REQUIRED FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF NEW SOUTH WALES. SYDNEY, 1963. I HEREBY CERTIFY THAT THIS WORK HAS NOT BEEN SUBMITTED FOR A HIGHER DEGREE TO ANY OTHER UNIVERSITY OF INSTITUTION. D. R. MOOREHEAD ABSTRACT In this thesis a study is reported of the kinetics of solution of quartz and the crystallization of calcium silicate hydrates during the hydrothermal treatment of quartz in saturated lime solutions. Single crystals of quartz were used to facilitate the examination of the interaction of the lime with the quartz surface. Sectioning of the product layer and subsequent microscopic investigation showed the calcium silicate hydrate to be of fibrous crystal habit and to grow radially from nucleating centres in a general direction away from the quartz surface. X-ray analysis showed the mineral formed to be mainly xonotlite. Measurements of the weight increase of a quartz crystal were made at various times during runs at 335°C. and 235°C. Plots of the quantity of xonotlite versus square root of time gave a straight line indicating that the process was diffusion controlled. The crystallization of the mineral did not follow the receding surface of the quartz. Extension of the product layer took place only on the surface in contact with the lime solution. The silicate ions apparently diffused readily through the product layer. Measurements were made to determine the selective action of the mineral membrane towards the diffusion of calcium ions in association with chloride and hydroxyl ions and of sodium ions in association with chloride ions. These results showed the membrane had no selective action towards the diffusion of calcium ions when associated with hydroxyl ions. The solubility data for calcium hydroxide in water was extended from 180° to 300°C. From this data, estimates -ii- of the diffusion rates were made for the species Ca and HoSi0,. These calculations showed the diffusion rate of the l^SiO^ species to be many orders greater than that of I | the Ca species. This information was used to account for extension of the product layer on the surface in contact with the lime solution. Other systems including barium hydroxide-quartz and calcite-silica gel were briefly investigated. TABLE OF CONTENTS CHAPTER (1) 1.10 Introduction 1 1.20 Literature Review 2 1.21 Structure and Morphology of Calcium Silicate-Hydrates 3 1.22 Effect of Temperature on Phases Produced 5 1.23 Some Proposed Mechanisms 9 1.24 Water of Hydration and Free Water 11 1.30 The Objective of the Work 14 CHAPTER (2) 2.00 Experimental Part 15 2.10 Reagents and Materials used 15 2.11 Preliminary Experiments 16 2.21 Physical and Chemical Examination of th^roduct Formed at 350°C 21 2.22 Microscopic Examination 23 2.23 Electronmicroscopic Study 28 2.24 Chemical Analysis 28 2.25 Thermogravimetric Study 32 2.26 Measurement of Porosity 34 2.27 Infra-Red Absorption Spectra 34 2.31 Kinetic Study of the Formation of the Product Mineral at 335° and 225°C 34 2.32 Microscopic Examination 42 2.33 X-Ray Analysis 42 2.40 Diffusion of Ions Through the Porous Product Material 44 2.50 Solubility of Calcium Hydroxide at 300°C 50 2.60 Work at Supercritical Temperatures 51 2.61 X-Ray Examination 51 2.62 Electronmicroscopic Examination 59 2.70 Experiments with Calcite Crystals 59 2.80 Barium Hydroxide-Quartz Hydrothermal Reaction 62 CHAPTER (3) 3.00 Discussion of Results 64 3.10 Preliminary Work using X-Ray Analysis 64 3.11 Products formed at 235°C 64 (a) X-Ray Analysis 66 (b) I.R. Study 67 3.12 Identification of Products formed at 335°C 69 (a) X-Ray Analysis 69 (b) I.R. Study 72 (c) Thermal Weight Loss 73 3.13 Products formed at Super Critical Temperatures 73 (a) X-Ray Analysis 3.21 Experiments with the Calcite-Silica System 74 3.22 Experiments with the Barium Hydroxide-Quartz System 75 3.30 Morphology and Macro Structure of Products 76 3.31 Microscopic Examination (a) Products formed at 235°C 76 (b) Products formed at 335°C 77 (c) Products formed at Super Critical Temperatures 77 3.32 Electronmicroscopic Study of Products 78 3.33 Macro Structure 80 (a) Pore size Distribution 80 (b) Ultrasonic Dispersion 80 3.40 Kinetics and Mechanism of Formation of Xonotlite 82 3.41 Activation Energy 82 3.42 (a) Mechanism at 335°C 82 (b) Mechanism at 500°C 83 3.43 Characteristics of the Dissolution of Quartz in Saturated Lime Solution 85 3.44 Diffusion of Ions through the Product Layer 86 3.45 Solubility of Calcium Hydroxide at 300°C 86 3.46 Diffusion Coefficients 88 CHAPTER (4) 4.00 Practical Implications 93 4.10 Further Work 94 4.20 Concluding Remarks 95 4.30 Acknowledgements 96 4.40 References 97 CHAPTER (5) 5.00 Other Publications submitted for Collateral Credit 103 5.10 Light weight Calcium Silicate Hydrate 104 5.11 Discussion of the Above Paper 111 5.12 The Sucrose Extraction Method for the Estimation of Available CaO in Hydrated Lime 112 1 1.10 INTRODUCTION: Calcium Silicate Hydrates are of considerable importance in pure and applied chemistry and in geochemistry if for no other reason than the widespread occurrence of the materials involved in their formation. Technologically they feature as the binding or cementing minerals formed during the hydration of Portland cement and during the autoclave treatment of building units made from mixtures of lime and siliceous materials. Previous work by Taylor^^ and the writer on some applied studies of the system CaO - SiC^ - ^0 as a cementing matrix led to this present work being under­ taken. These studies were concerned chiefly with the strength development of mixtures of crushed quartz and lime under a variety of hydrothermal conditions. It was found from this work that the depth of erosion on the quartz particles of these test pieces was less than one micron. The formation of a microcrystalline product surrounding the quartz particles was responsible for the development of a binding matrix. Compressive strengths in excess of 30,000 P.S.I. were achieved from some test pieces. With these facts in mind the obvious area to which to direct our efforts appeared to be towards the understanding of the mechanism of the reaction at the quartz-solution interface and the study of the thin layer of products surrounding the quartz particles. 2 1.20 LITERATURE REVIEW. The system CaO - SiC^ - H^O has been studied extensively and there now appear in the literature some well established data of phase compositions and equilibria that exist under a variety of conditions, and using a diversity of starting materials. Most of the workers have been motivated in their investigations by the need to acquire a more complete understanding of the physical and chemical changes taking place during the hydration of Portland cement and the hardening of silica- lime mixtures during autoclave treatment. The processes of cement hydration have been studied by many workers and although the mechanisms are related to some extent, only the specific interactions of the CaO - Si02 - H^O system will be dealt with in this thesis. (2) Steinourv has made an adequate review of the literature in this field up to 1947. Until relatively recent times, the work has been hindered by lack of suitable equipment to distinguish between and make quantitative estimates of the poorly crystalline phases that are produced by the interactions mentioned above. The application of newer techniques such as X-ray diffraction, electron microscopy, differential thermal analysis, thermal balances and infra-red spectroscopy have made the contribution of workers since this time more significant. 3 1.21 Structure and Morphology of the Calcium Silicate Hydrates These minerals have characteristic chain silicate structures which have been described by Taylor^ ' The SiO^ chains are kinked in such a way as to repeat at intervals of three tetrahedra and have therefore been called "Dreierketten". This distinguishes them from other kinked chains such as pyroxene chains or "Zweierketten". The semi-crystalline phases* C.S.H.(I) C.S.H. (II) and gels formed in the hydration of C^S and Portland cement are related structurally to tobermorite and also belong to this group. In the case of xonotlite the chains are condensed together in pairs giving an analogue of the double "Zweierketten" or amphibole chain. (Fig. 1.21 (a)) The structure of xonotlite has been determined by Mamedov^^ , and is illustrated by a photograph of the model (Fig. 1.21 (b)); it belongs to the monoclinic system with cell constants a = 16.95 b = 7.33 c = 7.03 R ft = 90°. Megaw and Kelsey^8) ^ave described the structure of tobermorite to be very nearly orthohombic with cell constants a = 11.3b = 7.33 c = 22.6 R, a layer structure with layers parallel to the (001) plane. On dehydration they noticed a shrinkage of the (002) spacing indicating * Cement chemical nomenclature C = CaO, S = Si02, H = H20 FIG.1.21(b) ItGDBL OF XONOTLITE STRUCTURE* Cft.(white). (OH).(Blue)• (G).(clear). (si).(red). 5 that the layers were packing more closely together when the water was driven off. The suggestion is made by (18 ^ Mamedov and Belov ' that there may be a close structural resemblance between what is designated as 11.3$ tobermorite and xonotlite. Taylor(v 9)J observed that a high degree of order is preserved in the transformation of tobermorite to xonotlite. The morphology of tobermorite apparently depends to a large extent on the conditions of formation.
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