Thermodynamics of Cement Hydration

UNIVERSITY OF ABERDEEN DEPARTMENT OF CHEMISTRY Thermodynamics of Cement Hydration by Thomas Matschei Dipl.-Ing., Bauhaus University Weimar A Thesis presented for the degree of Doctor of Philosophy at the University of Aberdeen Aberdeen, 06 December 2007 Declaration This Thesis is submitted to the University of Aberdeen for the degree of Doctor of Philosophy. It is a record of the research carried out by the author, under the supervision of Professor F.P. Glasser. It has not been submitted for any previous degree or award, and is believed to be wholly original, except where due acknowledgement is made. Thomas Matschei Aberdeen, December 2007 Abstract 3 Abstract The application of thermodynamic methods to cement science is not new. About 80 years ago, Bogue wrote a series of equations describing the relationship between clinker raw meal chemical composition and the mineralogy of the finished clinker. These enabled the amounts of minerals to be calculated from a bulk chemical composition. Fundamental to the equations was a precise description of the high temperature equilibrium achieved during clinkering. Bogue admitted four oxide components into the calculation; lime, alumina, silica and ferric oxide and assumed that equilibrium was attained (or very nearly attained) during clinkering. This approach, which is, with modifications, still a widely used tool to quantify cement clinkering, was one of the main motivations of this work. Thus the overall aim of this Thesis is to provide a generic toolkit, which enables the quantification of cement hydration. The use of thermodynamic methods in cement hydration was often doubted, as the water-cement system was considered to be too complex. Furthermore metastable features occur, e.g. C-S-H, which lead to the conclusion cement hydration is a “non-equilibrium” process. Nevertheless pioneering works, by Damidot and Glasser, as well as from other groups e.g. Reardon et al. and Berner et al. prove that cement hydration follows the basic principles of physical chemistry by minimisation of the free energy of an isochemical system. Hence these studies demonstrated the usefulness of thermodynamic equilibrium models in cement hydration. However the success and the accuracy of these predictions are strongly linked to a reliable thermodynamic database, including the standard state properties of the aqueous species and the cement hydrates. Whereas the thermodynamic properties of the aqueous ions are well described in the literature, the dataset for cement hydrates is incomplete or inconsistent, or both. Thus the main goal of this Thesis was to develop a consistent thermodynamic database, which enables the assessment of the constitution of hydrated Portland cements. Because hydrated concretes are exposed to different service temperatures, data were obtained in the range ~1°C to 99°C. The database is developed for commonly-encountered cement substances including C-S-H, Ca(OH)2, selected AFm, AFt and hydrogarnet compositions as well as solid solutions. Literature data were critically assessed and completed with own experiments. The tabulated thermodynamic properties were derived by a harmonisation of the available data. The new database enables the hydrate mineralogy to be calculated from the bulk chemical composition of the system: most solid assemblages, the persistence of C-S-H and failure to nucleate siliceous hydrogarnet apart, correspond closely to equilibrium. This realisation means that hydrate assemblages can be controlled. The development of a thermodynamic approach also enables a fresh look at how mineralogical changes occur as a function of cement composition as well as in response to environmentally-conditioned reactions. According to a literature review the constitution of the AFm phase in Portland cement is very sensitive with respect to its chemical environment. Except for limited replacement of sulfate by hydroxide, AFm phases do not form solid solutions and, from the mineralogical standpoint, behave as separate phases. Therefore, in dependence of the bulk chemical composition, many hydrated cements will contain mixtures of AFm phases rather than a single AFm solid solution. Relative to previous databases, sulfate-AFm is shown to have a definite range of stability at 25°C thus removing long-standing disagreement with theory about its persistence in hydrated cement pastes. - 2- Carbonate is shown to interact strongly with AFm and displaces OH and SO4 at species activities Abstract 4 commonly-encountered in cement systems across a broad range of temperatures ≤50°C. Many of the predicted reactions were confirmed by focussed experiments and literature studies. Possible anion substitutions in the AFt phase were investigated. Non-ideal thermodynamic models for SO4-CO3-AFt and ettringite-thaumasite solid solutions were derived from solubility experiments. Whereas at 25°C only minor anion substitution is likely, low temperatures tend to stabilise carbonate substituted AFt phases. Possible pathways of thaumasite formation were developed. It was concluded that there is no single route of thaumasite formation, but several pathways for thaumasite formation may occur simultaneously. Limestone, mainly consisting of calcite, is a permitted additive to Portland cements up to a 5 wt.-% limit under EN 197. The final chapter, on the impact of calcite addition upon cement hydration, enables a quantitative approach to its interaction with cement phases and prediction of space filling properties of pastes. The distribution of sulfate in AFt and AFm is much affected by the presence of carbonate. In the presence of portlandite the stabilisation of carboaluminate results in changes of the amounts of both portlandite and AFt: specimen calculations are presented to quantify these changes. Calculations of the specific volume of solids as a function of calcite addition suggest that the space filling ability of the paste is optimised when the calcite content is adjusted to maximise the AFt content. Additional calculations show how sulfate and carbonate distribution are affected by temperature. Carboaluminates become increasingly unstable at elevated temperatures, ≥ 50°C, whereas carbonate substitution in AFt is favoured at low temperatures in the presence of calcite. The resulting consequences of thermal cycles on the space filling properties of hydrated cements are discussed. Keywords: thermodynamics, thermodynamic data, modelling, cement hydration, AFm, AFt, sulfate, carbonate Acknowledgement 5 Acknowledgement Several people contributed to the successful completion of this Thesis, to whom I am grateful and indebted: Professor Fred Glasser for his excellent supervision of this project during my time in Aberdeen. I am most grateful for the advice and support he gave me throughout the duration of this Thesis. I enjoyed our long-lasting, motivating and academically stimulating discussions, mainly related to cement -surely one of the most fascinating manmade materials of this world-, but also to several other aspects of “daily life”. Dr. Barbara Lothenbach, EMPA Dűbendorf, my Thesis co-supervisor, for invaluable assistance with questions about thermodynamic modelling and guidance with GEMS-PSI. Her enthusiasm contributed to the completion of the database and related applications. My industrial advisors, Dr. Ellis Gartner, Lafarge Central Research, France, and Dr. Duncan Herfort, Aalborg Portland Group, Denmark, for stimulating discussions and guidance during this work. Nanocem, a research network of European cement producers and academic institutions, for funding this work and for giving me the opportunity to present and discuss the results at several intern meetings as well as at international conferences. Special thanks to Professor Karen Scrivener, representing members of Nanocem and the Nanocem steering-committee, for valuable discussions and helpful critics during the preparation of publications related to this Thesis. I would like to thank Marie-Alix Dalang-Secrétan for her help with administration throughout this project and for assistance with the preparation of the Workshop “Thermodynamic Modelling”. Dr. Dmitrii Kulik, PSI, Switzerland, for troubleshooting and assistance with GEMS-PSI and for helpful advice during the preparation of the thermodynamic database. In that respect I would also like to thank Dr. John Gisby, NPL, UK, for his comments. The staff of the Chemistry Department, University of Aberdeen, for technical assistance. I would like to thank Professor Jőrg Feldmann and his TESLA-team for invaluable guidance and introduction in “analytical methods for civil engineers” as well as for giving me the opportunity to participate in “various” group meetings. Thanks to Professor Donald Macphee for inspiring discussions about cement science, especially with respect to thaumasite formation. I enjoyed the refreshing discussions with the recently formed “cement-group” as well as with my colleagues from office “G 85”. The staff at EMPA Dűbendorf, for great technical and intellectual support during my stay in Switzerland. I would like to thank my examiners, Professor Denis Damidot, Ecole des Mines de Douai, France, and Professor Donald Macphee, University of Aberdeen, for critically reviewing this work. Finally, special thanks go to Kristina,… for her infinite patience with this “cement guy” my friends, whose support I appreciated throughout the years ein besonderes Dankeschőn gebűhrt meiner Familie in Deutschland, insbesondere meinen Eltern, Grosseltern, sowie allen Verwandten, die mich űber Jahre hinweg

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