The use of thermodynamic methods in cement hydration was often doubted, as the watercement system was considered to be too complex to model. Furthermore metastable features occur, e.g. C-S-H, which lead to the conclusion cement hydration is a 'non-equilibrium' process. Nevertheless several literature studies prove that cement hydration follows the basic principles of physical chemistry by minimisation of the free energy of an isochemical system. Hence thermodynamic equilibrium models are useful to assess and predict mineralogical changes during cement hydration. However the success and the accuracy of these predictions are strongly linked to a reliable thermodynalU'ic database, including the standard state properties ofthe 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(OHh 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 ofthe 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 hydrog~rnet 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. The constitution of the AFm phase in Portland cement is very sensitive with respect to its chemical environment. Carbonate is shown to interact strongly with stabilisation of AFm across a broad range oftemperatures and, at low temperatures, to substitute into AFt. Relative to previous databases, sulfate AFm is shown to have a defmite range of stability at 25°C thus removing long-standing doubts about its stability in normal hydrated cement pastes. Keywords: thermodynamics, thermodynamic data, modelling, cement hydration, AFm, AFt, sulfate, carbonate
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:485385 |
| Date | January 2007 |
| Creators | Matschei, Thomas |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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