Thermodynamic modelling of phase equilibria in cement systems: multiple sublattice model for solids in equilibrium with non-ideal aqueous phase

The thermodynamics of the CaO-SiO2-H2O (C-S-H) gel phase present in cement systems has traditionally been modelled in terms of one or more stoichiometric solid phases. We present a model for the C-S-H gel phase based on non-ideal solution of ions and/or molecular species on a number of sublattices similar to that used extensively in National Physical Laboratory's MTDATA software for modelling oxides and alloy phases. In application to the C-S-H system, aqueous phase compositions are represented within experimental uncertainties, and measured pH values are predicted exceptionally well although not included in the model parameterisation. Extensions to the base C-S-H model to account for the presence of Al2O3, modelling of the analogous M-S-H phase in MgO containing systems and an application to nuclear waste disposal technology where the portioning of uranium between the gel, aqueous and other phases is calculated, are also discussed.