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.