Aqueous solubility diagrams for cementitious waste stabilization systems. 1. The C-S-H solid-solution system

The major component in cementitious waste solidification/stabilization (S/S) systems is an amorphous calcium silicate hydroxide hydrate phase (C-S-H) phase according to cement chemistry nomenclature), but the behavior of this gel phase in aqueous solution is not well understood. In this paper, a thermodynamic model is suggested for predicting aqueous solubility of C-S-H as a function of the Ca/Si ratio based on a solid solution−aqueous solution (SSAS) subregular equilibrium model for a binary non-ideal mixing of the end-member phases calcium hydroxide and calcium silicate, with known pK values and a miscibility gap at Ca/Si > 3. A maximum in Gibbs free energy of mixing (GM) of −3.2 kJ/mol is predicted to occur at a Ca/Si ratio of 1.5. Predicted Ca and Si solubility, pH, and solid Ca/Si values agree with experimental data on C-S-H solubility reported from the literature. The thermodynamic basis of this SSAS model is superior to those based on empirical Ksp values vs Ca/Si ratio relationships or on ideal so...