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 Cal Si ratio based on a solid solution-aqueous solution (SSAS) subregular equilibrium model for a binary nonideal mixing of the end-member phases calcium hydroxide and calcium silicate, with known pi(values and a miscibility gap at Ca/Si > 3. A maximum in Gibbs free energy of mixing (G(M)) 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 K-sp values vs Ca/Si ratio relationships or on ideal solid solution models yet used in cement chemistry. It opens access to predicting the solubility of trace metals bound into the C-S-H host phase when the K-sp values and possible mixing gaps of the end-member phases are known.