The most common phases in cementitious waste solidification/stabilization (S/S) systems are amorphous calcium silicate hydoxide hydrate gels (C-S-H gel according to cement chemistry nomenclature), with both a high specific surface (> 100 square meter per gramm) and high trace metal sorption capacity. The behaviour of this C-S-H gel in aqueous solution is as yet not well understood. In the present paper, a thermodynamic model is suggested for predicting aqueous solubility of C-S-H as a function of the Ca/Si ratio based on Lippmanns solid solution - aqueous solution (SSAS) subregular equilibrium model for a binary
non-ideal mixing of the end-member phases calciumhydoxide and calciumsilicate, with known pK's and a miscibility gap at Ca/Si > 2.5. Predicted solubility data for Ca, Si and pH are in reasonable agreement with relevant experimental data reported from the literature. The thermodynamic basis of this SSAS model is superior to any of the various empirical fitting models used in cement chemistry, e.g. those based on variable Ksp values as a function of the Ca/Si ratio or on ideal solid solution models. It opens access to predicting the behavior of impurities in a multi-component system not previously accounted for by empirical models, that is of the cementitious S/S process. This will be exemplified for the solid solution of the pure trace-component end-member solid Zn(OH)2 by substitution in the octahedral Ca(OH)2-layers of CSH(I)-phases. The "solutus" plot of this non-ideal, pseudo-binary Zn/Ca-hydroxide system reveals a peritectic point due to a miscibility gap similar to that in the Zn/Ca-carbonate system. Aqueous solutions can achieve considerable undersaturation with respect to Zn-hydroxide for compositions below
that point, which has been verified experimentally. Comparison of the SI plots calculated for both systems predicts that
carbonatization will not enhance the Zn solubility in this CSH solid solution system. Such model results are important for predicting the long-term fate of hazardous waste upon carbonatization of the cementitious matrix.