Thermodynamic of incongruent solubility of C–S–H

In most situations, the pore solution of cement-based materials is in thermodynamic equilibrium with the calcium silicate hydrates resulting from the hydration of (tri–di)calcium silicates. This equilibrium is, however, difficult to characterise, the solution phase being incongruent with respect to the calcium silicate hydrate phase. A thermodynamic description of this equilibrium is proposed and some new relationships are derived involving the saturation indexes of dissolved pure substances and the calcium/silicon ratio of the calcium silicate hydrate. These relationships can be understood as a generalisation of the law of mass action for incongruent chemical reactions. The validity of these relationships was checked using measurements reported in the literature.