Free‐Energy‐Based Model of Chemical Equilibria in the CaO–SiO2‐H2O System

The chemical equilibrium between the C-S-H gel phase and the aqueous phase in the CaO-SiO2-H2O system was modeled using the free-energy-based Thermo-Calc commercial software system. Unlike programs that are based on solubility products, the minimization of free energy allows the composition and solubility of the C-S-H phase to vary, as in real cementitious systems. The C-S-H gel is modeled as a defect tobermorite structure, based on recent compositional data gathered using 29Si NMR. The composition is varied by removal of some of the bridging silica tetrahedra in the SiO layers and by incorporating different amounts of Ca in the interlayer spaces.