Non-ideal solid solution aqueous solution modeling of synthetic calcium silicate hydrate

New data relevant to calcium silicate hydrate (C–S–H) gels prepared at room temperature have been obtained over a time period of up to 112 weeks. X-ray diffraction (XRD) indicates equilibrium was attained after 64 weeks. Coupled with fourier transform infrared (FT-IR) spectroscopy, a phase change in C–S–H gel at Ca/Si ≈ 1.0 was identified and the occurrence of portlandite as a distinct phase for Ca/Si > 1.64. The incongruent dissolution of C–S–H gel was modeled as a non-ideal solid solution aqueous solution (SSAS) between the end-member components CaH2SiO4 (CSH) and Ca(OH)2 (CH) using equations defining the solidus and solutus curves on a Lippmann phase diagram. Despite being semi-empirical, the model provides a reasonable and consistent fit to the solubility data and can therefore be used to describe the incongruent dissolution of C–S–H gels with compositions Ca/Si ≥ 1.0.