Toward the formation mechanism of synthetic calcium silicate hydrate (C-S-H) - pH and kinetic considerations

The formation of C-S-H in cementitious materials is critical to their gain in strength. As one of the key parameters, pH is reported to affect the nucleation and growth of C-S-H; however, the underlying mechanism remains unclear. In this study, C-S-H was synthesized under different pH conditions, to elucidate the underlying mechanism. Changes in the chemical composition, morphology and structure depending on the reaction time and interactions between pH and C-S-H were tracked using a series of characterizations. Three types of C-S-H (amorphous calcium silicate, C-S-H precursor and semi-crystalline C-S-H) formed at different pH and time points. pH affected the formation of C-S-H mainly through the protonation of silicates, which determined the surface charge of silicate tetrahedra and consequent chemical composition of C-S-H. At high pH, the formation of CaOH+ as the first step before the formation of C-S-H was responsible for the high Ca/Si ratio under such conditions.