Heterogeneous equilibrium of calcium silicate hydrate in water at 30 °C

Abstract

Calcium silicate hydrates (C–S–H), prepared at 20 °C and 50 °C to have various CaO : SiO₂ molar ratios (n), were examined for their compositions with particular attention to the combined water contents and to their solubilities in water at 30 °C. The C–S–H gels may be described as solid solutions of the type nCaO·SiO₂·(n+ 0.8) H₂O, where n≳ 0.8. A thermodynamic treatment was developed which enabled n to be calculated from solubility data; the n values observed were compared with those calculated. The conclusion reached was that non-equilibrium was liable to occur owing to the immature state of C–S–H. Also observed was the unusual property that the response to the varying calcium ion concentrations in the solutions was limited to within the surface layer of the C–S–H particles. The possible phase relations under the state of equilibrium are presented.