Thermodynamic Analysis of Tricalcium Silicate Hydration

Abstract

Thermodynamic analysis of the hydration processes of tricalcium silicate 3CaO•SiO2 is difficult due to the unreliability of the initial data for hydration products. In addition, there are disagreements about the basicity of the hydration phases (3CaO•SiO2•3H2O or 2CaO•SiO2•2H2O). For the latter, there is no free energy of formation in the reference literature. There are also no data on the water solubility of these calcium hydrosilicates. The proposed values of ∆G0298 for these hydrosilicates, equal to 1064,3 and 639,7, as well as the enthalpies of formation (∆Н0298), equal to 1157,2 and 696,9 kcal/mol, re-spectively. Further thermodynamic calculations were performed using these values. To calculate the composition of the liquid phase, a simplified Born-Haber cycle is used. The values of the calculated heat release of tricalcium silicate with the formation of C3S2H3 and C2SH2, obtained using the pro-posed values of enthalpies, differ little from each other and are close to the experimental data. The calculated solubility of C3S2H3 is 0,7 g/l CaO, and C2SH2 is 0,92 g/l CaO. Since the solubility of C3S2H3 is much lower than of Ca(OH)2 (portlandite), which is formed during hydration of tricalcium silicate in large quantities, C3S2H3 is unstable under these conditions and its basicity increases. It is suggested that C3S2H3 is the main hydration product of CEM III and other cements with a high content of active mineral additives, and C2SH2 is CEM I and CEM II.