Fundamental Discrepancy of Chemical Reactivity of Tricalcium Oxy Silicate (alite), Dicalcium Silicate (Belite), and Their Polymorphs: A Density Functional Theory Study

Tricalcium oxy silicate (C₃S) and dicalcium silicate (C₂S) are the major constituents of cement. In this study, the reactivity of polymorphs of calcium silicates is quantitatively investigated using Density Functional Theory. The result of combining the DFT calculation and the Bader charge analysis elucidates that the main difference in reactivity between C₃S and C₂S is the presence of oxy ions in C₃S which has smaller partial charge compared to that of other oxygen in the crystals. For the C₃S, the reactivity of among different C₃S polymorphs is decisively affected by the Bader charge of oxy ions. In contrast, total internal energy of C₂S determines the quantitative chemical reactivity of C₂S polymorphs. This result suggests that oxy ion has more dominant impact on the thermodynamic stability of calcium silicates. Furthermore, total energy can be used to estimate the chemical reactivity of calcium silicates, where there is no oxy ion exists.