Mineral dissolution mechanism of alite polymorphs from ReaxFF molecular dynamics and ²⁹Si NMR investigations

The hydration of tricalcium silicate (C₃S, C = CaO, S = SiO₂) played a critical role in the mechanical property improvement in cement-based cementitious materials. In this study, we investigated the hydration mechanism of M3- and T1-C₃S polymorphs using molecular dynamics simulation with the ReaxFF force field. Six surfaces were selected by their low-index surface energy and surface areas of the Wulff structure to study their hydration process, including M3-C₃S (1 0 0), (0 0 1) and (1 1 1) surfaces, as well as T1-C₃S (0 0 1), (0 1 0) and (1 1 1) surfaces. Bonding formation kinetics and radial distribution functions were analyzed. Results show that water dissociation degrees on the studied surfaces were comparable, independent of the crystal polymorphs. The final bond formation percent of Ca-OwH and Os/Oi-H on all the surfaces exhibited little difference. However, we observed the formation of pentacoordinate silicon atoms with a [SiO₅]^6- trigonal bipyramid structure, occurring earlier in M3-C₃S than in T1-C₃S. Furthermore, the M3-C₃S exhibited a higher abundance of pentacoordinate silicon atoms compared to the T1 crystal, suggesting greater reactivity of M3-C₃S. This study provided valuable insights into enhancing the quality of cement clinker and activating other silicate minerals in supplementary cementitious materials.