Hydration Mechanism of Reactive and Passive Dicalcium Silicate Polymorphs from Molecular Simulations

Belite (C₂S, C=CaO, Si=SiO₂) based cements are promising low-CO₂ substitutes of ordinary Portland cement. The main drawback is their low hydration rates, which makes them unpractical for construction. Yet more disconcerting is the different reactivity between polymorphs of belite: while β-C₂S reacts slowly with water, γ-C₂S is almost inert. Due to the demand of improving C₂S reactivity, in this work we aim to understand the hydration mechanism of belite polymorphs by density functional theory and molecular dynamics simulations methods. We calculated the low-index cleavage energies, and the thermodynamic equilibrium structures were constructed through Wulff shape constructing method. We built the adsorption energy surface (AES) maps and found out the transition state structures for (chemi)sorption of water molecules. Finally molecular dynamics were employed to simulate the reactions taking place during 2 ns at room temperature. We found that water dissociation consists of three steps, rotation, dissociation, and diffusion, with different energy barriers. Considering the AES, DFT energy barriers, and the molecular dynamics simulations, the number of reactive sites is the key aspect that controls hydration; even though water reacts preferentially in γ-C₂S surfaces over in β-C₂S in terms of energy, a considerably lower number of reactive points in γ-C₂S would limit the surface hydration and dissolution. (Figure Presented).