Molecular dynamics study on structural characteristics of amorphous C-S-H with different Ca/Si ratios

As the major hydration product of cement, hydrated calcium silicate (C-S-H) governs the overall performance of cement-based materials. The molar ratio of CaO to SiO2 (Ca/Si ratio) significantly affects the structure and properties of C-S-H. This study analyzed the effect of Ca/Si ratios (0.83-2.0) on the structural morphology evolution, bond lengths and angles, polymerization process, and nanoporosity of amorphous C-S-H, with the help of the ReaxFF force field. The results showed that the reacted C-S-H tend to form a fibrous network-like morphology at low Ca/Si ratios, while the silicate chains are prone to accumulating at high Ca/Si ratios, forming a dense granular ovoid structure. Meanwhile, the Ca/Si ratio has no effect on the bond lengths and angles. In addition, the Ca2+ ions can interrupt the silicate chains during hydration, which leads to a decrease in the average silicate chain length with increasing Ca/Si ratio. The porosity of C-S-H decreases from 59.3% to 54.3% when the Ca/Si ratio increases from 0.83 to 2.0. It can be deduced from these findings that the increase in the Ca/Si ratio decreases the compressive strength of cement-based materials but increases their durability.