Abstract: Calcium silicate hydrate (C-S-H), as the main binding phase of Portland cement-based materials, significantly effects the durability, physical and mechanical properties of cement-based materials. In this paper, 5 models with different Ca/Si ratios from 1.1 to 1.9 were constructed, and molecular dynamics was carried out to simulate the nanoindentation test on C-S-H models along x, y and z directions. Then their indention modulus and hardness were calculated by the typical Oliver-Pharr method respectively. According to results from present simulation, as the Ca/Si ratio increases, the density and the average silicate chain length gradually decrease, but the W/Si ratio gradually increases. The mechanical properties are greatly influenced by the Ca/Si ratio, as the Ca/Si ratio increases, the degree of the defect of silicate chains increases, and the stability of calcium silicate sheets is correspondingly reduced, resulting in the weakening of the ability of C-S-H structure to resist deformation and the reduction of indention modulus and hardness. Indention modulus and hardness values in the direction parallel to calcium silicate layers are closed, while the values in the direction perpendicular to calcium silicate layers are slight lower, C-S-H is similar to transversely isotropic structure. As Ca/Si ratio increases, values of both three directions are gradually closed, C-S-H changes from transversely isotropic structure to isotropic structure.

Key words: calcium silicate hydrate, molecular dynamics, nanoindentation, mechanical property, indention modulus, hardness