Calcium silicate hydrate (C–S–H) gel contains almost 50% to 70% of cement paste, which has a significant effect on the cement paste properties. An attempt is made to assess the atomistic structure of calcium silicate hydrate and the impact of water on it through molecular dynamics simulation. The most similar compositions (tobermorite 14Å, tobermorite 11Å, and jennite) are chosen and the effect of water thereof is studied. This study is presented in two phases: (1) water removal from the structure and simulating difference of water-to-calcium ratios through grand canonical Monte Carlo method. Next, the effect of water on the mechanical properties of C–S–H gel is examined through molecular dynamics method. (2) Defects in the existing molecular structures were introduced, which lead to newer Ca–Si proportions followed by the examination of the effect of water on the new structures. The obtained results revealed that as W/Ca ratio increases, the Young's modulus decreases. An increase in the Ca/Si ratio, owing to the defects in the structure, leads to the reduction in the Young's modulus.
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