Molecular Dynamics to Understand the Mechanical Behavior of Cement Paste

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The tensile strength of cement paste is one of the most important mechanical properties that influence shrinkage cracks in cementitious materials. Cement pastes that exhibit low tensile strength tend to exhibit greater shrinkage crack potential and reduced durability. Increasing the tensile strength in cement paste can minimize the shrinkage cracking potential. It is believed that the strength and cohesion of cement paste are controlled by the formation of calcium silicate hydrate (C-S-H) gel. To enhance macroscopic mechanical properties (tensile strength), it is necessary to understand the structure and behavior of C-S-H gel at the atomic level. Previously, molecular statics was used to determine minimal potential energy and the mechanical properties of crystalline C-S-H structures. From this study, a plausible atomic structure of C-S-H gel is proposed. This research effort builds on the aforementioned work by using molecular dynamics to derive tensile and compressive strengths of C-S-H structures from uniaxial stress–strain data. The results from the molecular dynamics simulations showed that the maximum strengths (i.e., compressive and tensile) for the proposed C-S-H structures are three orders of magnitude higher than the strength at the macrolevel. However, the tensile strength of the proposed C-S-H gel is 23% of the compressive strength. This research also concludes that electrostatic forces and bond forces in the silicate chains are the main contributors to cement strength at the atomic level and that breakage in silicate chains leads to low tensile strength in C-S-H gel.