Molecular dynamics study on calcium silicate hydrate subjected to tension loading and water attack: structural evolution, dynamics degradation and reactivity mechanism

The coupled effects of mechanical loading and chemical attack can dramatically weaken the durability of a material. In this study, reactive force field molecular dynamics and the GCMC method were utilized to investigate the degradation mechanism for a deformed C-S-H gel subjected to water attack. In the elastic region of the stress-strain relation, no water molecules invaded the deformed C-S-H gel tensioned along the y-direction. On the other hand, in the failure stage, the tension loading stretched/broke the Si-O-Si bond, resulting in the distortion of the "dreierketten'' silicate chain distribution and ordered zigzag sheets built by the calcium oxygen octahedron. More water molecules penetrated into the defective silicate sheets and dissociated into the Si-OH and Ca-OH surrounding the highly reactive non-bridging oxygen sites induced by the silicate chain breakage. The water invasion and hydrolytic reaction reduced the cohesive stress of the tensioned C-S-H structure. Furthermore, the cracks in the calcium silicate sheet connected with the interlayer region, enhancing the channel connectivity for the water transport. This resulted in the water dynamic transformation from the cage stage to the diffusive stage. The high mobility of confined water molecules further weakened the stability of the hydrogen bonds in the calcium silicate skeleton. Moreover, the tensile loading and water attack contributed to the silicate morphological rearrangement. The long silicate chains were first destroyed to form shorter chains and then re-polymerized to form a branch and ring structure to strengthen the weak interlayer regions.