Molecular insights into pre-existing crack induced mechanical degradation of CSH gel.

Calcium silicate hydrate (CSH), the primary product formed during cement hydration, serves as the key binding phase and is characterized by a multiscale, heterogeneous, and highly disordered architecture. Within its intrinsic microstructure, detrimental pores and nonuniform cracks progressively aggravate the deterioration of interparticle bonding under service conditions. In this work, molecular dynamics (MD) simulations coupled with uniaxial tensile loading are employed to systematically investigate the intrinsic mechanical response of CSH and the degradation mechanisms associated with Mode I fracture defects. The results demonstrate that the Ca/Si ratio is a key governing factor controlling both the elastic properties and tensile behavior of CSH. Differences in mechanical performance among various compositions are primarily attributed to the progressive increase in silicate tetrahedral vacancies and the concomitant reduction of interlayer bridging sites along the silicate chains. Furthermore, a pronounced mechanical anisotropy is observed in CSH, wherein the load-bearing capacity normal to the layered configuration is particularly limited. Notably, the presence of a Mode I fracture induces severe stress concentration at the crack tip, resulting in a substantial redistribution of the originally quasi-uniform stress field within the matrix. Such a sharp escalation in local stress intensity promotes premature failure under comparatively low tensile loads, thereby significantly diminishing the overall load-carrying capacity of the CSH matrix.