Structural, dynamic and mechanical evolution of water confined in the nanopores of disordered calcium silicate sheets

Abstract

Water molecules confined in the interlayer region of calcium silicate hydrate (C–S–H) are closely related to the cohesion in the cementitious materials. In this research, molecular dynamics is employed to investigate the structure, dynamics and mechanical properties of water molecules in the nanopore of C–S–H gels at ambient temperature. In order to consider the confinement effect of calcium silicate sheets, the pore size expressed as interlayer distance changes from 13 to 22 Å. The water molecules near the C–S–H surface are strongly influenced by the O_NB atoms in the silicate chains and Ca_w atoms in the interlayer region. They demonstrate following structural features: layering in the water density profile, the orientation preference in the dipolar angle distribution and long special correlation in the radial distribution function. Dynamically, while the interlayer distance increases by 10 Å, the diffusion coefficient transfers through two orders of magnitude from 3.42 × 10⁻¹² to 5.3 × 10⁻¹⁰ m²/s². In the mean square displacement curves, the cage stage, a characteristic of the dynamics in the glasses, gradually disappears with increasing interlayer distance. The dynamic evolution of water molecules is primarily induced by the changes in the strength of H-bonds and Ca–O_w connections, which is consistent with lowering of the frequency in the simulated vibration spectrum. The strongly attractive interaction between water molecules and calcium silicate layer is weakened and the motion of water molecules is less restricted at large interlayer distance. Furthermore, the interlayer fluid pressure study shows that the cohesion in the C–S–H gel, contributed by the interlayer calcium atoms, progressively diminishes from 1.5 to 0 GPa, due to the enhancing disjoining effect of water molecules connected with calcium atoms with increasing interlayer distance.