Desorption of water from aqueous solution confined in C-S-H gel pore: A molecular dynamics study

The durability of concrete is significantly affected by the evaporation of salt solutions within its porous structure, particularly in harsh environments containing NaCl, which accelerates deterioration and impacts the long-term performance of the material. This study employs molecular dynamics (MD) simulations to investigate the desorption of water from NaCl solutions confined within calcium silicate hydrate (C-S-H) gel nanopores, a critical process in understanding drying shrinkage and salt crystallisation in cement-based materials. Two ensembles were considered: canonical (NVT), where the pore size remained fixed to analyse controlled water desorption, and isothermal-isobaric (NPT), where the nanopores shrank dynamically, simulating real-world drying-induced contraction. The results reveal that, when the pore size is fixed, evaporation occurs preferentially from the central region of the nanopore, leading to a density gradient within the confined water phase. As water is removed, Na⁺ and Cl^- ions migrate towards the C-S-H surface, forming a double ionic layer, which may have implications for salt precipitation. Under NPT ensemble conditions, it is observed that, despite significant pore shrinkage in the ensemble, the C-S-H structure remains stable. Additionally, the study finds that varying the Ca/Si ratio (1.2–2.0) in the C-S-H gel has a minimal impact on evaporation behaviour. These findings provide fundamental atomic-scale insights into the mechanisms governing water loss and ion redistribution in confined cementitious environments, contributing to a better understanding of concrete durability and degradation under drying conditions.