Coupled Effects of Pore Size and Salinity on Ionic Spatial Distribution and Transport in C-S-H Nanopores and Their Implications for Cement-Based Material Durability

The durability of cement-based materials is strongly affected by ionic ingress and transport within calcium silicate hydrate (C-S-H) nanopores, governing their long-term degradation in saline environments. However, the coupled effects of pore size and salinity on nanoscale ionic behaviors remain insufficiently understood, limiting the mechanistic interpretation of durability evolution in cementitious systems. Existing studies have mainly considered pore size and solution salinity separately, while a systematic understanding of their coupling effects on ionic spatial distribution, transport properties and regime transitions is still lacking. In this study, molecular dynamics simulations are performed for NaCl solutions confined in C-S-H nanopores with pore sizes of 2.5–12.5 nm and salinities of 0–2 M. Results show layered water and ion structures that become increasingly confined with decreasing pore size. Increasing salinity enhances ion accumulation while suppressing water mobility due to competitive adsorption. Ion diffusion is significantly lower than that of water molecules, while transport parallel to the C-S-H surface is much higher than in the perpendicular direction, indicating strong anisotropy. Regime-dependent diffusion behaviors are observed across pore size–salinity conditions. These findings deepen the understanding of water and ionic transport and adsorption, improving durability models for cement-based materials in construction engineering.