Insights into Ionic Diffusion in C-S-H Gel Pore from Molecular Dynamics Simulations: Spatial Distributions, Energy Barriers, and Structural Descriptor

Understanding transport behavior in nanoconfined environments is critical to many natural and engineering systems, including cementitious materials, yet its molecular-level mechanisms remain poorly understood. Here, molecular dynamics (MD) simulations were used to investigate Na, Cl, and water diffusion inside a 4 nm calcium-silicate-hydrate (C-S-H) pore channel over temperatures ranging from 300 K to 360 K. Spatially resolved analysis revealed strong suppression of diffusivity near the solid-liquid interface and gradual recovery toward the pore center. Arrhenius analysis further quantified the spatial variation of activation energy barriers and intrinsic mobilities across the pore channel, showing distinct confinement effects. The spatially resolved structural analysis uncovers a mechanistic transition from structure-controlled to hydrodynamics-controlled transport regimes with increasing distance from the pore surface. A structural descriptor, total coordination strength (TCS), was introduced, providing a predictive link between local liquid structure and molecular mobility within approximately 1 nm of the interface. Beyond approximately 1 nm, suppressed diffusivities were well captured by an empirical model inspired by the Darcy-Brinkman framework. To the best of our knowledge, this is the first MD study to comprehensively resolve the spatial heterogeneity of transport, thermal kinetics, and structure within cementitious nanopores. These findings deepen the fundamental understanding of nanoscale transport phenomena and suggest that tailoring the nanochannel structure and interfacial chemistry of cementitious gels, for example surface coordination environments, pore size distributions, and adsorption sites, may offer a promising strategy to suppress ionic ingress and enhance the durability of cement-based materials.