Evolution of moisture transport properties in cement mortar under marine salt spray environment

Chronic salt spray exposure induces microstructural evolution in historic building materials, critically altering their hygrothermal performance through modified pore networks and capillary dynamics. This study systematically investigates the humidity-regulated transition of the water vapor permeability coefficient ( δ _v )—a key parameter governing moisture transfer in building envelopes—under marine aerosol conditions. Through accelerated salt spray cycling (35 cycles, 5% NaCl solution) combined with dry-cup/wet-cup measurements across contrasting humidity gradients (0→50% vs. 50→98% RH), cement mortar exhibited distinctly opposing δ _v responses: a 31.3% reduction under low humidity due to crystalline pore blockage, contrasting with a 239.4% enhancement at high humidity caused by deliquescence-induced brine migration. A predictive piecewise model (R² > 0.95) based on salt influence factors and critical humidity thresholds was developed. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) analyses revealed NaCl crystallization preferentially occupying 0.02–0.12 μm pores within the top 2 mm, reducing effective porosity by 42% while establishing two distinct transport modes: pore occlusion by crystals (dry state) and interconnected brine networks (humid state). These findings provide a mechanistic framework for predicting moisture dynamics in porous building materials under salt spray environments, enabling accurate hygrothermal simulations that improve resilience and durability predictions for coastal historic buildings.