Fitting equations for the sorption isotherms of cement mortar with salt spray deposition

The sorption isotherm of porous building materials serves as a critical hygrothermal property that regulates coupled heat and moisture transfer and influences the energy efficiency of building envelopes. Coastal buildings endure chronic salt spray exposure, yet classical fitting equations neglect salt deposition effects. This study investigates cement mortar specimens subjected to accelerated salt spray tests (0–35 cycles). The salt content of the specimens was quantified via chloride ion analysis, and isothermal sorption tests were conducted under 33%–93% relative humidity (RH) using a static equilibrium method. A modified model integrating a salt influence factor (ηu) into classical equations was developed. Additionally, dual-regime sorption isotherm models were formulated based on deliquescence mechanisms of salt crystals above critical humidity, governed by the Robinson equation and Nielsen model, respectively. This framework enables accurate prediction of equilibrium moisture content under varying coupled humidity-salt conditions, significantly enhancing the reliability of hygrothermal simulations for coastal buildings in salt spray climates.