Investigation of hygroscopic properties and isothermal adsorption models of hydrophobic SiO ₂ aerogel cement-based composites

The hygroscopicity of building materials directly influences their thermal insulation performance, which is critical for achieving energy efficiency. Conventional materials, however, are limited by high moisture absorption, flammability, and poor thermal stability, rendering them inadequate for modern energy-saving needs. In this study, hydrophobic SiO ₂ aerogel was incorporated into cement-based matrices to fabricate SiO ₂ aerogel cement-based insulation boards (AIC) with enhanced thermal insulation properties. The investigation focused on how varying hydrophobic SiO ₂ aerogel (SA) concentrations (0–50 vol%) affect the hygroscopic behavior of AIC under relative humidity (RH) conditions ranging from 0% to 95%. Furthermore, an isothermal adsorption model for moisture in SA cement-based materials was also established. Experimental results showed that under low to moderate humidity (RH ≤ 50%), the hygroscopicity of AIC was proportional to the SA concentration. At RH 30%, the water content increased from 1.25% in AIC0 (0 vol% SA) to 1.46% in AIC50. When RH exceeded 50%, 30 vol% SA marked the threshold between moisture absorption and hydrophobicity. When SA concentration was below 30 vol%, the hygroscopicity of AIC increases. At RH 70%, AIC20 had a water content of 6.67%, a 21% increase over AIC0 (5.51%). When SA concentration was above 30 vol%, the hydrophobicity of AIC is enhanced. At RH 95%, AIC40 showed a water content of 10.85%, a 20% reduction compared to AIC0 (13.5%). In addition, the hygroscopic equilibrium process of AIC was divided into three stages: the slow growth of monomolecular layer adsorption, the steady-state growth of multimolecular water film formation, and the rapid growth of capillary adsorption. For the first time, the Lewicki model is validated for hydrophobic composites, achieving exceptional accuracy (R ²  > 99%, SSE: 0.001–0.204, RMSE: 0.0002–0.2032) by integrating SA-induced pore coarsening and hydrophobic effects. These findings provide a theoretical foundation and data support for the application of AIC in diverse humidity environments.