Asymmetric kinetics and entropy-driven activation mechanisms of water-vapor sorption in asphalt mortar: Effects of binder type, temperature, and humidity

Interactions between water vapor and solid surfaces control moisture transport and retention in asphalt mortar, serving as a trigger for moisture-induced damage. However, the kinetic and energetic characteristics of this interfacial process during humidity fluctuations remain insufficiently understood. In this study, isothermal gravimetric sorption measurements were utilized to characterize adsorption–desorption kinetics in asphalt mortar. Adsorption exhibited three distinct stages, whereas desorption was completed in two. Kinetic modeling showed that the pseudo-second-order (PSO) model outperformed the pseudo-first-order (PFO) model, indicating that surface interactions at the pore–vapor interface limit the overall rate. The Elovich model further indicated a heterogeneous distribution of active-site energies on the mortar surface. Material composition, temperature, and humidity exerted strongly asymmetric effects on adsorption and desorption. The SBS-modified binder increased the equilibrium adsorption capacity but reduced the initial desorption rate. Elevated temperature suppressed adsorption by up to 94.7% but accelerated desorption significantly beyond a critical threshold of approximately 30 °C, whereas high relative humidity promoted adsorption and inhibited desorption, thereby facilitating internal moisture accumulation. To clarify the energetic origin of this asymmetric interfacial behavior, an activation energy analysis was performed, which showed that the desorption barrier was 2.7–3.5 times higher than the adsorption barrier. Notably, desorption had a lower Gibbs free energy of activation because of its positive entropy of activation, indicating that moisture release is entropy-driven and controlled by the density of polar binding sites at the binder–vapor interface. This work elucidates the interfacial kinetic and activation mechanisms that control water-vapor sorption in asphalt mortar and provides fundamental insights for designing materials for environments with frequent humidity fluctuations.