Development of macro- and micro-models for moisture adsorption-desorption in cement-based materials

The moisture adsorption-desorption hysteresis in cement-based materials critically influences their durability. However, accurate prediction remains challenging due to limitations in existing macro-models and an insufficient microstructural understanding. Using a combined macro-micro approach, this study systematically investigates the adsorption-desorption characteristics of cement mortar under varying relative humidity conditions. Vapor isothermal adsorption-desorption tests and NMR experiments were conducted on specimens with different water-to-binder ratios (0.4, 0.5, 0.6) to obtain sorption hysteresis curves and pore structure parameters. Based on the experimental results, a novel two-stage model integrating the GAB and Freundlich models was proposed, effectively describing multilayer adsorption in the low-medium humidity region and capillary condensation in the high-humidity region. Furthermore, based on the NMR-derived parameters, 3D pore structure models were constructed using Gaussian random field theory and a biphasification technique. Finally, an adsorption-desorption simulation algorithm that accounts for the ink-bottle effect by integrating the Kelvin-Laplace equation with image analysis methods from mathematical morphology was developed. The results indicate that the hysteresis phenomenon is most significant within the 60%-78% RH range. The proposed two-stage macroscopic model demonstrated excellent fitting accuracy across the entire humidity range. The microscopic model simulations agreed well with experimental data, revealing that ink-bottle pores (approximately 20%~26% of the total) are the decisive factor causing adsorption-desorption hysteresis. The evolution of the pore network during drainage and filling, as visualized in the simulations, follows “large-to-small” and “small-to-large” sequences, respectively. This study provides reliable theoretical models and numerical tools for predicting the moisture transport behavior of cement-based materials across both macro- and micro- scales.