Advanced modelling of moisture transport process in unsaturated cementitious materials considering multi-modal pore size distributions

Moisture permeation in unsaturated cementitious materials is highly related to the pore structure characteristics. The present study carries out a novel pore-scale analysis to quantitatively establish the relationship between pore structure features and water vapour sorption isotherms, and develops a microstructure-based model for predicting the moisture transport in cementitious materials, considering multi-modal lognormal pore size distributions. A key innovation of this study is the introduction of a new S-shaped function to characterize the fraction of trapped water due to the pore-blocking effect during desorption, highlighting its strong dependence on the peak pore size. The predicted water vapour sorption isotherms—accounting for both pore-blocking and cavitation mechanisms—along with relative permeabilities and internal moisture distributions during adsorption and desorption, were validated against experimental data, showing good agreement. The findings reveal that a denser pore structure will contain a relatively higher water saturation degree at the same relative humidity. Furthermore, the results emphasize that a multi-modal pore size distribution model provides a more comprehensive representation of moisture transport behaviour compared to the conventional single-modal approach. By bridging pore-scale mechanisms with macro-scale transport modelling, this study proposes an advanced numerical approach for investigating moisture transport in cementitious materials.