A unified pore structure-based model for diffusion coefficient of unsaturated cementitious materials

The diffusion coefficient of unsaturated cementitious materials is a crucial factor in assessing their transport-related properties for durability evaluation. However, predicting the diffusion coefficient of unsaturated cementitious media through mathematical modeling is challenging due to the variability of fitting parameters in existing models. This work explores the impact of pore structure and hydration product composition on the diffusion coefficient and corrosion of steel in cementitious materials, particularly under an unsaturated state. The findings reveal that there is no substantial difference in the diffusion mechanisms between saturated and unsaturated cementitious materials, once the roles of capillary and gel liquids are distinguished. Therefore, the same Archie's equation, with consistent fitting parameters, can be used to determine the diffusion coefficient of both saturated and unsaturated cementitious materials. The variability of fitting parameters in the saturation function primarily arises from the varying percentage of capillary pores in different cementitious microstructures. Additionally, the new unified pore structure-based model of diffusion coefficient can be used to accurately predict the corrosion rate of steel embedded in unsaturated cementitious materials.