Modeling the dependence of capillary sorptivity on initial water content for cement-based materials in view of water sensitivity

As a broadly used transport property indicating the durability of cement-based materials (CBMs), capillary sorptivity strongly depends on initial water content. Based on the underlying assumption of static pore structure in classical unsaturated flow theory, several models have been proposed to quantify the dependence of unsaturated sorptivity on initial water content, but all fail to capture the measured unsaturated sorptivity of CBMs. Considering water sensitivity of C-S-H gels (swell upon wetting and contract upon drying), a new model for unsaturated sorptivity is theoretically developed according to the inherent relationships among sorptivity, hydraulic diffusivity and permeability, and then successfully validated with reported experimental data of various CBMs. It is found relative sorptivity is dominated by the shape parameter representing tortuosity of pore structure, which is crucial in describing unsaturated mass transport. When measuring sorptivity, special attention should be paid to the drying preconditioning of tested specimens with certain uniform water content.