Numerical investigations into the physical significance of sorptivity for cement-based materials considering water sensitivity

Capillary sorptivity precisely defined in classical unsaturated flow theory is challenged for cement-based materials (CBMs) due to unexpected deviation from the square root of time (SRT) law, which is attributed to the wetting expansion (water sensitivity) of C-S-H gels. To enhance the physical significance of sorptivity, long-term absorption of representative CBMs are predicted with time-dependent permeability accounting for the water sensitivity through numerically solving the modified Richards equation. It is found that capillary absorption of CBMs still follows SRT law in the first several hours, and then gradually transfers to another linear stage in several days. The initial and secondary sorptivity depending on time-dependent permeability, water retention behavior, porosity and tortuosity scales as the square root of initial and final permeability, respectively. Physically, initial sorptivity characterizes the ease of coarser dry CBM to transmit water under capillarity, whereas secondary sorptivity quantifies the resistance of finer re-saturated CBM to water penetration.