Dependence of unsaturated chloride diffusion on the pore structure in cementitious materials

Conceptual analysis is performed to examine the effects of pore features on the water continuity in unsaturated porous systems. The roles of pore features in relative chloride diffusion coefficient (D_rc) of mortar specimens at various degrees of water saturation (S_w) were studied based on mercury intrusion porosimetry and resistivity tests. It is found that the role of pore structure in the D_rc-S_w relationship is a result of its effect on the water continuity. Porosity and tortuosity are not relevant to the D_rc-S_w relationship. A finer pore size distribution or lower pore connectivity tends to result in a lower D_rc. The pore size effect on the D_rc is pronounced primarily at high S_w, while the D_rc is dominated by the pore connectivity at low S_w. Cement mortar with a higher water-to-binder ratio shows larger chloride diffusion at high relative humidity levels but smaller chloride diffusion at low relative humidity levels.