Capillary absorption of nitrate solutions into white cement mortars: mechanistic insights into water sensitivity

To examine the effects of cations on the moisture-induced swelling of pore structure of cement-based materials, long-term capillary absorption tests were conducted on white cement mortars using deionized water and aqueous solutions of NaNO3, KNO3, and Ca(NO3)2. The evolution of permeability and pore structure was characterized by steady-state permeation and low-field nuclear magnetic resonance relaxometry, respectively. The results indicate that water penetration induces C–S–H gel swelling and pore refinement, resulting in the deviation of water absorption from the square root of time law. Capillary absorption of these nitrate solutions similarly deviates from this law, but with a delayed deviation time. Both the initial sorptivity and the difference between initial and secondary sorptivity are reduced. The mechanisms are interpreted to involve increased osmotic pressure and decreased electrical double-layer repulsion between C–S–H sheets, which suppress C–S–H swelling, mitigate water sensitivity, and consequently increase liquid permeability. These findings provide insights into the transport properties of cement-based materials in contact with salt solutions.