The dependence of capillary sorptivity and gas permeability on initial water content for unsaturated cement mortars

Abstract Capillary sorptivity and gas permeability characterizing the penetration ease of water and gas deserve attention in the current attempts to quantify durability performance for cement-based materials. To investigate their dependences on water content, disk specimens of cement mortars are prepared and thoroughly preconditioned. At ambient condition, available models could quantify the measured dependence of gas permeability except capillary sorptivity. Surprisingly, it follows another bilinear law with a transitional saturation degree, beyond which capillary condensation occurs in gel pores at equilibrium. This extraordinary disagreement is ascribed to the dynamic structure of C–S–H gel and thus mortars, which are sensitive to water removal and regain. Physically, it is also responsible to several unexpected anomalous characteristics of water absorption. The unsaturated flow theory is no longer applicable if ignoring its sensitivity to water, which is the key to approach the transport mechanisms of water and other deleterious agents in common unsaturated cement-based materials.