Experimental insights and modeling on capillary pressure-saturation relationship in hardened mortar with effects of mix design parameters

The deterioration of cement-based structures under atmospheric conditions is significantly influenced by water transport governed by capillary pressure. This study combines experimental analysis and theoretical modeling to investigate the capillary pressure-saturation relationship in hardened mortar. Capillary pressure was derived from internal relative humidity (IRH) measurements, while pore size distribution (PSD) was characterized using low-field nuclear magnetic resonance (LF NMR) with varying water-binder (W/B) ratios, sand-cement (S/C) ratios, and fly ash (FA) dosages. A novel capillary pressure model is developed based on pore geometry, incorporating maximum equivalent pore radius ( R ) and fractal dimension ( D ) to capture the influence of PSD on capillary behavior. The proposed model shows excellent agreement with experimental data across various compositions and quantitatively links mix parameters to pore structure characteristics via functional relationships between W/B, S/C, FA dosage, and model parameters R and D . Results show that capillary pressure increases as saturation decreases and is strongly affected by mix composition, with lower W/B and S/C ratio yielding higher capillary pressures. Appropriate FA dosages promote a denser microstructure and higher capillary pressures through later-stage pozzolanic reactions, whereas excessive FA dosages increase the total porosity due to reduced early-stage calcium silicate hydrate (C-S-H) formation from lower cement content. This integrated experimental-modeling approach improves understanding of the interaction between mix design, pore structure, and capillary water transport, providing a robust tool for predicting durability-related moisture behavior in Portland cement-based materials.