Isothermal calorimetry and physics-informed modelling of capillary imbibition in pore-engineered cementitious materials

Rapid capillary water uptake is essential for evaporative pavements intended to mitigate urban heat-island effects, but the mechanisms governing water transport in pore-engineered cementitious materials remain insufficiently resolved. This study compares reference cement pastes, glass-sphere-modified composites, and raw- or calcined-clay-modified composites, with emphasis on the origin of their different capillary water absorption coefficients. Twenty specimens were characterised through open-porosity and density measurements, gravimetric imbibition tests, isothermal calorimetry, and physics-guided modelling incorporating pore connectivity and tortuosity. The reference pastes exhibited intermediate capillary water absorption coefficients of 5.60–5.94 kg·m^-2·h^-0.5, representing transport through the intrinsic capillary network of the cement matrix. Most glass-sphere composites showed lower values of 1.77–4.83 kg·m^-2·h^-0.5. Although packed glass spheres can generate interparticle capillarity, their smooth, nonporous spherical particles acted mainly as fillers after incorporation into cement paste, occupying packing voids, increasing matrix density, interrupting continuous capillary pathways, and increasing pore tortuosity and hydraulic resistance. In contrast, clay-modified composites achieved the highest coefficient, reaching 13.02 kg·m^-2·h^-0.5. Raw clay promoted water transport through its layered morphology, high specific surface area, hydrophilic hydroxyl-rich surfaces, interlayer microporosity, and possible swelling-induced microcracking. Calcined clay refined the pore system while increasing gel and fine-capillary pores and improving connectivity between macro- and micropores, thereby forming a hierarchical network that combined rapid uptake with water retention. These structural mechanisms produced the overall transport order clay-modified composites > reference pastes > glass-sphere composites. Calorimetric responses followed the same trend: clay-containing specimens reached a specific energy of 8.56 J·g^-1, whereas the lowest glass-sphere value was 0.70 J·g^-1. Peak heat flow correlated strongly with open porosity (R² = 0.92) and specific imbibition rate (R² = 0.85). The physics-guided model further captured the coupled effects of porosity, connectivity, tortuosity, and time. The results demonstrate that isothermal calorimetry, combined with pore-structure analysis, provides a sensitive framework for mechanistic evaluation and optimisation of water-transport performance in evaporative pavement materials.