Thermokinetics of capillary absorption and its application in cement-based materials

Capillary water can transmit in capillary tube induced by hydrostatic pressure, which has an important impact on the durability and shrinkage of concrete. In this study, the ultimate height of capillary rise under various conditions was analyzed based on thermodynamic equilibrium of capillary water where capillary water is subjected to six types of force. Subsequently, a logarithmic model of capillary rising with three parameters (Jurins's height hc, permeability coefficient k, characteristic time τ) was proposed according to the analytical solution of kinetics function of capillary absorption. To better understand capillary behavior in capillary bundle with different radius, the spatio-temporal distribution of saturation, and the variation of water content with time and saturation are predicted according to the kinetics of capillary rise in single capillary. Additionally, the kinetics of capillary absorption in porous materials was discussed. Finally, the developed model was verified by testing the true rising height of water in a mortar specimen using X ray computed tomography. The equivalent height of adsorbed water in mortar with varied saturation under different time was obtained by gravimetric method. The results show that the logarithmic model gives a good description of the kinetics data of water absorption in mortar.