Quantification of capillary pores and large pores with narrow throats in concrete materials by the hysteresis characteristics of water vapor

The hysteresis characteristics of gaseous water of concrete materials in the atmosphere environment are closely associated with the properties of its pore structure. Concrete exhibits variations between adsorbing and desorbing behavior of water vapor. This study conducted experiments on the adsorption and desorption of water vapor of concrete materials to investigate the respective processes. The hysteresis curve obtained from these tests provides insights into the characteristics of the pore structure of concrete material. The results indicate that the desorption activation energy of concrete material exceeds that of the first layer's adsorption energy, making water vapor desorption from inner pore walls more challenging than its adsorption. According to the isothermal adsorption and desorption hysteresis curves associated with humidity variations, the volumetric ratio of different size and shape pores can be determined. In concrete materials, capillary pores account for 22%–43% of the total pore volume, and large-volume pores with narrow throats range from 15% to 25%. For concrete with a certain water-binder ratio, an increase in fly ash content leads to a higher proportion of capillary pores. Besides, when the fly ash content remains constant, an increase in the water-binder ratio results in a decrease in the volume ratio of capillary pores and large pores with narrow throat.