Multiscale analysis of pore structure in clay due to freeze-thaw

The variation of pore structure due to freeze-thaw (F-T) plays a vital role in revealing the thaw settlement mechanism of soils. In this study, remolded saturated clay specimens experienced unidirectional freezing and natural thawing in an open or closed system (i.e., with or without water supply). X-ray computed tomography (CT) and mercury intrusion porosimetry (MIP) were conducted to analyze pore structural characteristics, including macropores, mesopores, and micropores, along the temperature gradient direction before and after F-T. The longitudinal pore binary images, obtained by processing CT images, were used to assess the changes in porosity, pore shape, and orientation of macropores and mesopores due to F-T. The results indicate that the total longitudinal-sectional porosity and macropore percentage increased, whereas the mesopore percentage decreased. F-T considerably influenced the shape and orientation of macropores but had minimal effects on mesopores. The most significant changes occurred in the unfrozen zone due to F-T in an open system but near the final freezing front in a closed system. Additionally, Micropore porosity, diameter, and thermal fractal dimension were analyzed based on MIP data. The results showed that micropore porosity and diameter decreased, with significant changes occurring in the unfrozen zone of the open and closed systems. The micropore porosity and diameter decreased more in the unfrozen zone and near the final freezing front after F-T in a closed system, whereas a smaller decrease occurred in the frozen zone. The thermal fractal dimension value increases after F-T, indicating that the micropore structure became complex. In general, the multiscale pore analysis demonstrated that pore structure, including macropores, mesopores, and micropores, is significantly affected by F-T in the open system compared to the closed system.