Study on the evolution of microscopic pore structure of sandstone under freeze-thaw cycles

In cold regions, rocks undergo freeze-thaw (FT) cycles, leading to damage caused by the evolution of pores. This phenomenon significantly influences the rock mass stability. A comprehensive and accurate study of FT damage evolution in rock pores was conducted in this research. The microscopic morphology and pore size distribution (PSD) of sandstone samples subjected to FT cycles were measured utilizing scanning electron microscopy (SEM), nitrogen adsorption/desorption (NAD) method, and mercury intrusion porosimetry (MIP). Multifractal analysis was applied to explore the evolution of PSD. A gradual weakening of the cementation degree of mineral particles under FT cycles was observed based on SEM results, resulting in the formation of larger connected pores. A decrease of 37.3% in the number of pores was recorded after 40 cycles. According to NAD and MIP results, a greater effect on mesopores and large pores were found, with a minimal impact on micropores and transition pores. Multifractal characteristics were identified in the PSD of sandstone samples with different FT cycles. Initially, the connectivity of sandstone pores was found to decrease and then increase gradually after 10 cycles. Uneven PSD was observed to concentrate stress among pores, exacerbate frost heave damage, and drive the PSD towards a more uniform direction. It was concluded that the non-uniform degree of PSD does not consistently increase with FT cycles. The findings of this study can serve as a reference for further research on rock FT damage mechanisms.