Freeze–Thaw Impact on the Mechanical Properties and Microstructure Changes of Sandstone with Varying Water Saturations

This experimental study focuses on the performance of sandstone under freeze–thaw (F-T) cycles with different water saturations. Sandstone is widely involved in engineering constructions such as tunnels, slopes, and foundations in cold regions, where F-T cycles induced by seasonal temperature variations frequently cause severe structural damage (e.g., spalling and cracking) and even engineering failures, posing significant threats to construction safety and service durability. Hence, investigating its performance under F-T cycles with different water saturations is of critical practical significance, addressing the urgent need for durability assessment of cold region rock engineering. Sandstone samples with water saturations of 0%, 25%, 50%, 75%, and 100% were prepared and then subjected to 0, 30, 60, and 90 F-T cycles, aiming to comprehensively investigate the changes in microstructure damage and mechanical properties. The nuclear magnetic resonance technique was used to analyze the microstructure. The T2 spectrum curve of the pore size distribution was obtained, and the pores were divided into micropores (T2 < 3 ms), mesopores (3 ms ≤ T2 ≤ 33 ms), and macropores (T2 > 33 ms). The research results show that with the increase in water saturation and F-T cycles, the pore structure has changed significantly. Higher water saturation and a larger number of F-T cycles result in more severe microstructure damage, specifically manifested as an increase in porosity. Uniaxial compressive strength (UCS) tests were also conducted. A prediction model for the UCS deterioration of F-T rocks under different water saturations was established and verified. It was found that there is a negative correlation between the UCS and water saturation as well as the F-T cycles. The model can well describe the mechanical deterioration characteristics of sandstone samples under different water saturations and F-T cycles. This study provides valuable insights into understanding the degradation mechanism of sandstone under different water saturations and offers crucial data support for the design of anti-freeze–thaw measures, durability evaluation, and risk mitigation of sandstone engineering in cold regions.