Evolution mechanism of pore structures in sandstone under coupled effect of hygrothermal cycles and Na2SO4 solution

Sandstone slopes, especially those subjected to a hygrothermal cycle of rainfall-sunshine-rainfall, experience significant reductions in stability and strength, particularly in saline environments represented by Na2SO4 solutions. In this study, sandstone samples were immersed in Na2SO4 solutions with concentrations of 28.01 mg/L, 40.38 mg/L, and 136.44 mg/L and subjected to 0, 10, 20, 30, 40, and 50 hygrothermal cycles. After every ten cycles, tests were conducted to measure the mass, surface hardness, nuclear magnetic resonance (NMR), and wave velocity of the samples. The results show that as the number of cycles increases, the mass loss of the sandstone will increase with the rise in the concentration of the sodium sulfate solution. After 50 cycles, the maximum quality loss rate can reach up to 4.5%, and the hardness can decrease by up to 10%. NMR experiment indicates that during 50 cycles and as the salt concentration increases, the number of medium-sized pores in the sandstone decreases, while the number of large pores and porosity increase. The ratio of medium-sized pores to large pores rises from 23.96% to 12.78% to 24.28% and 15.74% respectively. Before 20 cycles, the degree of salt crystallization is relatively low, and the wave velocity increases by 12.7%, 24.3%, and 39.1% with increasing salt concentration. After 20 cycles, the crystallization stress intensifies, leading to pore rupture and a subsequent decrease in wave velocity. This study provides experimental and theoretical basis for the construction and protection of slope engineering.