Coupled effects of chemical environments and freeze–thaw cycles on damage characteristics of red sandstone

To study the influence of freeze–thaw cycles under complex chemical environments on the red sandstone, nuclear magnetic resonance (NMR) technology was used to analyze the damage evolution law of red sandstone. The freeze–thaw cycles were carried out on four groups of samples soaked in sulfuric acid solution, sodium hydroxide solution, sodium chloride solution and pure water, respectively. Further, NMR tests were performed on the samples and the porosity change of samples, T₂ distribution and nuclear magnetic resonance images were analyzed. Results show that: (1) chemical environments have significant effects on the freezing-thawing damage, and the porosity increases linearly with increasing number of freeze–thaw cycles. (2) Under the frost-heaving force and ionic reactions, the T₂ spectrum will successively shift towards the left and then to the right with increasing number of freeze–thaw cycles. Micropores will constantly appear, and small pores will dynamically expand into macropores. (3) With increasing number of freeze–thaw cycles, the areas of light spots in NMR images will increase gradually, which means that the damage degree is aggravated with continuous development of internal pores. (4) The samples soaked in sodium hydroxide solution and sodium chloride solution are damaged more seriously than those soaked in sulfuric acid solution and pure water. By analyzing and comparing the NMR characteristics of rock under the coupled effects of chemical environments and freeze–thaw cycles, more reliable test data will be available to study the damage mechanism of rock.