Periglacial Structure Evolution of Loess-Like Sulfate Saline Soil under Freeze–Thaw Conditions

The study of the cryostructure of loess-like sulfate saline soil under freeze–thaw cycles is crucial for understanding its frost heave properties. To investigate this, closed-loop freeze–thaw simulation experiments were conducted on loess-like sulfate saline soil samples with varying salt content and top plate temperature. The experiments assessed the cryostructure, moisture migration, and salt migration under freeze–thaw conditions. The results showed that the variations in freeze–thaw temperature, moisture content, and electrical conductivity inside different samples were distinct. Regarding the freeze–thaw temperature inside the samples, a decrease in the top plate temperature changed the duration of the positive temperature phase during melting, while an increase in salt content inhibited the heat transfer within the soil. As for the moisture content inside the samples, changes in top plate temperature altered the moisture migration path, while an increase in salt content promoted the migration of moisture within the sample. With regard to electrical conductivity, a decrease in the top plate temperature inhibited the migration of salts within the sample, and an increase in salt content also hindered salt distribution under certain conditions. From the perspective of intuitive images, when the salt content of the sample was 0.3%, the ice layer was evenly distributed. When the salt content was 1.0%, the cryostructure was most pronounced, whereas when the salt content was 3%, no obvious cryostructure occurred. At salt contents of 0.3% and 1.0%, lower top plate temperatures led to more pronounced cryostructures. However, when the salt content was 3%, no obvious phase separation occurred in the sample. This indicates that under the prescribed low top plate temperatures, the effect of salt content on the cryostructure is nonmonotonic, as a 1.0% concentration paradoxically induced a more pronounced structure.