Preferential Freeze–Thaw Damage in Limestone Containing Stylolites and Calcite Veins from High-Altitude Mining Areas

Abstract Limestone slopes in high-altitude mining areas are subjected to strong diurnal temperature fluctuations and repeated freeze–thaw cycles, which can induce the development of internal pores and cracks and cause structural deterioration of the rock mass. However, where preferential damage occurs and how it affects the structural continuity of limestone remain unclear. To investigate how structural units, including stylolites, calcite veins, and the matrix, control the spatial distribution of freeze–thaw damage in natural limestone, this study examined limestone containing stylolites and calcite veins. The specimens were classified into sparse and dense structural-unit types according to the degree of structural-unit development. Low-field nuclear magnetic resonance T2 spectra, magnetic resonance imaging, scanning electron microscopy, energy-dispersive spectroscopy, and P-wave velocity tests were combined to reveal the evolution of the pore structure, microscopic damage, and structural continuity under freeze–thaw cycles. The results show that the dense structural-unit specimens exhibited a more pronounced increase in long-T2 components, clustering of high-signal MRI regions, and P-wave velocity attenuation. After 30 freeze–thaw cycles, the velocity attenuation rate of the dense specimens reached 28–30%, significantly higher than that of the sparse specimens, which was 10–12%. The absolute value of the fitted porosity–velocity slope for the dense specimens was approximately 2.17 times that of the sparse specimens. SEM-EDS results indicate that the matrix and calcite vein interiors showed limited changes, although localized crack-like features developed near some vein boundaries after 30 cycles, whereas stylolites gradually evolved into widened, deepened, and rough-bounded banded damage zones, accompanied by relative Ca depletion and local enrichment of Si and Al. These findings demonstrate that freeze–thaw damage in natural limestone is not a uniform accumulation process but a stylolite-dominated preferential damage process controlled by mineralogical-structural heterogeneity. This understanding provides experimental evidence for identifying freeze–thaw damage characteristics and determining key monitoring zones of limestone slopes containing natural structural units in high-altitude mining areas.