Structural damage mechanism of weakly cemented sandstone from the North Grotto Temple under freeze-thaw action

The North Grotto Temple, constructed on weakly cemented sandstone formations, has suffered from significant weathering due to prolonged freeze-thaw cycles. To explore the structural damage mechanisms, the saturated sandstone samples were used to simulate the external temperature variations endured by the grotto. After 0, 5, 10, 15, and 20 cycles, the mass, elastic wave velocity, dry density, relative density, porosity, X-ray diffraction (XRD) mineral analysis, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) were tested. The evolution of damage was analyzed from both macro-scale and micro-scale perspectives, clarifying the structural damage mechanisms The results indicate that: (1) The macroscopic deterioration of sandstone initially manifested as granular disintegration and the cracks development. In subsequent stages, the samples showed more severe disintegration, and local block detachment was attributed to the cracks coalesce. (2) The samples displayed a decrease of mass, wave velocity, and dry density with an increasing number of cycles, while relative density and porosity increased. (3) Freeze-thaw action induced fluctuations in the volume ratio of pores of different sizes. Compared to the initial state, the final volume ratio of small and medium pores decreased, whereas the ratio of large pores increased. (4) The freezing stages of sandstone progressed from the unfrozen stage to the supercooling stage, rapid freezing stage, and stable freezing stage. Freezing pressure promoted particle breakage, pore development, and crack propagation. The thawing stages followed a sequence from the unthawed stage to the slow thawing stage and rapid thawing stage. Upon thawing, the sandstone structure lost support from ice crystals, leading to structural failure. (5) The effective frost heaving pressure in sandstone increased with the number of freeze-thaw cycles, while crystallization pressure showed a significant negative correlation with pore size. (6) The accumulation of microstructural damage ultimately resulted in macroscopic failure and the degradation of physical properties. These research findings can serve as a reference for the protection of weakly cemented sandstone grotto.temples.