Sustainable Conservation of the Xumishan Grottoes: Weathering Characteristics and Causes of Red Sandstone Surfaces

The red sandstone rock mass and sculptures within the Xumishan Grottoes exhibit multiple forms of weathering damage, including flaking, hollows, efflorescence, and salt efflorescence. Among these, flaking and salt efflorescence are the most pronounced, posing a severe threat to the long-term preservation of the cave’s cultural relics. This study employs SEM-EDS, XRD, XRF and IC techniques to conduct scientific analysis and characterization of the weathering products on the surface of red sandstone. By integrating the lithology of the red sandstone itself with its depositional environment, the study investigates the causes of surface weathering in red sandstone. Research findings indicate that the lithology of the cave site consists primarily of muddy siltstone, with gypsum and mirabilite present within the weathered red sandstone, alongside minor quantities of Epsom salt and chloride salts. The migration and enrichment of these soluble salts within the rock mass constitute the key chemical factors triggering efflorescence and spalling in sandstone. Concurrently, influenced by the grottoes’ environmental conditions, periodic temperature fluctuations and moisture migration further promote freeze–thaw cycles and the crystallization–dissolution cycle of salts, accelerating the weathering process of the red sandstone within the Xumishan Grottoes.