The surface weathering characteristics of the statues at Sanqing Grotto in Nanshan of the Dazu Rock Carvings

The Dazu Rock Carvings are the largest group of grotto statues in southern China. The sandstone statues, located in the Sanqing ancient cave in the Nanshan Rock Carvings area (excavated during the Shaoxing period of the Southern Song Dynasty), exhibit typical disease characteristics, including matrix weathering and erosion of the painted layer, resulting from the long-term influence of geological forces and human factors. The weathering characteristics of the sandstone in the Sanqing ancient cave statues have been systematically studied using multiscale characterization techniques, such as scanning electron microscopy, X-ray diffraction, Raman spectroscopy, ion chromatography and laser particle size analysis. The results show that the mineral composition of relatively fresh sandstone is mainly quartz, calcite and feldspar, with secondary products such as clinochlore and raw gypsum generated after weathering of the sandstone. Microstructurally, weathered sandstone is characterized by the dissolution of cements to form a porous network structure, as well as the crystallization and enrichment of gypsum in the surface layer and within the pores of the rock. Compared with fresh sandstone, the content of SO₄^2- in weathered sandstone increased by about 214-526 times, and the content of Ca²⁺ increased by about 19-43 times. The proportion of sand particles (particle size >75 μm) decreased by 1.89%-26.68% (volume fraction), and the proportions of silt particles (particle size of 5-75 μm) and clay particles (particle size <5 μm)increased by 1.84%-19.47% and 0.06%-7.34%, respectively. The transformation and migration of sand particles into silt and clay particles confirms that the mineral transformation during weathering directly leads to the physical fragmentation and finegraining of sandstone. The results provide a reference for the analysis of the weathering mechanism of grotto statues and the formulation of protection strategies for similar stone cultural relics.