Abstract: The Yungang Grottoes, carved into sandstone strata, are culturally significant heritage sites. However, prolonged exposure to natural elements has led to considerable weathering. Focusing on the north wall of the Chanting Corridor in Cave 9,nondestructive testing was performed using a combination of microwave moisture measurement, microscopy, and X-ray fluorescence techniques. This study examines the spatiotemporal distribution of moisture, chemical properties of aqueous solutions, and characteristics of sandstone weathering on the walls. Considering environmental conditions,the study also discusses the deterioration mechanisms of sandstone resulting from waterrock interactions. The results showed that the moisture distribution on the wall exhibited spatial heterogeneity, with the lower regions being consistently more humid than the upper regions. Rainfall-induced fluctuations in the ambient relative humidity significantly affected the moisture content of the shallow sandstone layers, resulting in the formation of continuous water films and droplets on the surface. The weathered sandstone contained a high concentration of soluble salts, and the aqueous solution on the wall exhibited an alkaline nature. The primary chemical elements in the solution were Mg, S, Na, Ca, Cl,and K, with Mg and S concentrations being substantially higher than those of the other elements. Weathering pathologies were predominantly observed in powdery and flaky forms,characterized by porous and fractured microstructures, with color variations due to differences in material composition. Wetting-drying cycles and chemical dissolution emerged as key factors contributing to the weathering of the walls in the Chanting Corridor. These findings provide valuable data and a theoretical basis for both scientific understanding and preventive conservation efforts in similar stone cultural heritage sites.

Key words: grottoes, chemical solution, nondestructive testing, weathering pathologies, heritage conservation