The chemical weathering mechanism of sandstone in Yungang Grottoes Cave 9

To investigate the relationship between weathering processes, cementation loss, and pore penetration mechanisms in the sandstone of the Yungang Grottoes, this study analyzed sandstone of the cave body sample and surface exfoliation fragments from Cave 9. Characterization was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), confocal Raman microscopy, and X-ray diffraction (XRD). The results indicate that micromorphological alterations in diagenetic minerals are closely associated with weathering behavior, primarily manifested as changes in cementation, biotite alteration, and kaolinization of potassium feldspar. The synergistic effects of these microstructural mineral changes constitute the intrinsic mechanism underlying weathering pathologies observed in Cave 9. In minimally weathered sandstone, calcite(CaCO₃) serves as the primary cement, effectively binding diagenetic minerals； biotite[K(Mg,Fe)₃AlSi₃O₁₀(OH)₂] and potassium feldspar[KAl(Si₃O₈)] exhibit smooth surfaces and intact morphology. Conversely, weathered sandstone displays a loose structure, with the kaolinite predominating as thecementing agent. The relative calcite content is significantly reduced, while the kaolinite content increases. Biotite is present in only trace amounts, and the surfaces of potassium feldspar surfaces exhibit dissolution grooves with severe kaolinization. Under the influence of swelling stresses and other factors, cementation failure occurs, leading to particle collapse and consequent damage to the sandstone cultural relics.