Microscopic weathering characteristics and deterioration mechanisms of sandstone in the Nankan Grotto, northern Sichuan, China

The mineralogy, micro-structure, geochemical mass balance, and porosity of sandstone in the Nankan Grotto were analyzed to explore the weathering mechanism of typical deterioration patterns: scaling and disintegration. The results signify that the major elements Ca, Na, K, Al, and Si have been leached during the weathering of calcite, feldspar, and quartz. Fe and Mg exhibited different geochemical behaviors due to the weathering of ankerite and biotite and the enrichment of clay minerals formed from feldspar alteration. The Leeb hardness values (in HLDs) of samples in the scaling and disintegration sites both showed an obvious decreasing trend from Zone-4 to Zone-1. The porosity exhibited little variation at the early stage and then increased rapidly during the weathering system, which was attributed to the sharp increase in the abundance of the macropores. The physico-mechanical properties (porosity and hardness) were useful parameters in evaluating the deterioration status of sandstone in the Nankan Grotto. Based on the microscopic petrographic evidence, the stress imposed by the biotite oxidation was the primary stimuli of the scaling pattern. The disintegration was mainly attributed to the high content of calcite and its intense fragmentation. The mineralogical compositions (especially calcite and biotite contents) laid the foundation for the distinct development of scaling and disintegration. In addition, the swelling/shrinking stress of clay minerals during hydration-dehydration reactions was an indispensable cause for the formation of both scaling and disintegration patterns. Our research reveals the microscopic weathering mechanism of the sandstone deterioration in the Nankan Grotto, providing new insights for research on the deterioration mechanism of grottoes. Further research and subsequent conservation of the Nankan Grotto could be conducted from the engineering geology aspects.