8.2 Microstructure analysis
The SEM images at 2000 times (shown in Figs. 22, 23 and 24) reveal the microstructure of samples. The structure of fresh sandstone is characterized by its good homogeneity and compactness, but without apparent layered phenomenon. There are some clay minerals containing illite, smectite and chlorite. After the experiment, the sandstone starts to show obvious changes in its morphology from the surface to the inner structure. The surface structure deterioration is characterized by huge calcite loss, and secondary crack along the grain boundary, resulting in a loose and porous microstructure. The deterioration is particularly serious with corrosion holes in the acid rain cycle. The middle and inner structure deterioration is relatively minor, and the minerals between grain boundaries drift away due to reactions of water, generating pores and micro-cracks. There are a few clay minerals around the pores and micro-cracks, e.g., kaolinite, chlorite, illite, smectite, illite/smectite formation and chlorite/smectite formation. To sum up, the main mechanism involved in granular disintegration appears to be gradual widening of grain boundary micro-cracks and, to a lesser extent, development of other micro-cracks. The effects are: (1) it diminishes the degree to which the grains in the sandstone interlock, as reducing the mass, surface hardness and P-wave velocity; and (2) it facilitates the ingress of water, thereby accelerating the weathering rate (Sassoni & Franzoni, 2014).
By comparing the mineral and chemical composition and microstructure analysis of sandstone, we find that the surface deterioration of sandstone is more severe. This means that the WRI occurred from the surface to the inner structure. The statues in Niche of Sakyamuni Entering Nirvana are in the open air, hence the deterioration by water (including precipitation and capillary water) is serious and irreversible.