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.