Study on weathering mechanism of sandstone statues in Southwest China: example from the sandstone of Niche of Sakyamuni Entering Nirvana at Dazu Rock Carvings

To understand the weathering mechanism of sandstone statues in Southwest China, the sandstone of Niche of Sakyamuni Entering Nirvana at Dazu Rock Carvings was sampled. The three experiments were set up, including dry–wet cycle, freeze–thaw cycle and acid rain cycle. In this study, the mass, surface hardness, P-wave velocity were applied to explore the sandstone weathering degree over multiple cycles (for a total of 100 periods, 700 days). Various devices (i.e., X-ray diffraction, scanning electron microscope) were adopted to analyse the sandstone weathering mechanism. The experimental results showed that the deteriorated features of samples were the granular disintegration under the different experimental conditions, and the samples showed a decreased trend as a whole in the mass, surface hardness and P-wave velocity with the increase of periods. The sandstone deterioration by water occurred from the surface to the inner, and it was more serious with the increase of periods. The deterioration degrees affecting the granular disintegration of sandstone followed this order: acid rain cycle, freeze–thaw cycle, dry–wet cycle. Notably, higher H+, concentrations would enhance the sandstone deterioration. The sandstone of the bedding development was more easily to form the granular disintegration. The sandstone experienced the granular disintegration due to the combined effects of the internal factors (i.e., the dissolution of calcite, alteration of feldspar and water swelling and drying shrinkage of smectite on account of moisture) and external factors (i.e., the acid rain, temperature and humidity). The findings could provide useful insights for the protection of the studied statues, and the corresponding theories and methods had important guiding significance for the protection of grottoes in Southwest China.