Laboratory weathering experiment on sandstone of Niche of Sakyamuni Entering Nirvana at Dazu Rock Carvings, China
The sandstone of Niche of Sakyamuni Entering Nirvana statues at Dazu Rock Carvings in China has been experiencing surface weathering for an extensive period of time. To understand the weathering effects of the local environment, 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, surface microstructure, were applied to explore the sandstone weathering degree over multiple cycles (for a total of 50 periods, 350 days). Various devices (e.g., X-ray diffraction (XRD), scanning electron microscope (SEM)) were adopted to analyze the sandstone weathering mechanism. The experimental results indicate that the main sandstone deterioration is granular disintegration. For all experiments, the variation in mass decayed exponentially with the periods; while the variation in surface hardness and P-wave velocity decayed as a power function with the periods. The rankings of influencing environmental factors of sandstone weathering follow this order: acid rain cycle, freeze-thaw cycle, and dry-wet cycle. More generally, the water-rock interaction (WRI) occurs from the surface to the inner structure of the sandstone. Granular disintegration is driven by (1) the dissolution of calcite, alteration of feldspar, and water swelling and drying shrinkage of smectite; and (2) the widening inter-granular micro-cracks. The findings could provide useful insights for the protection of the studied statues.