Efflorescence, subflorescence, and crust weathering characteristics of sandstone in Nankan Grotto, China: insights into petrography and environment constraints

The different salt weathering issues of the sandstone in the Nankan Grotto have significantly influenced their preservation conditions. This work aimed to evaluate the petrography and environment constraints for salt weathering of three typical sandstones: yellow sandstone, cyan sandstone, and gray sandstone in the Nankan Grotto. Salt resistance test and acid leaching test were conducted on these three sandstones. In addition, the mineralogy, geochemical mass balance, micro-structure, and water transport properties of these sandstones were also analyzed. It is found that yellow sandstone had high quartz concentration, porosity, and water transport ability. Cyan sandstone had low calcite concentration, high porosity and medium water transport ability. Gray sandstone had high calcite concentration, low porosity and water transport ability. The decay of sandstones in the salt resistance test at 20 °C was faster than those at 5 °C and 35 °C. In addition, yellow sandstone was the most resistant to the salt resistance test, followed by cyan sandstone, and gray sandstone was the most vulnerable to the salt resistance test. In the H₂SO₄ acid leaching test, efflorescence pattern was the most likely type of decay in yellow and cyan sandstones. Thenardite was the exclusive salt in the yellow sandstone, while gypsum was the mainly salt in the cyan sandstone. Gypsum crust and subflorescence were the most common types of decay for gray sandstone. In the HNO₃ acid leaching test, yellow and cyan sandstones did not show obvious variations. For gray sandstone, the secondary minerals were gypsum and dolomite, and crust pattern was observed. In general, the salt weathering of gray sandstone was more severe than yellow and cyan sandstones. The differences in petrography (mainly calcite concentration) and micro-structure of sandstone and the external environment leaded to the differences in the types, amounts, and locations of the salt crystallization in the acid leaching tests. These contributed to the different development of efflorescence, subflorescence, and crust patterns. Our research reveals the petrographic, micro-structural, and environmental constraints for different salt weathering processes of sandstones in the Nankan Grotto.