Effect of Sulfate Crystallization on Sandstone in the Yungang Grottoes

Abstract Subjected to various weathering in nature over long time, sandstone has been deteriorated in the Yungang Grottoes. The repeated dissolution and crystallization of soluble salts are important factors that cause the deterioration of the grotto sandstone. In order to study the action mechanism of sulfate crystallization in the Yungang Grottoes sandstone, deterioration tests were carried out by the “soaking-evaporation” cycle of saturated magnesium sulfate (MgSO4) and saturated sodium sulfate (Na2SO4) solutions in laboratory. The test results show that the participation of sulfate greatly accelerated the weathering speed of sandstone. The ultrasonic velocity of the specimens circulated in both Na2SO4 and MgSO4 solutions show a trend of first increasing and then decreasing, and changes more obviously in Na2SO4 solution. In the evaporation stage of the circulation, the peeling mass of the specimens circulated in Na2SO4 solution is significantly greater than that in MgSO4 solution. From the evaporation tests on saturated Na2SO4 and MgSO4 solutions and observation with optical microscope and scanning electron microscope, it is found that a dense crystal layer was formed on the surface during the evaporation of saturated MgSO4 solution, and it gradually seals the internal solution, slowing down the further evaporation. During the solution evaporation, Na2SO4 crystallizes into powdery crystals in a flocculent and soft state, and there are pores between the crystal grains. The change in the ultrasonic velocity of specimens in the "soaking-evaporation" cycle tests can be explained by the evaporation test results.