The mechanisms of salt weathering responsible for sandstone deterioration in the Yungang Grottoes, China

Abstract

Salt weathering significantly contributes to the deterioration of porous building materials, particularly in stone cultural heritage. However, the origins of salts and water responsible for salt weathering remain poorly understood. This study focuses on the Yungang Grottoes, known for severe salt weathering. We collected 15 salt-bearing rock samples from 8 caves to determine the types of salts, 16 rock powder samples from two horizontal boreholes located in the interior and exterior walls of a seriously deteriorated cave to compare dissolved ions, and 7 rock samples from a horizontal borehole to determine the mineralogy. We found that epsomite (MgSO₄·7H₂O) is the dominant salt precipitated on the walls of many caves. The source of SO₄²⁻ is historical air pollution, while the source of Mg²⁺ is the weathering of ankerite and biotite. The comparable concentrations of dissolved K⁺ and Li⁺ in the interior and exterior walls indicate a similar degree of chemical weathering; however, the Ca²⁺ concentration in the interior wall is significantly lower, indicating processes that have removed Ca. PHREEQC simulation of the precipitation sequence of salts suggests that CaSO₄ may have already crystallized historically, which is supported by data reported in the literature. By monitoring vapor concentrations and wall temperatures, we infer that the interior walls are more prone to retain rock moisture, which contributes to both chemical and salt weathering, ultimately accelerating the deterioration of sandstone. These findings provide a scientific basis for the preservation of the grottoes and the mitigation of salt weathering.