Hygroscopic properties of salt-containing earthen plasters in the Mogao Grottoes: an experimental study

Salt damage poses a major threat to the preservation of earthen plaster at Mogao Grottoes. This study investigates the effects of sodium chloride and sodium sulfate on the hygroscopic properties of earthen plaster through isothermal moisture adsorption/desorption and water vapor permeability experiments. Results show that at high relative humidity (RH), salts substantially enhance the hygroscopic behavior of plaster materials, with a maximum salt influence factor of 1.52. Salts altered the pore structure, enhancing moisture uptake and desorption hysteresis, as revealed by SEM and nitrogen adsorption analyses. A multiparameter model considering coupled effects of RH and salt content for equilibrium moisture content in earthen plasters was successfully developed. For caves predominantly afflicted by salt efflorescence, ambient RH should be maintained below 75%. These findings clarify the mechanisms of salt-induced deterioration and suggest that controlling humidity fluctuations and reducing salt crystallization cycles are critical for the preventive conservation of mural heritage.