Influence of water vapor sorption of NaCl-Na2SO4-containing mural earthen plaster and preliminary deterioration analysis

Mud and earthen plaster murals are frequently found in cave temples and have tremendous historical and artistic importance, they are unfortunately susceptible to salt deterioration from environmental humidity. NaCl and Na₂SO₄ are among the most common soluble salts that induce salt deterioration in this type of murals. It is still unidentified how the combination of NaCl-Na₂SO₄ and environmental humidity causes salt deterioration. As a result, the current contribution elucidated the mechanism by which salt deteriorates under the influence of humidity using the concepts of moisture sorption and crystal deliquescence and dissolution. The moisture sorption process of the NaCl-Na₂SO₄ salt particles and earthen plaster samples containing NaCl-Na₂SO₄ mixes were comprehensively examined using the moisture uptake curve method. Thermogravimetric analysis was utilized to investigate the moisture characteristics caused by short-range adsorption and condensation in earthen plaster. Finally, based on the findings from ultra-depth-of-field microscopy, the change of the crystalline NaCl-Na₂SO₄ mixtures during the deliquescence-crystallization process was discussed. In the process of absorbing moisture, mixed-salt particles proceeded through two stages: deliquescence, where the rate of moisture absorption was constant, and osmotic suction, where the rate of moisture absorption was decreasing. Salt in plaster has a similar moisture sorption process, and the majority of the moisture in plaster caused by short-range adsorption and condensation participated in salt dissolution. The ion transport mechanism of NaCl-Na₂SO₄ under the influence of water vapor has been investigated, and the NaCl-Na₂SO₄ mixture yielded more intense efflorescence than a single salt in the same environment and salt content in plaster.