Salt weathering of sandstone under dehydration and moisture absorption cycles: An experimental study on the sandstone from Dazu rock carvings

Abstract

AbstractSalt crystallization and phase change in porous media cause significant deterioration of cultural heritage made of stones and bricks. In this study, laboratory experiments on salt weathering of sandstone from the Dazu rock carvings (DRC) in Chongqing, southwest China, were carried out by exposing them to cycles of temperature and humidity change. Mirabilite, the main component of the salts in DRC that readily undergoes phase change, was utilized for the study. Three groups of Dazu sandstone (DS) samples with high (2.4 mol/L), moderate (1.5 mol/L) and low concentrations (1.0 mol/L) of sodium sulphate were prepared; and two conditions, high‐temperature with low‐humidity and low‐temperature with high‐humidity, were designed to simulate the fluctuations of temperature and humidity in the Dazu area. After several cycles, the dehydration and moisture absorption of DS under these two conditions forces the phase change of sodium sulphate between thenardite (Na2SO4) and mirabilite (Na2SO4·10H2O), causing different levels of damage. A combination results of sample length, weight, microstructure, and mechanical property variations show that the damage effect of the thenardite formed during dehydration is more significant than that of mirabilite in moisture absorption. During these cycles, there is noticeable salt migration and the local concentration of salts is influenced by the microstructure of the stone and the distribution of clay minerals. Samples with high salt content show disintegration of the stone into powder, while those with moderate salt concentration show flaking and salt accumulation on the surface. These types of damage correspond with observations of natural weathering in the field. The experiment shows that no obvious damage takes place when the salt content is lower than 0.67%. These results contribute to a better understanding of the effects of salt weathering on sandstone and provide references for choosing correct prevention and protection measures for the world heritage site DRC.