The synergic impacts of salt mixture and frost damage on rock decay: implications for the deterioration of rock-hewn heritages

Abstract Soluble salt and freeze–thaw are two significant weathering factors that contribute to the deterioration of rock-hewn heritages. Previous research has predominantly focused on the separate influences of salt weathering and freeze–thaw. However, in a realistic scenario, these two processes can occur simultaneously in nature, leading to a more complex decay process for such heritage. The aim of this study was to assess the combined impact of salt weathering and freeze–thaw on the severity of deterioration in rock-hewn heritage through evaluation of the variation of stone samples’ appearance and physical–mechanical properties, such as deterioration patterns, pore structure, surface hardness and splitting tensile strength. A laboratory ageing test was conducted using materials and climatic conditions representative of the West Lake Cultural Landscape of Hangzhou (WLCL), a UNESCO World Heritage Site located in Southeast China. Five different types of stones were collected from the WLCL area and its vicinity and subjected to 25 ageing cycles, each consisting of three phases: salt impregnation, freezing-thaw, and evaporation. The mixture of $${{Na}_{2}SO}_{4}{-MgSO}_{4}$$ Na 2 S O 4 - M g S O 4 was used as the contaminant substance. Results show that stones with moderate porosity (~ 5%) and a higher proportion of micropores, like flint, were susceptible to the combined effects of salt weathering and freeze–thaw, exhibiting significant surface deformation and strength decline. The porous tuff was primarily impacted by the development of a hard and thick crust on its surface, along with noticeable pore fill. The compact carbonate rock exhibited limited dimensional expansion with no discernible signs of deterioration patterns. Most rock specimens, except tuff, experienced a shift in their pore size distribution towards larger magnitudes, resulting in an increased average pore radius. This suggests that the inherent properties of rock-hewn heritages play a significant role in determining their vulnerability to various weathering agents. Consequently, these properties should serve as the foundation for developing future preventive conservation measures. Graphical Abstract