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

Abstract

Abstract Soluble salt and freeze-thaw are two significant weathering factors inducing deterioration in rock-hewn heritages. While much research has focused on the influences of salt weathering and freeze-thaw separately, a realistic scenario reveals that the two processes often coincide in nature, making rock's decay process more complex and faster. We investigate how the synergic impact of salt weathering and freeze-thaw affect the severity of deterioration on rock-hewn heritage using materials and climatic conditions informed by rock-hewn heritage sites in SE China. The experiment was carried out on five types of stones sampled from the area of or close to the World Heritage Site, the West Lake Cultural Landscape of Hangzhou (WLCL), China. Samples were subject to 25 ageing cycles that comprised three phases: salt impregnation phase in 20℃/3hrs + freezing phase in -10℃/15hrs + evaporation phase in 50℃/6hrs). The mixture of Na2SO4-MgSO4 was utilised as the contamination substance based on the salt content determination of the stone heritage in the WLCL. Deterioration patterns, dimensional changes, pore structure and physic-mechanical properties, including surface hardness and splitting tensile strength, were determined. Results showed that the significant porous stone (porosity > 5%) with a higher proportion of micropores, e.g. flint, was more susceptible to the salt mixture, demonstrating pronounced surface detachment, deformation and strength decline. Dolomitic-rich carbonate stone with low porosity also shows deterioration, though their dimensional expansion and decline in mechanical properties are relatively minor. Most rock specimens' pore size distribution shifts to a higher magnitude, causing an enlargement in their average pore radius and an increase in mesopores. The relatively compact dolomitic limestone specimens developed new pores with a mean pore diameter of 6.5nm. This suggests that combining the two weathering factors would damage the appearance and nature of rock-hewn heritage by reshaping pore structure, weakening the cohesion between grains and consequently damaging the rock's initial fabric and strength.