A laboratory experiment on salt weathering by humidity change: salt damage induced by deliquescence and hydration

A laboratory experiment on salt weathering was carried out under changing humidity conditions. Three types of rock (tuff and dense and porous sandstone) with sodium chloride, sodium sulfate, or magnesium sulfate were subjected to humidity oscillations from 20%RH to 98%RH during 6h periods at 20 degrees C. Prior to the weathering experiment, the rock specimens were oven-dried, and the water supply was restricted to only that from the air during the experiment. Salt on the rock specimens was deliquesced/hydrated at almost 100%RH during the high-humidity period and crystallized/dehydrated during the low-humidity period. Sodium chloride, which has high deliquescence ability, caused the most intensive weathering. Porous sandstone with sodium chloride was completely broken down after 100cycles of humidity changes. The other two types of rock with sodium chloride showed flaking or swelling on their surfaces. For dense sandstone with sodium chloride, the Equotip rebound value decreased as the humidity cycles increased. Sodium chloride has a high susceptibility of deliquescence at humidities of more than 80%RH. Weathering by sodium chloride would occur extensively on the surfaces of stone heritages and geoheritages in humid coastal spray zones. Magnesium sulfate induced a weight loss in porous sandstone, flaking from the surface of tuff, and cracks on the surface of dense sandstone. Sodium sulfate had almost no effect in the humidity-change experiment, although salt efflorescence was obvious. The results for magnesium sulfate, salt deliquescence, hydration, and crystallization in the humidity-change experiment were easily repeated. Magnesium sulfate might have more influence than sodium sulfate when air humidity fluctuates in a short period.