Response Of Sandstone Salt Weathering To Water And Salt Migration In Grottoes

Under the influence of geological and climate environments in grottoes,open-air stone heritages suffer from different weathering diseases.Salt damage contributes to the stone heritage weathering in areas with abundant salt accumulation.Salt weathering is related to salt migration and precipitation in stone pores,and is mainly affected by stone properties,salt solution properties,and environmental conditions.Under different conditions,stone relics show different salt weathering patterns after a long stage of salt accumulation.In this study,two sandstones collected from the North Grotto Temple(Qingyang City,Gansu Province,China)and the Leshan Giant Buddha(Leshan City,Sichuan Province,China)were selected as the materials,and Na₂SO₄ solution was used as the salt contamination solution,to conduct the accelerated salt weathering tests and water-salt migration tests under constant temperature and variable humidity conditions.Based on the test results,the evolution of salt accumulation was analyzed;the relationship between salt damage patterns and water-salt migration was detailed;and measures to alleviate salt damage in stone heritages were suggested.The North Grotto Temple sandstone(NGT sandstone)and Leshan Giant Buddha sandstone(LGB sandstone)were used for the capillary rise test of Na₂SO₄ solution under conditions of RH=12%,33%,59%,and 76%,in order to investigate the relationship between salt damage characteristics and evaporation conditions.The capillary rise rate,evaporation rate,and macroscopic salt damage of sandstone were monitored during the capillary rise test.Salt content tests,elastic wave velocity tests,scanning electron microscopy(SEM)tests,and mercury intrusion porosimetry(MIP)tests were performed after the capillary rise test.The test results showed that:(1)After the capillary rise test,a salt layer appeared on the surface of NGT sandstone.As the relative humidity changed from high to low,the salt layer gradually became thicker.However,with the relative humidity from high to low,the macroscopic salt damage of LGB sandstone showed crack propagation,efflorescence,granular disintegration,and flaking successively.(2)Elastic wave velocity tests,MIP tests,and SEM tests showed that under the condition of RH=76%,deterioration occurred inside the NGT and LGB sandstone,while under the condition of RH<59%,no deterioration was found inside them.(3)Research results verify that the higher the evaporation rate is,the more efficiently the salt is carried to the surface,causing surface salt damage;while in the opposite condition,the salt is more likely to be trapped inside the sandstone,resulting in internal salt damage.The NGT sandstone and LGB sandstone were used for continuous evaporation tests of Na₂SO₄ solution under different humidity conditions,in order to simulate the transport of moisture and ions in sandstone and reveal the evolution law of salt accumulation.The sandstone evaporation process and macroscopic surface phenomena were monitored during the simulation test.After 10,20,40,80,and 160 days of salt solution evaporation,the salt efflorescence on stone surface was collected.Subsequently,ultrasonic wave velocity tests were conducted to determine salt damage within the sandstones.After that,the sandstone sample was sliced,and the moisture and salt content of the slice was tested.The test results showed that:(1)Salt accumulation interplayed with the evaporation of the liquid.On the one hand,high evaporation rates increased salt accumulation,resulting in significant efflorescence.On the other hand,salt accumulation decreased the evaporation rate.In general,the evaporation rate slowly decreased with an increasing salt concentration in the sandstone samples.In addition,in a few cases,efflorescence formed a compact salt crust and clogged the internal pores,resulting in a sharp decrease in the evaporation rate.(2)The mass fraction of efflorescence increased over time,indicating that efflorescence gradually became the dominant form of salt accumulation.(3)Salt accumulation increased along the direction of capillary rising height in both sandstone types,but in different increasing gradients.In LGB sandstone(having a smaller pore size),most of salts tended to accumulate in shallow areas below the stone surface(showing a rapid increase in salinity with rising height).In NGT sandstone(having a larger pore size),internal salts tended to be distributed throughout the depth of the samples(showing a slight increase in salinity with rising height).