Study On Damage Characteristics And Mechanism Of Sandstone Under Wetting-Drying Cycle Of Water-Salt

Stone relics,monuments,rock slopes are exposed to the natural environment for a long time,affected by the geological environment and climate environment,there will be various types of weathering damage.In places where water and salt are enriched,salt weathering is the main cause of destruction of these cultural relics,buildings and projects based on rocks.This paper takes sandstone as the research object,and uses 3%Na₂SO₄ water and salt solution as immersion solution to carry out water-salt wetting-drying cycle tests in the laboratory.Based on a series of macro and micro tests,the physical and mechanical damage characteristics and damage mechanism of sandstone are analyzed.Relevant results provide theoretical basis for the protection of stone relics and monuments in salt source areas.It is of great practical significance to provide test data for the construction and maintenance of civil engineering with rock as the main material.The main research results are as follows:（1）The change law of physical characteristics of sandstone under the action of water-salt wetting-drying cycle is clarified:the wetting-drying cycle of water-salt makes fresh and complete sandstone loose,fragile and porous,and its color becomes bright.The degradation modes can be summarized into three types:pulverizing and shedding mode,fissure formation and development mode and spalling mode.The number of cycles is negatively correlated with mass and p-wave velocity,and the mass change can be divided into three stages:fluctuation decline stage,uniform decline stage and rapid decline stage.It is considered that the deterioration of sandstone is a process of fatigue damage.The mineral composition of sandstones has been traced and determined.It is believed that chemical reaction does not control the deterioration of sandstones.（2）The variation law of mechanical properties of sandstone under the action of water-salt wetting-drying cycle is defined:the water-salt wetting-drying cycle has a significant influence on the failure form of uniaxial compression,but has little influence on the failure form of splitting.The compression failure form mainly includes single-slope shear failure and tensile failure,and the splitting failure form is split failure.The number of cycles is negatively correlated with the mechanical indexes of sandstone,and the most significant reduction is 20-30 cycles.After 50 cycles,the uniaxial compressive strength,elastic modulus and tensile strength of sandstone decrease by 52%,62%and 43%,respectively.The analysis of the stress-strain curve shows that with the increase of cycle times,the brittleness of sandstone decreases and the plasticity increases,and the strain corresponding to the peak stress of sandstone increases first and then decreases,with the maximum value at 30 cycles.After 30～50 cycles,different degrees of stress readjusting occurred in the loading process of the sample.（3）The coupled damage evolution model of sandstone under the water-salt wetting-drying cycling-load condition is established:By means of damage mechanics and statistical damage theory,based on Lemaitre’s equivalent strain principle and the equivalent strain theory popularized by Yang and Zhang,a coupled damage evolution model of sandstone under watersalt wetting-drying cycling-load conditions is established,and the model is verified to be able to describe the damage evolution process of sandstone well.（4）The variation rule of sandstone microstructure under the wetting-drying cycle of water and salt is clarified.The microstructure of sandstone at different locations with different cycles is observed by scanning electron microscopy.It is found that the damage of samples is mainly reflected in the loss of cement and clay minerals,the damage of mineral particle structural integrity,and the development and penetration of pores and cracks.The quantitative analysis of pores based on Image-Pro Plus software shows that the proportion of all types of pores in sandstone with different cycles is small pore>medium pore>large pore>micro pore from high to low.With the increase of cycles,the total number of pores decreases,while the total pore area and average pore area increase.Under the same number of cycles,the total area and average area of pores on the sample surface are much larger than the sample center.（5）The damage mechanism of sandstone under the action of water-salt wetting-drying cycle is analyzed.In the water-salt wetting-drying cycle test,sodium sulfate is mainly converted between solution,anhydrous phase Ⅲ,thenardite（Na₂SO₄）and mirabilite（10H₂O·Na₂SO₄）.In the process of repeated soaking and drying cycle,the complete mechanism of sodium sulfate damage to sandstone is the joint action of soaking and drying process,and attention should be paid to the damage caused by drying process.During the drying process,the salt solution concentration rises due to the evaporation of water on the surface of the sandstone,and the salt nucleates and grows in various pores on the surface of the sample.The internal pores act as the salt solution reservoir to provide ions needed for crystal growth.When the crystals grow large enough and are limited by the pore wall,large crystallization pressure is generated and the sandstone is damaged.In the process of wetting,when sandstone samples are re-wetted by sodium sulfate salt solution,salt enriched in pore network dissolves,resulting in supersaturation of mirabilite,rapid growth of mirabilite crystals and greater crystallization pressure resulting in sandstone damage.The crystallization growth characteristics of soluble salt were directly observed by capillary evaporation crystallization growth test,and the damage mechanism was confirmed.Under the action of long-term water and salt wetting-drying circulation,sandstone mineral particles such as quartz,feldspar,and calcite may undergo dissolution reactions in neutral solutions.