Study On Durability Deterioration Law And Mechanism Of Partially Immersed Recycled Aggregate Concrete Subjected With Composite Salt

Under the guidance of the national"carbon peaking and carbon neutrality"policy,recycled aggregate concrete has been used in new concrete structures.The climate in Northwest China is dry,and the soil and groundwater are rich in corrosive ions.The complex interface transition zone structure of recycled aggregate concrete makes it easy for corrosive ions to diffuse into the interior of the concrete and have a series of physical and chemical interactions with hydration products,reducing the durability of the recycled aggregate concrete structure.The root of the recycled aggregate concrete structure column is located in the junction area of high underground humidity and low air humidity.The“suction force”formed in the concrete capillary pores due to evaporation of the recycled aggregate concrete in the gas-liquid junction area and the atmospheric area accelerates the diffusion and transmission of corrosive ions.The concrete protection layer is peeled off and the steel bar is corroded,which seriously threatens the durability and safety of the structure.In view of this,it is of great scientific significance and engineering value to carry out research on composite salt corrosion durability of recycled aggregate concrete structure column roots,to reveal the law and mechanism of recycled aggregate concrete structure durability degradation,and to promote the scientific and rational application of recycled aggregate concrete in Northwest China.Based on the climatic and environmental characteristics of Northwest my country and the durability and failure characteristics of the roots of recycled aggregate concrete structural columns,the paper uses a 7.5%wt.Mg SO₄-7.5%wt.Na₂SO₄-5%wt.Na Cl composite solution to simulate environmental corrosion ions,and partial immersion to simulate structural columns In the root service environment,the system carries out the durability test of partially immersed composite salt corrosion recycled concrete.Taking the apparent morphology,relative longitudinal and transverse elastic modulus,quality and relative compressive strength of corroded recycled aggregate concrete as durability indicators,the durability degradation law of partially immersed composite salt corroded recycled aggregate concrete was analyzed,and the types and combinations of mineral admixtures were studied.The influence of the method on the durability of recycled concrete;X-ray diffraction,infrared spectroscopy,thermal analysis,scanning electron microscopy and other microscopic characterization methods are used to study the evolution process of the microstructure of damaged recycled concrete,and reveal the durability degradation mechanism of partially immersed composite salt corrosion recycled concrete.The research contents and main conclusions are as follows:（1）The surface morphology and macroscopic cracks of the corroded recycled aggregate concrete were observed and measured.Based on the macroscopic crack width,surface concrete spalling area,salt crystallization area and other indicators,the damage grading evaluation standard of the damaged recycled aggregate concrete surface morphology was proposed.The apparent damage degree of the double-mixed mineral admixture recycled aggregate concrete group is generally lower than that of the multi-mineral admixture recycled concrete.Among them,the damage degree of fly ash-slag double mixing is the lowest,while the damage degree of fly ash-silica fume double mixing is the highest,and the damage degree of slag-silica fume-metakaolin triple mixing of recycled aggregate concrete is relatively small.（2）The physical and mechanical properties of partially immersed composite salt corroded recycled aggregate concrete continued to decline with the corrosion age,showing three stages of slow decline,rapid decline and rapid decline according to the decline rate.By comprehensively analyzing the variation law of the physical properties of the corroded recycled concrete,the recycled aggregate concrete specimen can be divided into three parts from bottom to top along the length direction:the immersion area,the gas-liquid interface area and the atmospheric area.The relative dynamic elastic modulus of the soak area and the gas-liquid interface area varies with The increase of the corrosion age shows a great change,that is,the relative dynamic elastic modulus of the immersion zone changes from the initial stage of corrosion higher than that of the junction zone to a development trend that is lower than that of the junction zone.（3）The effects of mineral admixture types and matching methods on the durability of recycled aggregate concrete were studied.The results show that the durability of recycled aggregate concrete with double mineral admixture is better,while the durability of recycled aggregate concrete with multiple mineral admixture is poor.For the recycled aggregate concrete with double mineral admixtures,the physical and mechanical properties of the fly ash-slag double blend have the smallest degradation degree,and the fly ash-silica fume double blend has the largest degradation of the physical and mechanical properties.At the same time,the physical and mechanical properties of the slag-silica fume-metakaolin tri-doping are good after corrosion.（4）Comprehensive analysis of the physical and mechanical properties and microstructure characterization results of the corroded recycled aggregate concrete shows that the durability degradation process of the partially immersed composite salt corroded recycled aggregate concrete can be divided into three stages:slow performance degradation stage,rapid performance degradation stage and rapid performance degradation stage.In the stage of slow performance degradation,the surface of the recycled aggregate concrete in the soaking area and the gas-liquid junction area is filled with corrosion products such as brucite,gypsum,and vanadium,and the concrete density of the corrosion layer is improved,and the overall corrosion of the recycled aggregate concrete is slow.However,under the action of"suction force"formed by the evaporation of capillary water in the concrete in the atmospheric area,the corrosion rate of the regenerated concrete in the gas-liquid interface area is higher than that in the immersion area.In the stage of rapid performance degradation,mirabilite crystals gradually formed in the recycled aggregate concrete in the gas-liquid junction area,and its physical properties declined rapidly.At this time,the large amount of hydrated magnesium silicate in the concrete in the immersion zone causes its performance degradation rate to be higher than that in the gas-liquid interface zone.As the corrosion continued,the content of expanded crystals and non-cementable products in the recycled aggregate concrete increased rapidly,and the physical and mechanical properties of the concrete deteriorated rapidly.