Study On Salt Corrosion Resistance Of Fly Ash Concrete Under Compound Salt Erosion Environment

The degree of soil salinization in Aksu reclamation area is very serious,which causes serious corrosion damage to the concrete structures in the reclamation area,resulting in that the actual service life of the concrete project is much lower than its design service life,and at the same time,it brings great challenges to the long-term safe and durable operation of the structure.In this paper,closely combined with the characteristics of climate and soil environment in Aksu reclamation area,experiments on the effect of fly ash content on the basic mechanical properties and salt corrosion resistance of concrete were carried out by using the conventional raw materials used in concrete engineering in this area.Among them,in order to explore the effect of fly ash content on the basic mechanical properties of concrete,the cube compressive strength,splitting tensile strength and flexural strength of concrete specimens with fly ash content of 0,10%,20%,30% and 40% were designed at the age of 3,7 and 28 days respectively.In order to explore the effect of fly ash content on the salt corrosion resistance of concrete in different service environments,the durability tests of composite salt erosion-freeze-thaw cycle and composite salt erosion-dry-wet cycle were designed for concrete specimens with fly ash content of 0,10%,20%,30% and 40% respectively in different concentrations(clear water,reference concentration,5 times base concentration,10 times base concentration).According to the analysis of the test results of macroscopic performance and microstructure,the amount of fly ash corresponding to the best mechanical properties of concrete and the corrosion deterioration mechanism of fly ash concrete in compound salt erosion environment are determined.it provides a scientific basis for the wide application of fly ash in concrete engineering in southern Xinjiang.The main results are as follows:(1)The mechanical properties of early age concrete can be improved by adding a certain amount of fly ash.When the content of fly ash is 10%,the cube compressive strength,splitting tensile strength and flexural strength of concrete specimens all reach the maximum.Compared with the corresponding strength of the reference group without fly ash,it is increased by 18.19%,20.13% and 17.75%,respectively,and when the content is more than 10%,it will affect the increase of specimen strength.The results of SEM scanning electron microscope image analysis show that adding appropriate amount of fly ash can improve the internal microstructure of concrete and improve the compactness of the material.(2)After the erosion of composite salt solution with different concentrations is coupled with freeze-thaw cycle and dry-wet cycle,the apparent morphology damage deterioration of concrete specimens is positively correlated with the number of freeze-thaw cycles,dry-wet cycles and the concentration of erosion solution.Moreover,ettringite(AFt),a corrosion product dominated by chemical erosion,fills the initial microcracks in concrete at the initial stage of erosion,resulting in a temporary increase in the mass and relative dynamic elastic modulus of the specimen.With the increase of the number of erosion and the increase of solution concentration,the content of erosion products gradually increases,resulting in expansion stress,resulting in crack expansion and penetration of surface concrete,accompanied by new micro-cracks.It provides a new channel for the erosion solution to enter into the concrete,which speeds up the failure process of the specimen.Under the same corrosion deterioration condition,when the content of fly ash is 30%,the appearance damage of concrete specimen is the lightest,the mass reduction is the least,the relative dynamic elastic modulus is the largest,and the salt erosion resistance is the best.(3)From the SEM image analysis of the microstructure of the specimen,it is found that in the early stage of composite salt solution erosion,a large number of needle-like materials produced in the specimen are attached to the surface of fly ash particles and cement particles,which makes the specimen more dense as a whole.According to the XRD diffraction analysis,the acicular material is ettringite,and with the extension of erosion time,the volume expansion caused by the increasing number of needles accelerates the failure rate of the specimen.