Study On The Performance Of Recycled Concrete Under Freeze-thaw And Composite Salt Erosion Environments

Recycling waste concrete to prepare recycled aggregate not only alleviates the pressure of increasing natural aggregate shortage,but also reduces the environmental problems caused by construction waste stacking and burial,achieving the goal of energy conservation and emission reduction.However,compared to natural aggregates,recycled coarse aggregate has the characteristics of high water absorption and high crushing value,resulting in significant differences in mechanical and durability performance between recycled concrete and ordinary concrete.Moreover,in actual service environments,recycled concrete structures will not only be affected by freeze-thaw damage,but also suffer from sulfate and chloride salt erosion,causing recycled concrete to suffer from composite salt erosion damage at the same time.Therefore,studying the durability changes of recycled concrete under freeze-thaw environment and composite salt erosion has important theoretical significance and practical engineering value.This article studies the effects of replacement rate of recycled coarse aggregate and different amounts of fly ash on the durability of recycled concrete under freeze-thaw and composite salt erosion environments.We compared and analyzed the appearance evolution,compressive strength loss rate,relative dynamic elastic modulus loss rate,and quality loss rate of recycled concrete after 120 freeze-thaw and composite salt erosion cycles under different mix ratios.We also studied the sulfate ion content at different erosion depths of recycled concrete after 120 composite salt freeze-thaw cycles.We analyzed the same concentration of sodium sulfate solution in freeze-thaw erosion environments,The effect of chloride concentration on the change of sulfate ion content in recycled concrete was studied,and the microstructure and hydration products of recycled concrete after 120 cycles of composite salt freeze-thaw were analyzed using scanning electron microscopy(SEM)and XRD.The research results indicate that:(1)When the content of fly ash is the same,as the replacement rate of recycled coarse aggregate increases,the slump and 28 day compressive strength of concrete decrease.When the content of fly ash is 30%and the replacement rate of recycled coarse aggregate is 50%,the 28 day compressive strength of recycled concrete has the lowest value,which is 49.85MPa;When the replacement rate of recycled coarse aggregate is the same,the slump of concrete increases with the increase of fly ash content.With the increase of fly ash content,the compressive strength of recycled concrete at 28 days shows a trend of first increasing and then decreasing.When the content of fly ash is 15%and the replacement rate of recycled coarse aggregate is 25%,the compressive strength of recycled concrete with fly ash is the best,which is 58.19MPa;(2)When recycled concrete is subjected to 120 freeze-thaw cycles in different corrosive solutions(5%Na2SO4,3%NaCl+5%Na2SO4,5%NaCl+5%Na2SO4),the degradation pattern of recycled concrete is as follows:5%Na2SO4>3%NaCl+5%Na2SO4>5%NaCl+5%Na2SO4;In three different salt solutions of freeze-thaw,the highest degree of corrosion damage occurs in the 5%Na2SO4 solution.With the increase of chloride concentration,the sulfate resistance of recycled concrete increases;And it is concluded that the presence of chloride salts does not change the durability change pattern of recycled concrete when the concentration of sodium sulfate solution is the same;(3)When freeze-thawing in three different erosion solutions(5%Na2SO4,3%NaC1+5%Na2SO4,5%NaCl+5%Na2SO4),the amount of fly ash added is the same.As the replacement rate of recycled coarse aggregate increases,the salt frost resistance of recycled concrete decreases.When the content of fly ash is 30%and the replacement rate of recycled coarse aggregate is 50%,the durability of recycled concrete against salt freezing is the worst;Except for the benchmark group,when the fly ash content is 15%and the replacement rate of recycled coarse aggregate is 25%,the durability of recycled concrete with fly ash addition is better.(4)We studied the effects of recycled coarse aggregate replacement rate,fly ash content,and chloride solution concentration on sulfate ion content under freeze-thaw and composite salt erosion environments,with a constant concentration of sodium sulfate solution.The sulfate ion content in recycled concrete decreases with the increase of erosion depth and gradually flattens out;When the concentration of sodium sulfate solution is the same,as the concentration of chloride salt increases,the content of sulfate ions decreases at the same depth;(5)The micro properties of recycled concrete are studied:the concrete with poor durability such as macro compressive strength and relative dynamic elastic modulus,etc.The corresponding SEM analysis shows that the structure is loose,and the XRD analysis shows that the expansive erosion products ettringite and gypsum have strong diffraction peaks.(6)Analyzed the mechanisms of single sodium sulfate corrosion and sodium chloride sulfate corrosion,and fitted the diffusion coefficient of sulfate ions in concrete using Fick’s second law.The trend of sulfate ion content with depth was simulated using COMSOL.When the concrete mix ratio is the same,at the same erosion depth,the sulfate ion content in a single sodium sulfate(5%Na2SO4)solution is higher than that in a composite solution(3%NaCl+5%Na2SO4,5%NaCl+5%Na2SO4);When the chloride content in the composite solution is 5%,the sulfate ion content in the concrete has the lowest value,which is consistent with the change law of sulfate ion content in Chapter 5.