Study On The Influence Of Aggregate Carbonization And Nano-SiO₂ Strengthening On The Performance Of Recycled Concrete Under Ion Erosion Environment

China’s 14th Five-Year Plan puts forward the work plan of building a"waste-free city".At this stage,comprehensive treatment and reform of solid wastes is a powerful starting point to promote the construction of a"waste-free city".At present,the application of waste concrete is mostly to crush it into aggregate and then use it to prepare recycled concrete.As we all know,the service environment of concrete is complex,which puts forward higher requirements for the performance of concrete.However,the old slurry attached to the surface of recycled coarse aggregate seriously reduces the durability of recycled concrete,especially in different ion erosion environments,and the erosion medium is easily transmitted into the recycled concrete through weak multiple interface areas,which leads to the deterioration of recycled concrete.In view of this problem,it is necessary to strengthen recycled coarse aggregate and discuss the influence of ion erosion on modified recycled concrete.In order to explore the influence of corrosive ions on the macro and micro properties of the interface transition zone of the modified recycled concrete under different ionic corrosive environments,the samples of the modified recycled concrete were naturally soaked in 3.5%Na Cl solution and 5%Na₂SO₄solution.The macro performance of recycled concrete was studied by testing the compressive strength and ion erosion concentration,and the micro performance of interface transition zone of recycled concrete was studied by microhardness tester,scanning electron microscope and thermogravimetric analyzer.The experimental research contents and conclusions are as follows:（1）Firstly,the original concrete with different strength grades was simply crushed,and then the two types of recycled coarse aggregate were respectively treated by accelerated carbonization and nano-SiO₂soaking to obtain the repaired recycled coarse aggregate.Through the research on the basic physical properties of recycled coarse aggregate,the composition and micro-morphology of old mortar before and after repair,it is found that the basic physical properties of recycled coarse aggregate are improved after repair,and the crushing index value and water absorption value of recycled coarse aggregate repaired by nano-SiO₂are obviously reduced.Compared with the old mortar attached to the surface of low-strength recycled coarse aggregate,the hydration process of high-strength recycled coarse aggregate is more complete,and the microstructure is denser and smoother,which corresponds to its better basic physical properties.（2）The repaired recycled coarse aggregate is prepared into modified recycled concrete,which was soaked in 3.5%Na Cl solution for 45 days and 90 days.Before erosion and after the expiration of erosion age,the macro performance and micro interface transition zone performance were tested respectively to explore the influence of chloride ion erosion on modified recycled concrete.It is found that the content of free chloride ion in each layer of modified recycled concrete decreases obviously at different erosion ages,which can inhibit chloride ion transmission.Chloride ion erosion has the least influence on the interfacial mechanical properties of nano-SiO₂modified recycled concrete.The strength grade of recycled aggregate has influence on the width of Cl enrichment area at the old aggregate-old mortar-interface transition zone,Cl is easier to be enriched at the interface of recycled concrete prepared from low-strength coarse aggregate.（3）The repaired recycled coarse aggregate is prepared into modified recycled concrete,which was soaked in 5%Na₂SO₄solution for 45 days and 90 days.Before erosion and after the expiration of erosion age,the macro performance and micro interface transition zone performance were tested respectively to explore the influence of sulfate ion erosion on modified recycled concrete.It is found that with the increase of erosion age,the macro-mechanical properties and micro-mechanical properties of recycled concrete first improve and then deteriorate sharply.Modified recycled concrete has a good blocking effect on the transport of sulfate ions.Among them,the inhibition effect of nano-SiO₂modification is the most obvious.The content of ettringite in different modified recycled concrete is different.For recycled concrete prepared from low-strength coarse aggregate,aggregate strengthening has obvious inhibition effect on ettringite.The width of sulfur enrichment area at the old aggregate-old mortar-interface transition zone is consistent with the trend of microhardness test results.