Study On Strengthening Technology And Mechanism Of Recycled Concrete Aggregate

With the prosperous development of the construction industry,the consumption of construction resources is increasing year by year.At the same time,a large amount of construction waste has also brought enormous pressure on the environment,and the use of construction solid waste to prepare recycled aggregates（RCA）has become a hot issue worldwide.The RCA obtained after simple crushing of construction waste have a series of problems such as irregular shape,high water absorption,high crushing value and low apparent density,which limit their application in concrete.Therefore,it is important to enhance the performance of RCA,to clarify the reinforcement mechanism of RCA and to explore the connection between aggregate modification treatment and concrete performance for the recycling of RCA.This paper systematically investigates the strengthening effects of three RCA strengthening techniques,namely chemical strengthening,biological modification and chemical-biological composite strengthening,on RCA and recycled concrete（RAC）,and analyses their microstructure and properties using modern testing techniques such as XRD,SEM and microhardness,explores the strengthening mechanisms of different methods on RCA,and clarifies the effects of RCA strengthening treatment on the microstructure and mechanical properties of RCA and RAC.（1）All three means of RCA strengthening can improve the basic properties of RCA such as water absorption rate,apparent density and crushing index,among which chemical-biological composite strengthening technology is more effective and can make RCA obtain more superior basic properties.After chemical-biological composite strengthening treatment,the apparent density of RCA is increased by 2.1%,water absorption rate is reduced by 35.6%and crushing index is reduced by 23.5%,but the time and economic costs of composite reinforcement were the highest.（2）Chemical strengthening and biological modification technologies have different mechanisms for enhancing the performance of RCA.On the one hand,the cement slurry wrapped around the surface of the RCA hardens with the combined effect of hydration and carbonation as the age of maintenance increases,shaping the shape of the RCA;on the other hand,Ca²⁺in the cement slurry migrates and mineralises within the aggregate,improving the structure of the adhering mortar and interfacial transition zone and enhancing the performance of the recycled aggregate.When biologically modified,bacteria as living organisms tend to reside in the more defective areas within the RCA,where mineralisation deposition is induced by an alkaline environment rich in calcium and nutrients.As the bacteria grow and metabolise,they are able to steadily and continuously synthesise calcium carbonate precipitates that fill the interfacial transition zone of the RCA and pores larger in size than the transition pores.（3）All three strengthening techniques can enhance the performance of RAC.The bio-modification treatment was more effective than chemical strengthening in improving flowability,but the chemical strengthening treatment was more effective than bio-modification in improving mechanical properties and resistance to chloride ion penetration;composite strengthening showed better results in all properties of concrete,and the highest compressive strength of RAC prepared with composite strengthened recycled aggregates,compared with concrete prepared with unreinforced RCA,after The 7d and 28d compressive strengths of concrete with 50%and 100%recycled aggregate replacement were increased by 12.4%,55.0%and 24.8%and 22.8%respectively.（4）The results of this paper on the basic properties and microstructure of RAC and RCA confirm that chemical strengthening is more effective than biomodification in improving the properties of RAC.In addition,although the composite strengthening technology has shown obvious superiority in enhancing the performance of RCA and improving the performance of RAC,the composite strengthening technology is significantly higher than the two single strengthening technologies in terms of strengthening time,environmental impact and economic cost.The development of this technology from the laboratory to practical engineering applications still requires a large number of equipment,technology,cost and other aspects of optimization of experimental support.