Study On Efficient Peeling Of Recycled Aggregate Attached Mortar Based On Composite Freeze-Thaw Action

Crushing waste concrete into recycled concrete aggregates(RCA)for efficient resource utilization is an important approach to achieve green and sustainable development in the construction industry.However,the presence of attached mortar on RCA surfaces results in a large number of weak interfaces in newly cast recycled concrete(RA),leading to weaker mechanical and durability properties compared to conventional concrete.This severely hinders the application of RA in engineering.This article focuses on the research of coarse recycled concrete aggregates and proposes an efficient physical modification method to remove attached mortar on RCA based on multiple composite freeze-thaw actions.The specific research contents are as follows:(1)By changing parameters such as the lowest temperature of the freeze-thaw cycle,high-temperature treatment temperature,and high-temperature treatment times,the influence mechanism of high and low-temperature treatments on mortar detachment is investigated,and the freeze-thaw modification process is further optimized.The mercury intrusion method is used to analyze the degradation mechanism of aggregates under high-temperature treatment.X-ray computed tomography(X-CT)is used to study the pore size distribution of RCA before and after freeze-thaw modification.The properties of natural aggregates(NA)before and after modification are explored using mercury intrusion and microhardness tests.Energy consumption evaluations are performed for each combination of working conditions.The study shows that hightemperature treatment results in more large-size pores and microcracks at the interface of RCA,leading to a significant decrease in their freeze-thaw resistance.The freezethaw modification method leads to an increase in the size and number of internal pores under low-cycle fatigue loading.Freeze-thaw modification does not have a negative impact on NA.The optimal treatment condition is one high-temperature treatments(300℃)and three freeze-thaw cycles(-40℃～15℃),with three modification cycles to remove all adhering mortar and the lowest power consumption.(2)RCA with three different water-to-cement ratios of 0.35,0.5,and 0.65,with and without air entrainment,are prepared.Based on the optimal treatment condition obtained in Chapter 2,the performance of freeze-thaw modification in RCA with different initial frost resistance is investigated,and the feasibility of using vacuumsaturated method to enhance the freeze-thaw modification effect of air-entrained recycled concrete aggregates(ARAC)is explored.The study shows that freeze-thaw modification can rapidly remove attached mortar from all three types of non-airentrained recycled concrete aggregates.For ARAC,Vacuum water retention presaturation can render the air-entraining agent ineffective during the freeze-thaw process,while the amount of icing increases significantly,significantly weakening the material’s frost resistance and shortening the number of modifications to less than three.(3)Based on the thick-wall spherical shell assumption,a quantitative analysis model of composite freeze-thaw modification of RAC is established.The freezing force of pores in concrete during freezing and thawing is calculated by theories of porous media mechanics and thermodynamics.The main factors affecting the modification effect of RAC are analyzed,and the influence coefficients of each parameter on freezethaw damage are obtained.Combined with the results of the experiments,the mapping relationship between the theoretical model and the number of modifications required for actual mortar flaking is obtained by fitting,and the two have good correlation,which can provide theoretical reference for the engineering practice of disposal of recycled solid waste.