Study On The Influence Of Nano Attapulgite On Properties And Microstructure Of Recycled Aggregate Concrete

Many buildings have reached their service life,and a large amount of construction waste is generated when they are demolished and rebuilt.Scholars at home and abroad have broken,washed and graded it into recycled coarse aggregate（RA）to replace natural coarse aggregate（NA）,which is not only in line with the current green and sustainable development,but also the urgent need of replenishing natural resources and protecting the environment.However,the physical property of RA is worse than that of NA,and the performance of recycled aggregate concrete（RAC）decreases obviously.In order to enhance the performance of RAC,calcined nano attapulgite（NAT）was used to improve the working performance,mechanical properties and durability of RAC concrete.and combined with X-ray diffraction（XRD）and scanning electron microscopy（SEM）,Barret-Joyner-Halenda（BJH）methods analysis of the effect mechanism of calcined NAT on the microstructure of RAC,the main research contents and conclusions are as follows:（1）The effect of calcination at different temperatures on NAT activity was studied,and the changes of NAT at different temperatures were analyzed by thermogravimetric analysis（TGA）,SEM and XRD.The experimental results show that when the calcination temperature is 500℃,the NAT rod crystals are refined into smaller bundle of crystals,and a large amount of active Si O₂and Al₂O₃are generated,and the activity of NAT is improved.（2）The effects of different content of calcined NAT and replacement rate of RA on slump,compressive strength,splitting tensile strength and flexural strength of RAC.The results show that the slump of RAC increases with the increase of replacement rate of RA,but decreases with the increase of the content of calcined NAT.Most of the stress failure of RAC is bond failure in the interface transition zone（ITZ）,and fracture failure of new mortar and RA also exists.The mechanical strength of RAC decreases with the increase of replacement rate of RA.When the content of calcined NAT is 6%and the replacement rate of RA is 30%and 50%,the strength of RAC reaches the peak,and the compressive strength increases by 13.2%and 16.1%,respectively,compared with not mixed calcined NAT.The splitting tensile strength increased by 11.1%and 14.4%,and the flexural Strength increased by 11.7%and14.7%,respectively.Therefore,proper calcined NAT can effectively improve the working performance and mechanical properties of RAC.（3）The influence of different NAT content and replacement rate of RA on the durability of RAC was studied.The test results show that the durability of RAC mainly depends on the compactness of old mortar and new mortar.The chloride ion permeability resistance of RAC decreased with the increase of replacement rate of RA.When the content of calcined NAT was 6%,the replacement rate of RA was 30%,50%,minimum resistance to chloride ion penetration coefficient,compared with not mixed calcined NAT were reduced 21.1%and 22.1%,respectively;After 125freezing-thawing cycles,the mass loss rate of nano-recycled concrete（NRC6-30）is the minimum,and the mass loss rate is 2.49%less than that of the specimens with not mixed calcined NAT.The relative dynamic modulus of attapulgite is 26.10%higher than that of the specimen with not mixed calcined NAT.Therefore,proper calcined NAT can effectively improve the durability of RAC.（4）In order to study the effect mechanism of calcined NAT on the microstructure of recycled concrete,the microstructure of recycled concrete was analyzed by SEM,XRD and BJH methods.The test results show that with the addition of RA,the micro-cracks and harmful pores in RAC increase,and its mechanical properties and durability decrease.With the addition of RA,the micro cracks and harmful pores in RAC increase,and its mechanical properties and durability decrease.After adding proper amount of calcined NAT,it has filling effect,pozzolanic effect and surface effect,which can fill the cracks and harmful pores of hardened cement mortar.The bonding force of the ITZ is enhanced,and RAC becomes more dense.Therefore,calcined NAT can effectively improve the microstructure of RAC,thus improving the mechanical properties and durability of RAC.