Research For Microstructure Of Nano-Silica Modified Of Recycled Aggregate Concrete Based On The Carbonization Of Recycled Coarse Aggregate

A large amount of waste concrete is produced in our country every year.At the same time,a great deal of material such as sand and gravel is needed for the construction of the projects.It can effectively improve the utilization of waste concrete that process waste concrete into recycled aggregate.The microstructure of recycled aggregate is complex.It has multiple interfaces.The performance of recycled aggregate is worse than natural aggregate because that recycled aggregate is attached by the old mortar.In this paper,the properties of recycled coarse aggregate are improved by CO2 pretreatment,and the physical and mechanical properties of recycled concrete are improved by nano-Silica modification.Relevant experiment is designed to study the influence of CO2 pretreatment and nano-Silica modification to microstructure of recycled aggregate concrete.The main contents and results of this research are as follow:1.In this paper,the treatment of recycled coarse aggregate is carried out by using the method of vacuum pumping and high concentration CO2.The effect of carbonization time on microstructure of recycled coarse aggregate is investigated.Basic properties of recycled coarse aggregate with different carbonization time are comparative analysis.The X-ray powder diffractometer(XRD)and simultaneous thermal analyzer(STA)are used to investigate the content change of the Ca(OH)2 and CaCO3 in the old mortar and the orientation of Ca(OH)2.The scanning electron microscope(SEM)is used to analyse the microstructure of the old Interface Transition Zone(ITZ).The result shows that carbonization treatment can improve the apparent density of recycled coarse aggregate and reduce the water absorption and the crush index.The degree of density of the old Interface Transition Zone(ITZ)and the orientation of Ca(OH)2 are improved.The content of Ca(OH)2 decreases and the content of CaCO3 increases with the increase of carbonization time.However,the rate of change decreases after 24h carbonization.2.The recycled coarse aggregate before and after carbonization are used to mix concrete,separately.The orientation and content of Ca(OH)2,the degree of density of the old Interface Transition Zone(ITZ)and morphology of hydration products are discussed through recycled aggregate concrete cube compressive strength test and The X-ray powder diffractometer(XRD),simultaneous thermal analyzer(STA)and the scanning electron microscope(SEM).The experimental results show that the compressive strength of recycled aggregate concrete is improved after the carbonization of the recycled coarse aggregate.From the perspective of microstructure,the degree of hydration of the new Interface Transition Zone(ITZ)is higher,the size of hydration products is minified,the tigjtness degree between coarse aggregate and cement is improved after the carbonization of the recycled coarse aggregate.The comprehensive analysis shows that the optimal time of carbonization of recycled coarse aggregate is 24h.3.The nano-Silica is added into the high range water reducing agent to modify the recycled aggregate concrete on the basis of 24h carbonization of recycled coarse aggregate.The effect of the content of nano-Silica on the microstructure of the new Interface Transition Zone(ITZ)and the cement mortar matrix of recycled coarse aggregate.The result indicates that the content and orientation of Ca(OH)2 at the new Interface Transition Zone(ITZ)are significantly reduced when the content of nano-Silica is from 0.5%to 1%.The improvement of micro structure of cement mortar matrix gradually appears when the content of nano-Silica is from 1%to 2%.When the content of nano-Silica 3%,the agglomeration phenomenon occurs,and the microstructure of the interface transition zone and the cement mortar matrix is not improved.Therefore,in this paper,the optimal amount of nano-Silica is 1%.