Experimental Investigation On Performance Of Double Modified Recycled Coarse Aggregate And ITZ

Recycled coarse aggregate is the most important component of recycled concrete,which is mainly processed by crushing waste concrete.All aspects of its properties,especially the properties of the interface transition zone,have a direct impact on the strength and durability of recycled concrete.However,during the preparation process,the recycled aggregate is easily affected by external forces to produce pores and cracks,and a large amount of cement mortar will remain on the surface.These factors ultimately make the overall strength of the recycled coarse aggregate low and the water absorption rate large.In order to solve this problem,many domestic experts and scholars have carried out research on the modification of recycled aggregates,and conducted a large number of experiments on the modification of recycled aggregates.Although they have certain effects,they are still not ideal.In this test,double modification technology was used to treat recycled coarse aggregate.The first modification optimized the pore structure and filled the small pores and cracks in the aggregate.The second modification adds a protective film to the aggregate on the basis of the first modification to make the aggregate hydrophobic and improve the bonding ability with the cement mortar,and further improve the performance of the recycled aggregate.Four different methods were designed to treat recycled coarse aggregates:one modification of cement silica fume slurry,one modification of nano Si O2 solution,two modification of cement silica fume+water glass,and two modification of nano Si O2+PVA.The modified coarse aggregate has been studied on three scales:macroscopic,microscopic and mesoscopic,in order to find a cost-effective modification method.The main work conclusions are as follows:（1）The macroscopic performance test of recycled aggregate treated with different modification methods is carried out.The modification effect is comprehensively evaluated from the four aspects of aggregate bulk density,apparent density,water absorption,and crushing index.The experiment found that compared with the once-modified aggregate,the macroscopic properties of the double-modified recycled aggregate have been greatly improved.Among them,the effect of compound modification with cement silica fume+water glass dual treatment is the best.（2）In order to analyze the interface performance between recycled aggregate and cement mortar,a test model was designed to simulate the interface between aggregate and mortar.The test showed that the dual modification of cement silica fume+water glass can increase the interface bond strength the most Compared with the unmodified and one-time modified recycled aggregates of cement silica ash slurry,the increase was16.0%and 5.4%,respectively.The second is the double modification of nano-Si O₂+PVA,which is 3.6%and 10.9%higher than the unmodified and one-time-modified recycled aggregates of nano-Si O₂..（3）Microscopic experiments show that compared with unmodified and one-time modified recycled coarse aggregates,the new and old mortar ITZ area of recycled aggregates modified with cement silica fume+water glass has become significantly denser,and the microhardness is lower.The modified and one-time modified recycled aggregates of cement silica fume slurry increased by 51.9%and 14.0%,respectively.The double modification of nano-Si O₂+PVA also showed better effect than the one-time modification,the pores and cracks were greatly reduced,and the microhardness was increased by 45.0%and 15.2%respectively compared with the unmodified and the one-time modification of nano-Si O₂.