Effect Of Recycled Aggregate Properties On Carbonation Of Concrete

The carbonation reaction of alkaline substances such as calcium hydroxide(CH),hydrated calcium silicate(C-S-H)gel,ettringite(AFt)in the concrete structure is one of the important reasons for the durability of concrete.With the depletion of natural resources,recycled aggregate concrete(RAC)are used more widely.Recycled concrete aggregate(RCA)is inferior to natural aggregate(NA)due to its performance index,and many scholars have done a lot of research on modifying RCA performance.However,there are few studies on the anti-carbonation performance of modified RCA to RAC,and no studies have been made on the size and microstructure evolution of RAC partial carbonation zone.Therefore,this paper will carry out research from two aspects.Firstly,the effect of modified RCA on the carbonation resistance of RAC was studied.The order of ascending the anti-carbonation property of RAC is obtained and the strengthening mechanism is revealed.Secondly,the effects of different RCA replacement rates on the size and microstructure evolution of RAC partial carbonation zone are studied.The main research contents are as follows:Firstly,in order to solve the key problems of RAC sampling,the difference of pore structure between 0.83W/B,0.63W/B,0.53W/B,0.35W/B mortar mixed with iron oxide red group and blank group are studied by MIP method.The difference of solid phase composition between 0.53 W/B mixed iron oxide red group and blank group before and after carbonation is studied by TG method.SEM-EDS and NMR are used to study the variation difference of the characteristics of CSH in the process of 0.53 W/B mortar.The results of MIP study show that 0.5%iron oxide red has no effect on the total porosity of mortar.Regardless of the strength grade of the mortar,whether iron oxide red is added or not,the CO2 transport is mainly dominated by the diffusion of the Knudsen and the transition zone diffusion.From the diffusion point of view,Knudsen diffusion ratio decreased slightly,the transition zone diffusion ratio increased slightly.The results of TG-DSC study show that the CH content of 0.53 W/B mortar with iron oxide red increased by 1.6%compared with the non-added,and the loss of CaC03 is 3.8%less than that of the non-added.The results of SEM-EDS and NMR demonstrate that 0.5%iron oxide do not affect the C/S and the average chain length of C-S-H during carbonation.By analyzing TG,SEM-EDS,NMR results,we can find 0.5%iron oxide red will slightly affect the hydration of cement.Original recycled concrete aggregate(O-RCA)is strengthened by direct carbonation(C method),calcium-carbonation method(CC method),acetic acid impregnation method(A method)and water repellent soaking method(W method).The results show that the four kinds of strengthening methods can improve the performance of RCA,but each scheme has different effect on RCA performance.When the apparent density,bulk density,water absorption,crushing index are used as the evaluation index,the acetic acid impregnation is the best,modification way.Thirdly,the influence of RCA with different characteristics on RAC performance was studied.The results show that the order of ascending the RAC slump is:A method>W method>CC method>C method,the order of ascending the RAC compressive strength is:A method>CC method>C method>W method,the order of anti-carbonation property of RAC is:A method>CC method>C method>W method.Finally,the effect of RCA20 replacement rate on the size and microstructure evolution of RAC30 partial carbonation is studied by TG-DSC method.The results show that there are partial carbonation zone in RAC30,and the size of partial carbonation zone increases with the increase of RCA replacement rate and then reduce,finally increase.The contents of CH and CaCO3 in different depths of RAC30 are quantitatively characterized by TG-DSC method.The results show that when the old mortar was used as the sample for thermal analysis,the size of the carbonation zone of RAC30 becomes larger,and the specimen is completely carbonated,thus,there is no none-carbonated zone.The results show that the size of the complete carbonation zone of the RAC30 is equal to that of the new mortar,and the size of the partial carbonation zone is slightly larger than that of the new mortar when the new and old mortar samples with R = 100%and carbonated for 14 days are used to do the TGA.The experimental results reveal that the total porosity of the old mortar is larger than that of the new mortar,the volume fraction of the Knudsen diffusion hole and the diffusion pore of the old mortar is larger than that of the new mortar,and the interface theory is analyzed,CO2 gas is more likely to spread in the old mortar.