Study On The Mechanism Of Modifying The Performance Of Recycled Aggregate Concrete By Surface Treatment On Aggregate

The excessively discharged construction and demolition waste,as well as the shortage of non-renewable resources such as natural aggregates,has motivated the development of the recycling techniques of waste concrete.Compared to natural aggregate concrete（NAC）,the poorer mechanical properties and durability of recycled aggregate concrete（RAC）has limited RAC’s wide application in structural use.Therefore,the effective modification techniques on RAC should be key to promote the high-level application of RAC in practice and thereby promoting the sustainability of the construction industry.Since RAC contains more types of interface transition zones（ITZs）than NAC,while ITZs are widely known as the weak links in concrete,therefore,the modification of RAC may be realized by the enhancement of the the diverse types of ITZs.In light of this,this study aims at improving the properties of RAC by surface treating the recycled coarse aggregate（RCA）prior to concrete casting by using a sulphoaluminate cement（SAC）slurry and a water glass（WG）solution,respectively.The modification effects of the aggregate surface treatment on RAC were explored in terms of typical mechanical and durability properties.The properties of the various types of ITZs contained in RAC were also experimentally investigated,based on which the modification mechanism of the surface treatment taken by this study was discussed.The main contents and conclusions of this studied can be summarized as follows:1)The improvement in properties of RCA and the prepared RAC was different given the the water-to-binder（w/b）ratios of the SAC slurry or the concentration of WG solution were different.The improvement is more significant with a w/b ratio of0.8（for the SAC slurry）or a 3%concentration（for the WG solution,by mass）compared to other cases,in terms of the mechanical and durability performance of RAC on the macro scale.2)According to the BSEM photographs,the Ca（OH）₂crystals accumulating around the newly formed ITZs in RAC,i.e.,ITZ₁and ITZ₃,has been reduced by using the RA surface treated by the SAC slurry with a 0.8 w/b ratio or by the WG solution with a 3%concentration by mass.Besides,the microhardness measurements illustrate that the micro-mechanical properties of ITZ₁and ITZ₃are enhanced meanwhile the ITZ thickness is reduced.In contrast,the properties of the old ITZ₂and the old or new cement mortar are not obviously enhanced.3)The performance of RAC on both the micro and macro scales is first enhanced and then weakened with the increase in the erosion age under the external sulfate attacks,regardless of the surface treatment on RA or not.The deterioration of RAC modified by using the treated RA by the SAC slurry（0.8 w/b）or the WG solution（3%concentration）is less significant compared to that of the non-modified RAC,given a relatively longer erosion age.Therefore,the resistance of RAC to the external sulfate attacks has been improved by the surface treatment of RA.The microhardness test results suggest that the enhancement in ITZ₁and ITZ₃should be the predominant reason for such an improvement on the macro scale.4)The corrosion-induced cracking behavior of reinforced RAC blocks under the chloride attacks was monitored,through the process from steel depassivation to surface cracking initiation,till an average 0.3 mm surface crack width obtained.The electrochemical measurements show that the depassivation of steel bars under the chloride attacks has been delayed by using the surface treated RA,when compared to that using the non-treated RA.The width of the corrosion-induced cracks on the concrete surface is smaller for the modified RAC at the moment of surface cracking,compared to that for the control RAC.The width of these cracks is also better controlled in the modified RAC than in the control RAC after the surface cracking.Basically,the RA surface treatment by the SAC slurry or WG solution can improve the resistance of RAC to the chloride–induced corrosion and cracking.