Study On Green Chemical Methods To Improve The Quality Of Recycled Concrete Aggregates

The application of recycled concrete aggregate(RCA)can realize the effective recovery and reuse of waste concrete.However,properites of RCAs are inferior compared with natural aggregates,which mainly attributed to the porous nature of the cement mortar attached onto the RCA surface.Since the adhered mortar is responsible for the poorer quality of the RCA,we can improve the quality of the RCA by either removing this adhered mortar or strengthening it.This study proposes two green chemistry methods to treat RCAs so that the quality of the RCAs can be significant improved.The main research work and conclusions are as follows:The one approach is to use acetic acid to remove the residual cement mortar so that the quality of treated RCAs can be efficiently improved.Meanwhile,the waste solution can improve the strength of concrete and high value-added products can be produced form waste solution.In this approach,the RCAs are first soaked in the acetic acid(CH3COOH)solution,which the CH3COOH can react with cement hydration products(Ca(OH)2)in the adhered mortar attached on the surface of the RCAs.This soaking process can weaken the adhered mortar,making it easier to remove the adhered mortar on the surface of RCAs by using mechanical rubbing method in the next step.As a result,the water absorption of the treated RCAs can be reduced and less adhered mortar can be found.The compressive strength of the concrete prepared by using this treated RCAs can be improved by 25%at 28 days compared to untreated RCAs.It is safe and environmental to use this new approach compared to hydrochloric acid solution,because no hazardous chemical is used and hence no harmful chemicals are introduced.Moreover,no dangerous waste solution is produced since this waste solution can be re-used to create vaterite and calcium acetate and reproduced acetic acid.Testing results show that the waste solution can also be added into new concrete,and the compressive strength of produced concrete can be enhanced by 14%at 28 days.The other one is a new chemical process to treat demolished and waste concretes so that they can be recycled as aggregates for new applications and heavy metal adsorbent.This is done by immersing recycled concrete aggregates(RCAs)in diammonium hydrogen phosphate.The weak and loose mortar of the RCA can be consolidated by the DAP due to the reaction between the calcium-rich hydration products in the RCA and the DAP.Hydroxyapatite(HAP)precipitate can be produced by this reaction which fills pores and seals cracks of the RCA,significantly improving the quality of the RCA.As a result,the compressive strength of the new concrete made with the RCA as coarse aggregate was improved up to 19%by the proposed method.In addition,phosphate ions in the treated coarse RCA can inhibit the corrosion of reinforcing steels embedded within the concrete.This new method also c.onverts the fine RCA,which is unsuitable to be used as aggregate for new concrete,as adsorbent of heavy metal ions.This is because the proposed method produces a layer of HAP on the surface of the fine RCA,which has high surface area and excellent ion-exchange ability.Experiment results show that more than 85%of zinc ions were removed from 250 ml 0.025mol/L Zn(NO3)2 solution by adding 25 g treated fine RCA.In addition,the recycled concrete bricks,which have low strength and durability,can be enchanced by the similar to the DAP processing method of RCA.Experimental study shows that the water absorption of recycled bricks is efficiently decreased and up to 100%enhancement of the compressive strength of the concrete bricks can be achieved by this treatment.Moreover,HAP has significantly higher resistance to acid attack due to much lower solubility and rate of dissolution compared to CaCO3.