Effect and mechanism of surface-coating pozzalanics materials around aggregate on properties and ITZ microstructure of recycled aggregate concrete

A novel triple mixing method (TM) was developed to realize surface-coating of aggregate with pozzalanics materials for further improving microstructure of interfacial transition zone (ITZ) and properties of recycled aggregate concrete (RAC). Effects of mixing method and coating by various admixtures on compressive strength and chloride ions penetration resistance of RAC were investigated. Through preparing the new–old–new concrete sandwich specimen and SEM observation of the old concrete surface after fracture, effect of surface-coating of the recycled aggregate (RA) in RAC with various admixtures on ITZ microstructure was also studied. The results showed that properties of RAC can be further enhanced by using TM, as compared to that of using the double-mixing method (DM). Through SEM observation, it is revealed that the coated pozzalanic particles can consume CH accumulated in the pores and on the surface of the attached mortar to form new hydration products, which can not only in situ strengthen the RA, but further improve the microstructure of the ITZ, thus the strength and durability of the RAC was further enhanced.