Micro-structural analysis of recycled concretes made with recycled coarse concrete aggregates

Abstract

This paper investigates the microstructure of 100% recycled concrete (RC) containing recycled coarse concrete aggregates (RCA) using scanning electron microscopy (SEM). In this study, the effect of 100% replacement of RCA along with the use of cementitious materials (CM's) including micro-silica (SF), fly ash (FA) and natural zeolite (Z) was investigated. For a more accurate understanding of the changes in concrete microstructure, we performed compressive strength (CS), ultrasonic pulse velocity (UPV) and water absorption by immersion (WAI) tests. After 180 days of hydration, the results of SEM as well as the high qualitative loss of RC from the UPV test showed that, there was still a large amount of pores and discontinuties at the mortar surface and the interfacial transition zone (ITZ) of RC without pozzolan. In contrast, the use of 10% of SF in RC, significantly reduced the pores and compressed ITZ and by bridging the hydration products; the ITZ of this concrete was very close to that of conventional concrete (CC). The slow pozzolanic reactions in the FA as well as the low reactive capacity of the Z made the RC containing these two pozzolans have relatively porous and uncompressed microstructures. Consequently, beside the poor results of mechanical properties, excessive discontinuities were also observed in the ITZ of this type of concretes.