Influence of carbonation treatment on the properties of multiple interface transition zones and recycled aggregate concrete

Because of the carbonation treatment of recycled coarse aggregates (RCAs), the properties of the weak multiple interface transition zones (ITZs) in recycled aggregate concrete (RAC) can be improved, contributing to the enhanced behaviour of the RAC. In this study, different carbonation conditions (carbonation pressure, initial moisture contents of the RCAs and carbonation duration) were considered. A model of the RACs (MIRAC) was prepared to accurately locate multiple ITZs. Nanoindentation tests, scanning electron microscopy (SEM) tests were conducted to study the effect of carbonation treatment on the microproperties of RAC. At the same time, the influence of the carbonation treatment on the compressive strength and chloride ion penetration resistance of the RAC was evaluated. Moreover, the relationship between the microproperties and macroproperties of the RAC was discussed. The results showed that the modulus of the ITZs, the old mortar matrix and the new mortar matrix increased while the thickness of the ITZs decreased when the carbonation pressure or carbonation duration were increased. However, there was no further significant change after the carbonation pressure exceeded 1 bar, and the carbonation rate began to decrease significantly after 3 h of carbonation. The carbonation efficiency was superior when the moisture content was 1.81%. The effects of different carbonation parameters on the microproperties and macroproperties of the RAC were consistent. The compressive strength and chloride ion penetration resistance of the RAC had a linear correlation with the modulus of the ITZ between the aggregate and old mortar.