Improved concrete ITZ and performance via pre-soaking of inert recycled red brick aggregates in carbonation solution

Compared to recycled concrete aggregate (RCA), the high-water absorption of recycled red brick aggregate (RBA) has limited its utilization in concrete production. However, this study takes the advantage of its high-water absorption capacity as a carrier for carbonation solutions, aiming to compensate the strength loss by enhancing the interfacial transition zone (ITZ) properties of incorporated concrete. The effects of carbonation solutions (e.g., CO2 and NaHCO3) by varying the CO2 injection times (10 s, 60 s and 600 s) and NaHCO3 dosages (1%, 3% and 5%) on the physical (i.e. density and water absorption) and mechanical (i.e. compressive and tensile bond strengths) properties of concrete, in particular focus on the ITZ changes, were systematically investigated. In addition, the microstructure changes of concrete ITZ were studied using thermogravimetric analysis (TGA), scanning electron microscope-backscattered electron (SEM-BSE) image analysis and mercury intrusion porosimetry (MIP). The results indicated that it was feasible to enhance the ITZ properties by using CO2 solution as the pre-soaking agent of RBA. The adoption of CO2 solution could significantly improve the bond strength of ITZ by up to 22.5% and resulted 8.6% improvement in 28-day compressive strength. The CO2 solution contributed to the formation of CaCO3 can accelerate the cement hydration of surrounding matrix and hence densified the microstructure of ITZ. For the case of pre-soaking with NaHCO3, although the release of solution from RBA could improve the early strength of concrete, the later strength significantly reduced due to the effect of sodium ions in NaHCO3 on the growth of outer C–S–H gel.