Effects of varying grades/pretreatments of recycled aggregates on the development of pore structures and ITZs within reactive magnesia cement (RMC) concrete

This study explores the influence of incorporating recycled aggregate (RA) and CO₂-pretreated recycled aggregate (CRA) in reactive magnesia concrete (RMC) formulations. Through comprehensive microscopic characterizations, the phase composition, carbonation degree, pore structure, and interfacial transition zone (ITZ) characteristics are examined in both interior and exterior regions of specimens. The findings highlight that RA and CRA contribute to a 28 d strength in RMC formulations 14% and 25% higher than that with NA, respectively. This enhancement is attributed to internal curing and significantly increased channels for CO₂ diffusion within (C)RA specimens, leading to improvements in the modulus, hardness, and microstructures of the interior region—the traditionally weaker part in RMC-based concrete. Furthermore, a robust linear correlation is observed between ITZ characteristics, porosity, and compressive strength of CO₂-cured specimens. Finally, utilizing different grades of RA improves the performance of RMC-based concrete to varying degrees while reducing costs, thus potentially expanding the application of RA in practical RMC-based concrete scenarios, regardless of RA quality.