Elastic modulus recovery in concrete incorporating biomimetically mineralized recycled aggregates

Abstract

Abstract The structural application of recycled aggregate concrete (RAC) is severely hindered by its low elastic modulus, primarily caused by porous adhered mortar and defective interfacial transition zone (ITZ). This study proposes a novel biomimetic mineralization strategy to construct an enamel-like hydroxyapatite (HAp) coating on recycled coarse aggregates (RCA), restoring the macroscopic stiffness of RAC. The effects of mineralization temperature and duration on the elastic modulus of RAC were systematically investigated. The results showed that the optimal treatment at 60 °C and 24 h achieved exceptional elastic modulus recovery, increasing by 20.5% at 7 d and 17.4% at 28 d (reaching 28.4 GPa). Notably, this stiffness recovery significantly outpaced compressive strength enhancements. The stress–strain response showed that RAC changed from a deformable response to a stiffer load-bearing behavior after biomimetic mineralization, as evidenced by the reduced peak strain of 0.001 68 and the steeper post-peak descending branch. Microstructural and X-ray computed tomographic analyses revealed the underlying densification mechanisms. The biomimetic mineralized layer not only reduced the mesoscale pore fraction, but also effectively filled interfacial defects via localized Ca-O-P enrichment. This localized densification successfully narrowed the ITZ width from roughly 60 μm to 30–40 μm. Biomimetic mineralization transformed the deformation mode of RAC to a favorable stress-transfer behavior by densifying the adhered mortar and reinforcing ITZ between new mortar and RCA to form a continuous, rigid HAp-reinforced transition layer, providing an effective pathway for improving the deformation resistance and high-value utilization of RAC.