Microstructure and micromechanical properties of interfacial transition zone in green recycled aggregate concrete

Green recycled aggregate concrete (GRAC) is considered one of the sustainable building materials, which is made up of recycled aggregates (RAs) and green binding materials (alkali-activated materials, i.e., AAMs). However, the GRAC has not been used in building engineering due to its uncertain reaction mechanisms, especially the formation of interfacial transition zone (ITZ) between RAs and AAMs. Thus, the objective of this paper is to systematically investigate the microstructure and micromechanical properties of ITZ in GRAC. Backscattered electron microscopy (BSE) and energy dispersive spectroscopy (EDS) were used to investigate the microstructure and ITZ, whereas nanoindentation was used to study the micromechanical properties. The results indicate that the elastic modulus of ITZ in GRAC incorporated with crushed concrete (34 GPa) is higher than that with unbound stone (22 GPa) and crushed brick (24 GPa). This is due to the presence of C-A-S-H gels with superior micromechanical properties and compacted microstructure. Additionally, the elastic modulus and hardness of ITZ were found to be higher than the paste matrix in GRAC regardless of RAs due to the desired bonding between paste and RAs. It signifies the great potential of utilizing industrial by-products as well as construction and demolition waste to produce low-carbon and sustainable GRAC.