Effect of carbonated recycled coarse aggregates on the mechanical properties of 3D printed recycled concrete

A modification method of carbonation combined with presoaking in nano-SiO2 solution was employed to enhance the recycled coarse aggregates (RCA). The water absorption, apparent density and crushing index of RCA before and after carbonation were tested. The water absorption, compressive strength, flexural strength and splitting tensile strength of 3D printed recycled aggregate concrete (3DPRAC) with different RCA replacement rates were further investigated. Microscopic tests including X-ray diffraction and scanning electron microscopy were performed to analyze the changes in phase composition and microstructure of RCA, while X-ray computed tomography was utilized to explore the pore defect evolution of 3DPRAC. The results demonstrated the method of carbonation combined with presoaking in nano-SiO2 solution significantly improved the physical characteristics of RCA and densified the microstructure of 3DPRAC. With an increase in RCA replacement rate, there was a decrease in the mechanical properties in all directions. At the RCA replacement rate of 100 %, the compressive strength, flexural strength and splitting tensile strength increased by a maximum of 24.71 % in the X direction, 12.95 % in the Y direction and 18.27 % in the Z direction after RCA carbonation, respectively. The pore defects of 3DPRAC were effectively optimized by the inclusion of carbonated RCA. The modification mechanism was explained based on the enhanced performance of RCA and pore defect evolution.