Study on mechanical properties and microstructure of recycled composite micro-powder concrete

To enhance the utilization efficiency of recycled powder in concrete and address the research gap in traditional single-admixture studies of recycled concrete powder (RCP) and recycled brick powder (RBP), this study prepared RCP and RBP from construction waste (discarded concrete and bricks) and blended them at varying ratios (0:10, 2:8, 4:6, 6:4, 8:2, and 10:0) to produce recycled composite micro-powder (RCMP). The RCMP was then used to replace 20% (by mass) of cement in the production of C30-grade concrete. This research systematically investigated the effects of RCMP blending ratios on the workability and mechanical properties (at 3, 7, 14, 28, 56, and 90 days) of concrete, while also elucidating the underlying mechanisms from a microstructural perspective. The results indicate that RCP and RBP exhibit irregular geometries, rough surfaces, and high water absorption. Their chemical compositions are similar to ordinary Portland cement, with RBP demonstrating higher secondary hydration activity than RCP. The incorporation of RCMP reduced both the workability and mechanical performance of concrete. However, at an RCP:RBP ratio of 2:8, the composite powder effectively leveraged the filler effect and pozzolanic properties. Compared to the reference group, the 90-day compressive strength, splitting tensile strength, and flexural strength decreased by only 9.86%, 4.05%, and 1.75%, respectively, while the failure process and modes remained consistent with standard concrete. Furthermore, the T2 relaxation spectrum of RCMP-modified concrete resembled that of the reference group, primarily exhibiting four peaks. The addition of RCMP optimized the pore size distribution within the concrete matrix, enhancing its overall compactness.