Mechanical and microstructural properties of low-carbon engineered cementitious composites containing carbonated recycled fine aggregates

Recycled fine aggregate exhibits defects such as high porosity and the presence of micro-cracks on its surface. To overcome the performance limitations of recycled fine aggregate, carbonation treatment has emerged as a crucial method for enhancing its properties and promoting the production and application of recycled concrete. The use of carbonated recycled fine aggregate (CRFA) in concrete improves the sustainability of construction and demolition waste utilization. This study investigates the influence of CRFA replacement ratio (ranging from 0% to 100%) on the mechanical properties of the mortar and engineered cementitious composites (ECC), fiber bridging mechanisms, and microstructure. The results indicate that as the CRFA replacement ratio increases, the workability of the mixture decreases by 0.5% to 6.4%, and the compressive strength declines by 0.5% to 12.2% compared to the control mix with 100% quartz sand. In contrast, the ultimate strain increases by 4.5% to 33.4%, and the strain energy improves by 2.3% to 14%. Carbonation effectively repairs some micro-cracks in the CRFA; however, defects persist in the interfacial transition zone, leading to a reduction in matrix strength. These defective interfaces weaken the fiber-matrix bond strength, facilitating easier slipping and pulling out of fibers. This mechanism promotes multiple cracking and significantly enhances the strain-hardening behavior of the material. In conclusion, a 50% CRFA replacement ratio is identified as the optimal level, effectively balancing mechanical performance with waste utilization benefits, thereby contributing to the low-carbon design of ECC.