Utilizing carbonated recycled concrete fines to develop sustainable ultra-high-performance fiber-reinforced concrete

Ultra-high-performance fiber-reinforced concrete (UHPC) is known for its excellent mechanical properties and durability, yet its environmental impact remains a concern. This study investigates the potential of incorporating uncarbonated recycled concrete fines (URCF) and carbonated recycled concrete fines (CRCF) into UHPC to enhance sustainability while maintaining or improving its performance. The inclusion of URCF decreased UHPC flow by 4.57 %–19.63 %, indirect tensile strength (ITS) by 5.6 %–23.8 %, and compressive strength (CS) by 2.3 %–20.8 % at 28 days, indicating negative impacts on workability and strength. In contrast, CRCF improved workability by 2.87 %–4.55 %, ITS by 2.96 %–38.66 %, and CS by 1.73 %–3.69 %, demonstrating carbonation's positive influence on UHPC's mechanical properties. CRCF also improved resistance to acid attack, reducing weight loss by 16.5 %–41.8 % and enhancing residual compressive strength by 12.7 %–17.7 %. CRCF also demonstrated superior thermal stability, reducing weight loss by up to 33.3 % at elevated temperatures and improving pore structure refinement and shrinkage strain by 20 %–35 %. These findings highlight the considerable potential of CRCF to enhance UHPC's performance in aggressive environments, providing improved durability and dimensional stability under varying conditions. In addition to its mechanical and durability benefits, CRCF's role in pore structure refinement reduced cumulative pore volume by 25 %, contributing to better overall UHPC density. The hydration kinetics analysis revealed that CRCF-modified UHPC exhibited controlled hydration, peaking at 34–36 h, in contrast to the accelerated hydration observed in URCF mixes, which could lead to potential stability issues. This indicates CRCF's role in improving long-term structural integrity. The life cycle assessment (LCA) confirmed that both URCF and CRCF contributed to lower global warming potential (GWP), with the M5-URCF-25 mix reducing GWP by 11.7 % and the M10-CRCF-25 mix achieving an 8.0 % reduction, despite the energy costs associated with carbonation. Incorporating CRCF into UHPC thus offers a balanced solution, delivering mechanical performance improvements while addressing sustainability concerns by lowering the carbon footprint of UHPC. • CRCF enhances acid resistance, thermal stability, and strength retention of UHPFRC. • Controlled hydration and pore refinement by CRCF improve UHPFRC density and durability. • URCF accelerates hydration but causes stability issues, while CRCF ensures long-term durability. • Both CRCF and URCF reduce drying shrinkage strain, enhancing UHPFRC's dimensional stability.