Understanding the micro-macro properties of sustainable ultra-high performance concrete incorporating high-volume recycled brick powder as cement and silica fume replacement

Ultra-high performance concrete (UHPC) contains a high proportion of unhydrated cement and silica fume particles. Using recycled brick powder (RBP) as an alternative binder provides a feasible approach to achieving sustainable UHPC and high value-added recycling of clay brick waste. Therefore, this investigation elucidated the role of RBP in UHPC and studied the micro-macro performance of UHPC incorporating RBP as substitutes for cement and silica fume. RBP consists of quartz phases and amorphous components, showing favorable dilution, nucleation, filling and activity effects in UHPC mixture. Substituting 30-50 % of cement or silica fume with RBP can facilitate the hydration process and refine the microstructure; however, the replacement of a high-volume cement and silica fume with RBP is detrimental to the hydration process and microstructure of blended UHPC paste. Incorporating moderate RBP as cement and silica fume replacement can improve the drying shrinkage resistance of UHPC. The mechanical strength and permeability resistance of UHPC exhibit a trend of initially increasing and subsequently decreasing with the replacement of cement and silica fume by RBP. UHPC blended with a high-volume RBP still has satisfactory mechanical strength. The 28d compressive strength of UHPC with RBP replacing 0 %, 10 %, 30 %, 50 % and 70 % cement, and 100 % silica fume is 158.8, 165.6, 155.5, 149.7 and 129.1 MPa, and 140.8 MPa; besides, replacing 10 %, 30 %, 50 % and 70 % of cement with RBP causes 15.9 %, 39.9 %, 88.5 %, and 170.9 % rise in the compressive strength per unit cement. Optimizing the replacement ratio and replacing path of RBP can prepare a sustainable UHPC with satisfactory properties, and the simultaneous replacement of cement and silica fume with RBP in UHPC is also feasible.