Eco-friendly ultra-high performance concrete prepared with solid waste-based cementless binder and recycled coarse aggregate derived from demolished C30 concrete

This study demonstrated the feasibility of producing an eco-friendly ultra-high performance concrete (UHPC) using recycled coarse aggregate (RCA) derived from demolished C30 concrete and a cementless binder activated by calcium carbide residue (CCR). The mechanical properties and microstructure of the resulting cementless UHPC with RCA (CUHPC(RCA)) were investigated and compared with those of CUHPC incorporating natural coarse aggregate (CUHPC(NCA)). Under the CC150 regime (2 days of hot water curing at 90 °C followed by 3 days of dry-air heating at 150 °C), CUHPC(RCA) achieved compressive, splitting tensile, and flexural strengths of 131.6 MPa, 14.7 MPa, and 9.2 MPa, which were 9.6 %, 10.4 %, and 14.8 % lower than those of CUHPC(NCA), respectively. Notably, the compressive strength exceeded the 120 MPa threshold specified in GB/T 31387-2015. Microstructural analyses suggested that the ultra-high compressive strength of CUHPC(RCA) could be attributed to a strong confinement effect provided by the surrounding ultra-high-strength matrix, which restricts the lateral deformation of RCA under axial compressive stress and suppresses the adverse influence of RCA on CUHPC. Both RCA and NCA within the CUHPC matrix exhibited transgranular fracture, but the inherent microcracks and pores within RCA were the primary reasons for its 9.6-20.5 % lower compressive strength relative to CUHPC(NCA). Compared to cement-based UHPC, CUHPC demonstrates significantly superior environmental and cost performance. Furthermore, the incorporation of RCA adds no additional economic or environmental cost per unit of strength. Thus, this study provided a feasible approach for utilization of recycled coarse aggregate originating from C30 concrete to prepare UHPC.