Abstract: To promote the valorization of copper tailings, this study prepared alkali-activated ultra-high-performance concrete (UHPC) using a composite precursor system composed of slag, copper tailings, and silica fume. The influence of copper-tailings content on fresh workability and mechanical properties was systematically examined, and the hydration products and interfacial transition zone (ITZ) were characterized using XRD、FTIR、TG-DSC、MIP and SEM-EDS. The results show that increasing the copper-tailings content enhances flowability but delays setting, while compressive and flexural strengths decline overall, notably, the flexural performance is more sensitive to copper-tailings incorporation. When the copper-tailings replacement level does not exceed 20%, the 28-day compressive and flexural strengths of UHPC were maintained above 120 MPa and 20.01 MPa, respectively. Microstructural analyses revealed that the incorporation of copper tailings reduced the reaction degree of the system and suppressed the formation of C-(A)-S-H gel, thereby increasing the total porosity and promoting pore structure coarsening. Moreover, they weaken paste encapsulation and bonding at aggregate surfaces, degrade the ITZ, and ultimately impair mechanical performance. Overall, these findings indicate that copper tailings can be effectively incorporated as a component of the composite precursor in alkali-activated UHPC, providing a viable pathway for developing low-carbon cementitious materials. To ensure solid-waste valorization, high mechanical performance, and microstructural densification, the copper tailings content should not exceed 20%.