Ultra-high ductile concrete (UHDC) has gained significant attention ‎in modern construction due to its superior mechanical properties, ‎high ductility, and long-term durability. The incorporation of ‎recycled materials, particularly copper slag, offers a sustainable ‎approach to reduce the environmental footprint of the construction ‎industry, while potentially influencing the mechanical performance ‎of concrete. However, the effect of replacing natural aggregates with ‎varying proportions of copper slag on the compressive and tensile ‎performance of UHDC has not been comprehensively investigated.‎‏ ‏In this experimental study, UHDC specimens were prepared with 35, ‎‎70, and 100% replacement of natural aggregates by copper slag ‎and reinforced with 2 vol.% steel fibers. Mechanical properties, ‎including density, compressive strength, and splitting tensile ‎strength, were evaluated, with particular attention to particle ‎packing effects and the quality of the interfacial transition zone.‎‏ ‏The ‎results demonstrated that copper slag increased the concrete density ‎linearly by up to 10.4% and enhanced compressive strength by 6.8–‎‎12.2%, attributed to improved matrix packing and secondary ‎pozzolanic reactions. Conversely, splitting tensile strength decreased ‎significantly with higher slag content (11.9–34.9%) due to the ‎smooth, glassy texture of copper slag, which reduced mechanical ‎interlocking. In contrast, river sand, while providing similar ‎compressive benefits, improved tensile strength by 2.6% due to ‎better matrix bonding. The addition of steel fibers played a critical ‎role in transforming the failure mode from brittle to ductile, ‎preventing sudden collapse even in specimens containing 100% ‎slag. These findings indicate that an optimized combination of ‎copper slag and steel fibers can simultaneously enhance the ‎compressive strength and mechanical performance of UHDC, ‎offering a viable pathway for incorporating recycled materials in ‎advanced concrete systems.‎