The mining sector is in urgent need of circular economy strategies to manage its historical environmental liabilities. This study investigates the upcycling of mining legacy materials from the Iberian Pyrite Belt (IPB) as sustainable substitutes for natural fine aggregates in cement-based composites. Utilising a functional approach, four distinct waste samples (S1–S4) were used as aggregate replacement (10%, 50%, and 100%) in composites. The experimental campaign focused on the critical nexus between environmental safety and technological performance, evaluating leaching behaviour, water demand, hardened density, and mechanical strength. The results identify a threshold of 10% replacement as optimal, yielding high-performance composites with compressive strengths between 39 and 48 MPa at 28 days. Notably, even at 50% replacement, the mixtures exceeded the 20 MPa structural benchmark (22–37 MPa). Leaching tests confirmed that potentially pollutant elements remained well below European regulatory limits, demonstrating effective contaminant immobilisation. These findings validate a new starting point for valorisation routes for IPB mining legacies, proving that it is technically and environmentally feasible to manufacture composites that meet the basic fundamental industrial standards while promoting resource conservation.

The generation of slag from copper smelting is between 40 and 45 Mt per year. However, the utilisation of this material in large-scale applications remains limited. This study evaluates the combined use of copper slag as a fine natural aggregate substitute (50, 75, and 100%) and natural mordenite-type zeolite as a partial cement replacement (25, 40, and 55%) in the development of sustainable mortars. The samples were characterised using XRF, particle size distribution, and density analysis, and sixteen mixtures were produced. The consistency and water demand in fresh state, the flexural and compressive strength at 7 and 28 days, and the porosity measurements of the hardened mortars were analysed. The results demonstrate that zeolite significantly increases water demand, leading to higher porosity (27–34%) and reduced mechanical strength (14–31 MPa). Conversely, copper slag decreases the water-to-cement ratio and produces denser matrices with the lowest porosity values (8–13%), achieving compressive strengths at 28 days that are higher (53–58 MPa) than the reference mortar (50 MPa). In hybrid mixtures, slag partially mitigates the porosity increase induced by zeolite, revealing a favourable interaction between both materials. Mixtures containing 75–100% copper slag and 25% zeolite (MZ-8 and MZ-9) exhibited balanced porosity (18–21%) and mechanical performance (38–39 MPa), confirming their suitability for mortar applications. The findings demonstrate that the joint incorporation of copper slag and natural zeolite is a viable strategy for producing eco-efficient mortars while reducing clinker and natural aggregate consumption.