Improving the Circular Economy in the Iberian Pyrite Belt: Upcycling Mining Legacy Materials into Sustainable Cement-Based Composites

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.