The repurposing of gold (Au) mine tailings from artisanal and small-scale mining (ASGM) operations via alkali activation technology is a promising strategy for waste reduction in developing countries. Direct activation of mine tailings, however, is challenging because these materials contain relatively low aluminum (Al)-bearing minerals. In this study, palm oil fuel ash (POFA) was elucidated as a high Al-bearing waste derived-admixture for the synthesis of an ASGM tailings-based geopolymer composite. Semi-quantitative XRD analysis showed that the tailings contained quartz (SiO₂) (~58%), pyrite (FeS₂) (~20%) and calcite (CaCO₃) (~15%) with minor to trace amounts of aluminosilicates (~7%). Substantial amounts of environmentally regulated pollutants such as mercury (Hg) (40 mg/kg), lead (Pb) (8430 mg/kg) and arsenic (As) (300 mg/kg) were also found in the tailings. SEM-EDS, XRD and ATR-FTIR results showed the successful formation of a hybrid geopolymer-CASH matrix, which improved the unconfined compressive strength (UCS) of geopolymer composites from ~5 MPa to ~7 MPa. Furthermore, POFA did not significantly affect the thermal resistivity of geopolymer composites based on thermal analysis. Finally, the TCLP results showed that the Pb leaching concentrations from ASGM tailings exceeded environmental standards (~15,000 µg/L), which was suppressed after alkali activation to 300–500 µg/L. This means that POFA addition to ASGM tailings-based geopolymer composite improved not only its applicability as backfill, pavements and bricks but also its ability to immobilize toxic elements.

The rehabilitation of legacy mines continues to be a big challenge because of the difficulties in returning them to safe and stable conditions and ensuring that the mined-out areas become productive to support the economic activity of the host community. Previous efforts are often focused on purely technical and environmental aspects, leading to resistance from the local community due to their exclusion from the rehabilitation process. To address the issues associated with legacy mines and lack of participation of the community, we have developed a project, Biodiversity Positive Mining For The Net Zero Challenge (Bio + Mine), focusing on the abandoned Sto. Niño copper mine (Benguet, Philippines). The mine was closed in 1982 without a plan involving local stakeholders and leaving a significant ongoing negative legacy. Using the social-ecological-technological system framework, we will explore the intersections of the structure and functions of socio-economicdemographic, ecological, and technological data useful in devising a more inclusive mitigation strategy for the reconstruction of the supporting ecosystem. We aim to develop a site-specific system, underpinned by the local community's knowledge and practices, that can be a model for wider implementation in other legacy and active mines worldwide.