Chemically activated cementation of sulfidic waste rocks for preventing acid and metalliferous drainage pollution at mine sites

Sustainable strategies are urgently required to rehabilitate sulfidic waste rocks (W R ) and prevent acid and metalliferous drainage (AMD) into natural environments. This study aims to establish the proof of concept for transforming aluminosilicate-rich acidic mine wastes into a stable, consolidated, and cementitious structure (namely environmental geopolymer ) using alkali activation of W R and Class F fly ash (FA) at ambient temperature. The synthesised mineral gel effectively cemented W R -FA mixtures (up to 60 % W R ) into indurated structures. A mechanical strength of approximately 22 MPa was developed in 4 weeks, strengthening further to 29 MPa over 24 weeks. Mineralogical, geochemical, and micro-spectroscopic analysis revealed that acidic minerals in the W R materials were encapsulated and passivated by the cement-like amorphous sodium aluminosilicate hydrate (N-A-S-H) gel phase, which significantly immobilised toxic metal(loid)s. Further investigation into atomic coordination environment of the synthetic gel using Near-edge X-ray absorption fine structure (NEXAFS) at Si and Al K-edge suggested that tetrahedrally coordinated Si and Al form the 3D-extended network structure within the amorphous gel phase. This chemically activated cementation of polymineral wastes may offer a unique approach to valorise sulfidic W R and FA into a pseudo-sedimentary rock horizon for sustainable rehabilitation of sulfidic W R facilities through a combined mechanism of physical integration, as well as mineralogical and geochemical stabilisation. • Alkali activation transforms waste rock and fly ash into an indurated geopolymer. • The hardened geopolymer achieves ∼ 22 MPa strength after 4 weeks of ambient curing. • N-A-S-H gel encapsulated and passivated pyrite, preventing AMD generation. • Tetrahedral Si and Al polycondensation forms the 3D geopolymer network structure. • Polymineral gels immobilise the toxic metal(oid)s in the geopolymer samples.