Immobilisation of hazardous solid waste coming from phosphogypsum leachate treatment in slag-based alkali-activated mortars

This study explores the feasibility of immobilising hazardous solid waste generated during the neutralization of phosphogypsum leachate with calcium carbonate to pH 3.5, referred to as leachate decontamination waste (LDW), by incorporating it into copper slag-based alkali-activated materials (AAMs). Alkali-activated mortars were prepared with varying LDW additions (2, 4, and 6 wt.%) and characterised for mechanical properties, microstructure, and environmental safety. Results showed that the compressive strength remained stable (ca. 40 MPa) and the leaching tests showed effective immobilisation of heavy metals (> 96 %) and fluoride (92.3–94.8 %) in the matrix, although fluoride and chromium slightly exceeded the inert waste threshold in some cases. In addition, the resulting AAMs mortars showed negligible radiological impact. Overall, the results demonstrate that LDW can be safely immobilised in slag-based AAMs without affecting performance, offering a sustainable pathway for valorising this hazardous industrial residue.