Recycling pyrite tailings as sustainable fine aggregate in alkali-activated slag mortars: a comparative study with cement mortars

Pyrite tailings (PT), a kind of mineral processing solid waste generated from sulfuric acid production, was systematically evaluated as fine aggregate in alkali-activated slag (AAS) mortar through a comparative analysis with Portland cement (PC) mortar. The setting times, compressive strengths and autogenous shrinkage of mortars were measured, and the hydration kinetics, phase assemblage and microstructural evolution were characterised. Key findings revealed distinct responses between the two systems. The incorporation of PT sand in the PC mortar markedly delayed the setting times, reduced the compressive strengths and increased the porosity, mainly because the acidic nature of the PT sand suppressed PC hydration. Conversely, the AAS mortar exhibited superior compatibility with PT sand, mainly attributable to its inherently higher alkalinity and denser pore microstructure. The setting times were maintained within practical ranges (4.18 h and 6.14 h) and a 28-day compressive strength of 44.5 MPa was achieved. The inherent alkaline reservoir in AAS mortar effectively neutralised the acidic challenges, while residual pyrite was still detected. The results of this work demonstrate the feasibility of using PT sand in AAS mortar.