Sustainable alkaline cements with different Ca contents for stabilisation of copper mine tailings

The sustainable management of mine tailings is a growing challenge for the mining sector, with substantial environmental risks and long-term storage costs. This study explores the valorisation of copper mine tailings through the production of alkali-activated mortars (AAMs) using low-calcium fly ash and high-calcium blast furnace slag as precursors. A five-level Central Composite Design within the Response Surface Methodology framework was applied to optimise three key mix parameters: precursor/solid ratio, activator/precursor ratio, and sodium silicate/hydroxide ratio. Uniaxial compressive strength testing was supported by microstructural analyses (Scanning Electron Microscopy, Energy-Dispersive Spectroscopy, X-Ray Diffraction, Fourier Transform Infrared Spectroscopy) to correlate strength with gel composition and morphology. Optimal slag-based mixes achieved compressive strengths up to 15 MPa, driven by C-(A)-S-H and N-(C)-A-S-H gel formation, whereas ash-based systems reached 12 MPa, with performance strongly dependent on silicate/hydroxide ratio and N-A-S-H gel development. Response Surface modelling accurately predicted compressive strenght (R 2 > 0.87) and identified precursor chemistry as a key determinant of activator efficiency. These results demonstrate the feasibility of designing high-performance, tailings-based AAM, while reducing reliance on Portland cement, contributing to circular economy goals through CO 2 emission reduction and industrial waste reuse. A screening LCA indicates 0.24–0.37 kg CO 2 e kg −1 of paste for optimised mixes, 56–71 % lower than an OPC benchmark under declared assumptions. The work is framed as a screening-level optimisation and comparison (FA vs BFS) with explicit model limits and microstructural corroboration. The findings provide a robust optimisation approach for tailoring sustainable binder systems to diverse tailings chemistries. • Alkali-activated binders stabilise copper tailings using FA and BFS. • Precursor fraction and activator chemistry control early-age strength. • Inline figures and model diagnostics (incl. parity plots) improve clarity. • XRD, FTIR and SEM-EDS evidence support gel formation mechanisms. • Ambient validation, durability and leaching tests defined as next steps.