Sustainable alkali-activated materials from basalt mine tailings: Enhancing resource efficiency and performance

Basalt waste fine powder deriving from ore processes was tested as raw solid aluminosilicate precursor for sustainable alkali activated materials (AAMs) with prospects for expanding sustainable alternatives to conventional structural and non-structural materials. Different formulations able to guarantee high consolidation degrees were explored, varying liquid-to-solid ratios (0.35–0.5), alkaline activators (sodium hydroxides and/or silicate solutions at different concentrations) and the amount of a secondary highly reactive precursor (metakaolin) as partial replacement of basalt waste (0 %, 10 %, 15 % or 20 % by weight). The formulations were screened, and the obtained materials were characterized in terms of microstructural features, density, water stability, pore size distribution, mechanical strength and thermal behavior up to 1000 °C. Supplementary post-curing treatments by sodium silicate infiltrations were also explored, to optimize the water absorption and compressive strength of the obtained materials. The results highlighted that all the selected materials were characterized by a well reacted structure, with excellent thermal stability and tailorable pore size distribution in function of the basal-to-metakaolin relative amount and post-treatment steps. In particular, the basalt-only formulation activated with 8 M NaOH (B-H8) reached compressive strengths of 47.3 MPa, further increasing up to 63.7 MPa after silicate infiltration. The B/Mk formulation activated with sodium silicate (BMk-S) reached 29.1 MPa, increasing up to 59.2 MPa when using diluted silicate infiltration. Both systems exhibited low water absorption (≈10–15 %) and high thermal stability up to 1000 °C. By modulating the composition and curing procedure, valuable materials were obtained even from the alkaline activation of basalt powder alone. These findings identify B-H8 and BMk-S (after infiltration) as the most performant formulations under the investigated processing conditions. This study demonstrates that basalt-based AAMs offer a sustainable alternative for structural materials, with preliminary assessments indicating low production costs jointly with good mechanical properties.