Mechanical activation of sulfidic mine tailings for use as supplementary cementitious materials

Mine tailings (MTs) are hazardous waste generated in large volumes and can pose serious environmental risks if not effectively managed. They are typically stored as slurry in ponds, behind tailings dams, or storage facilities. In sulfidic MTs, the high concentration of sulfide minerals makes them prone to acid mine drainage, infiltrating soil and groundwater. Recently, producing supplementary cementitious materials (SCMs) from MTs has been noted as an effective approach for utilizing mining waste while reducing the carbon footprint of concrete. This study assesses the influence of four milling methods, rotary ball mill, planetary ball mill, vibratory ball mill, and vibratory disc mill, on the reactivity of sulfidic MTs. The vibratory-disc-milled MTs revealed higher reactivity with a heat release of 162 J/g, as quantified using the modified R³ test; this is likely due to their very fine particle size (d₅₀ = 9.8–14 μm). Owing to the presence of pyrite in raw materials, its impact on the activity of MTs in cement-tailings mixes was examined, focusing on heat release, hydration, and strength development through morphological, chemical, mineralogical, and mechanical characterization. Furthermore, hyphenated TGA and FTIR was employed to accurately identify the decomposition temperature ranges of chemically bound water and other evolved gas, in specimens made with tailings-based SCMs. A 10% cement substitution with tailings-based SCMs resulted in mortars with the highest strength at early ages, achieving 41.5 MPa at 3 days, which surpassed the 33 MPa strength of the control specimens. Although strength reduction with higher SCM content likely stemmed from pyrite oxidation and the formation of a secondary ettringite phase, up to 30% substitution achieved the required strength activity index per ASTM C618, indicating their potential as suitable SCMs.