Chemical stability and environmental characterization of alkali-activated mine tailings generated using a MgCO3/MgO industrial residue

Mining tailings are commonly combined with ordinary Portland cement (OPC) and water to form a paste used for mine gallery backfilling (MGBF). Although OPC remains the most frequent choice, alternative alkaline reagents such as Mg(OH)₂ are being investigated due to several limitations: its high cost, a significant carbon footprint associated with its production, and limited long-term durability, especially because of its vulnerability to sulfate attack. This study examines how the use of a MgCO₃/MgO industrial residue affects the environmental behavior of alkali-activated pastes in a wide range of mine tailings (MTs), considering the results obtained from different tests: ABA test, leaching test according to UNE 12457-4 and uniaxial compressive strength (UCS) tests. Various paste formulations were generated using different MgCO₃/MgO concentrations and six very different types of MTs spanning a wide range mineralogical, chemical and acid potential characteristic. As a main conclusion, all alkali-activated pastes, when compared with the original MTs, showed a very important improvement of their environmental behavior, marked by a consistent reduction of their acid generation potential, a water quality improvement of their leachates and their new consideration of inert wastes according to the European regulation for waste acceptance at landfills. While dynamic long-term leaching experiments and reactive transport geochemical models are advisable to better understand the behavior of these type of mine residues under real conditions and in the long term (decades to centuries); the present study shows how the combined application of ABA and UNE 12457-4 tests can offers a reliable initial environmental characterization of alkali-activated mine pastes.