Assessment of mine tailings as precursors for alkali-activated materials for on-site applications

Year after year the mining industry generates 5–7 billion metric tons of tailings worldwide. They are mostly disposed rather than valorised. The substantial amounts of tailings has led to growing concerns about their ecological and environmental impacts such as occupation of large areas of land, generation of windblown dust, and contamination of surface and underground water. Over the past years the mining industry has been subjected to increase environmental principles. As the consequence, besides waste rock and water management, tailings management becomes a progressively important factor for ecologic and economic reasons. Mine tailings from flotation processes are normally disposed in impoundments on the surface. This may cause environmental and safety problems including serious water pollution arising from contamination of surface water, groundwater, and soils due to the leaching of heavy metals, process reagents, and Sulphur compounds. Recently the process of alkali-activation and geopolymerisation has been explored as a potential method for tailings consolidation and heavy metal fixation. The tailings of many existing mining operations contain the aluminosilicate minerals needed for alkali-activation at least to some extent. From a sustainability point of view it makes sense to use alkali-activated products from tailings directly on the mine site as e.g. (1) backfill material applied to fill open cavities from the (underground) mining operation and (2) cover materials for surface deposits of tailings in order to store them ecologically safe and sealed off from the environment. Mine tailings from different mining operations around the world have been investigated regarding their potential use as precursors for alkali-activated materials. An assessment of the release of gel forming elements into alkaline solutions was performed by leaching experiments under various conditions (time, concentration of the solvent, and temperature). Compared to traditional alkali-activated precursor materials the release of aluminium and silicon is in general low for the mine tailings investigated. However, results indicate a favourable ratio of Si/Al for the formation of gels similar to other alkali-activated products. Additionally, the leached calcium can contribute to the strength of the resulting products by forming C-S-H gels when a considerable amount of Si is present in the pore solution either from the activator or dissolved from the material itself. The results of the leaching tests were used to facilitate the mix design for alkali-activated products. In order to increase the mechanical strength of the final product different approaches were considered: (1) addition of reactive materials (e.g. blast furnace slag), (2) increasing the reactivity of the tailings by grinding, and (3) increasing the reactivity by curing at elevated temperature. Depending on the tailings type, the mix design, and the curing conditions the final material shows a compressive strength in excess of 30 MPa even for curing at low temperatures. The investigated mine tailings can potentially be used for various applications on the mine site. With regard to specific target applications a further evaluation of long term properties of the resulting products (e.g. freeze–thaw resistance) has to be performed. Furthermore, results indicate that products with even higher strength can be manufactured by further reducing the particle size of the tailings or by the addition of other reactive materials.