In the Mittersill tungsten mine of Wolfram Bergbau und Hütten AG, large quantities of waste rock and tailings are produced during mining and processing of the ore. Most of the tailings from the flotation process is stored in tailings ponds. A smaller part is used as underground backfill. A small part of the waste rock is used as low-grade aggregates; the rest is deposited on site. To minimise the tailings volume and to extend the utilisation time of the approved tailings area, alternative valorisation options are under review. In the course of this work, it was investigated whether the flotation tailings can serve as a precursor for alkali-activated binders. For alkali-activated binder concepts and geopolymers, the solubility of aluminium and silicon from the solid material in alkaline solution provides a basis for the formation of new, strength-giving mineral phases. By alkaline activation, a building material can be produced, that is comparable to conventional concrete and also shows a similar structure. The solubility of relevant chemical elements from the solid depends on the mineral phase in which this element is incorporated. In addition, the specific surface (i.e. fineness) of the material also plays a decisive role. Flotation tailings usually have a relatively high fineness. They are - provided an appropriate chemical and mineralogical composition - suitable in many cases as a precursor for alkali-activated binders without further processing steps. The addition of other reactive components (for example residues from the mineral raw materials industry) can have a valuable contribution to the strength of such alternative building materials. The reactivity of the flotation tailings from the Mittersill mine in alkaline solution was investigated by solubility tests on samples of different fineness. The flotation tailings show moderate solubility of aluminium and silicon. As expected, a significant increase in solubility is seen when the surface area is increased by milling.
In order to investigate the effectiveness of the alkaline activation, cylindrical test specimens were produced from binder pastes of different compositions as well as mortar samples in accordance with EN 196 and their compressive strength was determined. In particular, the influence of the composition of the activator solution used, its concentration and its quantity, was determined. Furthermore, the influence of adding ground blast furnace slag (HOS) from the voestalpine Stahl GmbH plant in Linz on the strength was investigated. By adding HOS, both the early strength and the final strength could be increased. For the cylindrical specimens produced from binder paste a compressive strength >46 MPa could be measured for flotation tailings material mixed with HOS. For the mortar prisms mixtures of ground tailings from Mittersill (specific surface area: ~2600 cm²/g) and HOS (specific surface area: ~3100 cm@/g) were used and activated with a liquid solution of 10N NaOH and sodium silicate solution mixed at a ratio of 1:2,5 were prepared. On the one hand side standard sand accordin to EN 196 was used. Furthermore, a standard sand substitute derived from Mittersill waste rock material and adapted to the grain size distribution of the standard sand was tested. Results showed that the strength of the samples increase with increasing concentration of HOS. Samples prepared with tailings and HOS mixed for example at a ratio of 1:1 showed a maximum compressive strength of roughly 35 MPa. The compressive strength of mortar prisms with standard sand substitute showed was roughly 40 % lower. The results of this work show that an alkali-activated building material can be produced from the flotation tailings of the Mittersill mine. The utilisation of such a building material needs to be clarified by further tests to determine application-specific parameters. In addition, the tests with standard sand substitute should also be carried out with a larger specimen size using aggregates with a particle size distribution relevant for concrete, and properties such as freeze-thaw resistance should be determined. In addition, further investigations would be necessary to clarify the actual hardening mechanism of the binder concepts developed here.