Potential of inorganic polymers (geopolymers) made of halloysite and volcanic glass for the immobilisation of tailings from gold extraction in Ecuador

In the present study, inorganic polymers (geopolymers) were synthesised using volcanic glass, calcined halloysite-rich clay and mine tailings, all from Ecuador. In addition to the possibility of making a geopolymer binder from these materials, the influence of geopolymerisation on the immobilisation of the mine tailings was investigated. The research focused on the effect of synthesis conditions on the strength, microstructure and composition of the geopolymer binder and on the leaching behaviour of heavy metals and Arsenic (As). A water insoluble material with a compressive strength of 30 MPa was synthesised using a Na-silicate activator (composition in wt.%, Na2O: 15; SiO2: 13; H2O: 72) and using 110 kg of activating solution per tonne of raw material. It was shown that incorporation of mine tailings in the geopolymers did not have an effect on the mechanical properties. The production of geopolymer based tailing pastes thus shows potential for use on the mining site, for example as a capping material of tailing ponds. Geopolymerisation influences pH-dependent heavy metal leaching but does not cause a complete immobilisation of heavy metals and As. In general, the leaching of heavy metals is limited, below the regulatory concentrations for using the geopolymers in building materials. An exception on this are Cu and As, which show increased mobility due to the higher pH obtained in the geopolymerisation.