Thermally treated phlogopite as magnesium-rich precursor for alkali activation purpose

Abstract

Alkali-activated materials (AAMs), sometimes called geopolymers, have wide industrial relevance and are produced through alkaline activation of alumina and silica-rich precursors, including industrial side streams. It has been observed that MgO content in AAMs results in improved mechanical properties. However, most of the suitable MgO sources are either prohibitively expensive or carbonate-based, which leads to higher CO2 emissions. Phlogopite is an attractive source of MgO as it is carbonate-free and locally abundant. Besides MgO, phlogopite also contains significant quantities of silica and alumina, making it a viable candidate for alkali activation. However, an alkaline solubility test in NaOH revealed phlogopite to be highly inert, with both silicon (Si) and aluminum (Al) solubility at around 1wt% of the original material. In this work, thermal treatment was considered to improve the alkali activation potential of phlogopite. Thermal treatment was found to be successful; the resulting glassy material showed 45% Si and 33% Al release into the solution, on a par with highly soluble metakaolin (26% Si and 40% Al). During the solubility test, significant quantities of magnesium and iron were also released but they did not stay in the solution. Rather, they formed precipitated products, the main one of which was meixnerite (Mg6Al2(OH)18·4H2O), a hydrotalcite-like compound. The resulting alkali-soluble glass with phlogopite chemical composition can be used asa carbonate-free source of magnesium in alkali activation purposes. However, any commercial application should allow for due consideration of cost.