Valorization of phosphate by-products to produce acidic geopolymers

The present study evaluates the possibility of using a by-product of phosphate mines, such as red clay, as a precursor for the elaboration of acid geopolymers, using H3PO4 phosphoric acid (5M and 8M) as an activator. Considering the chemical composition of the clay, sodium aluminate (NaAlO2) was added as an aluminum corrector. To increase the reactivity of the precursor, the clay was thermally treated at 900°C for 2 hours. Pastes were prepared and characterized from the mechanical (compressive strengths), microstructural (BSEM/EDX and MIP), mineralogical (XRD and FTIR), and nanostructural (27Al, 29Si and 31P MAS NMR) point of view. The results showed that the type of reaction products is highly dependent on the chemical composition of the precursor and the acidic conditions. In addition, the presence of sodium aluminate improves the mechanical strengths in systems activated with 8M H3PO4.