Synthesis and characterization of new acid-activated red mud-metakaolin geopolymers and comparison with their alkaline counterparts

Red mud (RM), also known as bauxite residue, is the primary waste product generated by the Bayer process for alumina production from bauxite. Its disposal represents one of the biggest issues in the mining industry since the high amount produced every year. In the present paper, the synthesis and characterization of new materials obtained by reacting RM and RM/metakaolin mixtures with phosphoric acid (H3PO4) are reported. The obtained samples are characterized by excellent chemical and physical properties and superior compressive strength (up to 63 MPa), suggesting their application in the building sector. Moreover, a comparison with samples produced starting from a similar powder composition, but obtained in alkaline conditions, has been also performed. The proposed synthesis method enables the recovery of this industrial waste by producing objects with properties and characteristics comparable to existing construction materials. However, it is advisable to conduct life cycle assessment (LCA) and life cycle costing (LCC) analyses before undertaking large-scale implementation, even considering the possibility of using phosphoric acid activation procedure with the RM residual obtained after the extraction and recovery of metals and rare earth elements, widely studied for this type of waste.