A zero-waste strategy to synthesize geopolymer from iron-recovered Bayer red mud combined with fly ash: Roles of Fe, Al and Si

Bayer red mud (RM) was treated with carbothermal reduction for iron recovery, and then the fly ash-based geopolymer was synthesized from the iron-separated RM residue and fly ash with a mass ratio of 25:75. Under the optimized conditions, the recovery efficiency of iron and the total iron content in the iron concentrate could reach the maximum of 51.48 and 54.13 wt%, respectively. The leaching efficiencies of alumina and silica reached the maximum values of 11.45 wt% (Al2O3) and 2.62 wt% (SiO2) after the the iron recovery process, therefore, a compressive strength of 43.35 MPa of the geopolymer was achieved. The effects of Fe element in iron-separated RM residue as well as the transformation of Al and Si elements on the properties of geopolymer were analyzed by Mössbauer spectroscopy, X-ray diffraction, and linear regression model. The presence of iron had a negative impact on the strength of geopolymer, and the enhanced alumina and silica leaching efficiencies by the carbothermal reduction process had dominantly positive effects on the compressive strength. Density Functional Theory (DFT) calculation demonstrated that Fe3+ could substitute Al3+ in the geopolymer networks, and the intensity of the covalent interaction of Fe-O bond was lower than Al-O bond, which indicated that Al-O bond was more stable than Fe-O bond in geopolymer regarding to long-term stability.