Role of Fe species in geopolymer synthesized from alkali-thermal pretreated Fe-rich Bayer red mud

•The coordinated Fe3+ replaced Al3+ in the aluminosilicate structure of geopolymer.•Binding energies of Al-O and Si-O increased in geopolymerization.•Alkali-thermal activated Fe-rich Bayer red mud was used to synthesize geopolymer.•Dissolution efficiencies of Al2O3 and SiO2 in the RM were improved by pretreatment.•Synergistic effect of pretreated RM and fly ash on geopolymer strength was proved.