Electromagnetic wave absorption performance of fly ash based foamed geopolymers regulated by endogenous Fe-bearing phases

Endogenous Fe-bearing phases are recognized as important factors associated with the electromagnetic wave absorption performance of foamed geopolymers. A fly ash based foamed geopolymer exhibiting excellent broadband electromagnetic wave absorption capability is developed. The evolution of crystalline phases, restructuring of the aluminosilicate framework, and transformations in the valence state and coordination environment of endogenous Fe-bearing phases during alkali activation are elucidated through XRD, FTIR, XPS, and Mössbauer spectroscopic analyses. The results indicate that the Fe-bearing phase composition of the fly ash precursor is associated with differences in the electromagnetic response of foamed geopolymers. FA90 exhibited a minimum reflection loss of −38.77 dB and an effective absorption bandwidth below −10 dB of 15.47 GHz over 2–18 GHz. XRD analysis reveals that the original Fe-bearing crystalline phases in FA90 are partially preserved after alkali activation, whereas the intensity of several goethite associated reflections decreases, suggesting that the Fe species are not entirely inert. The shift of absorption bands in the low wavenumber region of the FTIR spectra indicates the involvement of Fe species in the restructuring of the aluminosilicate framework. XPS results further demonstrate variations in the binding energies and component distributions of Fe 2p spectra, implying a reconfiguration of the local chemical environment surrounding Fe. Mössbauer spectra show an increase in the fitted magnetically ordered Fe-bearing components from 33.40% to 40.70%. The coexistence of Fe 2 + and Fe 3+ becomes more pronounced, indicating that endogenous Fe is partially retained during dissolution, migration, and redistribution, and is subsequently reincorporated into the aluminosilicate network. These observations suggest that the reaction and evolution of endogenous Fe-bearing phases in fly ash may contribute to the enhanced electromagnetic wave absorption performance of the resulting foamed geopolymer.