Electromagnetic wave absorption performance of porous geopolymer: Dual effects of fly ash on alkali activation and microwave dissipation

The simultaneous occurrence of aggregation reactions in fly ash (FA) facilitates the uniform dispersion of internal ferrite into the geopolymer. Considering the excellent impedance matching and multiple reflection effects of foam concrete, this paper develops a ‘zero-absorber’ foam geopolymer-based electromagnetic absorbing material. The effects of different densities (400, 600, 800, 1000, 1200 kg/m3) and FA contents (40 %, 50 %, 60 %, 70 %, 80 %) on the electromagnetic absorption performance of foam geopolymer are investigated, respectively. Combined with the iron element distribution and 3D pore structure by SEM and X-CT analysis, the dissipation mechanism of electromagnetic energy by FA and pore structure in foam geopolymer are further explored. Experimental results show that FA can uniformly disperse inherent ferric ions from the matrix into the geopolymer during the polymerization reaction, leading to excellent magnetic losses through magnetic moment orientation adjustment and intermolecular friction interactions without adding external absorber. Additionally, the uniformly dispersed porous structure consumes incident electromagnetic energy through reflection, scattering, coherent attenuation, and interference resonance loss effects. It shows multi-scale electromagnetic absorption effects in the ‘zero-absorber’ foam geopolymer-based electromagnetic absorbing material, including pore structure multiple reflections (millimeter scale), inter-pore wall interference resonance (micrometer scale), and ferric ion magnetic loss (nanometer scale). For the foam geopolymer with 1000 kg/m3 density and 50 % FA, it achieves the optimal electromagnetic absorbing performance with the minimum reflection loss (RL) of −34.9 dB and the effective absorption bandwidth of 14.1 GHz (RL below −10 dB) in 2–18 GHz range. •A ‘zero-absorber’ electromagnetic absorbing foam geopolymer is developed.•Fly ash achieves dual effects on alkali activation and microwave dissipation.•‘Zero-absorber’ foam geopolymer shows multi-scale microwave absorption effects.•Foam concrete shows excellent impedance matching and multiple reflection effects.