The reaction products region formed between molten aluminium and dense, mainly amorphous, precipitator fly ash substrate, produced by hot pressing in argon, was investigated by transmission electron microscopy and energy-dispersive X-ray spectroscopy. The observations proved the oxy-redox reactions between Al and such fly ash constituents as SiO 2 , Fe 2 O 3 , Fe 3 O 4 and mullite. It results in the formation of two interpenetrating crystalline networks, i.e., α-Al 2 O 3 particles surrounded by a continuous metallic phase (Si or Al-Si alloy, reinforced with iron aluminade precipitates). The CaO does not react with Al and its fine crystallites were observed either inside or between the α-Al 2 O 3 particles.