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 SiO2, Fe2O3, Fe3O4 and mullite. It results in the formation of two interpenetrating crystalline networks, i.e., alpha-Al2O3 particles surrounded by a continuous metallic phase (Si or AI-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 alpha-Al2O3 particles. (C) 2003 Elsevier Science B.V All rights reserved.