The phase-mineral and chemical composition of heavy concentrates (HCc) and improved fly ash residues (IFAs) recovered from five fly ashes (FAs) produced in four large Spanish thermo-electric power stations was characterized. The HCs were separated by sink-float separation in bromoform. while the IFAs are residues after the isolation of ceramic cenosphere, salt, magnetic, char and heavy concentrates. The HCs recovered are in the range 0.02-0.28% and their phase-mineral composition (in decreasing order of significance) commonly includes aluminosilicate glass, hematite, magnetite, larnite, quartz, periclase, mullite, corundum, lime, char, melilite, rutile, plagioclase, wollastonite, ferrian spinel, and anhydrite. Numerous accessory minerals of Ba, Ce, Cl, Cr, Cu, F, Fe, La, Mn, P, Ph, Th, Ti, Y, and Zr, are also typical components of HCs. These fractions are enriched in As, Ca, Cr, Cu, Gd, Mg, Mn, Mo, Fe, Ni, Pb, S, Se, Ti, V, Zn, and Zr in comparison with the FAs. The IFAs isolated are in the range 71.5-97.1% and their phase-mineral composition (in decresing order of significance) normally includes aluminosilicate glass, mullite, quartz, char, kaolinite, plagioclase, cristobalite, wollastonite, hematite, anhydrite, K-feldspar, melilite, and corundum. The contents of most elements in IFAs are similar to FAs as Ag, Ba, Cl, Cs, Ge, Me, S, Sb, and Sc are significantly depleted in these residues. The IFAs have an improved composition in comparison with the FAs because they are: more homogeneous and finer-grained products; abundant in pozzolan and inert constituents; and depleted in harmful components. Some genetic features, properties, possible environmental concern, and potential utilization directions related to the HCs and IFAs are described. A basis for the advanced, multicomponent, wasteless, and environmentally safety utilization of various FAs is also discussed. (c) 2004 Elsevier Ltd. All rights reserved.