It is generally known that the ASTM LOI specification is not sufficient to identify the suitability of a fly ash for the cement industry, since this criterion only gives an approximation to the carbon content of a sample and in many instances provides a poor correlation with the capacity to adsorb air-entrainment admixtures (AEAs). Petrographic examinations have shown that the unburned carbon is not visually uniform and three microscopically distinct carbon types have been identified: inertinite, isotropic coke and anisotropic coke. Concentrates of these three carbon types have been generated from two high-carbon-content fly ash samples using the technique of density gradient centrifugation (DGC). The work reported here represents the first characterization of the bulk properties of these three carbon types, where the highest DGC purity fractions were extensively analyzed by several analytical techniques, including elemental and surface area analyses. The density of like carbon forms isolated from the two fly ashes appears to be quite similar, regardless of the source of the fly ash. The H/C atomic ratios are similar to0.03-0.02, indicating a high degree of condensation. The surface areas are relatively low (10-60 m(2) g(-1)), and most of the pores are in the mesopore range (2-50 nm in width). For both series of fly ash samples, the surface area was found to increase linearly with increasing particle density. On the basis of the distinctive physical and chemical properties (density, nitrogen and oxygen concentrations, as well as surface area and mesopore volume) of the three carbon types, it is anticipated that each carbon type will have specific adsorption behavior of AEAs. (C) 2001 Elsevier Science Ltd. All rights reserved.