Four fly ash samples, which had previously been found to effect concrete air entrainment in a manner inconsistent with their respective loss on ignition, were investigated using several physico-chemical techniques. This study focused on characterization of the high-carbon fraction of each fly ash, obtained by a triboelectric separation process. While the four samples displayed varying reactivities toward AEA adsorption, the BET specific surface area of all four samples was determined to be essentially the same. Thermal analysis and petrographic examination revealed that the higher demand for air entraining agents exhibited by two of the samples could be directly related to the presence of a higher proportion of optically isotropic, amorphous carbon. Liquid and vapor phase adsorption analysis suggested that the surface chemistry characteristics of the isotropic carbon resulted in a higher adsorption capacity for polar compounds such as air entraining surfactants. (C) 1997 Elsevier Science Ltd.
