Characterization and reactivity of size-fractionated unconventional fly ashes

Modification of fineness, including by grinding or classification, is a commonly used beneficiation method for improving fly ash performance. In this study, five fly ashes, including off-spec and reclaimed ashes, were size fractionated by sieving into the following fractions: < 20 µm, < 45 µm, and > 45 µm. The original fly ash and the size fractions were characterized, and their reactivity was measured. For Class C fly ashes, the CaO contents decreased and SiO 2 contents increased as the size fractions became coarser, with the > 45 µm size fractions having lower CaO contents that would allow them to be classified as Class F fly ashes. The > 45 µm size fractions had more angular or irregular particles (impurities and unburnt carbon), while the < 20 µm fractions had very limited amounts of such particles. There was a general reduction of reactivity from finer to coarser fractions as measured using a direct reactivity test. As expected, cement pastes made with finer fractions showed higher heat release and calcium hydroxide consumption. The magnitude of changes between the size fractions depended on the original fly ash composition and particle size distribution.