In this paper, we show a new technique to characterize fly ashes using scanning electron microscopy (SEM). Chemical composition of millions of points is measured by SEM-energy dispersive X-ray spectroscopy and plotted in a ternary frequency plot. This new and straightforward way of displaying microscope data makes it easy to understand and assess the number, the approximate chemical composition, and the relative proportions of populations that constitute the fly ash. For quantitative characterization of the raw fly ash, these populations are grouped as predominantly silicate, calcium-silicate, aluminosilicate, and calcium-rich aluminosilicate. The same technique is then used to track these anhydrous fly ash fractions in blended cements during hydration and thus provide crucial information on the reactivity of different glasses in cementitious systems.