MICROANALYTICAL CHARACTERIZATION (AEM) OF GLASSY SPHERES AND ANHYDRITE FROM A HIGH-CALCIUM LIGNITE FLY-ASH FROM GERMANY

Lignite fly ash is known to have cementing properties. The heterogeneous composition of lignite fly ash complicates its utilization as an admixture to concrete. The reactivity of lignite fly ash is related to the occurrence of reactive glassy components and several cement phases. Glassy spheres and anhydrite in the micrometer and submicrometer range of lignite fly ash were analyzed with AEM methods. The main components of the glassy spheres are Al2O3, SiO2 and CaO, The ratio of Al2O3/(Al2O3+SiO2) is constant for most analysis and indicates the origin of the grassy spheres from kaolinite. The calcium content of the spheres is rather variable. Leaching experiments led to the definition of three different compositional ranges of different reactivity. With decreasing CaO content the reactivity of the glassy spheres decreases. A major boundary at 25 wt-% CaO separates highly reactive glassy spheres from less reactive spheres. Calcium-free spheres with a Al2O3/(Al2O3+SiO2) ratio less than 0.45 show no reactivity. Anhydrite particles in the submicrometer range are often contaminated with aluminium in unknown binding form (presumable calcium aluminate, aluminium sulfate, or calcium aluminate sulfate). This aluminium is fixed in a highly soluble anhydrite environment and probably is the easy accessible elemental source for the first ettringite formation during the hydration reaction of lignite fly ashes.