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 heterogeneo us composition of lignite fly ash complicates its utilization as an ad mixture to concrete. The reactivity of lignite fly ash is related to t he occurrence of reactive glassy components and several cement phases. Glassy spheres and anhydrite in the micrometer and submicrometer rang e of lignite fly ash were analyzed with AEM methods. The main componen ts 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 thre e different compositional ranges of different reactivity. With decreas ing CaO content the reactivity of the glassy spheres decreases. A majo r boundary at 25 wt-% CaO separates highly reactive glassy spheres fro m 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 b inding form (presumable calcium aluminate, aluminium sulfate, or calci um aluminate sulfate). This aluminium is fixed in a highly soluble anh ydrite environment and probably is the easy accessible elemental sourc e for the first ettringite formation during the hydration reaction of lignite fly ashes.