The CaO distribution to mineral phases in a high calcium fly ash from eastern Germany

High Calcium Fly Ashes (HCFA) are increasingly important as an admixture to mortar and concrete. The reactivity of HCFA is related to mineral phases like free lime, glassy spheres, anhydrite and yee'liminite. The occurrence of CaO in several crystalline phases and at least one non-crystalline mineral phase complicates the quantification of any of those mineral phases. This study focuses on an direct assessment of free lime and an indirect method for determination of CaO bounded in glassy spheres and anhydrite. CaO is enriched in fine particle size ranges. Glassy spheres are the most important CaO bearing mineral phase in the examined HCFA. The maximum CaO content bound in a glassy matrix is close to the composition of anorthite (< 25 wt%). This is probably due to the restricted solubility of chemical species in a cooling melt of anorthitic composition. At the end of the high temperature process an equilibrium is attained between free lime, anhydrite and glassy spheres. The distribution of CaO among mineral phases is controlled by the availability of SO3 and free lime.