THE CAO DISTRIBUTION TO MINERAL PHASES IN A HIGH-CALCIUM FLY-ASH FROMEASTERN GERMANY

High Calcium Fly Ashes (HCFA) are increasingly important as an admixtu re to mortar and concrete. The reactivity of HCFA is related to minera l phases like free lime, glassy spheres, anhydrite and yee'liminite. T he occurrence of CaO in several crystalline phases and at least one no n-crystalline mineral phase complicates the quantification of any of t hose mineral phases. This study focuses on an direct assessment of fre e lime and an indirect method for determination of CaO bounded in glas sy spheres and anhydrite. CaO is enriched in fine particle size ranges . Glassy spheres are the most important CaO bearing mineral phase in t he 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 spher es. The distribution of CaO among mineral phases is controlled by the availability of SO3 and free lime.