Investigation of fly ash carbon by thermal analysis and optical microscopy

A previous study investigated various fly ashes that had comparable loss on ignition values, but significant differences with respect to air entrainment performance. Thermal analysis data suggested that a poorly performing fly ash, with respect to air entrainment, contained a higher proportion of carbon that gasifies (oxidizes) at comparatively low temperatures. A relatively high abundance of isotropic carbon was identified in the poor-performing ash using optical microscopy. The present investigation examined a larger collection of fly ash samples to determine if thermal analysis could be used as a prognostic tool for fly ash performance. An attempt was made to correlate mortar air and foam index values for each sample with differential thermal analysis (DTA) data. Optical microscopy and BET surface area analysis were used as supportive techniques. No clear relationship could be established with the thermal or optical methods, although fly ash performance did correlate well with BET surface area. A low temperature component of the DTA exotherms was considered to be a function of inorganic catalytic species that reside on the carbon surface and lower the ignition temperature.