Size distribution of unburned carbon in coal fly ash and its implications

A set of nine coal fly ashes, obtained from various US utilities, were fractionated by standard dry-sieving techniques. The carbon contents of the different size fractions were measured, and the nature of the carbon particles was microscopically examined. Significant differences were found in the distribution of carbon in class F and class C ashes. The ‘foam index’ test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. This test measures adsorption of air entraining admixtures (AEAs) by the carbon in the fly ash. Application of this test to the different ash fractions confirmed that the smallest particle size fractions of ash make the major contribution to AEA adsorption. The carbon from class F ash has a comparable capacity for AEA adsorption as carbon from class C ash, when compared on a surface area basis. What makes the class C carbons apparently ‘worse’ is the fact that they have much higher surface areas than class F carbons (and it is only by virtue of the low carbon mass in most class C ashes that problems with these ashes are not more common). The importance of accessibility of the surface is also clearly seen from these results.