Comparison of Methods to Measure Adsorptive Capacity of Coal Fly Ash

Although there are many benefits associated with the use of coal fly ash in portland cement concrete, a common issue is the adsorption of air-entraining admixtures by residual carbon, which adversely affects the process of air entrainment. Currently, loss on ignition (LOI) is employed to estimate the total amount of carbon present. The LOI test does not measure the adsorption properties of the carbon, which is the important characteristic. The carbon present in fly ash may or may not be activated and the degree of activation can vary. Therefore, two ashes with the same LOI can have very different adsorption capacities. This paper presents a comparison of methods that have been developed to measure the adsorption capacity of a coal fly ash. These include the iodine number, direct adsorption isotherm, and a fluorescence-based method (FBM). The results of these methods are compared and the relative merits of each methodology are discussed. Keywords: adsorption; air-entraining admixture; air entrainment; fly ash.