Abstract
A method using UV–Vis spectrophotometry for measuring the adsorption of air-entraining admixture (AEA) on the components of cements was optimized. The method was then used to measure the adsorption of AEA on a series of Class F fly ash samples with carbon concentrations of up to 14.6%. Class F fly ash carbons were found to have a very low capacity for AEA compared to Portland cement. The adsorption capacities of the fly ashes were compared to their carbon concentrations and foam index values. A lack of correlation between adsorption capacity and foam index was primarily due to differences in equilibration times used for the two measurements. Ultimately the Foam Index Test was found to be unsatisfactory for measuring the adsorption of AEA by fly ash carbon. The UV–Vis method is not necessarily a better choice for measuring AEA adsorption by carbon because soluble Ca 2+ and Mg 2+ ions from the fly ash were found to interfere with the UV–Vis method due to the formation of precipitates.
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