Air-Entraining Admixture Partitioning and Adsorption by Fly Ash in Concrete

The adsorption of air-entraining admixtures (AEAs) on coal fly ash was evaluated and quantified using adsorption isotherm experiments. The interaction between AEAs and aggregate, cement, and fly ash were studied, and the physical and chemical processes were identified and quantified. As a result, a method of fly ash adsorption capacity determination was developed to measure the residual AEA concentration in concrete mixtures. Fly ash adsorption capacity can be used for the purpose of characterization and specification of fly ash and to assess the suitability of fly ash for the use in concrete and other applications. In addition, AEA dosage adjustments can be made accordingly to compensate for the adsorbed AEA and to attain the AEA concentration required to produce the desired air void system. The procedure reported in this paper provides a solution to the AEA adsorption problem that hinders increased use of fly ash in concrete.