Adsorption of surfactants on unburned carbon in fly ash and development of a standardized foam index test

The foam index test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. Many foam index test procedures are in common use, and it is difficult to compare results between laboratories. The present paper explores the possibility of standardizing the test for use with coal fly ash pozzolans. It does so by establishing that a series of commercial air-entraining admixtures (AEAs) and pure anionic surfactants all behave in a well-correlated manner, when tested using the same protocol, on a suite of 29 fly ash samples (both class F and class C) obtained from utilities throughout the United States. A pure, reagent-grade surfactant can be chosen as the basis for a standardized test; it appears as though dodecyl benzene sulfonate (DBS) is a good candidate material for such testing. The present results also confirm that it is the carbon in coal fly ash that is the main sink for AEA adsorption in concrete mixtures containing significant amounts of ash. The solution chemistry in the testing mixture is important, and use of cement in the test mix is strongly recommended. Careful attention also needs to be paid to the relative amounts of different components in the mixture to be tested. (C) 2003 Elsevier Ltd. All rights reserved.