THE QUANTIFICATION OF THE FLY ASH ADSORPTION CAPACITY FOR THE PURPOSE OF CHARACTERIZATION AND USE IN CONCRETE
Fly ash has been shown to be an effective replacement for portland cement in concrete mixtures. However, many fly ash materials contain unburned carbon from the combustion process. Unburned carbon in fly ash adsorbs air entraining admixtures (AEAs) reducing their effectiveness in providing a specified air void system in concrete materials. Measurement tools and methods for characterization of the adsorption properties of fly ash materials are necessary for beneficial use of fly ash materials in concrete. In this research, two methods were developed to measure and quantify the adsorption capacity AEAs on fly ash materials. The first method is the fly ash iodine number, a simple laboratory procedure that measures the adsorption capacity of fly ash based on iodine adsorption. The second is the application of direct adsorption isotherms. This test can be used to quantify the amount of AEA adsorbed by fly ash in concrete.
When the iodine number test is combined with the direct adsorption isotherms, the AEAs dosage predictions can be made by simply measuring the fly ash iodine number of the fly ash, then use the fly ash iodine number-direct adsorption correlation to predict the amount of AEA adsorbed, which represent the required dosage adjustment.
These two tests provide a robust, simple, and practical methodology for engineers to use in the specification of AEA quantities required for concrete mixes when portland cement is replaced by fly ash.