Replacement of the foam index test with surface tension measurements

The foam index test is the method usually employed to determine the degree of fly ash interference with air entrainment agents in concrete. The test involves the use of commercial air-entraining agents (AEAs) and visual observation of foam stability. These facts reduce the reproducibility of the test, because commercially available AEAs vary in strength, and the criteria for foam stability are operator dependent. Therefore, it is of interest to develop a reproducible method, which is able to determine the fly ash quality with respect to air entrainment in concrete. This paper presents efforts toward the development of a new method based on dynamic surface tension measurements, using the bubble pressure method, on filtrate from a fly ash and cement suspension. A pure surfactant is added to the suspension as a substitute for a commercial AEA. The new method and the foam index test have been compared on fly ashes acquired from power plants in Denmark and the U.S. The results reveal a good relationship between the two methods, but the new method requires further work before a finished procedure is accomplished. Finally, it has been shown that changes in temperature affect both test methods.