Investigations on the influence of fly ash on the formation and stability of artificially entrained air voids in concrete

In concrete, fly ash is applied to a task-oriented improvement of different properties. Besides the advantages, e.g. the improvement of the rheology of the fresh concrete or the density of the hardened concrete, some investigations and the experience from practice indicate that some fly ashes probably influence the formation and stability of artificially entrained air voids. The reason lies presumably in the fraction of unburned carbon, a minor component of the fly ash. To identify the causes, seven fly ashes from European power plants were investigated. The fly ashes were characterized and mortar and concrete tests were conducted to identify specific fly ash parameters which might be responsible for the impaired formation and stability of the air voids. Furthermore, it was examined whether the foam index test is applicable for the assessment of the air entraining agent demand and whether an adequate accuracy of the results is given. On the basis of the results it was also examined whether the mortar tests or a fly ash specific parameter can be applied as an alternative prediction tool to assess the air entraining agent demand for an air entrained concrete.