Reactive alumina is a well-established group of raw materials for refractory castables to improve their rheological behavior. In this article the influence of bimodal reactive alumina and standard (<325#) calcined alumina on workability, packing density and strength of alumina based castables is examined. It will be shown that there is a significant effect on water demand and apparent density of the castable, which is depending on the percentage of reactive alumina on the one hand and on the type of reactive alumina on the other hand. The effect of the alumina on setting time and cold crushing strength has shown to be insignificant in this set of experiments.