The paper presents the results obtained with reference to the physical and mechanical properties of some composite cements containing fly ash (9% and 17% respectively). The paper focuses on the possibility of altering the characteristics of the fly ash so that it may show an as good as possible pozzolanic activity. It has been proven that the fine fraction of fly ash separated by sedimentation has mostly spherical particles and has a high content in vitreous phase. Such characteristics may substantiate the development of mortars with enhanced flowing properties and good strength, designed for developing grouting fluids.