Usage properties of powder mixtures like flowability and floodability depend strongly on their granular characteristics: size, shape, surface and spread of particle size. The products used are mixtures elaborated from two micron sized powders, made of synthetic minerals. Their particle size distributions are each initially mono-modal, respectively 5 mm and 70 mm. The mixture elaboration procedure consists in making several successive combined stages of co-grinding, mixing and surface treatment by addition of surfactants. We obtain thus several mixtures M having different rheological behaviours and particle size distributions-ascertained on a electron i beam microscope. The results permitted in understanding the influence of the granular macrostructure and the effect of a surface treatment on a powder's rheological behaviour.