Fly ash from power stations is used as concrete additive to improve strength and durability. Surprisingly, studies of ashes of identical mineralogical composition from two different places have reported different results in terms of the rheological properties of the fresh material. The viscosity of the pastes made from these different fly ashes seems to be linked to the proportion of spherical and smooth-shaped grains found in them. A quantitative image analysis was carried out to characterize the shape of the grains of these two ashes from different geographical origins. The main result proves that the higher the glassy particle content of the fly ash, the more the hydraulic matrix is fluid.