Rheological Behaviour of Bituminized Waste Products

Bitumen is used since the sixties as confinement matrix of radioactive wastes. The behavior of these bituminized waste products (BWPs) under thermal stress, under ionizing radiation or under leaching conditions has to be assessed. Especially, the determination of the rheological behavior is of primary of importance due to its impact on a lot of mechanisms such as the convection, the sedimentation or the migration of gaz bubbles generated under irradiation. BWP are elabrorated by twin screw extrusion and are mainly composed of 40% in weight of various inorganic salts presenting a spread granulometry, and of 60% in weight of 70/100 pur bitumen. BWPs are thus considered as a non brownian dispersion of salts suspended in a viscoelastic matrix highly thermodependant. Pur bitumen and model BWPs, containing various volume fraction of salts, were synthetized and characterized rheologicaly as a function of the temperature by using different rheological sequences (creep, flow curve and shear rate step). The results show that the pure bitumen is a newtonian fluid (in the range of 20-200°C) with a strong dependance with the temperature. On the other hand, the BWPs (with the higher load of salts) behave as a yield stress fluid that could be describe by a Herschel Bulkley model. The yield stress depends not only on the volume fraction of salts but also on the temperature as a power law. We will show that this yield stress is related to the adhesion force between the salts and by normalizing the stress by the yield stress, the relative viscosity can be scaled onto a single master curve. Finally, we will illustrate through some examples, that these rheological properties may induce some particular behavior in the flow of the materials under thermal sollicitation but also on the migration and on the bubble size distribution in the case of irradiated BWPs.