Development of calcium sulfoaluminate-belite cements for the encapsulation of radioactive waste

To demonstrate and evaluate the potential of calcium sulfoaluminate-belite (CSA) cements for the enhanced encapsulation of future higher activity waste treatment processes, a commercially available clinker was trialled on a 3 L scale, before being scaled up to 500 L scale, using a typical In-Drum mixing (IDM) methodology and equipment employed for the encapsulation of radioactive slurry wastes in the UK. The formulation envelope varied the gypsum addition, water to solid ratio, and mixing shear regime. Mixes were conducted neat, with no addition of waste material, additives, supplementary cementitious materials, or fillers. Conducting IDM standard mixes allowed for the trial of formulations in absence of waste loading during operations, subjecting products to hydration exotherm temperatures based upon the scale of the products cast in excess of normal operations, where waste incorporation would be expected to dilute the peak hydration exotherm obtained. This scenario allows for the behaviour of the main CSA strength giving phase ettringite to be evaluated, establishing a bounding case. Following a curing duration of 28 and 90 d, the phase assemblage between products subjected to different curing exotherms were indistinguishable, despite being subjected to a range of different temperature profiles and the structure of the 500 L products being compromised. Processing properties were consistent between mixing scales, with physical properties producing desirable results when not impacted by the high exotherm temperatures experienced by 500 L products.