Development of magnesium phosphate cements for encapsulation of radioactive waste

Radioactive waste streams that include metallic uranium are incompatible with conventional ordinary Portland cement (OPC)-based encapsulation matrices. These encapsulation systems are essentially composite materials that incorporate high replacement levels of pulverised fly ash (PFA) or blast furnace slag (BFS). A potential alternative encapsulant for the treatment of problematic waste streams is magnesium phosphate cement. This paper discusses the fundamental characterisation results obtained from two magnesium phosphate cement formulations being developed in the UK for the encapsulation of metallic intermediate level waste (ILW). When compared to conventional OPC based systems, the two magnesium phosphate cement formulations investigated have lower pH, are able to chemically combine more mix water into the system and provide sufficient workability at water/solid ratios close to the theoretical confines needed for paste saturation. The results presented have confirmed compliance of this material against NDA RWMD guidelines for strength and expansion. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results obtained for both formulations up to 360 days cure time have indicated that the cement system shows evidence of chemical stability.