The Development of Calcium Aluminate Phosphate Cement for Radioactive Waste Encapsulation

Reactive metals such as aluminium metal make up a significant proportion of the UK’s legacy radioactive waste. Current treatment methods – encapsulation in PC-based cementitious systems – do not perform optimally when applied to reactive metals. Corrosion of encapsulated aluminium, caused by the availability of free-water and highly alkaline pore solution, results in expansive corrosion products and the generation of significant quantities of hydrogen gas, which compromises the long-term performance of waste packages.
Calcium Aluminate Phosphate cements (CAP), formed from acid-base reaction between Calcium Aluminate Cements (CAC) and an acidic phosphate-based solution, were identified as alternative encapsulants that provide different internal chemistry i.e. pore solution of lower pH which may be advantageous when applied to the encapsulation of reactive metals. Various types of phosphates, monophosphates and polyphosphates, were assessed to identify suitable pre-cursor materials for producing a cementitious matrix when mixed with CAC, and a CAP formulation envelope suitable for the industry-defined processing and operational property requirements, was identified.
The corrosion behaviour of aluminium encapsulated in the CAP system was characterised by a dormant period, during which the corrosion and gas generation rates were very low, and a significant increase after the dormant period. The phase evolution of the CAP system altered not only the physico-mechanical properties of the system in longer-term but was also responsible for the latent corrosion behaviour of aluminium encapsulated in the CAP system.