Conventional Portland cement-based systems have been considered un-suitable for immobilising nuclear wastes containing reactive metals, such as alu-minium, due to the high pH of the pore solution (usually around 12.5) and free moisture. On the contrary, calcium sulfoaluminate cement (CSA) produces a low-er pH (10.5-12) environment and has an excellent water binding capability as a re-sult of the formation of its main hydration product, ettringite. Therefore, it offers a good potential to immobilise aluminium. However, the pore solution pH and ettringite formation depend largely on the raw materials used to formulate the CSA, which is usually a blend of 75%-85% of CSA clinker and 15-25% of calci-um sulfate (in the form of gypsum or anhydrite). In this paper, it was found that, compared to anhydrite, gypsum (15%wt of the blend) demonstrated the highest reduction in the corrosion of embedded Al, possibly due to its lower initial pH (around 10.5) and self-desiccating nature at the early stage of hydration. Whilst the CSA/anhydrite had a higher Al corrosion rate, the initial set was more ac-ceptable than CSA/gypsum. Nonetheless, overall, it was concluded that CSA with gypsum (15%wt) should be considered as a base formulation for the encapsulation of Al waste. The unfavorable rapid set and high heat generation, however, demon-strated that modifications are required, potentially by using mineral additions.
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