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An alternative to Portland Cement for waste encapsulation - The calcium sulfoaluminate cement system

Zhou, Q. / Milestone, N.B. / Hayes, M.

Currently, Portland Cement (PC) is used extensively in the solidification/stabilisation of a wide variety of wastes. In the nuclear industry, low and intermediate level radioactive wastes are encapsulated or immobilised within composite PC cement systems based on high replacement with blast furnace slag or fly ash. However, the high alkalinity of these PC-based systems will corrode reactive metals found in some wastes releasing hydrogen and forming expansive corrosion products. Alternative cement systems could provide a different hydration chemistry, which would allow wastes containing these metals to be encapsulated with lower reactivity. Calcium sulfoaluminate (CSA) cement is one such cement. It combines economy of cost and low emission of CO2 with rapid strength gain and compatibility with other construction materials. Hydration provides an internal pore solution where the pH is considerably lower than that of PC. The main hydration product, ettringite, can incorporate a number of ions into its crystal structure, making it an ideal candidate for waste immobilisation. This paper details some results from a commercial CSA system that examines aspects of mixing, hydration of different formulations and aluminium corrosion behaviour. The fluidity of mixes can be adjusted by changing the formulations. All designed mixes were set within 24 h with little bleeding and the pH values were in the range of 10 to 11.5. In addition, a significant reduction in Al corrosion was observed compared to a composite OPC system. Although these results provide encouragement for the idea that CSA cement can provide a possible alternative to PC in the immobilisation of difficult and reactive wastes, further investigation is needed.