Immobilisation of Fe floc: Part 2, encapsulation of floc in composite cement

Radioactive iron (Fe3+) hydroxide flocs are produced during reprocessing of nuclear fuel at Sellafield, UK. The flocs must be pre-treated with slaked lime before encapsulation in a pulverised fuel ash/ordinary Portland cement composite cement to produce a crack-free wasteform. This paper reports results obtained after investigating the fate of the iron in the floc during cementation. Results indicate that the iron in the floc reacts by substituting into and adsorbing onto the C-S-H phase formed during hydration. Additionally, a small quantity is substituted into a crystalline katoite phase, Ca3AlFe(SiO4)(OH)8. These substitutions are significant because the iron in the floc is rendered chemically immobilised within the cement rather than simply being physically encapsulated. No new phases are formed after twelve years cement hydration and examination of the twelve year old sample indicates that the durability of the wasteform appears to be high.