Immobilisation of Fe floc: Part 1, pre-treatment of floc with slaked lime

A series of radioactive iron (Fe3+) hydroxide flocs are produced during reprocessing of nuclear fuel at Sellafield, UK. Before the flocs can be successfully encapsulated in a pulverised fuel ash/ordinary Portland cement composite cement to form a durable cemented wasteform, they need to be pre-treated with slaked lime. This paper reports results after investigating the reaction between a simulant floc and the slaked lime and shows that the floc reacts readily to form an X-ray amorphous hydrated calcium ferrite. Calcite (CaCO3), formed from carbonation of Ca(OH)2, appears to contribute to the formation of a crystalline iron hydroxy carbonate phase (Fe6(OH)12(CO3)). This work concludes that pre-treatment allows a pozzolanic reaction to occur between the pulverised fuel ash and the Ca(OH)2 produced during cement hydration. This increases the quantity of CSH formed and increases the compressive strength of the hardened wasteform enough to withstand forces generated during curing and prevent cracking.