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

Abstract 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 C S H formed and increases the compressive strength of the hardened wasteform enough to withstand forces generated during curing and prevent cracking.