Matthew Isaacs

Organic superplasticisers improve the flow properties of cement, offering operational advantages for the disposal of radioactive wastes. However, there are concerns that they could increase the mobility of encapsulated contaminants significantly. The effect of polycarboxylate ether superplasticisers on the solubility of Ni(II), Am(III), Pu(IV) and U(VI) in two cement-equilibrated waters (ordinary Portland cement/pulverised fuel ash (OPC/PFA) and OPC/ground granulated blast-furnace slag (GGBS)) has been assessed. The study included four commercial superplasticisers, three adjuncts (a de-foaming agent, biocide and viscosity modifier) and a bespoke, synthesised superplasticiser from which residual monomer had been removed by dialysis. The commercial products (0·5% w/w dosage) had a much greater effect on metal solubility than the dialysed equivalent, increasing solubility by 2?3 orders of magnitude. As the adjuncts alone showed no effect, the difference between the commercial and synthesised superplasticisers is attributed to small molecules, primarily residual monomers, in the commercial formulations owing to incomplete polymerisation. The distribution of radionuclides in hardened cement pastes corresponds closely to the distribution of superplasticiser, as shown by 14C-labelling in combination with digital autoradiography and accounting for the bleed observed for certain slag-rich formulations.

Superplasticisers improve the flow properties of cement and offer a number of operational advantages to producers of radioactive waste. Research is underway to clarify their mode of interaction using a bespoke, purified polycarboxylate ether (PCE) superplasticiser in controlled trials. A large-scale (200 dm3) product was prepared with a mix of ground granulated blast-furnace slag and ordinary Portland cement (9:1 mix) and tested using methods established by the UK nuclear industry to assess grout performance. The product met the essential criteria for a radioactive waste encapsulation grout. Laboratory-scale studies utilising a ¹⁴C-labelled superplasticiser and its components were employed to better understand the mechanisms involved and to determine the location and distribution of the superplasticiser within cured cement monoliths. The results suggest that a PCE superplasticiser may enable the use of cement industry standard powders for encapsulation processes, allowing lower water content matrices and mitigating the risk of future powder supply issues.