Thorium and americium solubilities in cement pore water containing superplasticiser compared with thermodynamic calculations

Factors affecting the suitability of superplasticiser-amended cement for the encapsulation of radioactive waste

“…The measured baseline solubility of Am(III) compares well with the values reported by Greenfield et al (1998) in concrete water (5 Â 10 −11 M) and Kitamura et al (2013) in OPC systems (ca. 10 −11 M).…”

“…Similar increases in Pu(IV) solubility were measured in saturated calcium hydroxide in the presence of ADVA Cast 551, going from a baseline solubility of 1·2 Â 10 −10 to 2·2 Â 10 −7 M with 1% SP (Clacher et al, 2011). Although no results could be found in the literature for the solubility of Pu(IV) in the presence of PCE-SP under post-addition conditions, the work of Kitamura et al (2013) on Th(IV) showed only a slight increase in solubility from $ 2 Â 10 −10 M to around 10 −9 M with 1000 ppm TOC leached from an OPC matrix.…”

“…These authors attributed the observed increase of solubility to the formation of mixed complexes of the radionuclides with the SP additives stabilised by hydroxyl ions. It has been pointed out that this enhancement may not be realistic since at least 90% of the SP added to the mix is likely to be retained by the cement matrix (Fujita et al, 2008;Kitamura et al, 2013). Moreover, the composition of any organic compound leached from the cement may differ from the bulk SP, as only low-molecular-weight substances, either unreacted monomer or degradation products, could be detected in the pore water (Fujita et al, 2008).…”

“…Moreover, the composition of any organic compound leached from the cement may differ from the bulk SP, as only low-molecular-weight substances, either unreacted monomer or degradation products, could be detected in the pore water (Fujita et al, 2008). Studies undertaken with a commercial PCE (ADVA Cast 551) investigated its effects over a range of concentrations from 0·001 to 1% cement replacement (Clacher et al, 2011(Clacher et al, , 2013a(Clacher et al, , 2013bKitamura et al, 2013). These authors carried out ultrafiltration (10 and 30 kDa) in order to determine the distribution of Ni(II), Am (III), Th(IV) and U(VI) between the bulk SP and lower molecular weight components of the SP product.…”

“…As highlighted by Kitamura et al (2013), it is not only the concentration of the SP in the cement leachates that affects radionuclide solubility but also their speciation, that is the nature of the organic compounds leached into the pore solutions; this may or may not correspond to the composition of the original SP. Factors to consider are the purity of the SP polymer, potential degradation in high-pH cement pore waters and any adjuncts used to increase workability and shelf life.…”