Effects of Cement Additives on Radionuclide Mobility
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(IIHFWVRI&HPHQW$GGLWLYHVRQ5DGLRQXFOLGH0RELOLW\ Mårten Dario1, Mireia Molera2 and Bert Allard3 1 Dept of Water and Environmental Studies, Linköping Univ., SE-581 83 Linköping, SWEDEN 2 Dept of Chem./Nuclear Chem., Royal Inst. of Technology, SE-100 44 Stockholm, SWEDEN 3 MTM Research Centre, Örebro Univ., SE-701 82 Örebro, SWEDEN $%675$&7 The effects of organic cement additives (totally 7) on the sorption of Eu on cement and TiO2 were measured with variation of the concentration of the additive (0.001-10% of the water phase) and reaction time (up to 420 d). There was a significant reduction of the sorption of Eu on cement as well as on TiO2 after 24 h of exposure time in the presence of additives at levels that would be obtained in a cement/water system if the total load of the additive in the cement (up to 3%) would be released into the water phase. Sorption increased with time and would be expected to reach constant levels after 100-200 d. Storage of the additives at 60 and 120 oC, pH 12.5, for up to 110 d (in order to degrade them) did not lead to significantly altered sorption behaviour. ,1752'8&7,21 Cement and cement-based materials will be used as matrices and as containers for various categories of low-level and medium-level radioactive waste. Cement and concrete generally contain additives (up to 3%) of various kinds, usually organic polymers that will act as superplasticisers (denoted as water-reducers). It can not be ruled out that these agents, or their degradation products, may act as metal complexing agents with ability to enhance the solubility and mobility of radionuclides from the waste within or in contact with the cement. The effects of the presence of cement additives on the distribution of Eu(III) (model element for trivalent lanthanides and actinides) in a cement system are assessed in this study, and consequences for the safety of a cement-based waste deposition concept are discussed. (;3(5,0(17 Batch distribution studies of Eu (10-8 M, slightly below saturation) were conducted with Standard Portland cement and TiO2 as solid phases. TiO2 was selected in order to examine if it can be used as a representative analogue to cement, when estimating the influence of complexing agents on radionuclide sorption. All experiments were performed at pH 12.5, representing cement pore water in contact with Ca(OH)2 (see e.g. [1]). The ionic strength was held constant at 0.3 M, which is approximately the same as of cement pore waters (cf. the artificial pore waters used by [2,3]). Calcium was added to the TiO2 systems (0 or 2 mM Ca, representing the levels in a cement system). Seven cement additives were compared (Table I). The distribution of Eu between the solid and liquid phases was measured as a function of additive concentration and time. Distribution measurements were also performed using additives that had been stored at elevated temperatures (up to 120oC, pH 12.5) prior to the addition of the solid adsorbent and Eu.
1
7DEOH, Description of cement additives Cement additive Sikament 10 Si
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