Fundamental Study on Effects of Borate on the Sorption of Cesium ions to Calcium Silicate Hydrate - 18068

The effects of borate on the formation of calcium silicate hydrate (C-S-H) and the Cs sorption behavior were examined by the sorption experiments without drying process. C-S-H was prepared by mixing calcium oxide, fumed silica, and sodium-borate solution in a glove bag filled with nitrogen gas. The volume of sodium borate solution was 30 mL, in order to adjust the liquid/solid weight ratio to 20. The Ca/Si molar ratio of C-S-H was adjusted so as to be 0.4, 0.8, 1.2, and 1.6. The concentration of sodium borate was set to the range from 0 to 600 mM in the sample solution. The solution of cesium chloride was added to the each sample for setting the initial concentration of Cs ions to 1 mM. The samples were cured in a shaking chamber at 298 K and 120 strokes/min during 7 days. In addition, for estimating the curing-period dependencies, the curing period was set to the range from 0.5 to 28 days for the condition of 0 M or 400 M sodium borate. In the results, under the conditions of the borate concentration up to 200 mM, the sorption behavior of Cs ions to C-S-H was hardly different from those without borate, while the sorption ratio of Cs ions for the borate concentration of 400 mM or more decreased by down to 46% compared with the blank samples. However, the sorption ratio of Cs ions in the case of 400 mM borate increased and reached as much as that for the blank sample until 28 days. Besides, the Raman spectra of solid phase showed that the peak derived from boric acid or borate (calcium borate or calcium borosilicate hydrate) appeared under the high concentration conditions of borate, and the peak of the silica chain of C-S-H was weakened. Considering the practical concentration of borate and much longer period for the performance assessment of the solidified wastes, the immobilization properties for Cs ions of cementitious materials contained borate will be kept appropriately in the disposal system for low-level wastes. (authors)