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2 Publications
Leaching of U(VI) and Cm(III) doped calcium (aluminum) silicate hydrate gel (C-(A)-S-H) and tobermorite in saline brines
Wolter, J. M.; Schmeide, K.; Huittinen, N.; Bok, F.; Weiss, S.; Brendler, V.; Stumpf, T.;
To evaluate the retention potential of concrete inside a nuclear waste repository for actinides under saline and hyperalkaline conditions, leaching experiments with actinide doped cementitious phases were performed in repository-relevant brines. Therefore, U(VI) and Cm(III) doped calcium silicate hydrate (C-S-H) phases with different calcium-to-silicon (C/S) ratios (1.0−2.0) were synthesized directly in presence of either U(VI) or Cm(III) and characterized by time-resolved laser-induced luminescence spectroscopy (TRLFS), infrared (IR) spectroscopy, powder X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). The time-dependent release of Ca, Si, U or Cm from CSH phases into brines that contained either 2.5 M NaCl, 2.5 M NaCl/0.02 M Na₂;SO₄, 2.5 M NaCl/0.02 M NaHCO₃ or 0.02 M NaHCO₃ for U(VI) doped CSH phases or 2.5 M NaCl/0.02 M NaHCO₃ or 0.02 M NaHCO₃ for Cm(III) doped CSH phases was monitored in batch leaching experiments for 30 to 60 days. Subsequently, leaching induced changes of the C-S-H structure and of the U(VI) or Cm(III) coordination environment were investigated with TRLFS, IR spectroscopy and XRD. Results indicated that the U(VI) retention by C-S-H phases is maintained in the presence of NaCl rich solutions due to the formation of uranophane [1]. The presence of carbonate in saline leaching solutions increased the U(VI) mobility due to formation of Ca₂UO₂(CO₃)₃(aq) at moderate alkaline pH values [1]. Furthermore, an influence of the secondary CaCO₃ phases calcite, vaterite and aragonite was detected. Calcite contributed to the U(VI) retention which was shown with TRLFS [1]. The binding study of Cm(III) incorporated into C-S-H gel revealed at least two Cm(III) species: (i) Cm(III) substituted against Ca2+ from the C-S-H interlayer and (ii) Cm(III) incorporated in the polyhedral CaO plane of the C-S-H structure. Additionally, a luminescence line narrowing effect was observed indicating variations of the local surrounding of Cm(III) in C-S-H gel. Leaching experiments showed that Cm(III) is not mobilized by carbonate but becomes partially incorporated into secondary CaCO₃ phases. Recently, we started to investigate the Al and U(VI) incorporation into C-S-H phases at different Al/Si ratios (0.025−0.2) and synthesis temperatures (25°C or 200°C). The obtained phases were investigated with ² ⁷Al NMR, TRLFS, XRD and Raman microscopy. First results indicated an influence of the Al starting material and synthesis temperature on the Al incorporation.
Keywords: C-S-H, Aluminum, TRLFS, XRD, Raman, IR, Leaching, Saline, Carbonate
  • Contribution to proceedings
    5th International Workshop on Mechanisms and Modelling of Waste/Cement interactions, 25.-27.03.2019, Karlsruhe, Deutschland
    Proceedings of the 5th International Workshop on Mechanisms and Modelling of Waste/Cement interactions
  • Poster
    5th International Workshop on Mechanisms and Modelling of Waste/Cement interactions, 25.-27.03.2019, Karlsruhe, Deutschland

Publ.-Id: 28342 - Permalink