Comparison of the stability of U(VI) and Cm(III) doped calcium (aluminum) silicate hydrate (C-(A)-S-H) phases at saline conditions

Cementitious materials, used in a nuclear waste repository in the form of concrete or grout to ensure mechanical stability and sealing of disposal tunnels, constitute an important containment barrier for radionuclides in the event of water intrusion into a disposal site. The immobilization potential of hardened cement paste (HCP) as well as of calcium silicate hydrate (C-S-H), as main component of HCP, towards radionuclides such as Cm(III) or U(VI) has been demonstrated in a number of studies, e.g. [1-3]. To evaluate the retention potential of cementitious materials towards radionuclides at saline conditions, U(VI) and Cm(III) doped C-S-H phases were exposed to background electrolytes with salinities comparable to those reported for pore waters of North German clay formations, which are considered as potential host rocks.
U(VI) and Cm(III) doped C-S-H phases with calcium-to-silicon (C/S) ratios ranging from 2.0 to 1.0, representing a portlandite saturated C-S-H system as well as chemically degraded cement paste, were synthesized directly in presence of either U(VI) or Cm(III). These phases were characterized by time-resolved laser-induced luminescence spectroscopy (TRLFS), infrared (IR) spectroscopy, powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). Batch leaching experiments were performed for U(VI) doped CSH phases applying 2.5 M NaCl, 2.5 M NaCl/0.02 M Na2SO4, 2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3 and for Cm(III) doped CSH phases applying 2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3. The time-dependent release of Ca, Si, U or Cm from CSH phases into brines was followed up to 60 days. Leaching induced changes of the C-S-H structure and of the U(VI) or Cm(III) coordination environment were studied mainly with XRD and TRLFS or IR spectroscopy, respectively.
Generally, the high immobilization potential of C-S-H gel towards U(VI) and Cm(III), reported in the literature, was verified. In the presence of saline solutions, the C-S-H phases showed differences with regard to C-S-H stability and radionuclide release in dependence on the C/S ratio, the composition of the leaching solution and the immobilized radionuclide.
The leaching results for U(VI) doped C-S-H gel indicated that the U(VI) retention is maintained in the presence of NaCl rich solutions (2.5 M NaCl/0.02 M Na2SO4) due to the formation of a uranophane-like phase as detected by TRLFS [4]. The presence of carbonate (0.02 M) in the leaching solution, however, led in case of a C-S-H gel with a low C/S ratio (1.5, representing altered HCP) to some dissolution and thus, to a partial release of U(VI) whereby Ca2UO2(CO3)3(aq) is formed at moderate alkaline pH values. Part of the U(VI) is found to be retained in secondary CaCO3 phases after leaching. The release of U(VI) from C-S-H gel with a high C/S ratio (2.0, representing fresh HCP) due to carbonate was significantly smaller, only enhanced to a small extent due to the additional presence of 2.5 M NaCl.
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 (c.f. Fig. 1a), which is in accordance with the literature [5]. The luminescence line narrowing effect observed in the site-selective TRLFS measurements (c.f. Fig. 1b) indicates the presence of numerous, chemically similar sorption sites for Cm(III), which can be attributed to the amorphous to semi-crystalline structure of the C-S-H gel. In addition, C-S-H gel with a C/S ratio of 2.0 showed a co-incorporation of Cm(III) into portlandite. Leaching experiments showed that Cm(III) is not mobilized by solutions with increased salinities [6]. Results obtained by XRD showed that due to contact with carbonate-containing solutions part of the C-S-H gel is converted into calcite and aragonite (C/S 1.0) or calcite and vaterite (C/S 2.0). Site-selective TRLFS showed that Cm(III) was still incorporated in C-S-H gel and portlandite and in addition, partially incorporated in secondary CaCO3 phases.
Currently, the mobilization potential of low molecular weight organic ligands, which can be released due to leaching processes from cementitious materials or might occur as degradation products of polymeric cement additives, towards radionuclides retained by C-S-H gel is studied.
The utilization of Al-bearing additives in modern concrete and the usage of tobermorite as an ion exchanger justify the study of Al-containing C-S-H gel and tobermorite with regard to radionuclide retention. Thus, we investigated the Al and U(VI) incorporation into C-S-H phases and tobermorite at different Al/Si ratios (0.025−0.2) and synthesis temperatures (25°C or 200°C) using Al additives such as Al2O3 and Al(NO3)3. The obtained phases were characterized with solid state 27Al and 29Si NMR spectroscopy, TRLFS, XRD, IR and Raman spectroscopy. Subsequently, the synthesized U(VI) and Al containing samples (tobermorite and C-S-H) were exposed to leaching solutions (2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3) for 30 days to determine the U(VI) and Al release under degradation conditions. First results indicated a preferred synthesis of tobermorite over C-S-H at hydrothermal conditions while Al was found to enter the silica chain, cross-link the sheets of tobermorite and somewhat reduce the U(VI) retention capabilities of tobermorite in the presence of carbonate.