posted on 2007-03-15, 00:00authored byM. Vespa, R. Dähn, D. Grolimund, E. Wieland, A. M. Scheidegger
Cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposal
of hazardous and radioactive wastes. Cement is used to
condition and stabilize the waste materials and to construct
the engineered barrier systems (container, backfill and
liner materials) of repositories for radioactive waste. In this
study, Co uptake by hardened cement paste (HCP) has
been investigated with the aim of improving our understanding
of the immobilization process of heavy metals in cement
on the molecular level. X-ray-absorption spectroscopy (XAS)
on powder material (bulk-XAS) was used to determine
the local environment of Co in cement systems. Bulk-XAS
investigations were complemented with micro-beam
investigations to probe the inherent microscale heterogeneity
of cement by using μ-X-ray-fluorescence (μ-XRF) and
μ-XAS. μ-XRF was used to gain information on the spatial
heterogeneity of the Co distribution, whereas μ-XAS
was employed to determine the speciation of Co on the
microscale. The Co-doped HCP samples hydrated for time-scales from 1 hour up to 1 year were prepared under
normal atmosphere, to simulate similar conditions as for
waste packages. To investigate the role of oxygen, further
samples were prepared in the absence of oxygen. The
study showed that, for the samples prepared in air, Co(II)
is oxidized to Co(III) after 1 hour of hydration time.
Moreover, the relative amount of Co(III) increases with
increasing hydration time. The study further revealed that
Co(II) is predominately present as a Co−hydroxide-like
phase and/or Co−phyllosilicates, whereas Co(III) tends to
be incorporated into a CoOOH-like phase and/or Co−phyllomanganates. In contrast to samples prepared in air,
XAS experiments with samples prepared in the absence
of oxygen revealed solely the presence of Co(II). This finding
indicates that oxygen plays an important role for Co
oxidation in cement. Furthermore, the study suggests that
Co(III) species or Co(III)-containing phases should be
taken into account for an overall assessment of the Co
release from Co-containing cement-stabilized waste under
oxidizing conditions.