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Figure 4: SEM image of the alteration zone from a cement-bentonite diffusion experiment showing
the presence of brucite and tri-octahedral Mg-smectite [6].
4.3 Saline groundwaters
The interaction of waters of different salinities with MX-80 bentonite has minor effects on the mineralogical
composition of bentonite [8], with only minor amounts of montmorillonite being dissolved
leading to the precipitation of pyrophyllite. However, the most relevant changes occur in the
octahedral layer of smectite, where Mg is replaced by Al. This replacement has two effects: 1) a
decrease of the interlayer charge; and 2) an increase in Mg in solution leads to the replacement of
Na by Mg in the interlayer space. The swelling pressure of bentonite is expected to depend on the
ionic strength of pore water, as already identified by the results of other experiments prior to NF-
PRO. This was also recognised in NF-PRO, where experiments conducted with MX-80 bentonite
showed that swelling pressure decreases as ionic strength increases [8]. Moreover, a clear correlation
exists between interlayer charge and swelling pressure. In pure water, swelling pressure increases
with increasing interlayer charge, whereas in low salinity solutions, swelling pressure decreases
with interlayer charge (water uptake capacity is lower for multivalent cations than for Na).
Finally, in high salinity solutions, swelling pressure is subject to an initial increase, and then a decrease.
This different behaviour depends upon specific mineralogical transformations.
5. Radionuclide Transport
An experimental programme was designed in order to contribute to an improved understanding of
the basic processes underlying diffusion/retention of charged and uncharged radionuclides in compacted
bentonite. This programme included studies on pure clay minerals (montmorillonite, illite,
kaolinite) and bentonite in both dispersed and compacted form. From the behaviour of the radionuclides
in the pure clay systems, the behaviour in the complex systems was evaluated (‘bottom-up
approach’).
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