RD&D-Programme 2004 - SKB

those of montmorillonite and mica. Potassium ions are, for example, bound to some extent in an
illite clay, but not sodium or calcium ions. Binding of multivalent ions, usually iron or magnesium,
can also take place via a bridge of hydroxide, which gives a chlorite mineral.
Thus, in order for a montmorillonite to be transformed towards illite or chlorite, there must be
an increase in the layer charge, which can be brought about by:
• release of silicon,
• exchange of aluminium,
• change of valence in the structure (iron).
In the event of a transformation, secondary processes may be of importance for the performance
of the buffer. Release of silicon would probably lead to precipitations of different silicon
minerals, which can affect the rheological properties of the buffer (cementation).
Conclusions in RD&D 2001 and its review
Kasam considers that it would be valuable if SKB, in connection with its studies of other
compositions, also investigated how a limited illitization and cementation affects parameters
of importance for the buffer.
In SKI’s opinion, the mineral transformations should be studied in the laboratory, and
cementation during the unsaturated phase is an important process.
Newfound knowledge since RD&D 2001
Extensive laboratory experiments have been conducted to study the stability of the buffer at
very high pHs in the groundwater. The investigation was a part of the EU project Ecoclay,
whose purpose was to shed light on the effects of cement use in a final repository. The results
show that the montmorillonite is greatly affected by cement pore water from fresh Portland
cement, i.e. at pHs between 13 and 14. A heavy release of silicon was measured, causing an
increase in the layer charge and a loss of density in the bulk material. At lower pHs, the results
showed a significant difference between highly compacted bentonite and bentonite dispersed in
solution. A preliminary interpretation is that a favourable ion equilibrium is established between
highly compacted bentonite and the pH solution, see section 17.2.15.
Programme
The results of the Ecoclay project are in the process of being evaluated and published. The
results are judged to be very valuable. Further modelling and verification tests will probably be
carried out.
Laboratory tests with natural materials of lower quality than commercial bentonites are planned.
The investigations involve mineralogical description and determination of physical properties of
e.g. natural materials with different degrees of transformation, for example potassium bentonite
from Röstånga, Skåne.
SKB plans to study changes in the buffer properties as a result of reactions with iron in
bentonite with regard to two aspects:
• Valence change in the octahedral layer in the mineral structure will be studied by means
of diffusion tests with reducing and oxidizing solutions.
• Binding of iron as a charge-compensating counterion and/or as a central ion for a
hydroxide bridge between the mineral layers; this may lead to an increased corrosion
rate of surrounding iron.
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