RD&D-Programme 2004 - SKB

17.1.9 Hydrovariables
The hydrovariables are water flow, water pressure, gas flow and gas pressure. Initially it is
relevant to describe gas and water pressure. Flows do not occur initially in the buffer. At
emplacement of canister and buffer, the deposition holes will be kept drained and the repository
will be open to atmospheric pressure. This gives a gas pressure (air) of 1 atm (approx. 0.1 MPa)
and a water pressure of 0–0.1 MPa in the surrounding host rock. There will, however, be an
initial negative pore water pressure in the unsaturated bentonite blocks that drives the inward
transport of water. This pressure is on the order of 40 MPa.
17.1.10 Swelling pressure
The swelling pressure begins to build up when buffer and backfill come into contact with
external water, see sections 17.2.7 and 17.2.8. Initially there is no swelling pressure.
17.1.11 Montmorillonite composition
Conclusions in RD&D 2001 and its review
Further physical and chemical characterization was announced in RD&D 2001.
Newfound knowledge since RD&D 2001
A method for determining the average structural formula of montmorillonite has been developed
and has been used for the Wyoming and Milos materials. In the method, the montmorillonite is
separated from other material and ion-exchanged to sodium form, after which its metal content
is determined (ICP/AES). The cation exchange capacity (section 17.1.6) is determined for the
montmorillonite alone and used to calculate the total layer charge. A simple Excel macro has
been created to calculate the structure of the montmorillonite based on the basic formula for a
unit cell /17-4/.
Programme
A technique for determining the structural formula of the swelling mineral will be developed.
An essential point is to determine the valence state of the iron content, since this is decisive for
the charge distribution in determination of the structure. Furthermore, any changes due to e.g.
exposure to atmospheric oxygen and drying are of interest, since this leads to a change in the
total layer charge. Mössbauer analysis will be tried to begin with.
See also 17.1.6.
17.1.12 Pore water composition
Bentonite in nature contains water. After mining, the clay is dried and ground. At delivery the
water content is 12 percent maximum, according to the product specification. Prior to compaction
to blocks, distilled water is added to achieve a water content of 17 percent, which is
equivalent to a degree of saturation of 70–85 percent in the finished blocks. The concentration
of solutes in the pore water is thus dependent on the mineral composition of the buffer, as well
as on the water content at different times. Direct measurement of pressed-out pore water is not
suitable, due to the fact that a pressure-dependent ion equilibrium is developed, see section
17.2.15. When the buffer material is delivered, it will therefore be analyzed with respect to
constituent minerals and ions in the supernatant of dispersed material (aqueous solution on top
of slurried and centrifuged sample), which provides a good idea of the initial composition of the
pore water.
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