RD&D-Programme 2004 - SKB

Newfound knowledge since RD&D 2001
The effect of total ion exchange has been investigated for bentonite from Milos (Ibeco Deponit
CA-N) and from Wyoming (MX-80). The materials were investigated in their natural form,
which is primarily calcium for the Milos material and sodium for the Wyoming material. Both
materials were subsequently purified of accessory minerals and ion-exchanged to both calcium
and sodium form. The mineral structure of the montmorillonite was calculated, and swelling
pressure and hydraulic conductivity were determined for different densities. The results show
that the two bentonites have more or less identical properties after ion exchange, and that the
difference in properties at buffer density is minimal, regardless of ionic species. The swelling
potential of the materials that were ion-exchanged to calcium form was, however, appreciably
lower, which leads to poorer buffer properties at low densities.
Geometric modelling of ion exchange in an MX-80 material as a consequence of exposure to
groundwater from Äspö has been done in the Lot project /17-18/.
Programme
Effects due to ion exchange will be studied during the coming period by means of laboratory
tests with other potential buffer materials, and with materials of lower quality. Geometric
modelling will continue with the ultimate goal of being able to describe the effect of the
water saturation phase and of temperature gradients. Laboratory tests will probably have to be
performed to determine equilibrium constants for compacted bentonite at elevated temperatures.
17.2.17 Montmorillonite transformation
The desirable physical properties of the buffer, mainly swelling pressure and low hydraulic
conductivity, are due to interaction between water and the montmorillonite in the bentonite.
This interaction is affected by changes in the ion concentration in the groundwater, see section
17.2.15, and by changes in the mineral structure of the montmorillonite. The mineralogical
stability of the montmorillonite is therefore of crucial importance for the performance of the
buffer.
Montmorillonite can be stable for hundreds of millions of years in its formation environment,
but changes in the geochemical environment can lead to a relatively rapid change of the mineral
structure. The ideal structural formula of montmorillonite can be written:
(Al (4 – x) Mg x ) (Si (8 – y) Al y ) O 20 (OH) 4 k z (x + y)/z n(H 2 O)
The sum of x and y can vary by definition between 0.4 and 1.2 units (charge per O 20 (OH 4 ) unit),
and x > y. A certain fraction of the aluminium (Al) can be regarded as exchanged for (Mg), and
a smaller fraction of silicon (Si) is exchanged for aluminium (Al). The exchange of trivalent
aluminium for divalent magnesium leads to a negative net charge in the mineral layers, which is
balanced by exchangeable cations (k).
Minerals occur in nature with a similar structure but great differences in layer charge. If the
charge (x+y) is close to zero (for example pyrophyllite), the interaction with water is insignificant,
which results in radically different properties compared with montmorillonite. An increase
of the layer charge, and thereby more balanced cations, leads to greater interaction with water.
If the layer charge increases enough, the ions may be bound more tightly to the mineral, resulting
in less interaction with water. The end mineral in such a series has a layer charge of 2 per
O 20 (OH 4 ) unit (mica). The typical properties of the montmorillonite are thus a consequence of a
medium-high layer charge.
The binding of charge-balanced ions is due to a high degree to the properties of the ion.
Potassium ions, for example, are bound at a lower layer charge than sodium ions, which are in
turn bound at a lower charge than calcium ions. Illite is a material with a layer charge between
218 RD&D-Programme 2004