RD&D-Programme 2004 - SKB

Newfound knowledge since RD&D 2001
A fundamental requirement on the buffer is that it must have swelling and thereby sealing
properties. Montmorillonite with sodium as a counterion has a very high swelling potential
and is available in sufficient workable quantities to be a realistic buffer alternative. SKB has
therefore used such a bentonite grade from American Colloid Co (Wyoming) with the product
name MX-80 as a reference material for a long time. A comprehensive programme for studying
alternative buffer materials has been initiated and largely carried out. The main purpose is to
correlate physical and chemical properties to fundamental mineralogical properties. The project
includes:
• Products with a similar content of montmorillonite and a similar counterion distribution as
in MX-80, since these materials can be assumed to have equivalent sealing properties.
• Materials that have a high content of montmorillonite and are dominated by divalent ions.
The high densities of the buffer theoretically entail that such materials have sufficient
swellability. Divalent counterions (such as calcium) are furthermore expected to provide
advantages at low ionic strength in the groundwater.
• Materials that contain other swelling minerals.
• Materials that have a lower content of swelling minerals, principally for possible use as
tunnel backfill.
A large number of commercial bentonites from major producers have been investigated with
respect to detailed mineralogy and swelling properties. The investigations have so far been
performed on the reference material MX-80 from Wyoming (American Colloid), four samples
from India (Ashapura), and one sample from Greece (Silver & Baryte). All materials have a
high content of montmorillonite and represent different ratios between monovalent and divalent
cations. At present, three samples from Denmark (NCC) and three samples from the Czech
Republic (from a university in Prague) are being studied. Four of the samples are examples
of materials with a lower montmorillonite content.
It can be concluded from these investigations that materials with a high montmorillonite
content, similar charge distribution and sodium as a counterion exhibit negligible differences
in physical properties, regardless of where they come from. Calcium as a counterion entails a
lower swelling potential, but its sealing properties are equivalent to those of sodium bentonite
under buffer conditions. In terms of swelling properties, several of the investigated bentonites
are suitable as buffer materials without necessitating any changes in the dimensions of the
buffer. Other aspects, such as long-term stability and influence of accessory minerals, need to
be further investigated, however.
Programme
Ongoing laboratory investigations will continue for the purpose of correlating desirable physical
and chemical properties to fundamental mineralogical properties, for example relationships
between swelling pressure on the one hand and the clay mineral’s cation exchange capacity,
ionic species (sodium or calcium) and charge distribution on the other. The goal is to describe
crucial relationships so well that procurement of buffer material can be optimized with respect
to safety, availability and cost.
Field tests will be performed with materials shown by the laboratory investigations to be
potential buffer materials. The tests are planned to be conducted in accordance with the same
experimental principles and on the same scale as the ongoing Lot tests in the Äspö HRL. The
purpose is to verify the results from the laboratory investigations under more realistic conditions
with respect to temperature, scale and geometric conditions, and to discover any handling
problems. The results of investigations conducted to date show that materials from Milos,
Greece (S&B Industrial Minerals SA) and Buj, India (Ashapura Minechem Ltd) are suitable for
inclusion in the test series.
196 RD&D-Programme 2004