RD&D-Programme 2004 - SKB

observation is that isosaccharinic acid itself is strongly sorbed in concrete, which mitigates the
effect which complexation with ISA can have on the mobility of the radionuclides. The effect of
additives in concrete on radionuclide sorption has also been studied at the Paul Scherrer Institute
and at Linköping University /25-9, 25-10/.
Degradation of concrete
The waste will be encased to a large extent in concrete containers. The enclosure with its
backfill also consists of concrete. This concrete is an important barrier in the near field. Even
though we don’t believe that the concrete can prevent groundwater from entering the repository
after closure, it will retain radionuclides, partly by virtue of its low hydraulic conductivity and
partly by sorbing dissolved radionuclides. It is therefore important to understand how concrete
ages. It is not so critical if some new cracks should form with time. We assume that the structure
may have some cracks from the start. The hydraulic cage will work nevertheless, and the
predominant transport mechanism will be diffusion. However, it is important that the structure
does not collapse entirely and that its chemical properties are not completely altered during the
time the repository’s barriers are important for safety.
Water in contact with concrete will dissolve some of the components contained in the concrete.
This could lead to degraded properties in the long term. Models exist for calculating leaching
from concrete /25-11/. An urgent task has been to test these models. Samples of old concrete
that has been in a water-saturated environment have been collected and investigated for this
purpose /25-12, 25-13/. Although none of the samples is older than 90 years, it is nevertheless
possible to follow the changes by means of microscopic examination.
Cement analogue
Hyperalkaline water (pH about 12) and primary and secondary cement minerals present
naturally in certain areas in Jordan are being investigated in the Maqarin Project. The
investigations have been under way since 1990 and the project is now into its fourth phase.
Several organizations in different countries support the project. The final report for phase III
was published in 1998 /25-14/, and the final report for Phase IV is planned to be finished in late
2004. The following points are examples of valuable observations:
• Minerals formed in cement paste remain for more than 100,000 years, provided that the
hyperalkaline conditions persist.
• Hyperalkaline water reacts with the minerals in the rock. Secondary minerals are thereby
formed, which tend to clog fractures and prevent the flow of water.
• The surfaces of the water-bearing fractures react with hyperalkaline water, but the porosity
in adjacent rock is still available for inward diffusion of solutes (matrix diffusion).
• The colloid concentrations are low (like in ordinary deep groundwater) and we see no
tendency towards production of colloids.
• Solubilities of different trace metals can be calculated at high pHs with the aid of thermodynamic
constants, but the uncertainties are greater than otherwise. In general, however,
the calculated values are conservative.
• Microbes can grow and be active under aerobic and anaerobic hyperalkaline conditions,
but their growth and activity are very low.
• In Kushaym Matruk (in central Jordan), the illite and smectite minerals are more amorphous,
i.e. they have a lower swelling capacity, close to the cement limit. High pH has spread a few
metres into the sedimentary matrix by diffusion.
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