Where nowadays salt deposits are located in the subsurface of Northern and Central Germany, there was the Zechstein Sea more than 250 million years ago. Evaporation processes led to the formation of huge salt sediments that were covered by other sediments with higher rock density (e. g. sand, clay) in the course of the Earth’s history. Differences in density and the great thicknesses resulted in folding and the formation of salt domes over millions of years in Northern Germany.
Salt deposits have been used in Germany for various purposes for a long time. Potash salts are processed into fertiliser. Rock-salt serves as raw material for the chemical industry, as road salt, or as foodstuff. Cavities specially created in salt domes are used, among other things, as storage locations for natural gas and crude oil. Recently, the storage of hydrogen as an energy store in salt caverns has also been taken into consideration. Furthermore, the suitability of salt domes as repositories for radioactive waste is investigated. Especially in Germany, cavities already created by potash and rock salt mining are used for the underground disposal of chemical-toxic waste.
With the use of cavities in rock salt for the storage of hazardous waste, use is made of the special properties of the rock: It has, for example, a low permeability to liquids and a high thermal conductivity. In addition, salt has a property that scientists call "viscosity". Like a glacier, it can flow very slowly. The salt fills underground salt cavities, thus encloses the stored waste, thereby preventing the release of pollutants into groundwater and the environment.
Geological and technical barriers must complement each other
Salt rock, however, does not only have favourable properties. In particular, its solubility in water may cause problems. Underground waste disposal sites and repositories in salt rock must therefore be designed such that an adequate barrier function of the host rock salt is maintained in the future taking into account all conceivable geological developments.
The natural barrier function of the salt rock is supplemented by so-called technical barriers. The technical barriers are particularly important where man has disturbed the natural barriers. For example, the shafts sunk (built) in a salt dome, but also underground drifts and boreholes will therefore be backfilled and sealed after their use. For this purpose, these artificial cavities are filled, among other things, with so-called salt backfill, a treated, fine-grained salt. Together with the natural barrier, these measures are to ensure safe enclosure of the waste in the long term.
GRS patent for self-healing salt backfill
Since the late 1990s, GRS has been working on a "self-healing salt backfill” (selbstverheilender Salzversatz – SVV). The idea was to create a tight, stable and, above all, self-healing barrier against potential ingress of water into a
repository or an underground waste disposal site. GRS examined more than 200 recipes for the selection of suitable building materials. The most efficient building materials were those where ingressing water was bound in a mineral by chemical reactions. In 2004, the developments were advanced so far that GRS was awarded a patent for the SVV.
The developed building material consists of anhydrous magnesium sulfate. If a liquid enters - for example water or saline solution - the magnesium sulfate reacts. It absorbs the liquid, thereby doubles its volume and thus seals the backfilled cavity against the surrounding, natural salt rock. The resulting sealing body is nearly impermeable to water. Even pollutants that might have already been dissolved can be fixed in the self-healing salt backfill by the reaction and thus do no longer represent an environmental hazard.
Laboratory, pilot plant and in-situ investigations
The building material was first analysed in the geoscientific laboratory and its pilot plant. In small-scale experiments, the backfill material was filled into a column through which various solutions were pumped. The magnesium sulfate spontaneously reacted with the water of the solution. Already after a few days, the reaction was completed and the permeability of the backfill material was no longer different from the natural rock permeability.
In a next step, GRS performed so-called in-situ tests in salt mines. The researchers were interested in how the building material behaves under the local conditions (pressure, temperature). In the Teutschenthal disposal mine in Saxony-Anhalt, a horizontal borehole with a length of six meters and a diameter of 1.2 meters was filled with the self-healing salt backfill. In contact with a typical saline solution (water highly mineralised due to the saline environment), this building material sealed the borehole within a few weeks such that no more saline solution could enter.
Further investigations showed that the salt backfill that had reacted with saline solution exhibited mechanical properties very similar to those of natural rock salt. Long-term observations showed that the backfill that came into contact with water is transformed in kieserite over time. Kieserite is in chemical equilibrium with all surrounding salts and is therefore stable in the long term.
Find out more
GRS report: SVV2: Qualification of flow barriers in salt formations (in German)