Microstructural evolution of compacted granular salt: insights from 40-year-old backfill at a former potash mine (Sigmundshall, northern Germany)

Abstract

Compacted granular salt backfill is widely regarded as the most favorable geotechnical barrier for sealing a radioactive waste repository within a rock salt formation. However, the reduction of salt backfill porosity and permeability during compaction by slowly converging cavity walls is still a matter of on-going research, both in laboratory and underground experiments, as well as in computational forecasting. Here, we present an in-depth microstructural analysis of a dense, formerly deployed salt backfill material, recovered from the decommissioned salt mine Sigmundshall, Bokeloh, Germany. The backfill compacted over 40 years, resulting in as little as 1 % porosity (+4/-1 %). Some differences are inevitable compared to a potential future backfill emplaced in a radioactive repository, notably in grain size, moisture content and backfill height (178 m vs. ∼ 5 m). However, this valuable “natural laboratory” sampling opportunity has allowed microstructural evidence to be sought for the deformation mechanisms that control salt backfill compaction under in-situ deployment conditions and on timescales that cannot be achieved in laboratory tests. For the present example of grain sizes in the range of 50 μm to 3 mm, our results show that more or less complete densification of granular salt is feasible in a timeframe of decades (<40 y). Pressure solution is likely the main deformation mechanism along with limited cataclasis, which presumably occurred only in the early stages during/after emplacement. This conclusion is evident from tight, indenting, truncating and interpenetrating grain boundaries, as well as from the fact that almost all grains appear to be substructure-free, despite limited signs of recrystallization. The absence of intra-crystalline deformation indicators excludes dislocation creep as a compaction-contributing mechanism, which, on the other hand, is known to occur in many laboratory-based compaction tests. We outline the impact of this difference on the long-term in-situ compaction under repository conditions. Note: The Sigmundshall mine is not considered as a future repository for radioactive waste.