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. Repositories for high-level radioactive waste in geological formations requireknowledge on thermal, mechanical and fluid transport properties of the wholerepository system, including the engineered barriers and backfillmaterials. For about 30 years, crushed salt has been considered the mostsuitable geotechnical barrier material to backfill cavities and encapsulateradioactive waste in rock salt repository sites (e.g., Czaikowski et al.,2020). Over time, when the surrounding cavity walls converge by the creep ofsalt, it can become strongly compacted and safely encapsulates radioactivewaste from any fluid flow. Hence, crushed salt has been characterized indetail for its physical material properties and its response to environmentalcontrols (stress, temperature and moisture). This characterisation provides abasis for long-term numerical simulations (e.g., Liu et al., 2018), whichverify so-called safety cases in radioactive waste disposal. Displacement-controlled oedometric compaction tests mimic the long-term insitu behaviour of crushed salt. The tests show that it can be compacted to astate comprising physical rock properties similar to natural rock salt. Ingeneral, compaction is easier with an increase in humidity and temperature(e.g., Stührenberg, 2007; Kröhn, et al., 2017). Triaxial test seriesaddress the compactions' response to differing confining pressures and help toidentify generalized constitutive equations for crushed salt. Both BGRprocedures, the oedometric and the triaxial compaction, are verified by theGerman accreditation body (DAkkS). Figure 1 illustrates the history of oedometric tests at the BGR laboratorysince 1993, which examined crushed salt from various origins and differingtemperature conditions. Most tests focused on material from the Asse mine,revealing the compactions' response to the materials' humidity and to brineflow. Moreover, systematic test series with synthetic grain size distributionsand bentonite additives provided a basis for barrier material design. Morerecent tests on bedded salt formations (e.g., Teutschenthal and Sondershausenmines) allow the differentiation from characteristics from domal salt deposits(e.g. Gorleben). The current research continues the history of oedometric and triaxial tests,but has a new focus on late compaction stages with marginal remainingporosities (&lt;5 %). The approach of systematic material characterizationunder best-controlled conditions essentially benefits from the internationalresearch collaboration in the KOMPASS project (Czaikowski et al., 2020). Theaim of its current phase two is to synthetically generate, identify andquantify dominant grain-scale deformation processes in response to changes inenvironmental controls. Subsequently, these laboratory results will beembedded in numerical models on the long-term in situ rheology of crushedsalt.

Over 30 years of research on crushed salt as a barrier material: fundamental findings and open questions