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Cement-based materials for use as sealants in underground waste storages must be erosion-resistant and chemically stable. Placement of highly radioactive waste (HLW) in boreholes may require that the rock is cement-grouted and stabilized by constructing concrete plugs. Where smectitic clay seals are in contact with concrete there is mutual degradation, and low-pH cement with inorganic superplasticizers, like talc, are recommended for preparing the concrete. This paper reviews our current state-of-knowledge concerning the grout and concrete sealing very deep boreholes (DBD) for purpose of high-level radioactive waste disposal. In this concept, the lower 2 km section of 4 km deep holes bored in crystalline rock could host waste-containers while the upper parts are sealed by dense clay and concrete. 1 1 At present, continued R&D will show if the holes can be only 2.5 km deep with HLW in the lower 2 km parts. The parts of such a hole that intersect fracture-poor rock are sealed with dense expandable clay while concrete is cast where fracture zones are intersected. The paper summarizes the available experimental results concerning the performance of grouts and concrete with talc as superplasticizer in contact with smectitic clay. [ABSTRACT FROM AUTHOR]