Journal of Earth Sciences and Geotechnical Engineering

Mechanisms of Mineralogical Alteration of Dioctahedral Smectites in Contact with Water – A Review

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• Abstract

   

  Bentonites are a suitable buffer- and backfill-material in the multibarrier-concept for high level radioactive waste - repositories. The geotechnical performance of engineered barriers using bentonite is to predict for the next 1 million of years. This review of experiments and phase-analytical investigations on more than 25 different bentonites and clays lets recognize few mechanisms in mineralogical alteration processes of smectite, gives first indications as possible impact on geotechnical properties of these bentonites and offers further screening tools for bentonites concerning their rate of alteration (slow or fast reacting phases).

  Each bentonite and clay are characterized by a specific rate of alteration (sleeper or sprinter). This specific rate is determined by the geological background during the bentonite formation (parent rocks, marine or freshwater environment, heat impact during smectite formation). The geological background is controlling the active mechanisms of alteration by dissolution/precipitation and solid state-transformation. The composition of octahedral sheet (Al-rich bentonites have the lowest sheet stress by ion radius) and two interlayer mechanisms protect smectite aggregates (a: Na+-cations and/or b: Ca2+- and Mg2+-cations are responsible for stabilization). The reported mechanisms help to understand the variable response of bentonites in mineralogical and geotechnical performance in different zones in a repository like at interfaces “Canister / Bentonite”, “Host rock / Bentonite” as well as partially also for “Concrete /Bentonite”.