Alteration of expandable clays by reaction with iron while being percolated by high brine solutions

• Smectites were altered in reaction with iron and percolated by high brine solutions.
• Alteration depended on the iron activity in the system and on the hydraulic regime.
• IS-ml-bearing clay were higher capacity to convert into new montmorillonitic layers.
• Si-cementation of fully developed MX80 montmorillonite can reduce the quality of bentonite for being barrier materials.

Bentonites are suitable candidates as buffer and backfill materials in HLW-repositories. A target of this research is to define the mineralogical alteration of bentonite caused by its interaction with iron powder, which simulates the contact of bentonite with a steel container. Compacted MX80 bentonite and Friedland clay (raw density of 1.6 g/cm3) were used as the initial materials for clay/iron interaction experiments (10 wt.% of iron) involving percolation with NaCl or Mg-rich IP21 solution. These experiments were conducted at 25 °C, 60 °C, and 90 °C and a suite of analytical techniques was applied to identify the mineralogical transformations. Smectite was the main phase and was fully expandable in all reaction products, though the octahedral and tetrahedral compositions were altered. “Illitization” was found as a main process of smectite alteration and was caused by the percolation component of these experiments. Smectitization occurred only when Fe-oxidation was considerable. The alteration of smectite was mainly driven by the high alkaline pH-value resulting from Fe-corrosion. Different interstratified phases like kaolinite–smectite–dioctahedral vermiculite, berthierine–saponite, chlorite–saponite–trioctahedral vermiculite, and cronstedtite–saponite–trioctahedral vermiculite interstratifications were identified as neo-formed phases which were formed in minor amounts. Cementation of particles by Fe- or Si-precipitation was assumed to reduce swelling pressure and permeability.