Precipitation of chlorite-like structures during OPC porewater diffusion through compacted bentonite at 90 °C

• Alkaline diffusion experiments were performed in compacted bentonite at 90 °C.
• K–Na–OH fluid alters the mineralogy of bentonite at the alkaline interface.
• Formation of chlorite-like phases was detected.
• They appeared as brucite monolayers intercalated in the montmorillonite interlayer.
• Results provide confidence to include clinochlore or sudoite in modeling studies.

The reactivity between concrete and bentonite is a key issue in the performance of a deep geological repository for radioactive waste. The long-term reaction of alkaline solutions representative of early and late stages of concrete degradation (K–Na–OH, pH 13.5, and Ca–OH, pH 12.5, synthetic solutions) was studied by diffusion experiments with compacted bentonite columns at 90 °C. Relevant mineralogical alterations were produced with the K–Na–OH solution and consisted mainly of the precipitation of a non-expandable 14.4–7.4 Å chlorite-like phase at the alkaline interface and through the first 2–3 mm. The chlorite-like phase, a non-swelling layer silicate, was characterized as a brucite–montmorillonite complex produced by a brucite monolayer intercalation in the montmorillonite interlayer and its predicted formation would thus impact upon the safety functions of the bentonite buffer. The precipitation of discrete brucite was not significant, being this, the main difference found in comparison with previous studies at temperatures lower than 90 °C. The high intensity of the 7.4 Å X-ray diffraction peak, in the absence of iron components, and the rise of new hk0 trioctahedral bands, determined by X-ray diffraction and electron diffraction on plate-like mineral aggregates, indicate the possible formation of new Mg-layer silicates, although they were not clearly defined. In contrast to K–Na–OH experiments, mineralogical alterations with the Ca–OH solution were almost non-existent, including insignificant changes in the Ca/Mg cation exchange distribution. The buffer capacity of the bentonite against the migration of alkaline fluids is driven by mineralogical reactions affecting a few millimeters thickness from the alkaline solution–bentonite interface. The nature of these reactions is critically affected by the temperature change within the expected designed conditions for radioactive waste repositories.