Holmehus clay — A Tertiary smectitic clay of potential use for isolation of hazardous waste

The Danish Tertiary Holmehus clay belongs to the series of illite–smectite mixed-layer clays in northern Europe and has a potential to serve as an effective engineered barrier for isolating hazardous waste. Mineralogical specification was made of the components illite/smectite mixed-layer and dioctahedral vermiculite/smectite mixed-layer structures. The lower content of expandable phases than of smectite-rich clays like MX-80 gives somewhat higher hydraulic conductivity and somewhat lower swelling pressure. Both are deemed suitable for isolating canisters with high-level radioactive waste in deep repositories for densities at water saturation of at least 1900 kg/m3. Holmehus clay has fulfilled the demanded parameters for swelling pressure with > 0.3 MPa and hydraulic conductivity with < 10 E−12 m s− 1 under this density. The lower swelling pressure, the potential of being chemically more stable, and the high buffer capacity by providing dissolved Si makes Holmehus clay a candidate for use in both deep and near-surface repositories.