Interaction of clay and concrete relevant to the deep disposal of high-level radioactive waste

• Physical and chemical interaction of low pH concrete and dense clay has been investigated.
• XRD and SEM reveal a reduction in cation exchange at higher temperatures.
• Dissolution of illite/smectite is evident when the temperature is increased.
• Increase in strength of both clay and concrete takes place when the temperature is increased.
• The strength of concrete heated from 20 to 100 and 150 °C increases by 100%.

A concept for the disposal of highly radioactive waste at depth in the Earth's crust using very deep bore-holes requires that the upper 2 km's of the 800 mm diameter, steeply drilled holes, be effectively sealed. This can be achieved by using dense smectitic clay where the rock is weakly fractured and strengthening with concrete when fracture zones are encountered. Earlier investigations have shown that chemical reactions between the clay and concrete can be expected both in the upper part where the temperature is lower than 90 °C and in the deeper section where the temperature reaches up to 150 °C. To study further this interaction, hydrothermal experiments were conducted using mixed-layer (illite/smectite) Holmehus clay and a low pH slag-based concrete placed in contact under isothermal conditions at 21 °C, 100 °C and 150 °C for a period of 8 weeks. The sample sets, which consisted of two clay discs separated by concrete cast on the lower clay disc, were extracted in undisturbed form and exposed to uniaxial pressure for measuring the compressive strength at successively increasing pressures. Compression tests under enhanced thermal conditions led to strengthening of both the clay and concrete. X-ray diffraction and electron microscopy analysis of the material revealed an increasing degree of cation exchange at higher temperatures with the cement, whereby Ca replaced Na in the interlayer sites of smectite layers. Dissolution of illite/smectite was also evident occurring at enhanced temperatures, with a decrease in K, Mg and Fe content with advanced alteration. The enhanced strength of clay can be partly attributed to the precipitation of cement phases from circulating fluids, including precipitation of gypsum.