Self-sealing of an alkaline plume affected Boom Clay evidenced by microCT and hydraulic conductivity measurements

The self-sealing is an important property of clay host rocks considered for long-term geological disposal of high level radioactive waste and spent fuel. The self-sealing of Boom Clay has been demonstrated on artificially fractured fresh Boom Clay cores in the laboratory at ambient temperature and during heating and cooling cycles up to a temperature of 80 °C. As a geological disposal concept in Boom Clay includes components made of concrete, such as for instance the lining of galleries or the secondary packages for the wastes, it is important to evaluate if clay in contact with cementitious materials over a long period retains its self-sealing ability. For this purpose, we used Boom Clay samples that were percolated with young cement water (YCW, pH of 13.2, KOH dominated) and evolved cement water (ECW, pH of 12.5, Ca(OH)2 dominated) for about 15 years. After dismantling of the percolation cells, we created artificial planar fractures in the middle of the cores perpendicular to bedding planes. The self-sealing was followed by means of hydraulic conductivity measurements using constant volume permeameter cells and 3D imaging by micro computed tomography (micro-CT) scanning. In the course of the permeameter cell tests, the hydraulic conductivity decreases from 4.29  10-11 m/s as measured in fractured ECW affected Boom Clay sample to 1.99  10-12 m/s after self-sealing. In analogy, the initial (average) K of 4.84  10-11 m/s exponentially decreases with time to as low as 3.13  10-12 m/s in Boom Clay affected by YCW. The final K values are higher than those measured prior to artificial damage (1.67  10-12 m/s in the case of YCW and 7.35  10-13 m/s in the case of ECW). Although the relative decreases of the K values indicate self-sealing, the original hydraulic properties could not be fully recovered within the studied time frame. The comparison of µCT images after artificial fracturing and after a period of resaturation shows that fracture gaps tend to close and Boom Clay tend to attenuate density differences triggered by fracturing probably due to swelling behaviour of the clay minerals. The self-sealing behaviour documented here is in line with observations made by Chen et al. (2012) and Van Geet et al. (2008) on chemically undisturbed Boom Clay.