Self-sealing capacity of boom clay affected by an alkaline plume: Evidences from high resolution X-ray computed tomography and hydraulic conductivity measurements

This study aims to evaluate the self-sealing capacity of the Boom Clay when perturbed by an alkaline plume, by investigating the long-term evolution of hydraulic conductivity and mineralogical changes in fractured Boom Clay upon interaction with highly alkaline solutions. In a first step, percolation experiments were conducted on undisturbed clay samples using Young Cement Water (YCW, pH of 13.5) and Evolved Cement Water (ECW, pH of 12.6), both parallel and perpendicular to bedding planes. Hydraulic conductivity (K) in these alkaline environments were estimated, confirming the previous studies, with higher horizontal than vertical hydraulic conductivity as a result of the natural anisotropy of the Boom Clay. In a second step, clay samples were fractured. All samples experienced a significant increase in K, followed by a progressive decrease, indicative of an efficient self-sealing. This self-sealing process was visualized using X-ray micro computed tomography and spectral micro computed tomography analysis. In addition, mineralogical analyses, including specific surface area and pore size distributions from N₂-physisorption, (Quantitative) X-ray diffraction and Fourier Transform Infrared spectroscopy analysis were conducted. They revealed partial dissolution of smectite in YCW environments but no significant mineral alteration in ECW-percolated samples. N₂-physisorption experiments of the post-mortem samples indicated decrease of the specific surface area and concomitant decrease in the microporosity in all studied cases. Though the decrease in the specific surface area and microporosity can be explained by partial dissolution of smectite in the YCW environment, the shift of the pore size distribution towards larger pores can be linked to relative density changes in the vicinity of the fractures and overall increase in hydraulic conductivity. The results confirmed that Boom Clay retains its self-sealing capacity, even after prolonged exposure to high-pH conditions. This study provides valuable insights into the hydro-mechanical and mineralogical response of Boom Clay, essential for assessing its long-term behaviour as a geological barrier.