The self-sealing capacity of the Callovo-Oxfordian (COx) claystone contributes to the long-term safety of the deep geological disposal of radioactive waste. Resaturation of the excavation damaged zone promotes fracture closure through clay swelling, but this process can be impaired by the hyper-alkaline plumes released by cementitious materials, which raise the pore water pH up to 13.5 and increase its ionic strength. At the matrix scale, competing mechanisms govern the response. A higher pH promotes edge deprotonation and dispersion, whereas potassium fixation in the illite-smectite interlayers and compression of the diffuse double layer at high ionic strength reduce swelling. The outcome therefore depends on pH, ionic strength and the nature of the ions, not on pH alone. Water retention decreases as pH increases, and destructuration appears only under unconfined conditions, the specific surface area reaching about 55 m²/g at pH 7.5 against 21 m²/g at pH 13.5, confinement preserving the macroscopic integrity. Oedometer tests reveal the dependence on the chemical loading path, an alkaline-first path fixing potassium durably, whereas prior neutral hydration maintains a more open structure and limits its persistent fixation. Three independent approaches then converge on the same conclusion at the fracture scale. Multi-sensor tests show a slower and weaker development of pressures and displacement under alkaline hydration. X-ray micro-tomography shows a nearly complete closure after 370 days under synthetic pore water, with a self-sealing ratio above 99.6 %, against about 70 % under alkaline conditions. The intrinsic permeability drops within a few months under pore water but only after about 300 days under alkaline conditions, where it stabilizes around 400 days at about two orders of magnitude above the pore water value. The three methods point to the same mechanism, the loss of interlayer swelling preventing fracture closure, so that the matrix densifies while the fracture fails to seal. Under a hyper-alkaline perturbation, the process involves partial closure rather than effective self-sealing.