Observation of fracture sealing in high-strength and ultra-low-permeability concrete by micro-focus X-ray CT and SEM/EDX

Fracture sealing by precipitation is known to occur in high-strength and ultra-low-permeability concrete (HSULPC) immersed in water. Because a high ability to retard radionuclide migration is required for HSULPC, understanding both the sealing process and the composition of sealing deposits is important to identify optimum conditions for significant sealing. In this study, sealing of a macro-fractured HSULPC specimen with initial aperture of approximately 0.1 mm was investigated in simulated seawater over 49 days. The composition of sealing deposits at 49 days after immersion was clarified by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDX), and the progress of sealing during the 49 days was clarified by image analysis with micro-focus X-ray computed tomography (X-rayCT). Both the SEM/EDX and X-rayCT results showed that significant sealing was attained only near the outermost part of the specimen. The SEM/EDX results showed that a thin brucite layer formed on the entire specimen surface over which significant precipitation of calcium carbonate occurred and sealed the macro-fracture only near the outermost part of the specimen. The X-rayCT results indicated that the amount of sealing deposits in the macro-fracture ((P_mathrm{seal})) reached 70 % in the mostly sealed region at 49 days and the rate of change in (P_mathrm{seal}) became maximum ((3.7,\% hbox { day}^-1)) during 7–21 days after immersion, then decreased. In conclusion, the findings in this study represent an important clue in the search for optimum conditions to achieve fracture sealing in HSUPLC.