High-strength and ultra-low-permeability concrete (HSULPC) is thought to be useful as a radioactive waste package. Thus, a high confining ability is desirable.For cementitious materials, sealing of cracks may occur in water due to the precipitation of calcium compounds. This can affect the confining ability. In this study, the sealing of a crack in HSULPC in water was investigated using microfocus X-ray computed tomography (CT). The sealing by precipitation occurred only around the end of the specimen. Sealed regions of the crack were identified using three-dimensional image registration and CT image subtraction of images obtained for the specimen before and after it was immersed in water to evaluate temporal changes of the sealing deposits in the crack. The sealing deposits increased as the HSULPC specimen was kept in water longer. It was concluded that cracks in HSULPC in water are sealed by precipitation.
Fracture sealing by precipitation is known to occur in high-strength and ultra-lowpermeability 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 seal ) reached 70 % in the mostly sealed region at 49 days and the rate of change in P seal became maximum (3.7 % day -1 ) during 7-21 days after immersion, then decreased. In conclusion, Revised_Manuscript Click here to download Manuscript: IJF_manuscript_FRAC_Rev1.doc Click here to view linked References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 the findings in this study represent an important clue in the search for optimum conditions to achieve fracture sealing in HSUPLC.
It is very important to predict alterations in the concrete used for fabricating disposal containers for radioactive waste. Therefore, it is necessary to understand the alteration of cementitious materials caused by calcium leaching when they are in contact with ground water in the long term. To evaluate the long-term transport characteristics of cementitious materials, the microstructural behavior of these materials should be considered. However, many predictive models of transport characteristics focus on the pore structure, while only few such models consider both, the spatial distribution of calcium silicate hydrate (C-S-H), portlandite, and the pore spaces. This study focused on the spatial distribution of these cement phases. The auto-correlation function of each phase of cementitious materials was calculated from two-dimensional backscattered electron imaging, and the three-dimensional spatial image of the cementitious material was produced using these auto-correlation functions. An attempt was made to estimate the diffusion coefficient of chloride from the three-dimensional spatial image. The estimated diffusion coefficient of the altered sample from the three-dimensional spatial image was found to be comparable to the measured value. This demonstrated that it is possible to predict the diffusion coefficient of the altered cement paste by using the proposed model.
For cementitious composites and materials, the sealing of fractures can occur in water by the precipitation of calcium compounds. In this study, the sealing behavior in a macro-fractured high-strength and ultra-low-permeability concrete (HSULPC) specimen was investigated in simulated seawater using micro-focus X-ray computed tomography (CT). In particular, the influence of fracture width (0.10 and 0.25 mm) on fracture sealing was investigated. Precipitation occurred mainly at the outermost parts of the fractured surface of the specimen for both fracture widths. While significant sealing was observed for the fracture width of 0.10 mm, sealing was not attained for the fracture width of 0.25 mm within the observation period (49 days). Examination of the sealed regions on the macro-fracture was performed using a three-dimensional image registration technique and applying image subtraction between the CT images of the HSULPC specimen before and after maintaining the specimen in simulated seawater. The temporal change of the sealing deposits for the fracture width of 0.10 mm was much larger than that for the fracture width of 0.25 mm. Therefore, it is concluded that the sealability of the fracture in the HSULPC is affected by the fracture width.
Cement-based materials used at radioactive waste disposal sites are required to possess long-term stability. However, when these materials come in contact with groundwater, calcium leaching from the solid occurs, and the material becomes porous. The use of mineral admixtures is recommended to minimize porosity. However, few studies have focused on the diffusion performance of cement-based materials blended with mineral admixtures after leaching. Therefore, in this study, the diffusion performance of such materials using blended cement after leaching was evaluated. It was found that the diffusion coefficient of the blended cement increased with leaching, and when leaching progressed considerably, the diffusion coefficient of the blended cement was close to that of ordinary Portland cement. Furthermore, the diffusion coefficient after leaching demonstrated good correlation with the pore volume when the pore diameter was 50 nm or larger.
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