Deposition of Calcium-Silicate-Hydrate Gel on Rough Surface of Granite from Calcium-rich Highly Alkaline Plume
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Deposition of Calcium-Silicate-Hydrate Gel on Rough Surface of Granite from Calcium-rich Highly Alkaline Plume Yuichi Niibori, Kyo Komatsu and Hitoshi Mimura Dept of Quantum Science and Energy Engineering, Tohoku University, 6-6-01-2, Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8589 Japan. ABSTRACT Cement-based materials used in the construction of the repository for high/low level radioactive wastes may produce a highly alkaline calcium-rich groundwater (plume). The Ca ions react with soluble silicic acid, depositing calcium-silicate-hydrate (CSH) gel on the surfaces of the groundwater flow-paths and decreasing the permeability of the bedrock. Such a decrement of permeability may play a role in retarding the migration of radionuclides. In this study, the deposition behavior in a fracture was experimentally examined by using a micro flow-cell consisting of silicon plate (including a slit (60 mm×5 mm, or 60 mm×2 mm)) and granite-chip. The initial equivalent-aperture based on the square law was estimated in the range of 26 μm to 45 μm from the flow test of pure water. In the experiments, a Ca(OH)2 solution of 6.36 mM (pH: 12.2 to12.5, including NaOH) was continuously injected into the flow system at a constant flow rate of 1 or 2 ml/h. The solution flowed on the surface of the granite-chip. In this study, we prepared two kinds of chips that differed in the treatment of the surface. One chip was roughly ground with #2000 sandpaper (hereinafter referred to as rough surface) and another was polished to mirror-like surface. As a result, on the rough surface the deposits of CSH gel appeared along flow-channels across mineral grain-boundaries, while the deposits on the mirror-like surface were relatively uniform. Furthermore, the permeability in the case of rough surface became smaller than that in the case of mirror-like surface, showing the repeats of rapid decrement and increment due to the relatively large roughness of the surface. In order to estimate the decrement degrees of permeability, a simple, one-dimensional mathematical model is proposed in this study. INTRODUCTION The groundwater in the surroundings of the repository will include Na, K and Ca ions due to the partial dissolution of cementitious material used in the construction of the repository of radioactive wastes (e.g., [1-5]). Such a highly alkaline groundwater may dissolve some silica or silicate minerals and produce calcium-silicate-hydrate (CSH) with relatively low Ca/Si mol-ratio as a secondary mineral, reducing the permeability of the flow-paths around the repository. In fact, through HPF (Hyperalkaline Plume in Fractured rock) project in Grimsel Test Site, Mäder et al. [4] have reported the decrease of permeability resulted from continuous injection of the high alkaline solution of Na-Ca-OH system (pH 13.2). Komatsu et al. [5] have pointed out by the inroom experiment of packed beds that the change of permeability is caused by the deposits of CSH gels. However, so far, the details of the deposition of the CSH gels on the fracture surface have not been su
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