Mineralogical Changes of Cement and Bentonite Accompanied With Their Interactions
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Mineralogical Changes of Cement and Bentonite Accompanied With Their Interactions Seiichi Hoshino, Tetsuji Yamaguchi, Toshikatsu Maeda, Masayuki Mukai, Tadao Tanaka and Shinichi Nakayama Nuclear Safety Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
ABSTRACT Mineralogical changes of cement and bentonite accompanied with their interaction were experimentally studied by mixing granulated hardened cement paste and bentonite, and aging the mixture for 91 days at 50 C. Mineralogical changes of cement and bentonite were identified by XRD. Hydrated calcium-silicate phases (C-S-H), Ca(OH)2, ettringite and monosulfate were identified in the unaltered hardened cement. While Ca(OH)2 and monosulfate decreased with aging and disappeared after 91 days, calcite and katoite (Ca3Al2(SiO4)(OH)8) were formed concurrently. Montmorillonite, quartz (and/or chalcedony), clinoptilolite, plagioclase, calcite, analcime and pyrite were identified in the unaltered bentonite. The XRD pattern showed that diffraction intensities of these minerals decreased with aging. It seems that these primary minerals dissolved in the course of the alteration. C-S-H appeared in bentonite during the aging as secondary phases, indicating the participation of silicon dissolved from the bentonite and calcium from the cement formed the C-S-H. The formation of C-S-H that had been predicted by previous modeling studies was confirmed by the present experiments. In addition, diffusivity of tritiated water in mixed specimen with granulated hardened cement and bentonite was determined by a through-diffusion method. The effective diffusivity of tritiated water decreased with aging. The result suggests that the mass diffusivity in the interface of cement-bentonite system will decrease with their interactions. The results of the diffusion experiments are qualitatively consistent with the diffusivity change in cement-bentonite systems predicted by some computational studies. INTRODUCTION Understanding of cement-bentonite interactions is important in long-term performance assessments of engineered barriers in radioactive waste disposal system. The mineralogical changes in the course of the interactions of cement-bentonite or -clay system have been predicted through many computational model studies[1-3]. For example, we have shown that the hydrated calcium-silicate phases (refer to as “C-S-H”) are formed as secondary mineral phases in cement and in bentonite at their interface, and mass diffusivity in the interface of cement-bentonite system is lowered owing to decrease in porosity caused by the mineralogical changes[1]. The observations are common to some computational model studies[2]. On the other hand, some researchers have experimentally investigated the cement-bentonite or -clay interactions[4,5]. However, experimental knowledge to verify the computational analysis and to understand the phenomena is scarce. Experimental research to clarify the mineralogical changes and mass diffusivity changes takes a long time to yield detectable amounts of
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