Mineralogical Changes of Cement and Bentonite Accompanied ith Their Interactions

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 (Ca 3 Al 2 (SiO 4 )(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.

Mineralogical changes and associated decrease in tritiated water diffusivity after alteration of cement–bentonite interfaces

“…The changes in the diffusivity were smaller than those observed by Hoshino et al (2009). Alteration of interfaces was more intense in the experiments using ground, granulated, hardened cement paste (Hoshino et al, 2009) than the present experiments.…”

“…The changes in the diffusivity were smaller than those observed by Hoshino et al (2009). Alteration of interfaces was more intense in the experiments using ground, granulated, hardened cement paste (Hoshino et al, 2009) than the present experiments. The difference can be explained by the presence of different reactive surface areas: 14.13 cm 2 for the cement carbonation tests, 7.07 cm 2 for the cement-bentonite interface tests and 104 cm 2 for the experiment using ground, granulated hardened cement paste (Hoshino et al, 2009).…”

“…The effective diffusivity decreased to 71 ± 10% of the initial value after 600 days. The reason for the decrease in the diffusivity observed in the present two types of experiments should be the same as that in the experiments for mixed specimens of ground, granulated, hardened cement paste and bentonite (Hoshino et al, 2009), because the mineralogical modifications observed (dissolution of Ca(OH) 2 and formation of calcite and katoite in hardened cement paste) were common to all the experiments.…”

“…where Δc is equal to a sum of Δc alt and Δc 0 . Because the diffusivity can be assumed to decrease to 20% of the initial value in the experiments in which ground, granulated hardened cement paste and bentonite reacted thoroughly (Hoshino et al, 2009), D e,alt was replaced by 0.2 D e,0 and equation 4 was simplified as:…”

“…Fernández et al, 2013;Bartier et al, 2013), reliable evidence has not been given for the clogging of the pores due to the formation of secondary minerals. Hoshino et al (2009) studied the diffusivity of tritiated water through mixed specimens of ground, granulated hardened cement paste and bentonite, and suggested that the diffusivity decreased with aging to 20% of the initial value over 180 days. The relationship between such significant change in diffusivity and those occurring at interfaces between bentonite and intact cementitious materials, however, has not been clarified.…”