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Development of an analytical technique for the detection of alteration minerals formed in bentonite by reaction with alkaline solutions
Physics and Chemistry of the Earth 32 (2007) 311–319 www.elsevier.com/locate/pce
Development of an analytical technique for the detection of alteration minerals formed in bentonite by reaction with alkaline solutions H. Sakamoto b
a,1
, M. Shibata
a,1
, H. Owada
b,2
, M. Kaneko
b,3
, Y. Kuno
b,*
, H. Asano
b,4
a Taiheiyo Consultant Corporation, 2-4-2, Osaku, Sakura-shi, Chiba Pref., 285-8655, Japan Radioactive Waste Management Funding and Research Center, No. 15 Mori-bldg., 2-8-10, Toranomon, Minato-ku, Tokyo 105-0001, Japan
Received 22 April 2005; received in revised form 4 August 2005; accepted 25 October 2005 Available online 7 February 2007
Abstract A multibarrier system consisting of cement-based backfill, structures and support materials, and a bentonite-based buffer material has been studied for the TRU waste disposal concept being developed in Japan, the aim being to restrict the migration of radionuclides. Concern regarding bentonite-based materials in this disposal environment relates to long-term alteration under hyper-alkaline conditions due to the presence of cementitious materials. In tests simulating the interaction between bentonite and cement, formation of secondary minerals due to alteration reactions under the conditions expected for geological disposal of TRU waste (equilibrated water with cement at low liquid/solid ratio) has not been observed, although alteration was observed under extremely hyper-alkaline conditions with high temperatures. This was considered to be due to the fact that analysis of C–S–H gel formed at the interface as a secondary mineral was difficult using XRD, because of its low crystallinity and low content. This paper describes an analytical technique for the characterization of C–S–H gel using a heavy liquid separation method which separates C–S–H gel from Kunigel V1 bentonite (bentonite produced in Japan) based on the difference in specific gravity between the crystalline minerals constituting Kunigel V1 and the secondary C–S–H gel. For development of C–S–H gel separation methods, simulated alteration samples were prepared by mixing 990 mg of unaltered Kunigel V1 and 10 mg of C–S–H gel synthesized using pure chemicals at a ratio of Ca/Si = 1.2. The simulated alteration samples were dispersed in bromoform–methanol mixtures with specific gravities ranging from 2.00 to 2.57 g/cm3 and subjected to centrifuge separation to recover the light density fraction. Subsequent XRD analysis to identify the minerals was complemented by dissolution in 0.6 N hydrochloric acid to measure the Ca and Si contents. The primary peak (2h = 29.4, Cu Ka) and secondary peaks (2h = 32.1 and 50.1, Cu Ka) of the C–S–H gel, which could not be distinguished before the heavy liquid separation, were clearly identified by XRD after separation. The result of the analyses of the light density fraction indicates highest recovery of C–S–H gel and least inclusion of bentonite for separation using heavy liquid with a specific gravity of 2.10 g/cm3. The traces of bentonite minerals included in the suspension were identified to be montmorillonite, quartz, clinoptilolite, and calcite. The separation technique was also tested for Ca-bentonite prepared by passing a calcium hydroxide solution through a bentonite (Kunigel V1)-silica sand mixture. The results indicated that the technique would also be applicable to separation of C–S–H gel from Ca-bentonite. 2007 Published by Elsevier Ltd.
*
Corresponding author. Tel.: +81 3 3504 1505; fax: +81 3 3504 1297. E-mail addresses: [email protected] (H. Sakamoto), [email protected] (M. Shibata), [email protected] (H. Owada), [email protected] (M. Kaneko), [email protected] (Y. Kuno), [email protected] (H. Asano). 1 Tel.: +81 43 498 3858; fax: +81 43 498 3859. 2 Tel.: +81 3 3504 1582; fax: +81 3 3504 1297. 3 Tel.: +81 3 3504 1573; fax: +81 3 3504 1297. 4 Tel.: +81 3 3504 1485; fax: +81 3 3504 1297. 1474-7065/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.pce.2005.10.004
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Keywords: Radioactive waste disposal; Bentonite; Alteration; Cement/bentonite interaction; C–S–H gel; Analytical method
1. Introduction
Table 1 Minerals constituting Kunigel V1 and their specific gravities
A multibarrier system consisting of cement-based backfill materials, structures and support and a bentonite-based buffer material has been studied for the TRU waste5 disposal concept being developed in Japan. The expected functions of the bentonite buffer material include retardation of radionuclides during migration, restriction of flow of groundwater and filtration of colloids. One concern about this buffer material under disposal conditions is potential long-term alteration by hyper-alkaline fluids leached from cementitious material (Savage et al., 1992; Kubo et al., 1998). Long-term alteration of bentonite-based materials due to reaction with alkaline solutions has been investigated using analytical approaches based on coupled mass transport and chemical reaction studies, which suggest potentially significant changes in various properties of the buffer material associated with such alteration (Windt et al., 2002; Kato et al., 2004). Long-term assessment of the performance of the engineered barriers under disposal conditions is important for achieving a reasonable design and eliminating excessive conservatism in the safety assessment. The secondary minerals defined in the numerical analysis affect the evaluation of the evolution of barrier performance (sealing capability, etc.) expected for bentonite and have a significant effect on the performance of the entire system. Therefore, it will be essential for improving the reliability of the long-term performance of the bentonite-based material to verify model results by alteration tests; the aim is also to improve understanding of alteration phenomena under disposal conditions. In previous tests of the interaction between bentonite and cement, however, secondary minerals formed due to alteration reactions under the conditions expected for geological disposal of TRU waste (equilibrated water with cement at low liquid/solid ratio) were not observed. In order to clearly confirm the alteration behavior, accelerated conditions (i.e. extremely hyper-alkaline conditions and high temperatures) were usually used in the laboratory experiments. The reason for this was considered to be that analysis of C–S–H gel formed at the interface was difficult using XRD because of its low content and low crystallinity. This study was conducted with the aim of developing a method for identifying and quantifying secondary minerals, in order to examine their formation in bentonite-based material.
Mineral
Content (%)a
Specific gravity (g/cm3)
Smectite Quartz Chalcedony Calcite Plagioclase Dolomite Analcime Pyrite
46–49 0.5–0.7 37–38 2.1–2.6 2.7–5.5 2.0–2.8 3.0–3.5 0.5–0.7
2–3 (montmorillonite)b 2.65b 2.65b 2.71b 2.62–2.76c 2.85b 2.22–2.29b 5.00–5.03b
5 TRU waste: Radioactive waste generated by reprocessing of spent fuel and fabrication of MOX fuel, which contains significant amounts of transuranic nuclides, not including high-level waste. The quantitative ratio of cementitious materials to bentonite materials in the TRU waste repository is larger than that of the HLW repository in the Japanese concept.
a b c
Ito et al. (1994). Willard et al. (1992). Hatayama (1988).
The minerals constituting Kunigel V1 and their specific gravities are shown in Table 1 (Ito et al., 1994; Willard et al., 1992; Hatayama, 1988). The specific gravities are 2.6 g/cm3 or higher, except for montmorillonite which ranges from 2 to 3 g/cm3 and analcime from 2.22 to 2.29 g/cm3. The specific gravity of C–S–H gel is 2.0–2.2 g/cm3, depending on the drying conditions of the samples, which suggests that C–S–H gel formed as a secondary mineral in the bentonite could be separated and enriched under appropriate conditions (Taylor, 1997). Suzuki et al. (1990) reported on the heavy liquid separation technique using a bromoform–methanol solution for separating aggregate from cement hydrate in concrete. Shibata et al. (2005) identified the existence of C–S–H gel in altered Kunigel V1 using a bromoform–methanol solution with a specific gravity of 2.1 g/cm3. In this study, the separation characteristics of the technique using simulated alteration samples prepared by mixing Kunigel V1 with synthesized C–S–H gel were evaluated as a function of the specific gravity of the heavy liquid. In addition, the applicability of the analytical technique was studied for bentonite–sand mixture samples which had been subjected to calcium hydroxide solution flowing through them for 7.5 years.
2. Experimental details Kunigel V1 is considered as a potential buffer material radioactive waste disposal in Japan. Kunigel V1 is consists of montmorillonite, chalcedony, plagioclase, and calcite. To develop an analytical technique for the C–S–H gel formed in bentonite as a secondary mineral, Kunigel V1 was used in this work. In the XRD analysis of Kunigel V1, the primary peak of the calcite (2h = 29.4, Cu Ka) and that of the C–S–H gel overlap, which makes identification of the low content of C–S–H gel formed as a secondary mineral difficult. Thus, development of analytical
H. Sakamoto et al. / Physics and Chemistry of the Earth 32 (2007) 311–319
techniques for secondary minerals in bentonite is a key issue in improving understanding of alteration phenomena. Here, the focus is on utilizing the difference in specific gravities between crystalline minerals constituting Kunigel V1 and C–S–H gel in order to separate C–S–H gel from Kunigel V1. As the amount of suspension to be recovered is assumed to be as small as in the order of several tens of mg in this study, both XRD analysis for identification of the crystalline structure of the suspension and an acid dissolution test for evaluation of the chemical composition of the suspension were applied.
2.1. Study on concentrating secondary minerals Samples were prepared by mixing commercially available Kunigel V1 with a given quantity of C–S–H gel with a mole ratio of CaO/SiO2 = 1.2, which was synthesized using pure chemicals (Suzuki et al., 1990). The C–S–H gel was prepared as follows: a supersaturated solution of reagent quality calcium hydroxide was prepared, to which ethyl silicate solution was added at a mole ratio of CaO/ SiO2 = 1.2. The mixture was agitated for 48 h under a controlled atmosphere to prevent carbonization. As the calcium ions in the solution would react with CO2 in the air to form calcium carbonate, these operations were conducted in an atmosphere-controlled glove box (filled with Ar gas). Sediments formed were removed by filtration and vacuum-dried. Synthesized C–S–H gel was subjected to XRD analysis to check that there was no calcium hydroxide present (starting material) and was also dissolved with 0.6 N hydrochloric acid to verify the specified mole ratio. Loss on ignition of vacuum-dried synthesized C–S–H gel was 19.4%, while the oxide weight was 80.6%. Loss on ignition of similarly vacuum-dried Kunigel V1 was 6.3%, while the oxide weight was 93.7%. Heavy bromoform–methanol mixtures were prepared with specific gravities of 2.00–2.57 g/cm3. The specific gravity of the heavy liquid was measured by pycnometer according to the Japanese Industrial Standard JIS Z 8804 ‘‘Methods for measuring the specific gravity of liquid’’. The separation procedure involved dispersing 1 g of simulated alteration samples (prepared by mixing 1 wt% of synthesized C–S–H gel with Kunigel V1) in 40 ml of heavy liquid with a given specific gravity using ultrasonic agitation (28 kHz). Centrifuge separation was then conducted at 3000 rpm (maximum centrifugal acceleration 1630 G) for 10 min. The solid phase suspended in the heavy liquid (the ‘‘light density fraction’’) was subjected to filtration with a 0.45 lm membrane filter, washed with methanol and vacuum-dried to adjust its water content for analysis.
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was a bentonite–silica sand mixture prepared by mixing Kunigel V1 with silica sands (with a maximum particle size of 4 mm) at a ratio of 2:8, molded into a cylinder 100 mm in diameter and 100 mm in height with a dry density of 1.6 g/cm3. The column was infiltrated with 1413 ml of calcium hydroxide solution (12.5 mmol/l) over a period of 7.5 years. A core 2 cm in diameter was drilled parallel to the flow direction. Samples were collected by slicing every 4 mm from the flow surface of the core. As the samples consist of approximately 80% silica sands, these sands were removed through sedimentation using methanol. After that, vacuum drying was conducted for further analysis. After pretreatment to remove the silica sands, the sliced samples (0–4, 8–12 and 16–20 mm from the flow surface) were subjected to heavy liquid separation using bromoform–methanol mixtures with a specific gravity of 2.05 g/cm3. Also, with the objective of verifying the separation characteristics of C–S–H gel from alteration samples obtained through column tests, 10 mg of synthesized C–S–H gel was added to the sliced samples of 20–24 mm (2.17 g), and 20 mg synthesized C–S–H gel to those of 24–28 mm (1.91 g), and subjected to heavy liquid separation. To enhance separation efficiency, heavy liquid separation was carried out in a centrifuge separator at a rotation speed of 4000 rpm (maximum centrifugal acceleration of 2900G) for 20 min. Heavy liquid with a specific gravity of 2.05 g/cm3 was used, based on preliminary tests of the column samples. The separation procedure involves dispersing altered samples in 40 ml of heavy liquid with a given specific gravity using ultrasonic waves (28 kHz). The solid phase suspended in the heavy liquid was subjected to filtration with a 0.45 lm membrane filter, washed with methanol and vacuum-dried to adjust its water content for analysis. When analyzing alteration samples after the column test, Kunigel V1 used for the secondary mineral separation method was used as blank for XRD analysis because the original Kunigel V1 used in the column test was not available.
Slice of column experiment sample (Bentonite/sand mixture) CH3OH Pretreatment (gravity separation)
Sand
Altered bentonite
XRD analysis
Heavy liquid 40mL (CHBr3/CH3OH mixture, s. g. 2.05 g/cm3) Ultrasonic dispersion Centrifugation Filtration
4,000rpm, 20min Heavy liquid 0.45 μm
Vacuum drying
2.2. Analysis of alteration samples after column tests Fig. 1 shows the analytical procedure for alteration samples obtained through column tests. The starting material
Suspended particles
XRD analysis
Fig. 1. Analytical procedure for alteration samples obtained through column tests.
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100 Kunigel V1 99% + C-S-H gel 1%
Weight ratio of suspended particles (%)
90
Kunigel V1 100%
80 70 60 50 40 30 20 10 0 1.90
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
3
Specific gravity of heavy liquid (g/cm ) Fig. 2. Relationship between specific gravity of heavy liquid and suspension rate.
without C–S–H gel and simulated alteration samples prepared by mixing Kunigel V1 with 1 wt% synthesized C– S–H gel were tested. The suspension rate of the simple Kunigel V1 was observed to be extremely small in heavy liquid with a specific gravity of 2.19 g/cm3 or less, but increased with specific gravity of 2.24 g/cm3 or higher. A light density fraction phase was not recovered from simulated alteration samples containing 1 wt% C–S–H gel in heavy liquid with a specific gravity of 2.00 g/cm3, whereas approx. 1% was recovered at a specific gravity of 2.10–2.19 g/cm3 and the suspension rate increased with a specific gravity of 2.24 g/cm3 or higher.
3. Results 3.1. Examination of secondary mineral separation method 3.1.1. Relationship between specific gravity of the heavy liquid and ‘‘suspension rate’’ The suspension rate was defined as the ratio of the weight of the light density fraction in heavy liquid to that of simulated alteration sample. The relationship between the specific gravity of the heavy liquid and the suspension rate is shown in Fig. 2. The suspension rate of simulated alteration samples was obtained using heavy liquid with a specific gravity of 2.00–2.57 g/cm3. Simple Kunigel V1
10000 Sm : Smectite Q : Quartz (Quartz+Chalcedony) CSH : C-S-H gel
Intensity (cps)
8000
6000
4000 CSH Q
2000
Sm
CSH
Sm Q
CSH
0 0
5
10
15
20
25
30
35
40
45
50
2θ (CuKα) (°) Fig. 3. XRD analysis for the light density fraction (specific gravity of 2.10 g/cm3).
55
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3.1.2. XRD analysis of the light density fraction Light density fractions from simulated alteration samples using heavy liquid with a specific gravity of 2.10, 2.19 and 2.38 g/cm3 were measured by XRD analysis. The results of XRD analysis for the light density fraction in heavy liquid with specific gravities of 2.10, 2.19 and 2.38 g/cm3 are shown in Figs. 3–5, respectively. In terms of XRD peaks for the light density fraction in heavy liquid, with a specific gravity of 2.10 g/cm3, a primary peak (2h = 29.4, Cu Ka) and secondary peaks (2h = 32.1and 50.1, Cu Ka) of C–S–H gel were identified, as well as minor peaks of smectite and quartz. For a specific gravity of 2.19 g/cm3, the peak of clinoptilolite was the highest, while for a specific gravity of 2.38 g/cm3, the peaks of quartz and smectite were the highest. C–S–H gel
315
could not be identified by XRD analysis for specific gravities of either 2.19 or 2.38 g/cm3. 3.1.3. Mole ratio of CaO and SiO2 in the light density fraction The concentrations of CaO and SiO2 that can be dissolved from the light density fraction with 0.6 N hydrochloric acid are shown in Figs. 6 and 7, respectively. The mole ratios of such CaO and SiO2 are shown in Fig. 8. It was shown that the weight ratios of soluble CaO and SiO2 were extremely small for the light density fraction from the original Kunigel V1. However, when simulated alteration samples mixed with 1 wt% synthesized C–S–H gel were subjected to the same treatment, the weight ratios
10000 Cl
Sm : Smectite Cl : Clinoptilolite H : Heulandite Q : Quartz Quartz+Chalcedony D : Dolomite An : Analcime
Intensity (cps)
8000
Cl
6000
Cl
4000 Cl
Sm
Cl Q ClCl Cl
Cl Cl Cl An Cl H Cl Cl
2000
Cl
Cl Sm Q
Cl Cl Cl
Cl Cl D
Cl Sm Cl Cl
ClClQ
Q
Q
An
0 0
5
10
15
20
25
30
35
40
45
50
55
60
2θ (CuKα) (°)
Q
Fig. 4. XRD analysis for the light density fraction (specific gravity of 2.19 g/cm3).
10000
Q
Sm : Smectite Cl : Clinoptilolite Q : Quartz Quartz+Chalcedony Pl : Plagioclase C : Calcite D : Dolomite
Intensity (cps)
8000
6000 Q Sm
4000 Cl
2000 Sm Cl
Cl
Cl Cl Cl
Cl
0 0
5
10
15
20
25
Q
Cl C Cl Cl Cl Pl D
30
Sm
35
Q Q Q Q
40
Q Q
Q
45
50
2θ (CuKα) (°) Fig. 5. XRD analysis for the light density fraction (specific gravity of 2.38 g/cm3).
55
Q
60
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40
SiO2 content in suspended solid (wt%)
CaO content in suspended solid (wt%)
40 Kunigel V1 99% + C-S-H gel 1% Kunigel V1 100%
35 30 25 20 15 10 5 0 1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
30 25 20 15 10 5 0 1.9
2.7
3
Specific gravity of heavy liquid (g/cm )
Kunigel V1 99% + C-S-H gel 1% Kunigel V1 100%
35
2.0
2.1
2.2
2.3
2.4
Fig. 7. SiO2 content in the light density fraction (dissolved with 0.6 N hydrochloric acid).
3.2. Analysis of alteration samples from column tests 3.2.1. XRD analysis before heavy liquid separation The results of XRD analysis of samples before heavy liquid separation (after the pretreatment to remove silica sands) are shown in Fig. 9. The samples before heavy liquid separation show peaks of smectite and quartz, while Kunigel V1 used as a blank shows a peak of clinoptilolite as well as peaks of smectite and quartz. The results of XRD analysis for alteration samples indicate a shift of the peak near 2h = 7.5 (d value: ˚ ) due to reflection of the (0 0 1) plane of smectite in 11.8 A ˚ ) for samples Kunigel V1 to near 2h = 6 (d value: 14.7 A sliced from the flow surface at 0–4, 4–8, 8–12 and 12– 16 mm. Also, with samples sliced at 16 mm and beyond,
5.0
Measured value CaO/SiO2 molar ratio of the synthesized C-S-H gel
CaO/SiO 2 molar ratio
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 2.0
2.1
2.7
Specific gravity of heavy liquid (g/cm )
of CaO and SiO2 were higher in the light density fraction for heavy liquid with a specific gravity of 2.10– 2.19 g/cm3, and extremely high for heavy liquid with a specific gravity of 2.10–2.14 g/cm3. The mole ratio of CaO/SiO2 in the light density fraction after heavy liquid separation with a specific gravity of 2.10– 2.19 g/cm3 was 1.2, which was the same as the mole ratio of the synthesized C–S–H gel added to the sample. However, the mole ratio of CaO/SiO2 in the light density fraction after heavy liquid separation with a specific gravity of 2.24–2.57 g/cm3 increased with increasing specific gravity of the heavy liquid. This is believed to suggest the inclusion of fine particles of accessory minerals in the bentonite (calcite and dolomite) which were dissolved by the hydrochloric acid.
1.9
2.6 3
Fig. 6. CaO content in the light density fraction (dissolved with 0.6 N hydrochloric acid).
4.5
2.5
2.2
2.3
2.4
2.5
2.6
3
Specific gravity of heavy liquid (g/cm ) Fig. 8. CaO and SiO2 mole ratios (dissolved with 0.6 N hydrochloric acid).
2.7
H. Sakamoto et al. / Physics and Chemistry of the Earth 32 (2007) 311–319
317
15000
← Ca-type Depth (mm)
← Na-type
0-4
10000
Intensity (cps)
4-8 8-12 12-16 16-20 36-40
5000
56-60 76-80 96-100 BL
0 0
10
20
30
40
50
60
2θ (CuKα) (°) Fig. 9. XRD analysis of samples before heavy liquid separation (after removing of silica sands).
the peak was broader at the low-angle side than the Kunigel V1 blank. The basal spacing of Ca-smectite is reported to be wider than that of Na-smectite under relative humidity in the range of 20–50% (Sato et al., 1992). Although vacuumdried samples were used for XRD analysis in this study, the shift of the peak location might be attributable to the change from Na-type to Ca-type.
sized C–S–H gel added), at 24–28 mm (with 20 mg of synthesized C–S–H gel added) and for Kunigel V1 as a reference material. The XRD peaks of 0–4, 8–12 and 16–20 mm samples showed similar peaks to the Kunigel V1 blank and no additional new peaks due to the formation of secondary minerals were identified. In contrast, in the XRD analysis of the 20–24 and 24–28 mm samples, a peak resulting from the added C–S–H gel was identified. The Kunigel V1 used in the blank test was different from that used in the column tests and thus may contain more clinoptilolite than in that used in the column tests. However, as shown in Fig. 3, the light density fraction after
3.2.2. XRD analysis after heavy liquid separation Fig. 10 shows the results of XRD analysis for the light density fraction (specific gravity of 2.05 g/cm3) for samples sliced from the flow surface at 0–4, 8–12 and 16–20 mm (no C–S–H gel added), at 20 to 24 mm (with 10 mg of synthe10000
8000
Intensity (cps)
←
Depth (mm) 0-4
Montmorillonite
←
C-S-H gel
8 - 12
6000
16 - 20 20 - 24 + CSH10mg
4000
24 - 28 + CSH20mg BL
2000
C-S-H gel
0 20
25
30
35
2θ (CuKα) (°) Fig. 10. XRD analysis for the light density fraction (specific gravity of 2.05 g/cm3).
40
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heavy liquid separation did not contain any clinoptilolite and, therefore, it was used as a reference sample without C–S–H gel for the blank test. 4. Discussion 4.1. Separation method for secondary minerals Heavy liquid separation of simulated alteration samples mixed with 1 wt% synthesized C–S–H gel at specific gravities of 2.00–2.57 g/cm3 resulted in suspension of about 1% of solid phase for the heavy liquid with a specific gravity of 2.10–2.19 g/cm3 and a higher light density fraction portion suspension rate for the heavy liquid with a specific gravity of 2.24 g/cm3 or higher. This suggests that C–S–H gel added to these samples was suspended in heavy liquid with a specific gravity of 2.10 g/cm3 or higher, while additional light density fractions from Kunigel V1 are present for heavy liquid with a specific gravity of 2.24 g/cm3 or higher. This is consistent with the results of XRD analysis: the light density fraction after heavy liquid separation with a specific gravity of 2.10 g/cm3 shows a primary peak (2h = 29.4, Cu Ka) and secondary peaks (2h = 32.1 and 50.1, Cu Ka) for C–S–H gel. When using heavy liquid with a specific gravity of 2.19 g/cm3 or higher, the peak of C–S–H gel was not identified because of disturbance by peaks of crystalline bentonite minerals (mainly clinoptilolite, quartz, and chalcedony). The light density fraction was also dissolved with hydrochloric acid to measure Ca and Si contents and the weight ratio of soluble CaO and SiO2. The major minerals of Kunigel V1 are smectite, chalcedony, etc., which do not dissolve extensively in low concentrated hydrochloric acid – although this cannot be assumed for some minor minerals such as calcite and dolomite. The synthetic C–S–H gel (Ca/Si mole ratio of 1.2) added to simulated alteration samples was known to be dissolved completely by 0.6 N hydrochloric acid and thus the ratio of C–S–H gel in the light density fraction could be evaluated by dissolving it in hydrochloric acid. For the simulated alteration samples mixed with 1 wt% synthesized C–S–H gel, the weight ratio of soluble CaO and SiO2 in the light density fraction was very high for heavy liquid with a specific gravity of 2.10–2.14 g/cm3 and the inclusion of bentonite materials was low. This result is consistent with the results of XRD analysis. The above results suggest that heavy liquid with a specific gravity of around 2.10 g/cm3 is optimum for selectively separating C–S–H gel present in Kunigel V1.
˚) reflection of smectite to near 2h = 6 (d value: 14.7 A caused by uptake of calcium in samples from 0 to 16 mm from the surface. Also, for samples in this depth range, peaks were broader on the low-angle side than those in the Kunigel V1 blank. Although the samples had been subjected to calcium hydroxide solution flow for 7.5 years, C–S–H gel was not identified, even after heavy liquid separation. The CEC of Kunigel V1 is about 60 meq/100 g, and the initial amount of exchangeable cations in the column sample can therefore be calculated to be 150 meq: 785 cm3 (volume of the column sample) · 1.6 g/cm3 (density) · 0.2 (ratio of bentonite) · 0.6 meq/g (CEC). The amount of calcium ions provided to the column is 41.8 meq, as calculated from the volume of solution passed through, which indicates that only 1/3 of the exchangeable cations in the column sample were replaced with calcium ions. As the calcium ions provided to the column sample would exchange with interlayer sodium ions of montmorillonite and the concentration of calcium ions in the porewater would be kept low, sufficient C–S–H gel might not be formed to allow detection using the analysis method applied in this study. Sliced samples of core containing synthetic C–S–H gel were thus used to investigate the applicability and the detection limit of this analytical technique. XRD analysis of the samples after heavy liquid separation showed both a primary peak (2h = 29.4, Cu Ka) and a secondary peak (2h = 32.1, Cu Ka) of the C–S–H gel. This result indicates that the lower detection limit of this analytical technique would be less than 1 wt% of C–S–H gel for such altered bentonite samples. It was concluded from the above results that C–S–H gel formed in the bentonite by the reaction between cement and bentonite under simulated disposal conditions could be separated and refined by this analytical technique. However, the properties and the conditions of actual C–S–H gel formed by such reactions under real conditions (e.g. difference in Ca/Si mole ratio, etc.) are not sufficiently understood. In particular, the efficiency of mineral separation using the heavy liquid may have been affected if the C–S–H gel is formed and steady attached on the minerals constituting bentonite. Therefore, the applicability of this analytical technique to heavily altered bentonite needs to be verified for further work. 5. Conclusions An analytical technique for C–S–H gel that may be formed in bentonite by the reaction with cement was studied. The following conclusions were drawn:
4.2. Analysis of altered samples from column tests The analysis of secondary minerals (C–S–H gel) obtained by heavy liquid separation from column test samples (bentonite–silica sand mixture) showed a shift of the ˚ ) caused by the (0 0 1) peak near 2h = 7.5 (d value: 11.8 A
– C–S–H gel extraction by a heavy liquid separation method was demonstrated on Kunigel V1 with 1 wt% synthesized C–S–H gel (10 mg). It was found that bromoform–methanol mixtures with a specific gravity of 2.10–2.14 g/cm3 could separate and refine synthesized
H. Sakamoto et al. / Physics and Chemistry of the Earth 32 (2007) 311–319
C–S–H gel from Kunigel V1, which could be confirmed by XRD analysis. – Based on measured Ca and Si contents and the weight ratio of soluble CaO and SiO2 in the light density fraction, it could be shown that the content of C–S–H gel in the light density fraction was high in heavy liquid with a specific gravity of 2.10–2.14 g/cm3, while the inclusion of bentonite minerals was low. – XRD analysis of samples from column tests indicated only a shift of the peak near 2h = 7.5 (d value: ˚ ) representing reflection by the (0 0 1) plane of 11.8 A smectite in Kunigel V1 to near 2h = 6 (d value: ˚ ) for samples from 0 to 16 mm, attributed to 14.7 A uptake of calcium ions. – Column test samples were subjected to heavy liquid separation with a specific gravity of 2.05 g/cm3, but C–S–H gel was not identified due to very low content. The C–S– H gel artificially added to the bentonite was, however, detected by this method. The detection limit of this analytical technique is estimated as less than 1 wt%. This result implies that secondary C–S–H gel could be separated by the heavy liquid separation technique, as long as it is not attached to bentonite minerals. Acknowledgements This study is part of the project on ‘‘Long-term Verification Testing of Engineered Barriers’’ performed under contract from the Japanese Ministry of Economy, Trade and Industry (METI). We would like to express our appreciation to the members of the Examination Committee for Long-term Verification Testing of Engineered Barriers for the guidance they provided to us. We also express our special thanks to Prof. T. Ohe of Tokai University, Prof. A. Inoue of Chiba University and Prof. T. Sato of Kanazawa
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University for their guidance on research planning and report compilation.
References Hatayama, Y., 1988. Chigakujiten Zouho Kaiteiban. Heibonsha, Japan, p. 476 (in Japanese). Ito, M., Okamoto, M., Suzuki, K., Shibata, M., Sasaki, Y., 1994. Mineral composition analysis of bentonite. Transactions of the Atomic Energy Society of Japan 36-11, 1055–1058 (in Japanese). Kato, H., Asano, H. Yamada, N., 2004. A long-term performance assessment for engineered barriers – study on barrier materials alteration. In: Proceedings of the International Workshop on Bentonite–Cement Interaction in Repository Environments, NUMO-TR-0405, A3-70-73. Kubo, H., Kuroki, M., Mihara, M., 1998. Experimental investigation on alteration of bentonite by concrete pore fluids. Tsuchi-to-Kiso 46-10, 31–34 (in Japanese). Sato, T., Watanabe, T., Otsuka, R., 1992. Effects of layer charge, charge location, and energy change on expansion properties of dioctahedral smectites. Clays and Clay Minerals 40-1, 103–113. Savage, D., Bateman, K., Hill, O., Hughes, C., Milodowski, A., Pearce, J., Rage, E., Rochelle, C., 1992. Rate and mechanism of the reaction of silicates with cement pore fluids. Clay Science 7, 33–45. Shibata, M., Sutou, S., Sakamoto, H., Sato, T., 2005. Detection of C–S–H gel formed during bentonite–cement interactions. Transactions of the Atomic Energy Society of Japan 4-3, 227–231 (in Japanese). Suzuki, K., Nishikawa, T., Yamada, Y., Taniguchi, I., 1990. Analysis of hydrated phases for evaluation the durability of concrete. Concrete Research and Technology 1–2, 39–49 (in Japanese, with English Synopsis). Taylor, H.F.W., 1997. Cement Chemistry, second ed. Thomas Telford Services Ltd., Academic Press Inc., London, p. 119. Willard, L.R., George Jr., R.R., Julius, W., 1992. Encyclopedia of Minerals. Van Nostrand Reinhold Co., New York, p. 20, 101, 117, 177, 417, 498, 504. Windt, L.D., Pellegrini, D., Lee, J.v.d., 2002. Fully coupled modeling of radionuclide migration in a clayey rock disturbed by alkaline plume. In: TrePro-Workshop on Modelling of Coupled Transport Reaction Processes, Karlsruhe, pp. 5–8.
Development of an analytical technique for the detection of alteration minerals formed in bentonite by reaction with alkaline solutions H. Sakamoto b
a,1
, M. Shibata
a,1
, H. Owada
b,2
, M. Kaneko
b,3
, Y. Kuno
b,*
, H. Asano
b,4
a Taiheiyo Consultant Corporation, 2-4-2, Osaku, Sakura-shi, Chiba Pref., 285-8655, Japan Radioactive Waste Management Funding and Research Center, No. 15 Mori-bldg., 2-8-10, Toranomon, Minato-ku, Tokyo 105-0001, Japan
Received 22 April 2005; received in revised form 4 August 2005; accepted 25 October 2005 Available online 7 February 2007
Abstract A multibarrier system consisting of cement-based backfill, structures and support materials, and a bentonite-based buffer material has been studied for the TRU waste disposal concept being developed in Japan, the aim being to restrict the migration of radionuclides. Concern regarding bentonite-based materials in this disposal environment relates to long-term alteration under hyper-alkaline conditions due to the presence of cementitious materials. In tests simulating the interaction between bentonite and cement, formation of secondary minerals due to alteration reactions under the conditions expected for geological disposal of TRU waste (equilibrated water with cement at low liquid/solid ratio) has not been observed, although alteration was observed under extremely hyper-alkaline conditions with high temperatures. This was considered to be due to the fact that analysis of C–S–H gel formed at the interface as a secondary mineral was difficult using XRD, because of its low crystallinity and low content. This paper describes an analytical technique for the characterization of C–S–H gel using a heavy liquid separation method which separates C–S–H gel from Kunigel V1 bentonite (bentonite produced in Japan) based on the difference in specific gravity between the crystalline minerals constituting Kunigel V1 and the secondary C–S–H gel. For development of C–S–H gel separation methods, simulated alteration samples were prepared by mixing 990 mg of unaltered Kunigel V1 and 10 mg of C–S–H gel synthesized using pure chemicals at a ratio of Ca/Si = 1.2. The simulated alteration samples were dispersed in bromoform–methanol mixtures with specific gravities ranging from 2.00 to 2.57 g/cm3 and subjected to centrifuge separation to recover the light density fraction. Subsequent XRD analysis to identify the minerals was complemented by dissolution in 0.6 N hydrochloric acid to measure the Ca and Si contents. The primary peak (2h = 29.4, Cu Ka) and secondary peaks (2h = 32.1 and 50.1, Cu Ka) of the C–S–H gel, which could not be distinguished before the heavy liquid separation, were clearly identified by XRD after separation. The result of the analyses of the light density fraction indicates highest recovery of C–S–H gel and least inclusion of bentonite for separation using heavy liquid with a specific gravity of 2.10 g/cm3. The traces of bentonite minerals included in the suspension were identified to be montmorillonite, quartz, clinoptilolite, and calcite. The separation technique was also tested for Ca-bentonite prepared by passing a calcium hydroxide solution through a bentonite (Kunigel V1)-silica sand mixture. The results indicated that the technique would also be applicable to separation of C–S–H gel from Ca-bentonite. 2007 Published by Elsevier Ltd.
*
Corresponding author. Tel.: +81 3 3504 1505; fax: +81 3 3504 1297. E-mail addresses: [email protected] (H. Sakamoto), [email protected] (M. Shibata), [email protected] (H. Owada), [email protected] (M. Kaneko), [email protected] (Y. Kuno), [email protected] (H. Asano). 1 Tel.: +81 43 498 3858; fax: +81 43 498 3859. 2 Tel.: +81 3 3504 1582; fax: +81 3 3504 1297. 3 Tel.: +81 3 3504 1573; fax: +81 3 3504 1297. 4 Tel.: +81 3 3504 1485; fax: +81 3 3504 1297. 1474-7065/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.pce.2005.10.004
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Keywords: Radioactive waste disposal; Bentonite; Alteration; Cement/bentonite interaction; C–S–H gel; Analytical method
1. Introduction
Table 1 Minerals constituting Kunigel V1 and their specific gravities
A multibarrier system consisting of cement-based backfill materials, structures and support and a bentonite-based buffer material has been studied for the TRU waste5 disposal concept being developed in Japan. The expected functions of the bentonite buffer material include retardation of radionuclides during migration, restriction of flow of groundwater and filtration of colloids. One concern about this buffer material under disposal conditions is potential long-term alteration by hyper-alkaline fluids leached from cementitious material (Savage et al., 1992; Kubo et al., 1998). Long-term alteration of bentonite-based materials due to reaction with alkaline solutions has been investigated using analytical approaches based on coupled mass transport and chemical reaction studies, which suggest potentially significant changes in various properties of the buffer material associated with such alteration (Windt et al., 2002; Kato et al., 2004). Long-term assessment of the performance of the engineered barriers under disposal conditions is important for achieving a reasonable design and eliminating excessive conservatism in the safety assessment. The secondary minerals defined in the numerical analysis affect the evaluation of the evolution of barrier performance (sealing capability, etc.) expected for bentonite and have a significant effect on the performance of the entire system. Therefore, it will be essential for improving the reliability of the long-term performance of the bentonite-based material to verify model results by alteration tests; the aim is also to improve understanding of alteration phenomena under disposal conditions. In previous tests of the interaction between bentonite and cement, however, secondary minerals formed due to alteration reactions under the conditions expected for geological disposal of TRU waste (equilibrated water with cement at low liquid/solid ratio) were not observed. In order to clearly confirm the alteration behavior, accelerated conditions (i.e. extremely hyper-alkaline conditions and high temperatures) were usually used in the laboratory experiments. The reason for this was considered to be that analysis of C–S–H gel formed at the interface was difficult using XRD because of its low content and low crystallinity. This study was conducted with the aim of developing a method for identifying and quantifying secondary minerals, in order to examine their formation in bentonite-based material.
Mineral
Content (%)a
Specific gravity (g/cm3)
Smectite Quartz Chalcedony Calcite Plagioclase Dolomite Analcime Pyrite
46–49 0.5–0.7 37–38 2.1–2.6 2.7–5.5 2.0–2.8 3.0–3.5 0.5–0.7
2–3 (montmorillonite)b 2.65b 2.65b 2.71b 2.62–2.76c 2.85b 2.22–2.29b 5.00–5.03b
5 TRU waste: Radioactive waste generated by reprocessing of spent fuel and fabrication of MOX fuel, which contains significant amounts of transuranic nuclides, not including high-level waste. The quantitative ratio of cementitious materials to bentonite materials in the TRU waste repository is larger than that of the HLW repository in the Japanese concept.
a b c
Ito et al. (1994). Willard et al. (1992). Hatayama (1988).
The minerals constituting Kunigel V1 and their specific gravities are shown in Table 1 (Ito et al., 1994; Willard et al., 1992; Hatayama, 1988). The specific gravities are 2.6 g/cm3 or higher, except for montmorillonite which ranges from 2 to 3 g/cm3 and analcime from 2.22 to 2.29 g/cm3. The specific gravity of C–S–H gel is 2.0–2.2 g/cm3, depending on the drying conditions of the samples, which suggests that C–S–H gel formed as a secondary mineral in the bentonite could be separated and enriched under appropriate conditions (Taylor, 1997). Suzuki et al. (1990) reported on the heavy liquid separation technique using a bromoform–methanol solution for separating aggregate from cement hydrate in concrete. Shibata et al. (2005) identified the existence of C–S–H gel in altered Kunigel V1 using a bromoform–methanol solution with a specific gravity of 2.1 g/cm3. In this study, the separation characteristics of the technique using simulated alteration samples prepared by mixing Kunigel V1 with synthesized C–S–H gel were evaluated as a function of the specific gravity of the heavy liquid. In addition, the applicability of the analytical technique was studied for bentonite–sand mixture samples which had been subjected to calcium hydroxide solution flowing through them for 7.5 years.
2. Experimental details Kunigel V1 is considered as a potential buffer material radioactive waste disposal in Japan. Kunigel V1 is consists of montmorillonite, chalcedony, plagioclase, and calcite. To develop an analytical technique for the C–S–H gel formed in bentonite as a secondary mineral, Kunigel V1 was used in this work. In the XRD analysis of Kunigel V1, the primary peak of the calcite (2h = 29.4, Cu Ka) and that of the C–S–H gel overlap, which makes identification of the low content of C–S–H gel formed as a secondary mineral difficult. Thus, development of analytical
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techniques for secondary minerals in bentonite is a key issue in improving understanding of alteration phenomena. Here, the focus is on utilizing the difference in specific gravities between crystalline minerals constituting Kunigel V1 and C–S–H gel in order to separate C–S–H gel from Kunigel V1. As the amount of suspension to be recovered is assumed to be as small as in the order of several tens of mg in this study, both XRD analysis for identification of the crystalline structure of the suspension and an acid dissolution test for evaluation of the chemical composition of the suspension were applied.
2.1. Study on concentrating secondary minerals Samples were prepared by mixing commercially available Kunigel V1 with a given quantity of C–S–H gel with a mole ratio of CaO/SiO2 = 1.2, which was synthesized using pure chemicals (Suzuki et al., 1990). The C–S–H gel was prepared as follows: a supersaturated solution of reagent quality calcium hydroxide was prepared, to which ethyl silicate solution was added at a mole ratio of CaO/ SiO2 = 1.2. The mixture was agitated for 48 h under a controlled atmosphere to prevent carbonization. As the calcium ions in the solution would react with CO2 in the air to form calcium carbonate, these operations were conducted in an atmosphere-controlled glove box (filled with Ar gas). Sediments formed were removed by filtration and vacuum-dried. Synthesized C–S–H gel was subjected to XRD analysis to check that there was no calcium hydroxide present (starting material) and was also dissolved with 0.6 N hydrochloric acid to verify the specified mole ratio. Loss on ignition of vacuum-dried synthesized C–S–H gel was 19.4%, while the oxide weight was 80.6%. Loss on ignition of similarly vacuum-dried Kunigel V1 was 6.3%, while the oxide weight was 93.7%. Heavy bromoform–methanol mixtures were prepared with specific gravities of 2.00–2.57 g/cm3. The specific gravity of the heavy liquid was measured by pycnometer according to the Japanese Industrial Standard JIS Z 8804 ‘‘Methods for measuring the specific gravity of liquid’’. The separation procedure involved dispersing 1 g of simulated alteration samples (prepared by mixing 1 wt% of synthesized C–S–H gel with Kunigel V1) in 40 ml of heavy liquid with a given specific gravity using ultrasonic agitation (28 kHz). Centrifuge separation was then conducted at 3000 rpm (maximum centrifugal acceleration 1630 G) for 10 min. The solid phase suspended in the heavy liquid (the ‘‘light density fraction’’) was subjected to filtration with a 0.45 lm membrane filter, washed with methanol and vacuum-dried to adjust its water content for analysis.
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was a bentonite–silica sand mixture prepared by mixing Kunigel V1 with silica sands (with a maximum particle size of 4 mm) at a ratio of 2:8, molded into a cylinder 100 mm in diameter and 100 mm in height with a dry density of 1.6 g/cm3. The column was infiltrated with 1413 ml of calcium hydroxide solution (12.5 mmol/l) over a period of 7.5 years. A core 2 cm in diameter was drilled parallel to the flow direction. Samples were collected by slicing every 4 mm from the flow surface of the core. As the samples consist of approximately 80% silica sands, these sands were removed through sedimentation using methanol. After that, vacuum drying was conducted for further analysis. After pretreatment to remove the silica sands, the sliced samples (0–4, 8–12 and 16–20 mm from the flow surface) were subjected to heavy liquid separation using bromoform–methanol mixtures with a specific gravity of 2.05 g/cm3. Also, with the objective of verifying the separation characteristics of C–S–H gel from alteration samples obtained through column tests, 10 mg of synthesized C–S–H gel was added to the sliced samples of 20–24 mm (2.17 g), and 20 mg synthesized C–S–H gel to those of 24–28 mm (1.91 g), and subjected to heavy liquid separation. To enhance separation efficiency, heavy liquid separation was carried out in a centrifuge separator at a rotation speed of 4000 rpm (maximum centrifugal acceleration of 2900G) for 20 min. Heavy liquid with a specific gravity of 2.05 g/cm3 was used, based on preliminary tests of the column samples. The separation procedure involves dispersing altered samples in 40 ml of heavy liquid with a given specific gravity using ultrasonic waves (28 kHz). The solid phase suspended in the heavy liquid was subjected to filtration with a 0.45 lm membrane filter, washed with methanol and vacuum-dried to adjust its water content for analysis. When analyzing alteration samples after the column test, Kunigel V1 used for the secondary mineral separation method was used as blank for XRD analysis because the original Kunigel V1 used in the column test was not available.
Slice of column experiment sample (Bentonite/sand mixture) CH3OH Pretreatment (gravity separation)
Sand
Altered bentonite
XRD analysis
Heavy liquid 40mL (CHBr3/CH3OH mixture, s. g. 2.05 g/cm3) Ultrasonic dispersion Centrifugation Filtration
4,000rpm, 20min Heavy liquid 0.45 μm
Vacuum drying
2.2. Analysis of alteration samples after column tests Fig. 1 shows the analytical procedure for alteration samples obtained through column tests. The starting material
Suspended particles
XRD analysis
Fig. 1. Analytical procedure for alteration samples obtained through column tests.
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100 Kunigel V1 99% + C-S-H gel 1%
Weight ratio of suspended particles (%)
90
Kunigel V1 100%
80 70 60 50 40 30 20 10 0 1.90
2.00
2.10
2.20
2.30
2.40
2.50
2.60
2.70
3
Specific gravity of heavy liquid (g/cm ) Fig. 2. Relationship between specific gravity of heavy liquid and suspension rate.
without C–S–H gel and simulated alteration samples prepared by mixing Kunigel V1 with 1 wt% synthesized C– S–H gel were tested. The suspension rate of the simple Kunigel V1 was observed to be extremely small in heavy liquid with a specific gravity of 2.19 g/cm3 or less, but increased with specific gravity of 2.24 g/cm3 or higher. A light density fraction phase was not recovered from simulated alteration samples containing 1 wt% C–S–H gel in heavy liquid with a specific gravity of 2.00 g/cm3, whereas approx. 1% was recovered at a specific gravity of 2.10–2.19 g/cm3 and the suspension rate increased with a specific gravity of 2.24 g/cm3 or higher.
3. Results 3.1. Examination of secondary mineral separation method 3.1.1. Relationship between specific gravity of the heavy liquid and ‘‘suspension rate’’ The suspension rate was defined as the ratio of the weight of the light density fraction in heavy liquid to that of simulated alteration sample. The relationship between the specific gravity of the heavy liquid and the suspension rate is shown in Fig. 2. The suspension rate of simulated alteration samples was obtained using heavy liquid with a specific gravity of 2.00–2.57 g/cm3. Simple Kunigel V1
10000 Sm : Smectite Q : Quartz (Quartz+Chalcedony) CSH : C-S-H gel
Intensity (cps)
8000
6000
4000 CSH Q
2000
Sm
CSH
Sm Q
CSH
0 0
5
10
15
20
25
30
35
40
45
50
2θ (CuKα) (°) Fig. 3. XRD analysis for the light density fraction (specific gravity of 2.10 g/cm3).
55
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3.1.2. XRD analysis of the light density fraction Light density fractions from simulated alteration samples using heavy liquid with a specific gravity of 2.10, 2.19 and 2.38 g/cm3 were measured by XRD analysis. The results of XRD analysis for the light density fraction in heavy liquid with specific gravities of 2.10, 2.19 and 2.38 g/cm3 are shown in Figs. 3–5, respectively. In terms of XRD peaks for the light density fraction in heavy liquid, with a specific gravity of 2.10 g/cm3, a primary peak (2h = 29.4, Cu Ka) and secondary peaks (2h = 32.1and 50.1, Cu Ka) of C–S–H gel were identified, as well as minor peaks of smectite and quartz. For a specific gravity of 2.19 g/cm3, the peak of clinoptilolite was the highest, while for a specific gravity of 2.38 g/cm3, the peaks of quartz and smectite were the highest. C–S–H gel
315
could not be identified by XRD analysis for specific gravities of either 2.19 or 2.38 g/cm3. 3.1.3. Mole ratio of CaO and SiO2 in the light density fraction The concentrations of CaO and SiO2 that can be dissolved from the light density fraction with 0.6 N hydrochloric acid are shown in Figs. 6 and 7, respectively. The mole ratios of such CaO and SiO2 are shown in Fig. 8. It was shown that the weight ratios of soluble CaO and SiO2 were extremely small for the light density fraction from the original Kunigel V1. However, when simulated alteration samples mixed with 1 wt% synthesized C–S–H gel were subjected to the same treatment, the weight ratios
10000 Cl
Sm : Smectite Cl : Clinoptilolite H : Heulandite Q : Quartz Quartz+Chalcedony D : Dolomite An : Analcime
Intensity (cps)
8000
Cl
6000
Cl
4000 Cl
Sm
Cl Q ClCl Cl
Cl Cl Cl An Cl H Cl Cl
2000
Cl
Cl Sm Q
Cl Cl Cl
Cl Cl D
Cl Sm Cl Cl
ClClQ
Q
Q
An
0 0
5
10
15
20
25
30
35
40
45
50
55
60
2θ (CuKα) (°)
Q
Fig. 4. XRD analysis for the light density fraction (specific gravity of 2.19 g/cm3).
10000
Q
Sm : Smectite Cl : Clinoptilolite Q : Quartz Quartz+Chalcedony Pl : Plagioclase C : Calcite D : Dolomite
Intensity (cps)
8000
6000 Q Sm
4000 Cl
2000 Sm Cl
Cl
Cl Cl Cl
Cl
0 0
5
10
15
20
25
Q
Cl C Cl Cl Cl Pl D
30
Sm
35
Q Q Q Q
40
Q Q
Q
45
50
2θ (CuKα) (°) Fig. 5. XRD analysis for the light density fraction (specific gravity of 2.38 g/cm3).
55
Q
60
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40
SiO2 content in suspended solid (wt%)
CaO content in suspended solid (wt%)
40 Kunigel V1 99% + C-S-H gel 1% Kunigel V1 100%
35 30 25 20 15 10 5 0 1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
30 25 20 15 10 5 0 1.9
2.7
3
Specific gravity of heavy liquid (g/cm )
Kunigel V1 99% + C-S-H gel 1% Kunigel V1 100%
35
2.0
2.1
2.2
2.3
2.4
Fig. 7. SiO2 content in the light density fraction (dissolved with 0.6 N hydrochloric acid).
3.2. Analysis of alteration samples from column tests 3.2.1. XRD analysis before heavy liquid separation The results of XRD analysis of samples before heavy liquid separation (after the pretreatment to remove silica sands) are shown in Fig. 9. The samples before heavy liquid separation show peaks of smectite and quartz, while Kunigel V1 used as a blank shows a peak of clinoptilolite as well as peaks of smectite and quartz. The results of XRD analysis for alteration samples indicate a shift of the peak near 2h = 7.5 (d value: ˚ ) due to reflection of the (0 0 1) plane of smectite in 11.8 A ˚ ) for samples Kunigel V1 to near 2h = 6 (d value: 14.7 A sliced from the flow surface at 0–4, 4–8, 8–12 and 12– 16 mm. Also, with samples sliced at 16 mm and beyond,
5.0
Measured value CaO/SiO2 molar ratio of the synthesized C-S-H gel
CaO/SiO 2 molar ratio
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 2.0
2.1
2.7
Specific gravity of heavy liquid (g/cm )
of CaO and SiO2 were higher in the light density fraction for heavy liquid with a specific gravity of 2.10– 2.19 g/cm3, and extremely high for heavy liquid with a specific gravity of 2.10–2.14 g/cm3. The mole ratio of CaO/SiO2 in the light density fraction after heavy liquid separation with a specific gravity of 2.10– 2.19 g/cm3 was 1.2, which was the same as the mole ratio of the synthesized C–S–H gel added to the sample. However, the mole ratio of CaO/SiO2 in the light density fraction after heavy liquid separation with a specific gravity of 2.24–2.57 g/cm3 increased with increasing specific gravity of the heavy liquid. This is believed to suggest the inclusion of fine particles of accessory minerals in the bentonite (calcite and dolomite) which were dissolved by the hydrochloric acid.
1.9
2.6 3
Fig. 6. CaO content in the light density fraction (dissolved with 0.6 N hydrochloric acid).
4.5
2.5
2.2
2.3
2.4
2.5
2.6
3
Specific gravity of heavy liquid (g/cm ) Fig. 8. CaO and SiO2 mole ratios (dissolved with 0.6 N hydrochloric acid).
2.7
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317
15000
← Ca-type Depth (mm)
← Na-type
0-4
10000
Intensity (cps)
4-8 8-12 12-16 16-20 36-40
5000
56-60 76-80 96-100 BL
0 0
10
20
30
40
50
60
2θ (CuKα) (°) Fig. 9. XRD analysis of samples before heavy liquid separation (after removing of silica sands).
the peak was broader at the low-angle side than the Kunigel V1 blank. The basal spacing of Ca-smectite is reported to be wider than that of Na-smectite under relative humidity in the range of 20–50% (Sato et al., 1992). Although vacuumdried samples were used for XRD analysis in this study, the shift of the peak location might be attributable to the change from Na-type to Ca-type.
sized C–S–H gel added), at 24–28 mm (with 20 mg of synthesized C–S–H gel added) and for Kunigel V1 as a reference material. The XRD peaks of 0–4, 8–12 and 16–20 mm samples showed similar peaks to the Kunigel V1 blank and no additional new peaks due to the formation of secondary minerals were identified. In contrast, in the XRD analysis of the 20–24 and 24–28 mm samples, a peak resulting from the added C–S–H gel was identified. The Kunigel V1 used in the blank test was different from that used in the column tests and thus may contain more clinoptilolite than in that used in the column tests. However, as shown in Fig. 3, the light density fraction after
3.2.2. XRD analysis after heavy liquid separation Fig. 10 shows the results of XRD analysis for the light density fraction (specific gravity of 2.05 g/cm3) for samples sliced from the flow surface at 0–4, 8–12 and 16–20 mm (no C–S–H gel added), at 20 to 24 mm (with 10 mg of synthe10000
8000
Intensity (cps)
←
Depth (mm) 0-4
Montmorillonite
←
C-S-H gel
8 - 12
6000
16 - 20 20 - 24 + CSH10mg
4000
24 - 28 + CSH20mg BL
2000
C-S-H gel
0 20
25
30
35
2θ (CuKα) (°) Fig. 10. XRD analysis for the light density fraction (specific gravity of 2.05 g/cm3).
40
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heavy liquid separation did not contain any clinoptilolite and, therefore, it was used as a reference sample without C–S–H gel for the blank test. 4. Discussion 4.1. Separation method for secondary minerals Heavy liquid separation of simulated alteration samples mixed with 1 wt% synthesized C–S–H gel at specific gravities of 2.00–2.57 g/cm3 resulted in suspension of about 1% of solid phase for the heavy liquid with a specific gravity of 2.10–2.19 g/cm3 and a higher light density fraction portion suspension rate for the heavy liquid with a specific gravity of 2.24 g/cm3 or higher. This suggests that C–S–H gel added to these samples was suspended in heavy liquid with a specific gravity of 2.10 g/cm3 or higher, while additional light density fractions from Kunigel V1 are present for heavy liquid with a specific gravity of 2.24 g/cm3 or higher. This is consistent with the results of XRD analysis: the light density fraction after heavy liquid separation with a specific gravity of 2.10 g/cm3 shows a primary peak (2h = 29.4, Cu Ka) and secondary peaks (2h = 32.1 and 50.1, Cu Ka) for C–S–H gel. When using heavy liquid with a specific gravity of 2.19 g/cm3 or higher, the peak of C–S–H gel was not identified because of disturbance by peaks of crystalline bentonite minerals (mainly clinoptilolite, quartz, and chalcedony). The light density fraction was also dissolved with hydrochloric acid to measure Ca and Si contents and the weight ratio of soluble CaO and SiO2. The major minerals of Kunigel V1 are smectite, chalcedony, etc., which do not dissolve extensively in low concentrated hydrochloric acid – although this cannot be assumed for some minor minerals such as calcite and dolomite. The synthetic C–S–H gel (Ca/Si mole ratio of 1.2) added to simulated alteration samples was known to be dissolved completely by 0.6 N hydrochloric acid and thus the ratio of C–S–H gel in the light density fraction could be evaluated by dissolving it in hydrochloric acid. For the simulated alteration samples mixed with 1 wt% synthesized C–S–H gel, the weight ratio of soluble CaO and SiO2 in the light density fraction was very high for heavy liquid with a specific gravity of 2.10–2.14 g/cm3 and the inclusion of bentonite materials was low. This result is consistent with the results of XRD analysis. The above results suggest that heavy liquid with a specific gravity of around 2.10 g/cm3 is optimum for selectively separating C–S–H gel present in Kunigel V1.
˚) reflection of smectite to near 2h = 6 (d value: 14.7 A caused by uptake of calcium in samples from 0 to 16 mm from the surface. Also, for samples in this depth range, peaks were broader on the low-angle side than those in the Kunigel V1 blank. Although the samples had been subjected to calcium hydroxide solution flow for 7.5 years, C–S–H gel was not identified, even after heavy liquid separation. The CEC of Kunigel V1 is about 60 meq/100 g, and the initial amount of exchangeable cations in the column sample can therefore be calculated to be 150 meq: 785 cm3 (volume of the column sample) · 1.6 g/cm3 (density) · 0.2 (ratio of bentonite) · 0.6 meq/g (CEC). The amount of calcium ions provided to the column is 41.8 meq, as calculated from the volume of solution passed through, which indicates that only 1/3 of the exchangeable cations in the column sample were replaced with calcium ions. As the calcium ions provided to the column sample would exchange with interlayer sodium ions of montmorillonite and the concentration of calcium ions in the porewater would be kept low, sufficient C–S–H gel might not be formed to allow detection using the analysis method applied in this study. Sliced samples of core containing synthetic C–S–H gel were thus used to investigate the applicability and the detection limit of this analytical technique. XRD analysis of the samples after heavy liquid separation showed both a primary peak (2h = 29.4, Cu Ka) and a secondary peak (2h = 32.1, Cu Ka) of the C–S–H gel. This result indicates that the lower detection limit of this analytical technique would be less than 1 wt% of C–S–H gel for such altered bentonite samples. It was concluded from the above results that C–S–H gel formed in the bentonite by the reaction between cement and bentonite under simulated disposal conditions could be separated and refined by this analytical technique. However, the properties and the conditions of actual C–S–H gel formed by such reactions under real conditions (e.g. difference in Ca/Si mole ratio, etc.) are not sufficiently understood. In particular, the efficiency of mineral separation using the heavy liquid may have been affected if the C–S–H gel is formed and steady attached on the minerals constituting bentonite. Therefore, the applicability of this analytical technique to heavily altered bentonite needs to be verified for further work. 5. Conclusions An analytical technique for C–S–H gel that may be formed in bentonite by the reaction with cement was studied. The following conclusions were drawn:
4.2. Analysis of altered samples from column tests The analysis of secondary minerals (C–S–H gel) obtained by heavy liquid separation from column test samples (bentonite–silica sand mixture) showed a shift of the ˚ ) caused by the (0 0 1) peak near 2h = 7.5 (d value: 11.8 A
– C–S–H gel extraction by a heavy liquid separation method was demonstrated on Kunigel V1 with 1 wt% synthesized C–S–H gel (10 mg). It was found that bromoform–methanol mixtures with a specific gravity of 2.10–2.14 g/cm3 could separate and refine synthesized
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C–S–H gel from Kunigel V1, which could be confirmed by XRD analysis. – Based on measured Ca and Si contents and the weight ratio of soluble CaO and SiO2 in the light density fraction, it could be shown that the content of C–S–H gel in the light density fraction was high in heavy liquid with a specific gravity of 2.10–2.14 g/cm3, while the inclusion of bentonite minerals was low. – XRD analysis of samples from column tests indicated only a shift of the peak near 2h = 7.5 (d value: ˚ ) representing reflection by the (0 0 1) plane of 11.8 A smectite in Kunigel V1 to near 2h = 6 (d value: ˚ ) for samples from 0 to 16 mm, attributed to 14.7 A uptake of calcium ions. – Column test samples were subjected to heavy liquid separation with a specific gravity of 2.05 g/cm3, but C–S–H gel was not identified due to very low content. The C–S– H gel artificially added to the bentonite was, however, detected by this method. The detection limit of this analytical technique is estimated as less than 1 wt%. This result implies that secondary C–S–H gel could be separated by the heavy liquid separation technique, as long as it is not attached to bentonite minerals. Acknowledgements This study is part of the project on ‘‘Long-term Verification Testing of Engineered Barriers’’ performed under contract from the Japanese Ministry of Economy, Trade and Industry (METI). We would like to express our appreciation to the members of the Examination Committee for Long-term Verification Testing of Engineered Barriers for the guidance they provided to us. We also express our special thanks to Prof. T. Ohe of Tokai University, Prof. A. Inoue of Chiba University and Prof. T. Sato of Kanazawa
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University for their guidance on research planning and report compilation.
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