Effect of aging on thermal conductivity of compacted bentonites

Engineering Geology 253 (2019) 55–63

Contents lists available at ScienceDirect

Engineering Geology journal homepage: www.elsevier.com/locate/enggeo

Effect of aging on thermal conductivity of compacted bentonites a

a,⁎

Yunshan Xu , De’an Sun , Zhaotian Zeng a b

b,⁎⁎

T

b

, Haibo Lv

Department of Civil Engineering, Shanghai University, Shanghai 200444, China Guangxi Key Laboratory of New Energy and Building Energy Saving, Guilin University of Technology, Guilin 541004, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Aging Thermal conductivity Bentonite Microstructure Smectite hydration

The thermal conductivity of compacted bentonite is one of the most crucial parameters for the safe operation of high-level radioactive waste (HLW) repositories where bentonite can act as a feasible buffer. In this study, the potential effects of aging on the thermal conductivity of Gaomiaozi (GMZ07) and Wyoming (MX80) bentonites were investigated experimentally. The bentonite specimens were prepared under different conditions, such as varying water content and dry density, through the static compaction method in a special mould. The water content of the compacted bentonite specimens was kept constant during curing periods of 0, 5, 15, 30, 60, and 100 d under constant volume conditions, and the thermal conductivity of the two bentonites was then measured using the thermal probe method. The test results showed that the thermal conductivity decreased with increasing aging time for both of the compacted bentonites, with a trend that decreased significantly at the early aging periods and then tended to be constant when the aging time exceeded 60 d. The effects of aging are more pronounced for specimens with higher dry densities and water contents. Mercury intrusion porosimetry (MIP) test results confirmed that the reduction in thermal conductivity with aging time could be attributed to smectite hydration within the bentonites during the aging process. With this hydration, part of the soil water moves into the interlayer spaces of smectite crystals, resulting in a decrease in the pore water outside the bentonite particles, which decreases the thermal conductivity of the bentonites.

1. Introduction

to the surrounding rock. This will help to prevent the peak temperature in the buffer from exceeding the thermal design criterion (100 °C) (SKBF, 1983; Cho et al., 1999), adopted because the bentonite used for the buffer may undergo mineralogical alteration (e.g. illitisation, silicification, etc.) if exposed to alkaline/saline pore fluids at high temperature (Eberl, 1978; Lee et al., 2010). In this study, the thermal conductivity of bentonites under potential disposal conditions is investigated experimentally. In the past few decades, the thermal conductivity of bentonite buffers has been measured by a number of researchers (JNC, 2000; Ould-Lahoucine et al., 2002; Cote and Konrad, 2005; Ye et al., 2010; Cho et al., 2011; Lee et al., 2016) to investigate its dependence on various conditions such as the water content, dry density, mineralogical composition, and additives; in addition, relationships with these parameters were developed to predict the thermal conductivity. Although the effects of the dry density and water content on the thermal conductivity have been extensively investigated, most measurements of the thermal conductivity have been conducted immediately after specimen preparation (i.e., compaction of bentonite). Practically, several to thousands of years may elapse from the preparation of the bentonite

Deep geological repositories 500–1000 m underground have been widely accepted as the best possible method for the permanent disposal of high-level radioactive waste (HLW) generated in nuclear reactors. The design concept of HLW repositories (Madsen, 1998) consists of a multi-barrier system including vitrified waste, a canister, buffer material, and a natural geological barrier (i.e. the surrounding rock). These barriers are designed to protect the bio-geosphere from the nuclear waste for at least 100,000 y. The buffer material, which provides environmental protection, is usually composed of compacted bentonite (Yong, 1999; Ye et al., 2009). Several bentonites, such as MX80 from USA (Madsen, 1998; Tang and Cui, 2010), Kunigel V1 from Japan (Sakashita and Kumada, 1998; Komine, 2004), FEBEX from Spain (Tripathy et al., 2004; Romero et al., 2005), FoCa from France (Marcial et al., 2002; Imbert et al., 2005), and GMZ bentonite from China (Wang et al., 2006; Ye et al., 2010; Sun et al., 2013, 2015) have been extensively studied as potential buffer materials. In an HLW repository, the buffer material should have a satisfactory thermal conductivity to quickly dissipate the decay heat from the HLW

⁎

Correspondence to: De’an Sun, 99 Shangda Road, Shanghai 200444, China. Correspondence to: Zhaotian Zeng, 12 Jiangan Road, Guilin, Guangxi Province 541004, China. E-mail addresses: [email protected] (D. Sun), [email protected] (Z. Zeng).

⁎⁎

https://doi.org/10.1016/j.enggeo.2019.03.010 Received 7 November 2018; Received in revised form 6 March 2019; Accepted 13 March 2019 Available online 14 March 2019 0013-7952/ © 2019 Elsevier B.V. All rights reserved.

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

2.2. Specimen preparation

brick to the time it experiences heating from the HLW in the repository. During this elapsed time period, the compacted bentonite may undergo changes in its pore structure and pore water distribution, which may affect its thermal properties. A problem can arise if the thermal conductivity changes during this period. In fact, the aging time is an important factor when studying bentonite behaviour. The microstructures of bentonite specimens change with the aging time, which significantly affects both the swelling potential and the ion exchange capacity of the bentonite (Day, 1994; Subba Rao and Tripathy, 2003; Delage et al., 2006). However, there is a lack of experimental data on how aging affects the thermal conductivity of bentonites. In China, GMZ bentonite, produced in the Gaomiaozi (GMZ) area of Inner Mongolia has been considered as a possible matrix for a buffer material in HLW deep geological disposal projects (Liu and Wen, 2003). The GMZ01 bentonite used in previous studies was extracted in Gaomiaozi area in 2000 (Wang, 2010). Because the GMZ01 bentonite has been almost exhausted, GMZ07 bentonite was extracted from the same stratum horizon in 2014 (Zhang et al., 2016). In this study, the effect of aging on the thermal conductivity of GMZ07 and MX80 bentonites was investigated. Samples were first compacted at target dry densities and water contents and were aged at a constant water content and constant volume for different curing time periods (i.e. 0, 5, 15, 30, 60, and 100 d). After aging, the thermal conductivity of the specimens was measured using the thermal probe method. Several factors were considered, including the dry density and water content of the aged specimens. Finally, mercury intrusion porosimetry (MIP) tests were performed to investigate changes in microstructures of compacted bentonite specimens with aging time.

To obtain specimens of two bentonites (GMZ07 and MX80) with specific water contents, powdered samples were first wetted by spraying with the required amount of distilled water to achieve the desired water content. The mixtures were then kept in closed plastic bags for 5 d. This allowed for the uniform distribution of moisture throughout the soil mass. The specimens were then prepared by static compaction of the wetted soil in a stainless-steel ring with an inner diameter of 70 mm and height of 52 mm. After compaction, the soil specimens in the stainless-steel ring were packaged in plastic wrap and sealed with adhesive tape to prevent evaporation. The constant volume holder consists of two plates and four columns. The two plates are clamped to the columns with nuts to restrict their movement, and were employed to fix the two ends (top and bottom surfaces) of the sealed bentonite specimens in order to maintain a constant volume. To eliminate the influence of fluctuations in the ambient temperature on the thermal conductivity, specimens were kept at a constant temperature (22 ± 0.5 °C) during the curing and testing periods. Changes in the weight (obtained with a balance) and volume (measured with a Vernier calliper) of each specimen were measured before and after aging. Fig. 1 shows the changes in the weight and volume of the specimens with the aging time; it is clear that the weight and volume of the specimens remained approximately unchanged during the aging process.

650

(a)

Weight of specimen (g)

2. Materials and methods 2.1. Materials The materials tested in this study are GMZ07 and MX80 bentonites. The GMZ07 bentonite was sampled from the Gaomiaozi area in Inner Mongolia, China, and MX80 bentonite was extracted from Wyoming, USA. The mineral compositions of the bentonites were determined using X-ray diffraction (XRD), and included montmorillonite, quartz, calcite, and feldspar. The main mineral compositions and physical property indexes of the bentonites tested in this study, as well as the properties of GMZ01 bentonite presented by Liu et al. (2007) and MX80L bentonite presented by Madsen (1998), are listed in Table 1. It appears that montmorillonite is the dominant mineral in the four bentonites. However, a significant difference in the proportion of quartz was observed between the two GMZ bentonites (GMZ01 and GMZ07) and the Wyoming bentonites (MX80 and MX80L): the GMZ01 tested by Liu et al. (2007) contained 15% quartz by weight, while the GMZ07 tested in the present study contained only 10.3% quartz. Madsen (1998) studied MX80L bentonite provided by Bentonite International GmbH, Duisburg, Germany and found a quartz content of 15%, whereas the MX80 bentonite tested in the present work (provided by WXDLM China LTD) contained only 7% quartz.

MX80

GMZ01a

MX80Lb

Smectite (%) Quartz (%) Specific gravity Liquid limit (%) Plastic limit (%) Plastic index

62 10 2.76 163 32 131

77 7 2.70 310 29 281

75 15 2.76 360 38 322

76 15 2.76 – – –

a b

1.8

15

1.6

25

1.6

15

1.6

5

1.4

15

1.2

15

550

500

0

20

40

60

80

100

120

Aging time (days) 198.0

(b) 3

Dry density (g/cm )

3

Volume of specimen (cm )

GMZ07

Water content (%)

600

450

Table 1 Primary mineral content and physical parameters of bentonites. Clay

3

Dry density (g/cm )

Water content (%) 15

197.9

1.2 1.6 1.6

25

197.8

1.6

15

1.8

15

1.4

15

197.7

197.6

0

20

40

5

60

80

100

120

Aging time (days) Fig. 1. Changes in the (a) weight and (b) volume of compacted bentonite specimens with aging time.

Data from Liu et al. (2007). Data from Madsen (1998). 56

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

2.3. Measurement of thermal conductivity The apparatus used to measure the thermal conductivity was a commercial thermal properties analyser, KD2 Pro (Decagon Devices, Pullman, Washington), which was equipped with three types of thermal probes (i.e. KS-1, TR-1, and SH-1) for different sample tests. The operating principle of this instrument is based on the hot wire method, which largely overcomes the disadvantages of the traditional steady state method, which can easily cause water redistribution during the testing process. The resolution of the KD2 Pro is 0.001 W/mK for measurements of the thermal conductivity. The SH-1 thermal probe used in this study is widely used for measuring the thermal conductivity of soils with an accuracy of better than 5% in the range of 0.02 to 2 W/ mK. This probe consists of dual needles 30 mm long and 1.28 mm in diameter, spaced 6 mm apart. The probe includes a heater inside one of the needles and a temperature sensor inside the other. During the testing process, a current first passes through the heater in one of the needles for a set amount of time (30 s), and the change in temperature is measured using the thermocouple in the other needle; a cooling period of 30 s follows the heating. By monitoring the heat dissipation of the linear heat resource in the specimen, the thermal conductivity of the specimen can be calculated and is displayed on the controller's screen. Before testing, the SH-1 sensor was calibrated using a standard block. Two holes 30 mm in depth and 1.3 mm in diameter with a spacing of 6 mm were drilled into the centre of the compacted bentonite specimens to ensure smooth insertion of the thermal probe (i.e. SH-1). To avoid a gap between the specimen and the thermal probe that may influence the heat conduction, a thin layer of thermal grease was smeared on the surface of the thermal probe. The thermal probe was then inserted in the hole to measure the thermal conductivity of the bentonite specimens. The results of repeated tests measuring the thermal conductivity under the same conditions are listed in Table 2. It can be seen from Table 2 that the variability in the measured values is within 0.7%. To improve the measurement accuracy further, the values obtained from repeated measurements of the thermal conductivity were averaged and reported as the final value. Fig. 2 shows a schematic diagram of the measurement of thermal conductivity by using the KD2 Pro analyser apparatus.

Constant temperature room Cable Thermal probe Specimen

Support

approximately 24 h. Then, MIP tests were performed using a PoreMaster GT33 (Quantachrome, Boynton Beach, FL, USA) with a maximum intrusion pressure of 231 MPa. From the results of the MIP tests, mercury intrusion–extrusion curves and the pore size distribution can be obtained. 3. Results 3.1. Comparison of the thermal properties of different bentonites Fig. 3 shows the measured thermal conductivity of GMZ07 and MX80 bentonites versus the water content (w) and degree of saturation (Sr) immediately after the compaction of the bentonites (i.e. at an aging time of 0 d). The results reported by Liu et al. (2007) and Madsen (1998) are also included in the figure. Liu et al. (2007) used a dual probe to measure the thermal conductivity of GMZ01 bentonite, while the thermal conductivity of MX80L bentonite was measured by Madsen (1998). It is clear that for compacted bentonite specimens with a dry density of 1.6 g/cm3, the thermal conductivity increased with increasing water content, and tended to be constant when the degree of saturation was near 100% (Fig. 3(b)). It can also be seen from Fig. 3 that immediately after compaction (0 d aging time), the thermal conductivity of the GMZ01 bentonite measured by Liu et al. (2007) was higher than that of the GMZ07 bentonite obtained in the present study at the same dry density and water content, and the thermal conductivity of the GMZ07 bentonite was higher than that of MX80 bentonite. Moreover, the MX80L bentonite had a higher thermal conductivity than MX80 bentonite. The thermal conductivities of the four bentonites are plotted versus the dry density in Fig. 4. It is clear that the thermal conductivity increased with increasing dry density. Comparing Fig. 4(a) and (b) shows that the higher the water content, the more obvious the effect of the dry density on the thermal conductivity will be.

Mercury intrusion porosimetry (MIP) tests were performed to investigate the effect of aging on the thermal conductivity of GMZ07 and MX80 bentonites from the perspective of their microstructures. Freeze-drying was used to dehydrate small blocks of compacted bentonite (approximately 1 g in weight), which were cut from the specimens after measurement of their thermal conductivity. This method can avoid changes in the soil structure during specimen preparation for the MIP tests and was performed as follows. Small sticks of wet samples were rapidly frozen using liquid nitrogen for approximately 30 min. After that, the sticks were vacuum-cooled until the water in the soils was completely dehydrated, often taking

3.2. Effect of aging on the thermal conductivity Fig. 5 shows the change in the thermal conductivity of GMZ07 and MX80 bentonites with varying aging time (i.e. 0, 5, 15, 30, 60, and 100 d) at the same dry density (ρd = 1.6 g/cm3) and different water contents. It is clear that the thermal conductivity changes with the aging time for both soil samples with the same dry density and water content. As shown in Fig. 5, the thermal conductivities of GMZ07 and MX80 bentonites decreased with increasing aging time. The thermal conductivity decreased significantly during the early curing periods, and tended to be constant when the aging time exceeded 60 d. It can also be seen from Fig. 5 that the change in thermal conductivity with the aging time was related to the water content (w). For the drier samples (w < 5%), the thermal conductivity at curing time of 0 d (immediately after compaction) is almost the same as that at 100 d, whereas the effect

Table 2 Results of repeated measurements of the thermal conductivity of GMZ07 bentonite under the same conditions.

1.6 1.6 1.6 1.6 1.6 1.6

Water content (%)

Aging time (d)

10 10 10 10 10 10

0 5 15 30 60 100

Thermal conductivity (W·(m·K)−1) No. 1

No. 2

No. 3

No. 4

Average

0.708 0.683 0.641 0.602 0.596 0.586

0.706 0.684 0.642 0.606 0.596 0.588

0.707 0.683 0.640 0.604 0.597 0.586

0.708 0.685 0.641 0.603 0.596 0.587

0.707 0.684 0.641 0.604 0.596 0.587

KD2 Pro

Fig. 2. Measurement of the thermal conductivity.

2.4. MIP test

Dry density (g·cm−3)

Steel ring

57

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

1.5

GMZ01[Liu et al., 2007] GMZ07[Present work] L MX80 [Madsen, 1998] MX80[Present work]

1.8 1.5

Thermal conductivity (W/mK)

Thermal conductivity (W/mK)

2.1

1.2 0.9 0.6 0.3

(a) 0

5

10

15

20

25

30

1.2

0.9

0.6

0.3

(a) 0.0 1.0

Water content (%)

GMZ01[Liu et al., 2007] GMZ07[Present work] L MX80 [Madsen, 1998] MX80[Present work]

1.2

1.4

1.6

1.8

2.0

2.2

3

Dry density (g/cm ) 1.8

GMZ01[Liu et al., 2007] GMZ07[Present work] L MX80 [Madsen, 1998] MX80[Present work]

1.8 1.5

Thermal conductivity (W/mK)

Thermal conductivity (W/mK)

2.1

1.2 0.9 0.6 0.3

(b) 0

20

40

60

80

0.9

0.6

(b) 0.3 1.0

1.2

1.4

1.6

1.8

2.0

2.2

3

Dry density (g/cm )

Fig. 3. Changes in thermal conductivity with varying (a) water content and (b) degree of saturation for bentonites with a dry density of 1.6 g/cm3 immediately after compaction (aging time = 0 d).

Fig. 4. Change in thermal conductivity with varying dry density for bentonites with water contents of (a) 7.0% and (b) 14.5% immediately after compaction (aging time = 0 d).

of the aging time on the thermal conductivity becomes significant when w > 5%. To quantify the decrease in thermal conductivity with aging time, a reduction ratio for the thermal conductivity is defined as follows:

λ 0 − λi × 100% λ0

1.2

100

Degree of saturation (%)

ηλi =

1.5

GMZ01[Liu et al., 2007] GMZ07[Present work] L MX80 [Madsen, 1998] MX80[Present work]

Fig. 5. It can also be seen from Fig. 7 that the effect of the aging time was more significant for samples with higher dry densities. For example, after 100 d, the thermal conductivity of GMZ07 bentonite with a low dry density (ρd = 1.2 g/cm3) changed from 0.421 (0 d aging time) to 0.348 W/mK (100 d aging time), a decrease of approximately 17.3%, which was less than the decrease observed with a dry density of 1.4 g/ cm3 (approximately 18.6%). The thermal conductivity of GMZ07 bentonite changed from 0.885 (0 d aging time) to 0.708 W/mK (100 d aging time) at a high dry density (ρd = 1.6 g/cm3), a decrease of approximately 20%, which was greater than the decrease observed at a dry density of 1.4 g/cm3. Comparing Figs. 6 and 7 shows that the reduction in thermal conductivity due to aging was more pronounced for specimens with higher water contents and high dry densities.

(1)

in which λ0 and λi are the thermal conductivities at aging times of 0 and i d, respectively, with i ranging from 0 to 100 d in this study. The relationship between the reduction ratio, ηλi, and the aging time for GMZ07 and MX80 specimens with different water contents is shown in Fig. 6. It can be seen that the reduction ratio, ηλi, increases with aging time for all specimens, and the higher the water content, the greater the reduction ratio will be. In addition, for a given aging time, ηλi is larger for the GMZ07 bentonite than that for MX80 bentonite under the same conditions (i.e. the same water content and dry density). The change in thermal conductivity with aging time for a given water content (w = 14%) and different dry densities (ranging from 1.2 to 1.6 g/cm3) is shown in Fig. 7. It is clear that the thermal conductivities of GMZ07 and MX80 bentonites decreased with increasing aging time; the thermal conductivities decreased significantly in the early aging periods and then tended to be constant when the aging time exceeded 60 d. This result is in agreement with the results shown in

3.3. Microstructure investigation For the same type of soil, the soil microstructure is an important factor affecting the time-dependence of the thermal conductivity. The soil microstructure can be simply represented by the pore size distribution (PSD), which can be determined using MIP tests. The literature indicates that in compacted clays, the primary particles are bonded 58

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

1.2

(a)

Water content (%) 1 5 10 14 20 24

1.2

Thermal conductivity (W/mK)

Thermal conductivity (W/mK)

1.5

0.9

0.6

0.3

(a) 1.0 0.8 0.6 0.4 0.2

0

20

40

60

80

3

Dry density (g/cm ) 1.2 1.4 1.6

100

0

20

80

100

0.8

1.2

(b)

Water content (%) 1 5 10 15 20 24

1.0

Thermal conductivity (W/mK)

Thermal conductivity (W/mK)

60

Aging time (days)

Aging time (days)

0.8 0.6 0.4 0.2

40

(b) 0.7 0.6 0.5 0.4 0.3 0.2

0

20

40

60

80

100

3

Dry density (g/cm ) 1.2 1.4 1.6

0

20

40

60

80

100

Aging time (days)

Aging time (days)

Fig. 7. Change in thermal conductivity with aging time at different dry densities for (a) GMZ07 and (b) MX80 bentonites with a water content of 14%.

Fig. 5. Change in the thermal conductivity with aging time at different water contents for compacted (a) GMZ07 and (b) MX80 bentonites with a dry density of 1.6 g/cm3.

together to form aggregates with different shapes and sizes (Diamond, 1971; Delage et al., 1996). The pore system can be simply separated into two types: intra-aggregate pores (pore diameter, d < 0.15 μm) within aggregates, and relatively larger inter-aggregate pores (d > 0.15 μm) between aggregates (Lloret and Villar, 2007). In this study, the total void ratio is denoted as etotal, which is determined from the soil physical properties. The void ratio corresponding to inter-aggregate pores (d > 0.15 μm) is denoted as emla, and the void ratio corresponding to the intra-aggregate pores (d < 0.15 μm) is denoted as ems, which is equal to etotal minus emla. All of these values can be obtained from the cumulative mercury intrusion curves. Fig. 8 shows the pore size distribution curves for two compacted bentonites with a dry density of 1.6 g/cm3 and initial water content of 14% immediately after compaction (0 d aging time). It is clear that both the compacted GMZ07 and MX80 bentonites exhibit bimodal PSDs, and the intra- aggregate and inter-aggregate pores can be distinguished by a pore diameter of d = 0.15 μm. Importantly, for compacted specimens with the same dry density and water content, the amount of relatively larger inter-aggregate pores in the MX80 specimen was greater than that in the GMZ07 specimen. Fig. 9 shows the cumulative intrusion curves for the compacted GMZ07 bentonite with a dry density ρd = 1.6 g/cm3 and different water contents (i.e. 5%, 14%, and 24%) at different aging times (i.e. 0, 30, and 100 d). In Fig. 9(a), after curing for 100 d, the mercury intrusion void ratio (emla) corresponding to the inter-aggregate pores of the

Fig. 6. Reduction ratio of the thermal conductivity versus the aging time (dry density = 1.6 g/cm3).

59

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

1.0

(a)

GMZ07 MX80

0.15

Cumulative void ratio em

Differential pore volume (mL/g)

0.20

0.10

0.05

0.00 -3 10

10

-2

10

-1

10

0

10

1

10

2

10

0.8 0.6

em100 em0

0.4 0.2 0.0 -3 10

3

0 day 30 days 100 days

etotal=0.725

10

-2

Pore diameter (um)

10

-1

10

0

10

1

10

2

10

3

Pore diameter (µm) 1.0

Fig. 8. Pore size distributions for two compacted bentonites with a dry density of 1.6 g/cm3 immediately after compaction (aging time = 0 d).

Cumulative void ratio em

(b)

compacted specimen with a water content of 5% was reduced from 0.371 (0 d aging time) to 0.366 (100 d aging time), a decrease of 1.3%. Because the total void ratio (etotal) remained unchanged during aging (etotal = 0.725), the void ratio corresponding to the intra-aggregate pores (ems) thus increased from 0.354 (em0) to 0.359 (em100), which is an increase of 1.4%. For the specimen with a water content of 14%, emla changed from 0.368 (0 d aging time) to 0.313 (100 d aging time), which is a decreased of 14.9%, while ems increased from 0.357 (em0) to 0.412 (em100), which is an increase of 15.4% (Fig. 9(b)). As shown in Fig. 9(c), similar changes in the inter- and intra-aggregate pores were observed for the samples with a water content of 24%, i.e. emla decreased by 30.9% and ems increased by 33.1%. Globally, after curing for 100 d, the inter-aggregate pores decreased, whereas the intra-aggregate pores increased. These results agree with those reported by Delage et al. (2006), who observed a significant change in the microstructure with aging time, characterised by a decrease in the inter-aggregate pores and an increase in the very thin pores not intruded by mercury. Comparing Figs. 9(a), (b), and (c) shows that for the same dry density, the greater the water content, the more significant the change in the microstructure with aging time will be. Fig. 10 shows the cumulative intrusion curves for compacted GMZ07 specimens with w = 14% and different dry densities (1.2, 1.4, and 1.6 g/cm3) at different aging periods (i.e. 0, 30, and 100 d). The cumulative intrusion curves for compacted GMZ07 bentonites with different aging times were similar. With increasing aging time, the inter-aggregate pores decreased while the intra-aggregate pores increased, which agrees with the results shown in Fig. 9. Importantly, it can also be seen from Fig. 10 that for similar aging times, the higher the dry density, the more significant the change in the microstructure will be. For example, for the specimen with a lower dry density (1.2 g/cm3), emla changed from 0.795 (0 d aging time) to 0.782 (100 d aging time), which is a decrease of 1.6%, while ems increased from 0.505 (em0) to 0.518 (em100), which is an increase of 2.6% (Fig. 10a). Fig. 10(b) shows the cumulative intrusion curves obtained at the same aging times with the same water content (14%) and a higher dry density (1.4 g/cm3). It can be seen that after aging for 100 d, emla was decreased by 6.2%, and ems was increased by 7%, and thus the changes in both the inter- and intra-aggregate pores are greater than those observed at a dry density of 1.2 g/cm3. As shown in Fig. 10(c), greater changes in the void ratios with aging time were observed at a dry density of ρd = 1.6 g/cm3, with a significant decrease (14.9%) in the inter-aggregate pores (d > 0.15 μm) from 0.368 (0 d) to 0.313 (100 d) and a clear increase (approximately 15.4%) in the intra-aggregate pores.

0.8

0 day 30 days 100 days

etotal=0.725

0.6

em0

em100

0.4 0.2 0.0 -3 10

10

-2

10

-1

10

0

10

1

10

2

10

3

Pore diameter (µm) 1.0

Cumulative void ratio em

(c) 0.8

0 day 30 days 100 days

etotal=0.725

0.6

em100 em0

0.4 0.2 0.0 -3 10

10

-2

10

-1

10

0

10

1

10

2

10

3

Pore diameter (µm) Fig. 9. Relationship between the pore diameter and cumulative intruded void ratio for compacted GMZ07 bentonite with a dry density of 1.6 g/cm3 and water contents of (a) 5%, (b) 14%, and (c) 24%.

4. Discussion Comparison of the thermal conductivities of different bentonites immediately after compaction (i.e. at an aging time of 0 d) indicated that the thermal conductivity of the bentonites increased with increasing water content and dry density, and the higher the water 60

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

bentonite particles was improved, resulting in better heat conduction and increased thermal conductivity. On the other hand, as the water content increased, the additional increase in the water modified the extent or quality of the contacts between bentonite particles and thus created more effective heat flow paths. Immediately after compaction (0 d aging time), for the same dry density and water content, GMZ01 had higher thermal conductivity values than those of GMZ07, while the thermal conductivity of the latter was higher than that of MX80. A possible explanation is that the difference in the mineralogical composition of bentonites results in the observed difference. Although Madsen (1998) did not report any details of the measurement methods used in that study, other studies have shown that the thermal conductivity measured using the thermal probe method is consistent with that reported by Madsen (Tang et al., 2008). Tang et al. (2008) also observed that the proportion of quartz significantly affected the thermal conductivity of bentonites, because the thermal conductivity of quartz (7.7 W/mK) is much higher than that of other minerals (2 W/mK). Accordingly, it was suggested that the higher the proportion of quartz, the higher the thermal conductivity of bentonites would be. As indicated in Table 1, the GMZ01 bentonite used by Liu et al. (2007) contained 15% quartz, while the GMZ07 and MX80 bentonites used in this study contained 10% and 7% quartz, respectively. This can explain the difference in the measured thermal conductivities for different bentonites (Figs. 3 and 4). In addition, the difference in the quartz contents of the MX80 (present study) and MX80L (Madsen, 1998) bentonites can explain the higher thermal conductivity of the MX80L compared to MX80 bentonite. Previous studies have shown that the thermal conductivity of aggregated samples decreased with increasing aggregate size (Ju et al., 2011), and a decrease in the contact area between heat-conductive solids may reduce the thermal conductivity (Usowicz et al., 2013). As shown in Fig. 8, the measured pore size distribution curves for the two compacted bentonites immediately after compaction condition indicates that the amount of inter-aggregate pores in the MX80 specimen was greater than that in the GMZ07 specimen. This implies that for GMZ07 bentonite, the contact area between the bentonite particles was larger, leading to better heat conduction and increased thermal conductivity. Therefore, this can partly explain the higher thermal conductivity of compacted GMZ07 bentonite than that of compacted MX80 bentonite (Fig. 3). The results in Figs. 5 and 7 show that the thermal conductivity of compacted GMZ07 and MX80 specimens decreased with increasing aging time, and the effects of aging on the thermal conductivity were more pronounced in specimens with higher dry densities and water contents. The results of the microstructure investigation (Figs. 9 and 10) showed that during the aging process, the microstructures of the aged GMZ07 specimens changed with increasing aging time: (i) the interaggregate porosity decreased and the intra-aggregate porosity increased; (ii) at the same water content, the greater the dry density the more significant the change in the microstructure would be; and (iii) at the same dry density, the higher the water content, the more significant the change in the microstructure would be. Actually, as described in the previous sections, the microstructure (e.g. the aggregates size) also affected the thermal conductivity of the compacted bentonites. Therefore, the reduction in thermal conductivity of bentonite specimens with aging time is thought to be related to changes in the microstructures with aging time. As reported by Delage et al. (2006), changes in the microstructure with aging time may be attributed to smectite hydration in the compacted specimens during aging. They suggested that changes in the microstructures of bentonites with aging time can characterise the extent of the smectite hydration. Detailed investigations conducted on the hydration of smectites (e.g. Norrish, 1954; Sposito and Prost, 1982; Bird, 1984; Saiyouri et al., 2000) have shown that as the hydration proceeds, part of the soil water moves toward the interlayer spaces of the smectite crystals, resulting in a decrease in the pore water outside

1.6

Cumulative void ratio em

(a)

etotal=1.3

0 day 30 days 100 days

1.2

em100 em0 0.8

0.4

0.0 -3 10

10

-2

10

-1

10

0

10

1

10

2

10

3

10

3

Pore diameter (µm)

Cumulative void ratio em

1.2

(b)

etotal=0.971

1.0 0.8

0 day 30 days 100 days

em0

em100 0.6 0.4 0.2 0.0 -3 10

10

-2

10

-1

10

0

10

1

10

2

Pore diameter (µm) 1.0

Cumulative void ratio em

(c) 0.8

0 day 30 days 100 days

etotal=0.725 em100 em0

0.6 0.4 0.2 0.0 -3 10

10

-2

10

-1

10

0

10

1

10

2

10

3

Pore diameter (µm) Fig. 10. Relationship between the pore diameter and cumulative intruded void ratio for compacted GMZ07 bentonite with a water content of 14% and dry densities of (a) 1.2 g/cm3, (b) 1.4 g/cm3, and (c) 1.6 g/cm3.

content, the more obvious the effects of the dry density on the thermal conductivity would be. This is because the thermal conductivity of the water in soils is much higher than that of the air in the voids of the unsaturated bentonite, leading to an increase in the thermal conductivity as water replaces the air with increasing water content of the bentonites. As the dry density increased, the contact between the 61

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

microstructure will be. iv) The reduction in thermal conductivity of bentonite specimens with aging time is thought to be mainly attributed to smectite hydration within the compacted bentonites. With this hydration, part of the soil water moves into the interlayer spaces of smectite crystals, resulting in a decrease in the pore water outside the bentonite particles, which decreases the thermal conductivity of the bentonites. This inference is confirmed by the microstructure investigation of specimens during the aging process, i.e. the inter-aggregate pores decrease and the intra-aggregate pores increase with aging time.

the bentonite particles. It has been established that the thermal conductivity of bentonites increases with increasing water content (Madsen, 1998; Liu et al., 2007; Tang et al., 2008; Ye et al., 2010; Lee et al., 2016). In fact, the pore water outside the bentonite particles plays a major role in the heat conduction of water in soil. Ju et al. (2011) suggested that water was held tightly in the intra-aggregate pores, effectively connecting the primary particles, while water in the interaggregate pores contributed mainly to the formation of a water film, which significantly increased the contact points and area per unit volume. Accordingly, it was suggested that the movement of water from the inter- or intra- aggregate pores to the bentonite particle pores would disrupt the continuity of the water film, thus significantly reducing the contacts between the soil aggregates or particles. On the other hand, as reported by Chen et al. (2015), the effect of the water in inter-aggregate pores on the heat conduction of unsaturated compacted bentonites is dominant, compared to that of water in the smaller pores (e.g. intraaggregate pores). Therefore, it is considered that the effect of aging on the thermal conductivity is mainly attributed to smectite hydration during aging. With hydration, the pore water outside the bentonite particles decreases, which reduces the thermal conductivity of the bentonites. This is capable of explaining to observed decrease in the thermal conductivity of bentonites with increasing aging time (Fig. 5). In addition, this can explain that for similar aging times, the greater the water content, the more significant the effect of aging on the thermal conductivity would be (Fig. 6). As discussed above, the decrease in thermal conductivity with aging time is mainly attributed to smectite hydration within the bentonites. With this hydration, part of the soil water moved into the interlayer spaces of smectite crystals, resulting in decreased pore water outside the bentonite particles, which decreased the thermal conductivity of the bentonites. The greater the content of the smectite hydration, the greater the reduction in the thermal conductivity of aged bentonite specimens will be. The results in Fig. 10 show that the higher the dry density, the more significant the changes in the microstructure will be. Therefore, the above phenomenon can be used to explain why the effect of aging on the thermal conductivity was more significant for a denser specimen (1.6 g/cm3) than for a looser one (1.2 g/cm3), as shown in Fig. 8.

Conflict of interest The authors declare that there are no conflicts of interest associated with publication. Acknowledgements This work was supported by the Project of Guangxi Key Laboratory of New Energy and Building Energy Saving [Grant Nos. 17-J-22-1, 17-J21-2], the National Natural Science Foundation of China [Grant No. 51568014 and 41630633] and the Natural Science Foundation of Guangxi Province of China [Grant No. 2018GXNSFAA138182 and 41630633]. References Bird, P., 1984. Hydration phase diagrams and friction of montmorillonite under laboratory and geologic conditions with implications for shale compaction, slope stability and strength of fault gauge. Tectonophysics 107 (3–4), 235–260. Chen, Y.F., Wang, M., Zhou, S., Hu, R., Zhou, C.B., 2015. An effective thermal conductivity model for unsaturated compacted bentonites with consideration of bimodal shape of pore size distribution. Sci. China Technol. Sc. 58 (2), 369–380. Cho, W.J., Lee, J.O., Kang, C.H., Chun, K.S., 1999. Basic physicochemical and mechanical properties of domestic bentonite for use as a buffer material in a high-level radioactive waste repository. J. Korean Soc. 31 (6), 39–50. Cho, W.J., Lee, J.O., Kwon, S., 2011. An empirical model for the thermal conductivity of compacted bentonite and a bentonite-sand mixture. Heat Mass Transf. 47 (11), 1385–1393. Cote, J., Konrad, J., 2005. A generalized thermal conductivity model for soils and construction materials. Can. Geotech. J. 42 (42), 443–458. Day, R.W., 1994. Swell-shrink behaviour of expansive compacted clay. J. Geotech. Eng. ASCE 120 (3), 618–623. Delage, P., Audiguier, M., Cui, Y.J., Howat, M.D., 1996. Microstructure of a compacted silt. Can. Geotech. J. 33, 150–158. Delage, P., Marcial, D., Cui, Y.J., Ruiz, X., 2006. Aging effects in a compacted bentonite: a microstructure approach. Géotechnique 56 (5), 291–304. Diamond, S., 1971. Microstructure and pore structure of impact-compacted clays. Clay Clay Miner. 19 (4), 239–249. Eberl, D.D., 1978. The reaction of montmorillonite to mixed-layer clay: the effect of interlayer alkali and alkaline-earth cations. Geochim. Cosmochim. Ac. 42 (1), 1–7. Imbert, C., Olchitzky, E., Lassabatère, T., Dangla, P., Courtois, A., 2005. Evaluation of a thermal criterion for an engineered barrier system. Eng. Geol. 81 (3), 269–283. JNC, 2000. H12 Project to Establish Technical Basis for HLW Disposal in Japan, Support Report 2. Japan Nuclear Cycle Development Institute. Ju, Z.Q., Ren, T.S., Hu, C.S., 2011. Soil thermal conductivity as influenced by aggregation at intermediate water contents. Soil Sci. Soc. Am. J. 75 (1), 26–29. Komine, H., 2004. Simplified evaluation for swelling characteristics of bentonites. Eng. Geol. 71 (3), 265–279. Lee, J.O., Kang, I.M., Cho, W.J., 2010. Smectite alteration and its influence on the barrier properties of smectite clay for a repository. Appl. Clay Sci. 47 (1), 99–104. Lee, J.O., Choi, H., Lee, J.Y., 2016. Thermal conductivity of compacted bentonite as a buffer material for a high-level radioactive waste repository. Ann. Nucl. Energy 94, 848–855. Liu, Y.M., Wen, Z.J., 2003. Study on clay-based materials for the repository of high-level radioactive waste. J. Miner. Petrol. 23 (4), 42–45 (in Chinese with English abstract). Liu, Y.M., Cai, M.F., Wang, J., 2007. Thermal conductivity of buffer material for highlevel waste disposal. Chin. J. Rock Mech. Eng. 26 (S2), 3891–3896 (in Chinese with English abstract). Lloret, A., Villar, M.V., 2007. Advances on the knowledge of the thermo-hydro-mechanical behaviour of heavily compacted FEBEX bentonite. Phys. Chem. Earth 32 (8–14), 701–715. Madsen, F.T., 1998. Clay mineralogical investigations related to nuclear waste disposal. Clay Miner. 33 (1), 109–129. Marcial, D., Delage, P., Yu, J.C., 2002. On the high stress compression of bentonites. Can. Geotech. J. 39 (4), 812–820.

5. Conclusions The effects of the aging time on the thermal conductivity of compacted Gaomiaozi (GMZ07) and Wyoming (MX80) bentonites were investigated experimentally. The influences of the dry density and water content on the effects of aging were also investigated. MIP tests were performed to study the changes in the microstructures of compacted bentonite specimens with aging time. Knowledge regarding the effects of aging on the thermal properties of buffer materials is helpful to accurately assess the thermal performance of HLW repositories and is useful for the buffer design of the repository. The following conclusions can be drawn from this study: i) The thermal conductivity of compacted GMZ07 bentonite is higher than that of compacted MX80 bentonite immediately after compaction of the bentonite. Moreover, after the same aging time, the compacted GMZ07 bentonite exhibits a more significant decrease in thermal conductivity than compacted MX80 bentonite. ii) The thermal conductivities of the compacted GMZ07 and MX80 specimens decrease with increasing aging time; the thermal conductivity decreases significantly at early aging periods, and then tends to be constant when the aging time exceeds 60 d. The effects of aging on the thermal conductivity are more pronounced in specimens with higher dry densities and water contents. iii) Inter-aggregate pores decrease and intra-aggregate pores increase with increasing aging time. The higher the water content and dry density, the more obvious the effects of aging on the change in the 62

Engineering Geology 253 (2019) 55–63

Y. Xu, et al.

Tang, A.M., Cui, Y.J., 2010. Effects of mineralogy on thermo-hydro-mechanical parameters of MX80 bentonite. J. Rock Mech. Geotech. Eng. 2 (1), 91–96. Tang, A.M., Cui, Y.J., Le, T.T., 2008. A study on the thermal conductivity of compacted bentonites. Appl. Clay Sci. 41 (3), 181–189. Tripathy, S., Sridharan, A., Schanz, T., 2004. Swelling pressures of compacted bentonites from diffuse double layer theory. Can. Geotech. J. 41 (3), 437–450. Usowicz, B., Lipiec, J., Usowicz, J.B., Marczewski, W., 2013. Effects of aggregate size on soil thermal conductivity: comparison of measured and model-predicted data. Int. J. Heat Mass Tran. 57 (2), 536–541. Wang, J., 2010. High-level radioactive waste disposal in China: update 2010. J. Rock Mech. Rock Eng. 2 (1), 1–11. Wang, J., Su, R., Chen, W.M., Guo, Y.H., Wen, Z.J., Liu, Y.M., 2006. Deep geological disposal of high-level radioactive wastes in China. Chin. J. Rock Mech. Eng. 25 (4), 649–658. Ye, W.M., Cui, Y.J., Qian, L.X., Chen, B., 2009. An experimental study of the water transfer through confined compacted GMZ bentonite. Eng. Geol. 108 (3), 169–176. Ye, W.M., Chen, Y.G., Chen, B., Wang, Q., Wang, J., 2010. Advances on the knowledge of the buffer/backfill properties of heavily-compacted GMZ bentonite. Eng. Geol. 116 (1) 21–20. Yong, R.N., 1999. Soil suction and soil-water potentials in swelling clays in engineered clay barriers. Eng. Geol. 54 (1–2), 3–13. Zhang, L., Sun, D.A., Jia, D., 2016. Shear strength of GMZ07 bentonite and its mixture with sand saturated with saline solution. Appl. Clay Sci. 132, 24–32.

Norrish, K., 1954. Crystalline swelling Of montmorillonite: manner of swelling of montmorillonite. Nature 173 (4397), 256–257. Ould-Lahoucine, C., Sakashita, H., Kumada, T., 2002. Measurement of thermal conductivity of buffer materials and evaluation of existing correlations predicting it. Nucl. Eng. Des. 216 (1), 1–11. Romero, E., Villar, M.V., Lloret, A., 2005. Thermo-hydro-mechanical behaviour of two heavily overconsolidated clays. Eng. Geol. 81 (3), 255–268. Saiyouri, N., Hicher, P.Y., Tessier, D., 2000. Microstructural approach and transfer water modelling in highly compacted unsaturated swelling clays. Mech. Cohes. Frict. Mat. 5, 41–60. Sakashita, H., Kumada, T., 1998. Heat transfer model for predicting thermal conductivity of highly compacted bentonite. J. Atom. Energy Soc. Jpn. 40 (3), 235–240. SKBF, K.B.S., 1983. Final Storage of Spent Nuclear Fuel-KBS-3. Swedish Nuclear Fuel Supply Company, Stockholm. Sposito, G., Prost, R., 1982. Structure of water adsorbed on smectites. Chem. Rev. 82 (6), 552–573. Subba Rao, K.S., Tripathy, S., 2003. Effect of aging on swelling and swell-shrink behaviour of compacted expansive soil. ASTM Geotech. Test J. 26 (1), 36–46. Sun, D.A., Zhang, J.Y., Zhang, J.R., Zhang, L., 2013. Swelling characteristics of GMZ bentonite and its mixtures with sand. Appl. Clay Sci. 83, 224–230. Sun, D.A., Zhang, L., Li, J., Zhang, B.C., 2015. Evaluation and prediction of the swelling pressures of GMZ bentonites saturated with saline solution. Appl. Clay Sci. 105, 207–216.

63