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ICE-BENTONITE POWDER MIXING METHOD TO IMPROVE THE HOMOGENEITY

OF COMPACTED BENTONITE IN AN INITIAL SAMPLE PREPARATION STAGE

YU PENG, HUYUAN ZHANG*, BINGZHUO YANG, XUEWEN WANG, XIANXIAN SHAO, AND PING LIU

Key Laboratory of Mechanics on Disaster and Environment in Western China (Lanzhou University), Ministry of Education,Lanzhou, 730000, China

Abstract—Bentonite is considered as an ideal buffer/backfill material for preparing an engineering barrierfor high-level radioactive waste (HLW) disposal. During initial sample preparation, the tendency of wetbentonite powder to gather into large agglomerates and the water to be spread unevenly in the traditionalwater content adjustment process decreases the homogeneity of compacted bentonite. The main purpose ofthis study was to solve this problem by applying a new wetting method, which mixes ice powder withbentonite powder (the ice-bentonite mixing method). This new method was used to adjust the waterdistribution in Gaomiaozi County, China (GMZ) bentonite powder and was compared to the traditionalspray method. The screening method was used to separate macro-agglomerates (5 0.25 mm) from thewater and bentonite mixture. The properties, the content of the various size agglomerates in loose mixtures,and the heterogeneity defects observed in compacted bentonite were compared. An index (P) was definedto quantitatively evaluate the water distribution in a loose bentonite/water mixture. Macro-agglomerates inloose mixtures produced heterogeneities in water content, density, and shrinkage. By using the ice-bentonite mixing method, fewer macro-agglomerates were formed and a homogeneous distribution ofwater was produced in the compacted bentonite. A homogeneous water distribution had the tendency todecrease the number of shrinkage cracks after the drying process and to maintain high mechanical strengthin the compacted bentonite. Although the production of ice powder was laborious, the ice-bentonite mixingmethod has workability advantages: (i) a high mixing efficiency, (ii) a low mass loss rate, and (iii) a smalldeviation between measured water content and target water content. The low thawing efficiency of ice-bentonite mixtures can be solved by using a microwave-assisted thawing method. This research canimprove the sample preparation method used to produce compacted buffer/backfill materials for HLWdisposal.

Key Words—Agglomerates, Bentonite, Compacted Buffer/Backfill Materials, Homogeneity, Ice-Bentonite Mixing Method, Water Content.

INTRODUCTION

Deep geological disposal is considered to be an

effective method for the disposal of high-level radio-

active waste (HLW) in many countries (Marsily et al.,

1977; Pusch, 1983; KBS-3 1983; Neretnieks, 1985;

Birkholzer et al., 2012; Dohrmann et al., 2013; Van

Geet and Dohrmann, 2016; Kaufhold et al., 2016), using

compacted bentonite as the buffer/backfill material

(Pusch, 1979; Ye et al., 2010; Sellin and Leupin, 2014).

The compacted bentonite provides both mechanical and

chemical protection around the canister, retards the

outward movement of any radionuclides that might

escape from the canister, and conducts heat from the

canister to the surrounding rock (Pusch, 1983; Chapman

and McKinley, 1989; Oscarson et al., 1994; Vieno and

Ikonen, 2005; Chegbeleh et al., 2008; Zhou et al., 2013;

Hedin and Andersson, 2014). In China, research into

buffer/backfill materials is currently in a transition stage

from small-scale specimens to large-scale blocks (Luo et

al., 2004; Wang, 2010), and scientists are facing the

challenge of how to produce a homogeneous compacted

bentonite in the initial sample preparation stage.

‘‘Homogeneity’’ is defined as the state of having identical

properties in terms of composition and structure.

Generally, an inhomogeneous composition can lead to

mechanically and chemically weak areas in the bentonite

barrier, which can induce structural defects, such as

crevices and flow channels (Becher, 1995; Laaksonen,

2010; Dixon et al., 2011). Heterogeneous structural

defects (crevices, joints, and flow channels) in the

bentonite buffer could obviously degrade the mechanical

and chemical barrier around the canister (KBS-3, 1983;

Pusch et al., 1985; Inspectorate et al., 2003). Moreover,

the nuclides and gases derived from radionuclide decay

could migrate away from the canister along the hetero-

geneous defect areas (Harrington and Horseman, 2003;

Laaksonen, 2010). With respect to compacted bentonites

used as buffer in HLW disposal, heterogeneities in

* E-mail address of corresponding author:

[email protected]

DOI: 10.1346/CCMN.2016.064039

Clays and Clay Minerals, Vol. 64, No. 6, 706–718, 2016.

This paper is published as part of a special section on the

subject of ‘Clay and fine particle-based materials for

environmental technologies and clean up,’ arising out of

presentations made during the 2015 Clay Minerals

Society-Euroclay Conference held in Edinburgh, UK..

composition (density, water content) or structure (cracks,

joints, and flow channels) have been studied in the

process of transforming loose material into the initial

compacted bentonite, from the initial compacted bento-

nite to integrated buffer, and subsequent operation of the

barrier (see Table 1) (Ichikawa et al., 1999; Bastiaens et

al., 2007; Muurinen et al., 2007; Johannesson et al.,

2008; Laaksonen, 2010). Many factors (time, hydraulics,

chemistry, radiation, and mechanics) and associated

actions (radionuclide migration and corrosion by water,

heat, and gases) can work together to promote the

formation and development of key homogeneity defects,

such as flow channels that degrade the effectiveness of

the buffer (Inspectorate et al., 2003; Rasilainen, 2004;

Karnland et al., 2000; Reid et al., 2015). To decrease

defects during bentonite barrier operation, the hetero-

geneity of buffer materials before use in initial sample

preparation, sealing, and homogenization stages of

compacted bentonite production should also be thor-

oughly examined (Johanesson et al., 1999; Bastiaens et

al., 2007; Kobayashi et al., 2008; Kaufhold et al., 2010;

Zhang et al., 2012). Studies in joint sealing and block

homogenization showed that hydraulic condition, time,

and density were the main factors. Associated actions,

such as permeation, swelling, and joint filling could work

together to integrate buffers by homogenizing block

density and sealing defects, such as cracks and joints

(Ichikawa et al., 1999; Bastiaens et al., 2007; Murinen et

al., 2007; Koch, 2008). Studies of the initial sample

preparation stage showed that the two-phase, liquid-solid

differences and the high plasticity of bentonites cause

bentonite powders to aggregate easily into macro-

agglomerates (Kobayashi et al., 2008; Kaufhold et al.,

2010; Zhang et al., 2012). The key causes for bentonite

heterogeneity in initial sample preparation were due to

differences between the macro-agglomerate properties

(water content, hardness) and the properties of the

surrounding bentonite powder (Becher et al., 1995; Cui

et al., 2012; Zhang et al., 2012), followed by the

homogeneous densi ty induced by compact ion

(Johanesson et al., 1999; Ritola and Pyy, 2012). The

spray method to wet bentonite has been widely used to

prevent the aggregation of fine bentonite powder into

macro-agglomerates, but it is limited by a low mixing

efficiency and bentonite adherence to the mixing machine

(Nienow et al., 1997; Atiemo-Obeng et al., 2004;

Kobayashi et al., 2008; Zhang et al., 2012). Another

method to wet bentonite efficiently and homogeneously

as an industrial process is to mix ice powder with

bentonite powder and is termed the ‘‘ice-bentonite mixing

method’’ as proposed by Kobayashi et al. (2008). Studies

by Kobayashi et al. (2008) showed that the ice-bentonite

mixing method leads to less agglomeration and an ice-

bentonite mixture of lower viscosity; thereby providing

better miscibility than the traditional spray method. The

important effects of the ice-bentonite mixing method on

bentonite homogeneity (water content distribution, den-

sity, shrinkage, mechanical strength), however, were not

reported because the macro-agglomerates produced by

adjusting the water content of the mixture were not

separated for further testing (Kobayashi et al., 2008;

Kaufhold et al., 2010; Zhang et al., 2012). Traditionally,

interest has focused on natural agglomerates or agglom-

erates produced by additives, not on agglomerates

produced by adjusting the water content during sample

preparation (Murungu et al., 2003; Lado et al., 2004;

Camprubı et al., 2014). In the present comparative study,

both loose mixtures and compacted specimens were

tested to evaluate the effects of different wetting methods

on compacted bentonite. The purpose of this paper was,

therefore, to verify that the ice-bentonite mixing method

can improve the homogeneity of compacted bentonite and

provide a better material to prepare buffer/backfill

samples for HLW disposal in China.

Table 1. Heterogeneity study of bentonite buffer material in different key stages.

ProcessKey heterogeneity

study stagesKey heterogeneity

defectsMain associated

actionMain influence factors

Loose material tocompacted bentonite

Initial samplepreparation

Agglomerates,Uneven density

Mixing,Compacting

Water content,Plasticity of bentonite,Liquid-solid phase differences,Compaction technology

Compacted bentonite tointegrate buffer

Sealing andhomogenization

Joints,Cracks

Permeation,Swelling,Joints filling

Hydraulic condition,Time,Density

Operation Long-term working Flow channel

Corrosion,Migration (Radio-nuclide, water,heat, and gas)

Time,Hydraulic condition,Chemical condition,Radiation condition,Mechanical condition

Vol. 64, No. 6, 2016 Ice-bentonite powder mixing method 707

MATERIALS AND METHODS

Materials

The Gaomiozi (GMZ) bentonite used in this test is a

natural Na-bentonite, which was mined near the town of

Gaomiaozi in Inner Mongolia, China (Zhang et al.,

2009). The bentonite powder was sieved through a

0.10 mm soil sieve (HKZY-1, Beijing Centrwin

Technologic Corporation, Beijing, China) to remove

the larger macro-agglomerates from the bentonite

powder. The bentonite consists of approximately 75%

smectite, 11% quartz, 4% feldspar, 7% a-cristobalite,1% kaolinite, and 1% illite as determined by quantitative

X-ray diffraction (XRD) analysis, and the specific

surface area measured using ethylene glycol monoethyl

ether adsorption was approximately 570 m2/g (Liu et al.,

2001; Wen, 2005). The air-dried water content of the

bentonite was 10.53%, the cation exchange capacity

(CEC) was approximately 760 mmol/kg (Liu et al.,

2001; Qin et al., 2008), the plastic limit was 32.34%, and

the liquid limit was 228.00% (Zhang et al., 2009). The

water used was distilled water with a resistivity of

approximately 200 Om. The bentonite grain size dis-

tribution was measured using a Mastersizer 2000

(Malvern Instruments Ltd, Malvern, Worcestershire,

UK) laser particle size analyzer (Figure 1).

Bentonite wetting (water content adjustment) methods

The water content was adjusted using either the ice-

bentonite mixing method or the traditional spray method.

Two kg of dried GMZ bentonite sample was mixed with

water to reach different water contents, such as 5%,

10%, 15%, 20%, 25%, and 30% to bentonite by w/w. In

the ice-bentonite mixing method, before mixing, both

the liquid water and the bentonite powder were frozen in

a freezer at �25ºC for 24 h. The ice from the water was

then shattered into ice powder using an electronic ice

crusher. The shattered ice was sieved to <1 mm and was

uniformly mixed with bentonite powder in a container

under an ambient temperature of �8ºC and a relative

humidity of 41%. Afterward, two methods were used to

thaw the ice-bentonite mixtures. Method 1: The frozen

mixture was stored in plastic bags in a closed glass

humidifier with a constant air humidity controlled by a

saturated NaCl solution for ~60 h at room temperature

(21�23ºC). Method 2: Alternatively, the ice-bentonite

mixture was thawed using a 350W Midea MM721AAU-

PU microwave oven (Midea Corporation, Foshan,

Guangdong Province, China). For the traditional spray

method of wetting, small amounts of water were first

sprayed onto the surfaces of the bentonite powder then

the mixture was stirred for about 60 s. The above steps

were repeated for each bentonite sample until the desired

water content was reached.

For simplicity, bentonite specimens were named

using two letters: S indicates the spray method, while I

indicates the ice-bentonite mixing method. The symbol

os was chosen to represent the water content of bentonite

adjusted using the spray method while oI represents the

water content of bentonite adjusted using the ice-

bentonite mixing method. The number after the speci-

men name indicates the target water content. For

example, the symbol S-5 means a specimen prepared

by the spray method with a target water content of

5 wt.%. Similarly, I-10 means a specimen prepared by

the ice-bentonite mixing method with a target water

content of 10 wt.%.

Separation method for macro-agglomerates

Macro-agglomerates were separated by sieve screening

(Yoder 1936; Horn et al., 1995; Shi et al., 1998). A top to

bottom nest of sieves with opening diameters of 10, 5, 2,

1, 0.50, and 0.25 mm was selected based on other studies

(Becher et al., 1995; Ma et al., 2014). The smallest size

screen of 0.25 mm was included in this test because

agglomerates <0.25 mm apparently have little influence

on compacted soil properties (Candan and Broquen, 2009;

Stavi et al., 2010; Wuddivira et al., 2010). Because

evaporation has a large impact on water content

measurements, the screening was, therefore, conducted

at an air temperature of 5ºC and a relative humidity of

92%, which was produced using an air humidifier in a

closed room without air circulation. To further minimize

evaporation, a large plastic bag was used to cover the

uppermost sieve in the nest and another plastic bag was

used to surround the sieves to minimize evaporation; then,

as soon as approximately 450 g of macro-agglomerates

was accumulated to obtain a weight measurement, it was

transferred into a small plastic zipper-seal bag and sealed.

After finishing the screening, the remaining macro-

agglomerates that accumulated on the different sieves

were collected into large plastic bags and sealed. The

water content of each sample, from both the large and

small bags, was determined gravimetrically.

Test of macro-agglomerate properties

Different size agglomerates were inevitably gener-

ated during the process of adjusting water content. SuchFigure 1. Cumulative grain size distribution curve of bentonite

powder.

708 Peng et al. Clays and Clay Minerals

macro-agglomerates are considered to be the major

factor responsible for the heterogeneities in compacted

expansive soils (Barden and Sides, 1970; Cheng et al.,

2008; Kobayashi et al., 2008; Zhang et al., 2012), which

is assumed to be the case here also. The water contents,

dry densities, and shrinkage ratios of agglomerates of

different sizes were measured.

Water contents (o, refers to the %moisture content)

were measured using the oven dry method using an oven

temperature of 105ºC.

o ¼ m�md

m� 100 ð1Þ

where o is water content (%); m is initial wet weight (g);

and md is oven dry weight (g).

Dry densities were determined by dividing the sample

weight by the volume measured using a graduated

cylinder partly filled with liquid paraffin. The %volume

shrinkage of agglomerates was determined by the

volume change that occurred after wet agglomerates

were dried. The volume and weight parameters of the

agglomerates were defined using the following equa-

tions:

rd0 ¼md

V0¼ mð1þ o=100Þ �

1V0

ð2Þ

dv ¼V0 � V1

V0� 100 ð3Þ

where rd0 is the dry density of wet agglomerates

(g/cm3); md is the dry weight of agglomerates (g); V0

is the volume of wet agglomerates (cm3); m is the weight

of wet agglomerates (g); o is the water content of

agglomerates (%); dv is the %volume shrinkage (%); and

V1 is the volume of dry agglomerates (cm3).

Observed homogeneity of compacted bentonite

specimens

The heterogeneity of a loose bentonite mixture could

affect the heterogeneity of the compacted bentonite

produced. This hypothesis was tested using the methods

of Cai et al. (2005) and Hoffmann et al. (2007) in which

mixtures containing agglomerates were statically com-

pacted in a 25 mm high by 72.4 mm diameter confining

ring to a target dry density of 1.50 g/cm3 and a target

water content of 20%. After compacting, the bentonite

was stored in plastic bags in a closed glass humidifier

with a constant air humidity controlled by a saturated

NaCl solution for about 24 h before the water content

was measured.

During static compaction, surface desiccation cracks

were photographed using a model XQ1 Fujifilm camera

(Fujifilm, Shanghai, China) with color film. Each

specimen was evenly split into two parts. In one part,

the homogeneity of the water hydration, the shapes of

vertical section faces, and the presence of air dry cracks

in vertical sections at 25�27% relative humidity and at

21�24ºC were determined. The second part of the

specimen was dried for 24 h at 105ºC in an oven. After

observing the surface cracks of compacted specimens

dried in an oven, a hammer was dropped from a height of

4 cm onto the specimen (Figure 2a) to expand cracks

(heterogeneity defects).

A Hitachi SU-1500 scanning electron microscope

(Hitachi Corporation, Hitachinaka, Japan) was used

Figure 2. Impact test on dry compacted bentonite specimens.


under low vacuum conditions to investigate micro cracks

(<2 mm long and <0.20 mm wide) within compacted

bentonites prepared using different wetting methods.

Microwave-assisted thawing method

To produce large-scale compacted bentonite blocks,

the low temperatures and large amounts of bentonite

would result in obvious differences in internal and

external melting of an ice-bentonite mixture if the ice-

bentonite mixing method were used. The microwave-

assisted thawing method is believed to improve the

thawing efficiency and produce greater homogeneity

(Venkatesh and Raghavan, 2004). A 350W Midea

MM721AAU-PU microwave oven (Midea Corporation,

Foshan, Guangdong Province, China) was used to assist

in the thawing of two mixtures with target water contents

of 5% and 25%, respectively.

As an alternative test, the traditional thawing method

was used for other ice-bentonite mixtures to let the

mixture thaw naturally for 60 h at temperatures of

21�23ºC. The microwave-assisted thawing was con-

ducted as follows. At first, the ice-bentonite mixture was

heated in a closed plastic bag by turning the oven on for

60 s, then, the heating process was interrupted to ensure

a slow temperature increase. After measuring the

temperature of the mixture, the oven was turned on

again immediately. The above steps were repeated until

all ice had melted by observing the color of the ice-

bentonite mixture which changed from white to a dark

color when it was wetted by the melted ice.

Workability test of the ice-bentonite mixing method

The workability in this test refers to the capability of

being effectively wetted with a minimum loss of

homogeneity due to a loose mixture. The capability of

effectively wetting the bentonite mainly includes the

following aspects: the mixing efficiency, mass loss due

to adhesion to the mixing equipment, and differences in

the water contents of loose mixtures.

The mixing time was measured using a Pursun PS538

stopwatch (Pursun Corporation, Shenzhen, China). In the

water content deviation test, the water contents of

25 specimens with 25% target water content were tested

using the two wetting methods. The losses in sample

mass were evaluated by calculating the mass loss rate as

shown in equation 5.

md1 ¼m1

ð1þ o=100Þ ð4Þ

Q ¼ md0 �md1

md1� 100 ð5Þ

where md1 is the dry weight (g) of the mixture after

mixing; m1 is the wet weight (g) of the mixture after

mixing; o is the water content (%) of the mixture after

mixing; Q is the mass lost after mixing (%); and md0 is

the dry weight (g) of the mixture before mixing.

RESULTS AND DISCUSSION

Differences in the properties of various size macro-

agglomerates

Compared with the ice-bentonite mixing method, the

spray method produced many macro-agglomerates

(Figure 3a) and much of the bentonite powder adhered

to the mixing basin (Figure 3b). Agglomerate size in the

mixtures was varied over a wide range (Figure 3c). The

agglomerate water contents (Figure 4) in the bentonite/

water mixture (target water content was 25%) increased

as agglomerate size increased regardless of which water

content adjustment method was used. The coarse

agglomerates were wetter and the fine agglomerates

were drier than the average water content of the

bentonite/water mixture. This might reasonably be

explained by noting that the interiors of macro-

agglomerates were much wetter than the surfaces

(Figure 3) and the size of the wet cores increased as

agglomerate size increased.

The dry densities of agglomerates (Figure 5)

decreased as the agglomerate size increased. The volume

shrinkage (Figure 5) values increased as the agglomerate

size increased. The low dry density of macro-agglom-

erates retarded bentonite compaction and high shrinkage

produced inhomogeneous shrinkage and deformation in

compacted bentonite. The homogeneity of the loose

bentonite mixtures was limited by the different water

contents, dry densities, and shrinkage values of the

different size agglomerates.

Size distribution of agglomerates

The size distribution of agglomerates was another

important soil parameter (Hillel et al. , 1998).

Measurement of the size distribution helped to evaluate

the homogeneity of compacted bentonite. The percent

macro-agglomerates in the bentonite/water mixture

(Figure 6) revealed that no matter which water content

adjustment method was used, the percent macro-

agglomerates in the bentonite/water mixtures increased

with increases in target water content. This means that

higher target water contents inevitably produced more

macro-agglomerates. Using the ice-bentonite mixing

method, the percent total macro-agglomerates were

always <30% and slowly increased with increased target

water contents. Additionally, fine particles constituted a

greater proportion of total macro-agglomerates than

coarse particles. This indicates that fine agglomerates

were the dominant component in all macro-agglomer-

ates. Using the spray method, on the other hand, macro-

agglomerates increased rapidly with increases in the

target water content. When the water contents were more

than 20%, the %macro-agglomerates had a normal

distribution. The correlation coefficient (R2) of the

normal distribution and the peak macro-agglomerate

size was concentrated around 2�5 mm (Figures 6d, 6e,

6f). When the water content reached 29.61%, the macro-


agglomerates made up >90% of the mixture. The content

and distribution of agglomerates indicated that the ice-

bentonite mixing method could improve the homogene-

ity of compacted bentonite by sharply decreasing both

the total amount and the particle size of macro-

agglomerates in the mixture in comparison to the spray

method.

Dispersion of agglomerate water contents

Test results of the agglomerate size analysis indicated

that the water content distribution was influenced by

both agglomerate size (Figure 4) and quantity (Figure 6).

To quantitatively evaluate the homogeneity of water

contents in bentonite mixtures prepared using the two

mixing methods, a P parameter (dispersion of agglom-

erate water contents), was introduced based on statistics.

The dispersion of the water contents in the bentonite

mixture was determined by three factors: (1) percent

agglomerates of certain sizes; (2) average water contents

in agglomerates of certain sizes; and (3) water content

differences between agglomerates of various size ranges.

In statistics, the variation coefficient (equation 7) is

used to evaluate the discrete degree of variables at

different average levels (Breusch and Pagan, 1979;

Bedeian and Mossholder, 2000). The derivation of the

variation coefficient is defined as follows:

Figure 4. Water content vs. agglomerate size. The dashed and

solid lines indicate the average water contents of bentonite/

water mixtures prepared using the spray method and the ice-

bentonite mixing method, respectively. Figure 5. Dry density and%volume shrinkage of wet agglomerates.

Figure 3. Different size agglomerates produced by wetting.


s ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXNi¼1ðxi � xÞ2 � 1

N

� �vuut ð6Þ

CV ¼ sx� 100 ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXNi¼1ðxi � xÞ2 � 1

N

� �vuut � 1x� 100 ð7Þ

where s is the standard deviation; N is the total number

of samples; xi is the sample value; x is the sample mean;

and CV is the variation coefficient (%).

The dispersion of the agglomerate water contents is

derived from the differences in the water contents

between the different size-range agglomerates in a

loose mixture. The water content in a mixture, therefore,

could be divided into many groups according to

agglomerate size. Similar to equation 7, an equation

can be developed to quantitatively evaluate the disper-

sion of water contents in a bentonite mixture by

Figure 6. Percent agglomerates of different sizes with target water contents of 5�30%.


introducing the following factors: (1) average water

content (oi) of agglomerates for a certain size range

(segment i) which is analogous with sample (xi);

(2) percent (mi/ma) of agglomerates in a certain size

range (segment i) which is analogous with 1/N; (3) water

content difference (oi�o) between agglomerates in

various size ranges which is analogous with xi�x.Hence, equation 7 could be re-written as follows:

P ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXNi¼1ðoi � oÞ2 � mi

ma

� �vuut � 1o� 100 ð8Þ

where P (%) is defined as an index of the dispersion of

water and mass in the subfractions of a water/bentonite

mixture; N is the number of segments; oi is the water

content (%) of particles with a certain size range

(segment i) of the mixture; o is the average water content

(%) of the mixture; mi is the dry weight (g) of particles of

a certain size range (segment i) in the mixture; and ma is

the dry weight (g) of the water/bentonite mixture. A water/

bentonite mixture with water and mass equally divided

between the subfractions (i.e. oi = o, mi = ma) would have

a P value of zero. A larger P value means a more

heterogeneous distribution of water and mass in the

subfractions.

The particle size of a mixture can be divided into

many segments according to step length, with the

average water content in one segment being considered

as a sample (oi). Theoretically, the variation coefficient

can be more precise with a decrease in step length. Step

lengths, however, were determined by the sieve sizes

used in this test, i.e. 10, 5, 2, 1, 0.50, and 0.25 mm. The

P value = 0 if the total water and mass in a bentonite

mixture are equally distributed among the >10, 10�5,5�2, 2�1, 1�0.5, 0.5�0.25, and <0.25 mm fractions.

The dispersion of the water and mass (Figure 7) in

mixtures with different target water contents was

calculated using equation 8. In addition, the percent

macro-agglomerates of all sizes (Figure 7) were included

as a reference index to reflect the homogeneity of water

contents from another viewpoint.

Both the P value (the dispersion of the water and

mass) and the percent macro-agglomerates (Figure 7)

increased as the target water contents increased for the

two different wetting methods. For any water content,

the P values for bentonite/water samples prepared using

the ice-bentonite mixing method were always less than

the P values for the spray method. This indicates that the

ice-bentonite mixing method can produce a more

homogeneous distribution of water than the spray

method, especially for higher target water contents.

Evaluation of the heterogeneity of compacted bentonite

Specimen sections prepared using the spray method

had many dark, wet macro-agglomerates with jagged

surfaces. Moreover, after a section was exposed to air for

about 2 h, the water evaporated and cracks formed

around or cut through the macro-agglomerates, which

can be seen at labeled points , , , in the

micrograph (Figure 8a). In contrast, using the ice-

bentonite mixing method, no macro-agglomerates were

visible by naked eye observation in the flat section of a

compacted bentonite and the section had a homogeneous

water hydration. Section observations demonstrated that

the heterogeneity of water contents seen in the loose

mixture persisted in the compacted bentonite and that

the homogeneity of water contents in compacted

bentonites can be improved by the ice-bentonite mixing

method.

Using the spray method, many obvious cracks were

observed on the surface of compacted bentonite after the

drying process (Figure 2, top left) and the compacted

specimen was damaged after the hammer was dropped

only once. The fact that zig-zag failure crevices

corresponded to the surface shrinkage cracks indicates

that cracks were the weak parts that affected the

mechanical strength. In contrast, using the ice-bentonite

mixing method, no visible cracks were observed on the

surface of compacted bentonite and the compacted

specimen was only damaged after the hammer was

dropped three times and produced only one smooth

crevice. The results from the crack observations revealed

that by using the ice-bentonite mixing method, the

number of cracks produced by shrinkage in compacted

bentonite could be reduced and the mechanical strength,

therefore, could be improved by decreasing the number

of weak parts in compacted bentonite.

Scanning electron microscope (SEM) observations of

representative areas (Figure 9) allowed the identification

of small numbers of micro cracks in a compacted

bentonite that was wetted using the ice-bentonite mixing

method. For compacted bentonite specimens produced

using the spray method, plenty of micro cracks appeared

around the macro-agglomerate. These optical features

confirmed the macro crack observations (Figure 8).

Micro cracks could be explained by the higher water

1 2 3 4

Figure 7. P values of bentonite/water mixtures vs. target water

content.


contents of macro-agglomerates that cause greater

shrinkage in comparison to the surrounding bentonite

powder and leads to circumjacent cracks around the

macro-agglomerates. The cracked areas in compacted

bentonites demonstrate that macro-agglomerates that

form in loose mixtures due to heterogeneity can later

produce defects (cracks) in the compacted bentonite.

Using the ice-bentonite mixing method, a more homo-

geneous distribution of water and fewer macro-agglom-

erates decrease the number of cracks produced by

uneven dry shrinkage and the mechanically weak parts

of compacted bentonites, therefore, can be reduced.

Workability analyses of the ice-bentonite mixing method

Mixing efficiency. Time records showed that as the target

water content was increased from 5% to 30%, the mixing

time increased from 3.70 min to 24.20 min using the water

spray method, while the constant mixing time was shorter

than 3.00 min using the ice-bentonite mixing method.

When high target water contents were used, the ice-

bentonite mixing method exhibited an obvious advantage

in mixing efficiency compared to the spray method.

Mass loss. In the spray method, the mass loss for

bentonite/water mixtures increased from 0.89% to 9.12%

when the target water content was increased from 5 to

30% (Table 2). In the ice-bentonite mixing method, the

mass loss was invariably below 0.50% (Table 2). The

solid ice in the loose mixture led to less mass loss from

the part of the mixture that adhered to the mixing

container (Figure 3). The mass loss results demonstrated

that the ice-bentonite mixing method preserves the

mixture composition with a minimum loss of water and

bentonite.

Figure 8. Photographs of compacted bentonite sections (o = 20%).


Water content deviations in bentonite/water mixtures.

The actual measured water content (Table 3) indicated

that the average water content (25.28%) was much closer

to the target water content (25%) for a sample prepared

using the ice-bentonite mixing method, while the

average water content (23.57%) was significantly

lower than the target water content for a sample prepared

using the spray method. Using the ice-bentonite mixing

method, the water content standard deviation values in

both this test (0.61) and Kobayashi’s test (0.80) were

smaller than the standard deviation value (0.93) in the

spray method (Kobayashi et al., 2008). The average

water contents and standard deviation values of bento-

nite/water mixtures demonstrated that the ice-bentonite

mixing method produced water contents much closer to

the target value than the traditional spray method. Three

factors might be responsible for the lower water contents

of mixtures prepared using the spray method: (i) the

evaporation during a long mixing time, (ii) part of the

material sputtered out during the spraying, and (iii) the

water lost that adhered to the container walls. The small

difference between measured water content and the

target water content using the ice-bentonite mixing

method was attributed to solid-solid mixing in a short

time at a low temperature.

Limitations of ice-bentonite mixing method. The work-

ability advantages of the ice-bentonite mixing method as

shown in the previous section were due to the mixing

process which had a high mixing efficiency, a low mass

loss rate, and water contents close to the target value.

Drawbacks were found, however, in the preparation

process before mixing and in the thawing process after

mixing. The thawing efficiency of ice-bentonite mixtures

was low. This, however, was partially solved by using the

microwave-assisted thawing method. The total thawing

time was 420 s in this microwave-assisted thawing test.

After 6 h cooling, the water contents and particle size of

macro-agglomerates in the mixtures were measured.

Regardless of the thawing method used, no noticeable

differences were observed in either the macro-agglomer-

ates or the water contents of mixtures with target water

contents of 5% and 25% (Figure 10). This test demon-

strated that the microwave-assisted thawing method could

significantly accelerate the thawing rate without affecting

the percent macro-agglomerates and the water contents of

bentonite/water mixtures in comparison to traditional

thawing at room temperature.

Another drawback to the ice-bentonite mixing

process was the relatively laborious procedure, which

required cooling liquid water to make ice, crushing and

sieving the ice, and the need for a low-temperature

environment. With respect to efficiency, the low

efficiency due to the laborious procedure could outweigh

the advantage of the greater mixing efficiency when the

weight of bentonite is relatively small. For an industrial

size test of HLW disposal in China, the greater mixing

efficiency, however, would outweigh drawbacks of the

laborious ice-bentonite mixing procedure because the

use of a large mass of bentonite would accomplish a

Figure 9. SEM of compacted bentonite internal micro-cracks.

Table 2. Mass loss of mixtures with different target water contents.

Water content adjustment ——— Mass loss (%) of mixtures with different target water contents ———methods o = 5% o = 10% o = 15% o = 20% o = 25% o = 30%

Spray method 0.89 1.30 2.29 4.35 7.50 9.12Ice-bentonite mixing method 0.46 0.49 0.50 0.36 0.41 0.37


major part of the total process. Compared with the

traditional spray method, adjusting the water content of

bentonite using the ice-bentonite mixing method could

improve the homogeneity of compacted bentonite and

yield workability advantages in the mixing process, but

would also bring some disadvantages from the prepara-

tions needed before mixing. A low-temperature labora-

tory with the machinery to crush ice and to mix and thaw

an ice-bentonite mixture might be needed for the

industrial production of buffer/backfill material in

China.

CONCLUSIONS

The following conclusions can be drawn from this

experimental study.

(1) Macro-agglomerates have a higher water content,

higher shrinkage, and lower dry density than small

particles of bentonite powder in a loose bentonite and

water mixture. The size distribution of agglomerates

indicated that the ice-bentonite mixing method can

improve the homogeneity of loose mixtures by sharply

decreasing the number of macro-agglomerates.

(2) By analogy with the variation coefficient in

statistics, a P index to consider the effects of agglom-

eration was proposed to evaluate the dispersion of water

in the agglomerate size fractions. The low P value

indicated that the ice-bentonite mixing method led to a

more homogeneous water content distribution in the

bentonite/water mixture than the spray method.

(3) The homogeneity analyses of compacted speci-

mens showed that shrinkage cracks, which surround the

macro-agglomerates in compacted specimens, were

readily produced using the spray method. Using the

ice-bentonite mixing method, the bentonite hydration

was more homogeneous in vertical sections than with the

spray method. High homogeneity in loose bentonites

wetted using the ice-bentonite mixing method could

improve the homogeneity of compacted bentonites.

Table

3.Param

etersofmixturesunder

twodifferentwater

contentadjustmentmethods.

Water

contentadjustment

methods

Sam

ple

number

Standard

deviation

Targetwater

content(%

)Maxim

um

(%)

Minim

um

(%)

Averagevalue

(%)

Range

(%)

Dates

come

from

Spraymethod

25

0.93

25.00

25.18

22.13

23.77

3.05

This

study

Ice-bentonitemixingmethod

25

0.61

25.00

26.52

24.38

25.28

2.14

This

study

Ice-bentonitemixingmethod

69

0.80

21.00

23.40

19.60

21.30

3.80

Kobayashiet

al.

(2008)

Figure 10. Percent agglomerates after thawing ice-bentonite

mixtures at room temperature and by microwave heating.


(4) Workability analyses showed that the workability

advantages of the ice-bentonite mixing method were

reflected in the high mixing efficiency, low mass loss,

and low water content deviations in the water/bentonite

mixtures, and the microwave-assisted thawing method

can be used to improve the thawing efficiency of the

mixtures. Limitations of the ice-bentonite mixing

method, however, are the low temperatures required

and the relatively laborious procedure.

Test results verified that the ice-bentonite mixing

method was much better than the traditional spray

method in that the prepared bentonite mixture had

fewer macro-agglomerates and lower water content

deviations during initial sample preparation. The ice-

bentonite mixture after compaction had a distinct

homogeneity. In this test, the results of the ice-bentonite

mixing method largely depended on the composition of

the GMZ bentonite. Future studies should address the

influence of bentonite type and, possibly also, the

influence of bentonite mineralogical composition.

ACKNOWLEDGMENTS

This work was supported by the National NaturalScience Foundation of China (No: 41672261), the Funda-mental Research Funds for the Central Universities(lzujbky-2016-k15) and the Project "Compacted Buffer/Backfilling Blocks for HLW Disposal-Preparation Tech-nology and Engineering Property Measurement" sponsoredby the State Administration of Science, Technology andIndustry for National Defense, PRC.

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