Post on 31-Mar-2020
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:
zhanghuyuan@lzu.edu.cn
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.
Vol. 64, No. 6, 2016 Ice-bentonite powder mixing method 709
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-
710 Peng et al. Clays and Clay Minerals
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.
Vol. 64, No. 6, 2016 Ice-bentonite powder mixing method 711
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%.
712 Peng et al. Clays and Clay Minerals
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.
Vol. 64, No. 6, 2016 Ice-bentonite powder mixing method 713
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%).
714 Peng et al. Clays and Clay Minerals
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
Vol. 64, No. 6, 2016 Ice-bentonite powder mixing method 715
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.
716 Peng et al. Clays and Clay Minerals
(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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