GEOLOGICA CARPATHICA, OCTOBER 2009, 60, 5, 431—436 doi: 10.2478/v10096-009-0031-2
The decaying power from the spent fuel in the high level
waste canisters will increase temperature and initially give
rise to a thermal gradient over the bentonite buffer by which
original water will be redistributed parallel to an uptake of
water from the surrounding rock. A number of laboratory
test series, made by different research groups, have resulted
in various buffer alteration models. The most popular are
large laboratory and in-situ tests, done by radioactive waste
management agencies such as: ANDRA (France; Delay et al.
2007); ENRESA (Spain; EUR, ENRESA 2005), NAGRA
(Switzerland; Villar et al. 2005); ONDRAF/NIRAS (Bel-
gium; ONDRAF/NIRAS 2001); SKB (Sweden; Karnland et
al. 2000) in Europe, and many others in the world. The main
objective of these studies is to identify the processes occur-
ring in the Mock-Up tests during the heating and water satu-
ration of bentonites. The quantitative analyses of the changes
are instrumental for understanding the nature and potential
consequences for future repositories. The Belgian Mock-Up
experiment OPHELIE (Verstricht & Dereeper 2002, 2003).
and Spanish experiment FEBEX (EUR 2000; 2005) are very
good examples of large scale projects based on several heat-
ing tests simulating the conditions of a radioactive waste re-
pository, and reproducing the thermo-hydro-mechanical pro-
cesses that could eventually occur in the repositories using
the bentonitic barriers.
A similar experiment – Mock-Up-CZ was built by the
team of the Czech University of Technology in Prague (Pa-
covský 2004; Pacovský et al. 2007). The main aim was to test
the stability of the local bentonite buffer material for the even-
Mineral stability of Fe-rich bentonite in the Mock-Up-CZ
, VLADIMÍR ŠUCHA
, PETER UHLÍK
, JANA MADEJOVÁ
and IGOR GALKO
Department of Geology of Mineral Deposits, Comenius University, Mlynská dolina G, 842 15 Bratislava, Slovak Republic;
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovak Republic;
G55 s.r.o., Bakossova 8, 974 01 Banská Bystrica, Slovak Republic; email@example.com
(Manuscript received September 4, 2008; accepted in revised form December 18, 2008)
Abstract: Bentonite is a basic component of most concepts of multibarrier systems in underground radioactive waste
repositories. It is important to determine the bentonite stability under the conditions close to the future real situation.
The paper brings the detailed mineral and structural analyses of smectites from the bentonitic material exposed to the
long term Mock-Up-CZ experiment. The compacted barrier blocks and residual filling contained 85 % of bentonite
from the Rokle deposit, 10 % of quartz sand and 5 % of graphite. They were exposed to temperatures of up to 90 °C for
almost 4 years. Quantitative mineral analyses, crystal size distributions, FTIR spectra, as well as cation exchange capacity
and layer charge density show high mineral stability of the Rokle bentonite under the conditions of Mock-Up-CZ
experiment. Small changes in the crystal sizes and slight change in the layer charge as a consequence of the experimen-
tal alteration could be linked to the hydration and the variation of the geochemical environment of the experiment.
Key words: Mock-Up experiment, long-term bentonite stability, bentonite barrier, Rokle bentonite, Fe-rich montmorillonite.
tual construction of the repository. In this paper we bring the
results of detailed mineral and structural analyses of smectites
from the bentonitic material used for the experiment.
Materials and methods
The material used for this study is part of an experimental
barrier exposed to the experimental conditions of a large phys-
ical model Mock-Up-CZ (Pacovský 2004). This experiment
simulated the vertical placement of a container with radioac-
tive waste according to the Swedish KBS-3 system. The mod-
el consisted of a barrier of bentonite blocks and the heater.
Bentonite barrier was closed in a steel tank with a cylinder
diameter 800 mm and height 2230 mm and wall thickness of
8 mm. The top and bottom covers were made of 50 mm thick
steel. These individual components were connected using 16
bolts. The compacted blocks (
= 1700 kg/m
) and filling of
residual free space contained 85 % of bentonite (from the
Rokle deposit), 10 % of quartz sand and 5 % of graphite (from
the conditioning plant at Netolice) (Pacovský et al. 1998,
2007). The model was saturated with synthetic granitic water
(Svoboda & Vašíček 2009). The maximum temperature
reached in bentonite buffer was 90 °C. Temperature, swelling
pressure and hydration measurements were taken continuous-
ly from inside the bentonite barrier throughout the entire dura-
tion of the experiment, a period of 3 years and 9 months.
The Rokle bentonite deposit is part of an accumulation of
argillized volcaniclastic rocks in the Tertiary stratovolcanic
complex of the Doupovské hory Mountains. The main miner-
al phase – Fe-rich montmorillonite was produced by the al-
STRÍČEK, ŠUCHA, UHLÍK, MADEJOVÁ and GALKO
teration of basaltic ash in a stagnant, lacustrine environment.
Biotite was apparently stable during the alteration of the
hyaloclasts. Anatase and possible accessory heulandite-cli-
noptilolite were also formed in small amounts. Goethite is the
youngest oxidation product in some parts of the bentonite.
Minute fragments of sodium-rich plagioclase, potassium feld-
spar, quartz, and muscovite are ubiquitous accessories of the
original hyaloclasts. Together with kaolinite, they formed from
the underlying fresh or kaolinized orthogneiss (Konta 1986;
Hradil et al. 2004).
A series of samples were taken from three different distanc-
es with regard to the position of the block in the experimental
vessel: close to the heater (T), from the block centre (C), close
to the hydration system (O) and the last one from the hand-
compacted mixture (N). Samples were also divided in vertical
orientation into upper part (V), central part (C), and bottom
part (S) with regard to their position in the block (Fig. 1).
Mineral and crystallochemical changes after the experi-
ment were tested by the following techniques:
The XRD analyses of oriented (clay fraction) and random
specimens (bulk fraction) were carried out using a Philips
PW 1710 (Cu
Κα radiation with graphite monochromator)
Samples for quantitative analyses were milled in a McCrone
Micronizing Mill with internal standard ZnO to < 20 µm size.
The XRD data were converted into wt. % minerals using the
RockJock software (Eberl 2003). The program fits the sum
of stored XRD patterns of standard, pure minerals and amor-
phous phases (the calculated pattern) to the measured pattern
by varying the fraction of each standard pattern, by using the
Solver function in Microsoft Excel to minimize the degree of
fit parameter between the calculated and measured pattern.
Data were normalized to 100 % at the end of analysis.
XRD patterns with longer exposure times were used for
calculations of particle crystal thicknesses using the Bertaut-
Warren-Averbach (BWA) technique of Drits et al. (1998)
implemented in the MudMaster program (Eberl et al. 1996).
MudMaster is a program used to calculate crystal thickness
from the interference function, which is extracted from in-
tensities of basal XRD reflections by dividing them by the
Lorentz-polarization and structure factors. The measure-
ments of the smectite crystal thickness were performed on
001 reflections, recorded for clay samples saturated with
The FTIR (Fourier Transform Infrared) spectra were ob-
tained using a Nicolet Magna 750 spectrometer. The KBr
pressed pellets technique (1 mg of sample and 200 mg of
KBr) was used for the transmission measurements.
The determination method of the CEC (cation exchange ca-
pacity) is based on the complete exchange of the naturally oc-
curring cations against a copper triethylenetetramine complex
(Meier & Kahr 1999). Smectite samples (80, 100
and 120 mg) were added to 50 ml distilled water and 10 ml
solution of [Cu (trien)]
. The suspensions were dispersed by
an ultrasonic treatment. After 5 min, the suspension was cen-
trifuged. The supernatant solution was separated and the con-
centration of the Cu(II) complex was determined by spectro-
photometry (Cary 100, Varian). A standard deviation of the
measurements is 4 % (Ammann et al. 2005).
The UV-Vis spectra of the R6G/clay dispersion were mea-
sured using the same spectrophotometer. The final concen-
tration and loading of the dye solution in the dispersions
were always 1
M and 0.05 mol · g
of clay, respective-
ly. Visible spectra were measured 1 min after mixing the
clay dispersions with the solution of R6G. The prepared
R6G/clay dispersions were then shaken for 24 hours to
achieve equilibrium in the systems. Another series of spectra
for aged dispersions was taken. The spectra of the clay dis-
persions without dye, related to light scattering from solid
particles, were subtracted from the spectra of the dye/clay
dispersions in order to obtain the pure spectra of the dye spe-
cies. Second derivative spectroscopy was used for a better
resolution of the individual bands in dye absorption spectra,
for the estimation of the peaks’ position of arisen forms of
the dye, and for the comparison of the amounts of the species
All analyses (except quantitative mineral analyses) were
made with < 2 µm fractions separated from bulk samples by
sedimentation in distilled water. Prior to fractionation, the
samples were dispersed in an ultrasonic bath and subse-
quently treated with sodium acetate buffer, hydrogen perox-
ide and sodium dithionite (partially modified Jackson 1975
in Šucha et al. 1991). Excess soluble salts were removed by
centrifugation followed by dialysis.
Results and discussion
Smectite is the main mineral phase in all samples accord-
ing to the quantitative XRD analyses with contents varying
between 42 and 46 % as determined by the RockJock soft-
ware (Table 1). The quartz content is between 16 and 22 %,
volcanic glass up to 9 %, calcite 8 %, and graphite 4 %. Oth-
er accessory minerals (up to 5 %) were biotite, muscovite,
Fig. 1. Position of samples in the compacted block. The distance be-
tween the heater and the hydration system was 210 mm.
MINERAL STABILITY OF Fe-RICH BENTONITE IN THE MOCK-Up-CZ EXPERIMENT
kaolinite, illite, goethite and in some samples traces of the
anatase were detected. Vejsada et al. (2005) identified vari-
able amounts of possibly vermiculite in the Rokle bentonite
as a product of biotite alteration. No vermiculite was detect-
ed in the present study which can be explained by the high
heterogeneity of the material in the deposit. The quantitative
mineralogical composition of the experimentally altered
bentonite material is stable and does not show any variabili-
ty. It is almost identical with the original buffer material
used for the experiment. This means that no significant mod-
ifications occurred in the bentonite during the experiment
(Fig. 2). The presence of neoformed calcium sulphate de-
scribed in other studies (Humbeeck et al. 2005; Karnland &
Birgersson 2006; Vinšová et al. 2008) was not confirmed in
the samples examined in this paper. The precipitation of cal-
cium sulphate in the Mock-Up-CZ test could be the result of
dissolution of pyrite (trace amount in Rokle bentonite; Vin-
šová et al. 2008) or presence of Ca
granitic water. The amount of gypsum is probably so small
that it is under the detection limit of the techniques used in
Clay fraction analyses
Since the smectite is the most important and the most sen-
sitive component of the buffer material, we focused our in-
terest more closely on the clay fraction ( < 2 µm) which is
largely dominated by smectite. Crystallochemical character-
istic was determined by the FTIR. The spectrum of original
unaffected buffer material and the spectra of samples after
the experiment are very similar (Fig. 3). The OH deformation
bands at 917 cm
(AlAlOH) and near 880 cm
show significant substitution of octahedral Al by Fe. Other
features in the spectrum, common to all dioctahedral mont-
morillonites, include a complex Si—O stretching band at
and Si—O—Al and Si—O—Si deformations at 526 cm
Table 1: Quantitative mineral composition of samples.
feldspar goethite SUM %
46 16 4
4 2 6 4 9 3 2 100
45 20 2
4 2 8 4 6 3 2 100
44 19 3
5 3 8 3 7 3 2 100
43 19 4
5 2 7 3 9 2 3 100
42 19 3
5 2 8 3 9 3 2 100
46 17 4
5 3 6 2 8 3 2 100
45 21 3
4 1 7 2 7 3 3 100
43 22 3
4 3 8 3 6 4 2 100
Fig. 2. XRD patterns of unoriented specimens with inner standard ZnO. (S – smectite, M – mica, K – kaolinite, I – clays, Q – quartz,
Cc – calcite, G – graphite).
STRÍČEK, ŠUCHA, UHLÍK, MADEJOVÁ and GALKO
and 470 cm
respectively. The band at
indicates the presence of an amor-
phous silica admixture. The weak diagnostic
bands of kaolinite at 3696 and 698 cm
the quartz doublet at 802 and 780 cm
also determined in the IR spectra. No changes
in the intensities or positions of the character-
istic OH and Si—O bands of montmorillonites
can be recognized in the IR spectra of the
samples taken from different parts of the
compacted block (Fig. 3). Thus the IR spec-
troscopy confirms the crystallochemical sta-
bility of the bentonite Rokle upon the select-
ed experimental conditions.
The stability of smectite crystals (coher-
ently diffracting 1 nm thick 2 : 1 layers) was
determined using XRD based BWA tech-
nique. This technique is able to detect the
mean thickness of the crystals and the distri-
bution of their sizes. The distribution
(Fig. 4) is almost the same for all the mea-
sured sample and is of an asymptotic shape.
The shape of the distribution is a good pa-
rameter to test the differences in the origin
or evolution of the samples (Eberl et al.
1998; Šucha et al. 2001; Honty et al. 2004).
The similar shape of the crystal distribution
in the studied set of samples indicates the
environment with no or little influence on
the system. However some differences were
detected in the mean crystal sizes (Fig. 5).
There is a systematic decrease of the mean
thickness towards the edge of the experi-
mental vessel where the source of hydration
is situated. We suspect that the intensive hy-
dration may have a slight impact on the
number of coherently diffracting smectite
layers. Water content rises gradually from
15 % near the centre of experimental vessel
to 40 % close to the hydration system. The
swelling phenomenon in highly compacted
smectite clay was studied by Saiyouri et al.
(2000). They found that a slight decrease in
the thickness of the smectite crystals can be
attributed to a partial splitting of them, prob-
ably caused by swelling under high pressure
conditions. This could easily be the case of
Systematic changes in CEC values were
observed neither among the Mock-Up-CZ
samples, nor in comparison with the initial
sample (Fig. 6). All differences can be as-
signed to the error of the CEC determination
(Ammann et al. 2005) and to the small varia-
tions in smectite content.
The molecular aggregation of cationic
dyes on the surface of smectites should be
able to distinguish the changes in the layer
charge density (Bujdák et al. 2003; Czi-
Fig. 3. FTIR spectra of samples.
Fig. 4. Particle thickness distribution of smectite crystals.
Fig. 5. Mean thickness of smectite particles.
MINERAL STABILITY OF Fe-RICH BENTONITE IN THE MOCK-Up-CZ EXPERIMENT
Fig. 6. Cation exchange capacity of <2 µm fraction.
Fig. 7. Second-derivative spectra calculated from the absorption spectra of R6G/clay sys-
tems measured 24 hours after mixing the components.
The detailed analyses of the bentonite
based material used in the Mock-Up-CZ
revealed stability of the mineral compo-
sition and the crystallochemical charac-
teristics of smectite which is the main
barrier component. We can conclude that
no deterioration of the smectite’s stabili-
ty occurred during the experiment. No
recrystallizations have been observed in-
side the buffer block, nor in the backfill
material. A slight decrease in the crystal
sizes could be assigned to the impact of
hydration and the slight change in the
layer charge could be due to change of
geochemical environment inside the ex-
periment. High iron content smectite is
very sensitive to such a change.
According to the obtained data the
mineral stability of the Rokle bentonite
can be ensured over a long period of
time if the conditions are similar to those
of the Mock-Up-CZ experiment.
Acknowledgments: We thank the Centre
of Experimental Geotechnics (CEG), Fac-
ulty of Civil Engineering, CTU for pro-
viding us with the samples, Dr. Adriana
Czimerová, Institute of Inorganic Chem-
istry, Slovak Academy of Sciences for
R6G spectra analyses and Dr.
Puškelová, Geological Institute, Slovak
Academy of Sciences for XRD analyses.
Financial support from the Slovak Grant
Agency VEGA (Project 1/3072/06) is ac-
merová et al. 2006). Šucha et al. (2009) demonstrated the re-
lation between Rhodamine 6G (R6G) spectra and low/high
charge smectites. According to these findings the bands at
about 551 nm could be assigned to the monomers related to
low charge smectites and the band at 468 nm to H-aggre-
gates which are formed at the sites with the high layer charge
density. Overall it seems that the original smectite represent-
ed here by the buffer material is of low layer charge. The
R6G spectra changed slightly during the Mock-Up test. The
intensity of the band related to H-aggregates increased and the
band of monomers decreased as demonstrated by Fig. 7. Tak-
ing into account the findings referred to above, this would
mean a very small increase in the layer charge during the test.
The observation of Vinšová et al. (2008) indicating illitiza-
tion and beidelitization of smectite in some local parts of the
Mock-Up-CZ experimetal column, were not confirmed even us-
ing extremely detailed mineralogical examination. Unfortunate-
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