# Rheological Behavior of Smectite Dispersions: The Influence  of Suspension Concentration and Exchangeable Cation

Authors: S. Paumier; P. Monnet; A. Pantet
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

The mechanical behaviors of smectite materials in water are very sensitive to many parameters, such as concentration,
particle size and morphology, exchangeable cation nature, and chemical environment (pH, ionic strength). The literature
usually provides general rules for sodium and calcium bentonites. However, it appears that the quantitative results present
a large range of values. This study has established the relationships between the smectite nature (mineralogy, morphology,
reactivity) and its rheological behavior according to the exchangeable cation. To develop a complete rheological study,
we need 800 g of montmorillonite purified and saturated withNa+ or Ca2+. Huge production necessitates optimized process
with accurate control of the powder quality and the chemical environment. Smectite particles have been extracted from
a natural bentonite and then saturated with sodium or calcium. Then, each sample was characterized with classical
methods like X-ray diffractometry (XRD), and laser granulometry but also  cutting-edge technologies like scanning
transmission electron microscopy on wet samples (Wet STEM). The Na-smectite dispersion contains a lot of small
units whereas the Ca-smectite dispersion contains a few large units. Rheological study consists in flow curves obtained
with a high-resolution rheometer. Logarithmic representation of the rheograms permits detecting a yield stress,
it separates solid and fluid behavior. The rheological datas (viscosity, consistency, and yield stress) allow defining the
limits depending on the concentration and on the saturation state. A Newtonian law could be applied to around 4% for
Na- and Ca-smectites. Seaward of these concentrations, the dispersions are shear thinning and modelled by Ostwald law.
Finally a yield stress appears at 6% for Na-smectite and 11% for Ca-smectite, the flow curves could be modelled by
Hershel-Bukley law. Results are in accordance with the bibliography.


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## Body

## 1. Introduction

The specific properties of bentonite (hydration, swelling, water adsorption,
viscosity, and thixotropy) make it a valuable material in the form of mineral
powder for a wide range of uses; agronomy, cosmetics, civil engineering, as a
geoliner in landfills, sewage lagoons, or as a compacted barrier for nuclear
waste repositories [1]. Bentonite contains primarily smectite clays
(mainly montmorillonite) and other secondary minerals, like feldspar, quartz,
calcite, and iron oxides. Bentonite suspensions exhibit strong colloidal
properties with yield stress and viscoelastic behavior, because of three-dimensional
organization of the smectite clay crystallites in water. They display varying rheological properties that are
highly dependent on the nature of the smectite (layer charge, localization of
the layer charge, nature of exchangeable cations, particle size, etc.),
suspension preparation, concentration, pH, ionic strength, and so forth [2].Sodium-saturated smectite dispersions have a more pronounced yield stress than calcium-smectite
dispersions. The exchangeable cation effect results in contrasted modes of
layer linkage. Smectite layers, that have negative charge, are surrounded by a
double diffuse layer (DDL), containing cations to balance the negative charge.
The calcium smectite platelets are aggregated into quasicrystals with a thin DDL,
in contrast sodium saturated smectites often are isolated platelets with large DDL
[2–11].Despite this knowledge, the comparison of rheological parameters, even
for similar materials like sodium bentonite, as determined by different authors
is difficult because experimental conditions are slightly different and also
because the cation exchange is not a simple answer. Experimental conditions are
different to the extent
that it is difficult to correlate data measured by various authors. This
divergence originates from differences in pretreatment and preparation as well
as from different modes of quantification  [12, 13]. In this study, we focused on the
unit characterization of homoionic smectite
dispersions (Na- and Ca-smectites) and many mixtures (Na-Ca), Coupling rheological techniques, which allows exploring how the
structures or the networks of particles inside smectite suspensions are
affected by modifications of the clay properties.


## 2. Material and Methods

### 2.1. Extraction Process Development

First,
the raw bentonite from the BENTOFRANCE (200 g) was dispersed in distilled water (500 mL) using a
strainer and sieved at 80 μm. Then, the fine fraction is extracted from the
infra 80 μm fraction by controlled centrifugation according to the usual stokes
relationship. In order to optimize the extraction process of the fine fraction
(named currently infra 2 μm fraction), we stopped the process at the seventh step. This process was repeated many times and the resulting materials were
mixed, dried, and crushed in order to obtain 800 g of homogenized smectite in
five weeks. The mineralogy [14] was determined by X-ray diffraction (XRD) with Philips X'Pert PRO PW3050/6x goniometer (using Fe-filtered CuKα radiation, 40 kV, 40 mA) over randomly oriented powders, analyzed in 2–65°2θ range with astep of 0.05°2θ, counting the time of 3 second
per step. The clay mineralogy of the fine fraction was analyzed by XRD on
randow powders and compared with the raw bentonite (Figure 1). The part of
secondary minerals has slightly decreased with the extraction process by
controlled centrifugation.Figure 1
Powder  patterns of the raw bentonite (upper diffractogram)
and the fine fraction (lower diffractogram)-smectite (bold values),
quartz (4.260 Å, 3.350 Å), anorthite (3.224 Å, 1.542 Å), opal CT (4.069 Å), mica (3.651 Å), and
magnetite (2.520 Å).


### 2.2. Preparation of Calcium and Sodium Exchanged Smectite

The
usual exchange process consists in contacting the clay with a saturating
solution, like NaCl or CaCl2, according to the expected saturation,
during five periods of one hour. However, its efficiency may vary according to
the clay properties [9, 15]. The
saturation rates were deduced from the exchangeable cations, by comparing the content
of the saturating cation with the sum of all cations. The cationic exchange
capacity (CEC) and exchangeable cations (Σ) were measured using the ammonium method
(AFNOR standard X 31,130 November 1985) and
Flame Atomic Absorption Spectrometry (FAAS). The optimized saturating process, by increasing the concentration of sodium ions in the
saturating solution and the contact period, allows to increase the ratio Na/Σcations near 1, indicating a
satisfied saturation.


### 2.3. Characterization Methods to Compare the Two Saturated Materials

In order to investigate the
impact of saturation on dispersions, we have characterized Na-smectite and
Ca-smectite. Unit sizes were measured with laser diffraction in dilute
dispersions (almost 0.05%). This method was
used to evaluate the size of the suspended unit after aging without
disintegration treatment (hexametaphosphate sodium adding). The distribution of Na-smectite powders is bimodal with the first mode at 0.6 μm and the second at 2.1 μm. The index curves are d10 = 0.3 μm, d90 = 3 μm, and d50 is 1 μm. The Ca-smectite dispersion seems to be bidispersed (5.3 μm mode—not determined with the focal lense) with a
broad distribution: d10 is 1.9 μm, d50 is 7.1 μm, and d90 is 51.8 μm.The Wet STEM observation method was developed on an
environmental scanning electron microscopy (ESEM) that enables wet samples to be observed without potential damage through
the use of partial water vapor pressure in the microscope specimen chamber. The
special device was developed by Bogner et al. [16]. The Na-smectite dispersion needed higher dilution to meet clarity requirements.


### 2.4. Rheological Behavior of the Homoionic and Mixed Suspensions

Dispersions were prepared by adding 0.2%–12% (w/w) of fine fraction to distilled water
(10 mL). The flasks were briefly hand-shaken, and dispersions were performed
for 3 minutes at 16 000 rpm with a homogenizer (IKA Ultra-Turrax T25). Then,
the resulting dispersions were slowly homogenized in a rotating shaker (50 rpm,
Prolabo) for 9 hours on average, and were let at rest for 15 hours at 20°C.
The equilibrium pH 8.4±0.1 was not modified. Shaking time and shaking energy
were cautiously checked during the dispersion preparation in order to ensure
reproducibility and to allow comparisons of the rheological tests. According to
Peysson [17], “the dispersion is stable if the mean unit size distribution is
stationary in time and space over a rest period longer than the testing
period.”The
rheological measurements were performed with a Gemini HR stress-controlled
rheometer (Malvern instrument). The rheological methods are sensitive enough to
quantify accurately variations in the microstructuring of the smectite
suspensions [18]. Experimental flow curves were obtained with
rough parallel-plate geometry (40 mm) spaced 500 μm apart. The mechanical
stability of dispersions was verified by agreement of successive tests. Then,
the dispersions were subjected to increasing and decreasing shear rates for 7 minutes by 20 steps from 0.1 s-1 to 150 s-1. A preshear at 100 s-1 was imposed for 60 seconds,
followed by a rest period of 5 s before each test, in order to place the
dispersion in an initial structural state.Three models were used to characterize the following behavior:(1)
Newton:τ=η×γ⋅;(2)
Ostwald:τ=kγ⋅n;(3)
Herschel-Bulkley:τ=τ0+kγ⋅n,
where τ (Pa) is the shear stress, η (Pa⋅s) is the
Newtonian viscosity, and γ⋅ (s-1) is the shear rate. τ0 (Pa) is the yield stress (empirical yield
stress), k is the consistency, and n is the phenomenological coefficient.


## 2.1. Extraction Process Development

First,
the raw bentonite from the BENTOFRANCE (200 g) was dispersed in distilled water (500 mL) using a
strainer and sieved at 80 μm. Then, the fine fraction is extracted from the
infra 80 μm fraction by controlled centrifugation according to the usual stokes
relationship. In order to optimize the extraction process of the fine fraction
(named currently infra 2 μm fraction), we stopped the process at the seventh step. This process was repeated many times and the resulting materials were
mixed, dried, and crushed in order to obtain 800 g of homogenized smectite in
five weeks. The mineralogy [14] was determined by X-ray diffraction (XRD) with Philips X'Pert PRO PW3050/6x goniometer (using Fe-filtered CuKα radiation, 40 kV, 40 mA) over randomly oriented powders, analyzed in 2–65°2θ range with astep of 0.05°2θ, counting the time of 3 second
per step. The clay mineralogy of the fine fraction was analyzed by XRD on
randow powders and compared with the raw bentonite (Figure 1). The part of
secondary minerals has slightly decreased with the extraction process by
controlled centrifugation.Figure 1
Powder  patterns of the raw bentonite (upper diffractogram)
and the fine fraction (lower diffractogram)-smectite (bold values),
quartz (4.260 Å, 3.350 Å), anorthite (3.224 Å, 1.542 Å), opal CT (4.069 Å), mica (3.651 Å), and
magnetite (2.520 Å).


## 2.2. Preparation of Calcium and Sodium Exchanged Smectite

The
usual exchange process consists in contacting the clay with a saturating
solution, like NaCl or CaCl2, according to the expected saturation,
during five periods of one hour. However, its efficiency may vary according to
the clay properties [9, 15]. The
saturation rates were deduced from the exchangeable cations, by comparing the content
of the saturating cation with the sum of all cations. The cationic exchange
capacity (CEC) and exchangeable cations (Σ) were measured using the ammonium method
(AFNOR standard X 31,130 November 1985) and
Flame Atomic Absorption Spectrometry (FAAS). The optimized saturating process, by increasing the concentration of sodium ions in the
saturating solution and the contact period, allows to increase the ratio Na/Σcations near 1, indicating a
satisfied saturation.


## 2.3. Characterization Methods to Compare the Two Saturated Materials

In order to investigate the
impact of saturation on dispersions, we have characterized Na-smectite and
Ca-smectite. Unit sizes were measured with laser diffraction in dilute
dispersions (almost 0.05%). This method was
used to evaluate the size of the suspended unit after aging without
disintegration treatment (hexametaphosphate sodium adding). The distribution of Na-smectite powders is bimodal with the first mode at 0.6 μm and the second at 2.1 μm. The index curves are d10 = 0.3 μm, d90 = 3 μm, and d50 is 1 μm. The Ca-smectite dispersion seems to be bidispersed (5.3 μm mode—not determined with the focal lense) with a
broad distribution: d10 is 1.9 μm, d50 is 7.1 μm, and d90 is 51.8 μm.The Wet STEM observation method was developed on an
environmental scanning electron microscopy (ESEM) that enables wet samples to be observed without potential damage through
the use of partial water vapor pressure in the microscope specimen chamber. The
special device was developed by Bogner et al. [16]. The Na-smectite dispersion needed higher dilution to meet clarity requirements.


## 2.4. Rheological Behavior of the Homoionic and Mixed Suspensions

Dispersions were prepared by adding 0.2%–12% (w/w) of fine fraction to distilled water
(10 mL). The flasks were briefly hand-shaken, and dispersions were performed
for 3 minutes at 16 000 rpm with a homogenizer (IKA Ultra-Turrax T25). Then,
the resulting dispersions were slowly homogenized in a rotating shaker (50 rpm,
Prolabo) for 9 hours on average, and were let at rest for 15 hours at 20°C.
The equilibrium pH 8.4±0.1 was not modified. Shaking time and shaking energy
were cautiously checked during the dispersion preparation in order to ensure
reproducibility and to allow comparisons of the rheological tests. According to
Peysson [17], “the dispersion is stable if the mean unit size distribution is
stationary in time and space over a rest period longer than the testing
period.”The
rheological measurements were performed with a Gemini HR stress-controlled
rheometer (Malvern instrument). The rheological methods are sensitive enough to
quantify accurately variations in the microstructuring of the smectite
suspensions [18]. Experimental flow curves were obtained with
rough parallel-plate geometry (40 mm) spaced 500 μm apart. The mechanical
stability of dispersions was verified by agreement of successive tests. Then,
the dispersions were subjected to increasing and decreasing shear rates for 7 minutes by 20 steps from 0.1 s-1 to 150 s-1. A preshear at 100 s-1 was imposed for 60 seconds,
followed by a rest period of 5 s before each test, in order to place the
dispersion in an initial structural state.Three models were used to characterize the following behavior:(1)
Newton:τ=η×γ⋅;(2)
Ostwald:τ=kγ⋅n;(3)
Herschel-Bulkley:τ=τ0+kγ⋅n,
where τ (Pa) is the shear stress, η (Pa⋅s) is the
Newtonian viscosity, and γ⋅ (s-1) is the shear rate. τ0 (Pa) is the yield stress (empirical yield
stress), k is the consistency, and n is the phenomenological coefficient.


## 3. Results

### 3.1. Observations by wet STEM

Each sample was observed by wet STEM, and 10
photographs of each sample were analyzed with the NIS software. Each unit was
approximatively defined and assimilated to an elliptical measurable area. Ca-smectite particle sizes
vary from 0.07 μm² (d10)
to 0.77 μm² (d90),
the particles  are dispersed (Figure 2(a)).
Particles are dense and seem to be composed of linked entities. Small units
could result from the aggregate break-up. Large spaces are occupied only by
water. Na-smectite units seem to fill all the available space (Figure 2(b)). This
observation could result from the three-dimensional superimposition or from the
particles settling down in the droplet. Particles sizes are quite homogeneous
with a mean area of 0.13 μm². Particles
distribution ranges from 0.035 μm² (d10)
to 0.3 μm² (d90),
although a few large particles were observed. The surface areas of the
Ca-sm units are approximatively 2.5 times larger than those of the
Na-sm units.(a), (b) Microscopic view (wet STEM) for diluted suspensions: Sm-Ca (0,1 g/l) and Sm-Na  (0,01 g/l) (photo: A. Bogner and G. Thollet, INSA Lyon).
(a)(b)


### 3.2. Rheological Comparison Tests

The mechanical behavior of the clay dispersions varies with
concentration and Na-content.(1)
At suspension concentrations lower than 4%, dispersions display
Newtonian behavior. Rheograms of Na-smectite, and Ca-smectite dispersions are
shown on Figure3. Newtonian viscosity values of each sample were calculated
for each concentration. The Newtonian behavior implies that units flow in water
without contact.(2)
At higher concentration, dispersions display shear-thinning behavior
which would indicate brittle organization through three-dimensional clay
particle association. The flow curves are modelled with Ostwald equation. The
increasing concentrations cause structure strengthening, finally resulting in a
yield value.(3)
At highest
concentrations and sodium contents, the rheograms are complex. At lower stress
values, a linear relation (A-B) can be seen in a log-lin plot of stress versus
strain (Figure4). Viscoelastic deformations of the gel, although the yield
stress is not reached, have been studied by creep and oscillatory tests. The part (B-C)
distinguishes these two regions, it represents the transition from elastic
solid to viscous liquid, which is associated with the yielding process.
The fluid is not completely sheared, so the shear rate values indicated by the
rheometer are smaller than the real ones. Two yield stresses, “static” and “dynamic,”
can be defined, respectively, at the points B and C [19]. The
parts C-D and D-E-F describe the shear-shinning behavior by increasing and
decreasing the shear rate. The Herschel-Bulkley parameters are defined
from the down curve (D-E-F).Flow curves for Ca-sm (a) and Na-sm (b) at
four concentrations (0.2%, 2%, 3%, and 4%).
(a)(b)Figure 4
Shear stress-shear rate
curve for Na-smectite suspension at 90 g⋅L-1.Figure5 shows the mechanical behavior of the
dispersions according to their concentration and smectite composition. Over the
X-axis, the suspension concentration varies from very dilute dispersions (2 g/l) to concentrated pastes (120 g/l). Over the Y-axis, the Na-content varies from
0% (Ca-smectite) to 100% (Na-smectite). The
rheological data are defined for each suspension and allow to give the limits
depending on the concentration and on the saturation state. Krigging
method from Surfer software allowed this representation with isovalues lines
for characteristic parameters (μ,
newtonian viscosity-k, consistency and  τ0,
yield
stress)  of the corresponding model.Figure 5
Isovalues
lines giving the parameters of flow tests on homionic and mixed suspensions at
different concentrations (isolines for each
domains: μ, Newtonian viscosity-k, consistency, and  τ0,
yield
stress).The Newtonian area
terminates approximately at the same concentration for Ca and Na but viscosity
evolution is quite different. We could consider first a sodium influence area,
where viscosity increase with increasing concentration and sodium content and
second a calcium influence area, where viscosity increases with increasing
concentration but decreases with the sodium content. This observation could be
partially explained by the polydisperse effect between Na- and Ca-platelets,
which have different sizes. In granular or colloïdal media, it is known, that
adding fine particles between coarse particles allows a better compactness, but
also increases fludity [20, 21].At higher
concentrations, sodium hydration capacity could develop and viscosity increases
continuously. The yield stress evolution is consistent and gradual with
increasing sodium percent but sharp with the dispersion concentration of the
sodium clay.


## 3.1. Observations by wet STEM

Each sample was observed by wet STEM, and 10
photographs of each sample were analyzed with the NIS software. Each unit was
approximatively defined and assimilated to an elliptical measurable area. Ca-smectite particle sizes
vary from 0.07 μm² (d10)
to 0.77 μm² (d90),
the particles  are dispersed (Figure 2(a)).
Particles are dense and seem to be composed of linked entities. Small units
could result from the aggregate break-up. Large spaces are occupied only by
water. Na-smectite units seem to fill all the available space (Figure 2(b)). This
observation could result from the three-dimensional superimposition or from the
particles settling down in the droplet. Particles sizes are quite homogeneous
with a mean area of 0.13 μm². Particles
distribution ranges from 0.035 μm² (d10)
to 0.3 μm² (d90),
although a few large particles were observed. The surface areas of the
Ca-sm units are approximatively 2.5 times larger than those of the
Na-sm units.(a), (b) Microscopic view (wet STEM) for diluted suspensions: Sm-Ca (0,1 g/l) and Sm-Na  (0,01 g/l) (photo: A. Bogner and G. Thollet, INSA Lyon).
(a)(b)


## 3.2. Rheological Comparison Tests

The mechanical behavior of the clay dispersions varies with
concentration and Na-content.(1)
At suspension concentrations lower than 4%, dispersions display
Newtonian behavior. Rheograms of Na-smectite, and Ca-smectite dispersions are
shown on Figure3. Newtonian viscosity values of each sample were calculated
for each concentration. The Newtonian behavior implies that units flow in water
without contact.(2)
At higher concentration, dispersions display shear-thinning behavior
which would indicate brittle organization through three-dimensional clay
particle association. The flow curves are modelled with Ostwald equation. The
increasing concentrations cause structure strengthening, finally resulting in a
yield value.(3)
At highest
concentrations and sodium contents, the rheograms are complex. At lower stress
values, a linear relation (A-B) can be seen in a log-lin plot of stress versus
strain (Figure4). Viscoelastic deformations of the gel, although the yield
stress is not reached, have been studied by creep and oscillatory tests. The part (B-C)
distinguishes these two regions, it represents the transition from elastic
solid to viscous liquid, which is associated with the yielding process.
The fluid is not completely sheared, so the shear rate values indicated by the
rheometer are smaller than the real ones. Two yield stresses, “static” and “dynamic,”
can be defined, respectively, at the points B and C [19]. The
parts C-D and D-E-F describe the shear-shinning behavior by increasing and
decreasing the shear rate. The Herschel-Bulkley parameters are defined
from the down curve (D-E-F).Flow curves for Ca-sm (a) and Na-sm (b) at
four concentrations (0.2%, 2%, 3%, and 4%).
(a)(b)Figure 4
Shear stress-shear rate
curve for Na-smectite suspension at 90 g⋅L-1.Figure5 shows the mechanical behavior of the
dispersions according to their concentration and smectite composition. Over the
X-axis, the suspension concentration varies from very dilute dispersions (2 g/l) to concentrated pastes (120 g/l). Over the Y-axis, the Na-content varies from
0% (Ca-smectite) to 100% (Na-smectite). The
rheological data are defined for each suspension and allow to give the limits
depending on the concentration and on the saturation state. Krigging
method from Surfer software allowed this representation with isovalues lines
for characteristic parameters (μ,
newtonian viscosity-k, consistency and  τ0,
yield
stress)  of the corresponding model.Figure 5
Isovalues
lines giving the parameters of flow tests on homionic and mixed suspensions at
different concentrations (isolines for each
domains: μ, Newtonian viscosity-k, consistency, and  τ0,
yield
stress).The Newtonian area
terminates approximately at the same concentration for Ca and Na but viscosity
evolution is quite different. We could consider first a sodium influence area,
where viscosity increase with increasing concentration and sodium content and
second a calcium influence area, where viscosity increases with increasing
concentration but decreases with the sodium content. This observation could be
partially explained by the polydisperse effect between Na- and Ca-platelets,
which have different sizes. In granular or colloïdal media, it is known, that
adding fine particles between coarse particles allows a better compactness, but
also increases fludity [20, 21].At higher
concentrations, sodium hydration capacity could develop and viscosity increases
continuously. The yield stress evolution is consistent and gradual with
increasing sodium percent but sharp with the dispersion concentration of the
sodium clay.


## 4. Conclusion

This study has shown that the rheological behavior of smectite suspensions,
prepared from a bentonite, is complex,
depending on the smectite concentration and Na-content. Smectite
was extracted from a natural bentonite, saturated with sodium or calcium, and its
mineralogy was determined with XRD. The samples were dispersed in water at low
concentration and studied with laser granulometry and wet STEM. The results
showed that the suspended Na-smectite particles are smaller and more dispersed
than the Ca-smectite units. At higher concentration, the suspensions develop yield
stress when the Na-content is 20%. It appears that the rheological measurements
are very sensitive to the platelets organization in a complex network,
depending on many parameters. When comparing rheological data of bentonite,
attention is drawn to the necessity of describing accurately the pretreatment
processes, especially when procedures for exchanging interlayer cations are
used, as they significantly influence the structural properties of the clay and
hence its technical properties. Associating fine mineralogical techniques with
rheological measurements is probably a way to understand the organization clay
suspensions.



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*Source: 856714-2008-02-26.xml*