Bentonite Mineralogy Literature Review
Preferential Modification of Bentonite Structure
MSc.
By Luke Molloy
Supervised by Dr Michael McLaughlin
Luke Molloy
lmolloy@research.ait.ie
Contents
Contents ................................................................................................................................ i
1
Background and uses .................................................................................................... 1
2
Mineralogy and molecular structure ............................................................................ 3
3
Particle size and stability in suspension ........................................................................ 6
3.1
4
Nano bentonite...................................................................................................... 6
Swelling and related information ................................................................................. 6
4.1
Barrier property enhancement ............................................................................. 8
5
Particle orientation ....................................................................................................... 8
6
The law of mass action.................................................................................................. 8
7
Isomorphous substation ............................................................................................... 9
8
Cation exchange capacity ........................................................................................... 10
9
Van der Waals Attractive Forces ................................................................................. 11
10
Particle associations in clay suspensions ................................................................ 11
11
Particle size and size distribution ............................................................................ 11
12
Permeability, ........................................................................................................... 12
13
Polymer/clay nano-composites ref: 13-028 ............................................................ 12
13.1
Other ................................................................................................................ 12
13.2
Diffusion ........................................................................................................... 15
13.3
Long term permeation ..................................................................................... 16
14
Bentonite products ................................................................................................. 16
15
Bentonite polymer nanocomposites BPN ............................................................... 17
16
Radox potential ....................................................................................................... 17
17
References ............................................................................................................... 19
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1 Background and uses
Clays can be defined as hydrous alumionsilicate minerals that make up a colloidal
fraction of soils with particle sizes generally less than 2 microns in length. Soils maybe
made up of sediments, rocks and water and contain clay minerals and clay size crystals
of other minerals such as quartz, carbonate and metal oxide, depending on the origin
and geological influences during its composition.
Clay barriers which are used to contain high level waste are susceptible to mechanical
property changes from internal multivalent cations from leachate solutions and
externally from changes in saline conditions of the encasements surrounding
groundwater. To be effective the engineered clay barrier must maintain its low hydraulic
conductivity value in order to protect the immediate environment. The engineered
barriers should consist of naturally occurring materials that can be shown to be stable in
the repository environment. There should exist a scientific knowledge of the processes
that can affect the barriers in a long term perspective (SKB 2010)
Figure 1 – Barrier assembly
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Figure 2 – KBS-3 repository
Figure 3 – Containment system
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Bentonite clays are used in a verity of applications including the sealing of leachate from
landfill sites to the containment of spent nuclear fuels. Bentonite clays have certain
desirable material properties such as the ability to swelling and high sorption capacity
including adsorption and ion exchange (Carlson 2004). The properties exhibited by
Bentonite clays are dependent on the mineralogy, geochemistry and chemical
composition of the material which are the result of varying geological history and source
locations.
The swelling capacity of bentonite, which has many commercial advantages, is
dependent on the proportion of smectite within the bulk material. Desired material
properties such as swelling capacity, cation exchange capacity and plasticity are
dependent not only on the proportion of smectite but the smectite species and the value
of exchangeable cations between the layer spacings, see Figure 2.1.
2 Mineralogy and molecular structure
Bentonite is a naturally occurring mineral mostly composed of the clay mineral smectite.
These clays are formed by the alteration of volcanic ash which is laid down in marine
environments which gets slotted between other types of rocks. Most of the smectite in
the clay is made up of montmorillonite, which is a dioctahedral smectite but occasionally
other types of smectite may be present. (Carlson 2004)
Montmorillonite is constructed of layers 1nm (nanometre, 1 x 10-9m) thick. The structure
of montmorillonite is that of an octahedral layer containing aluminium, magnesium,
oxygen and hydroxyl ions sandwiched between two tetrahedral layers of silicon, oxygen
and hydroxyl ions as seen in Figure 2.1 below.
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Figure 2.1 – Montmorillonite lattice structure (Poerpressure 2013)
In montmorillonite, about one in eight of the octahedral aluminium ions, Al3+, is replaced
by a magnesium ion, Mg2+. This results in a charge imbalance which draws any water
present into the interlayer space between the sheets. This causes the clay to swell
dramatically (Poerpressure 2013) as the net charge on the clay mineral becomes
negative thus attracting the H+ ion from the water (sorption). Adsorption of water
molecules is more intense near the surface of the clay particle with decreasing intensity
a function of distance (Cernica 1995). Two adjacent particles with like negative charges
will experience repulsion.
The ratio of 1:3 cation to anion ratio of montmorillonite makes it dioctahedral, only 2
out of every 3 octahedral sites around each hydroxyl needs to be filled to obtain
electrical neutrality. Other forms exist such as potassium (K), sodium (Na), calcium (Ca)
bentonites.
The clay mineral montmorillonite is part of a basic 3 group of minerals also containing
kaolinites and illites with the lattice structure of the minerals being the basis of their
classification.
Under a scanning electron microscope bentonite particles can be almost
indistinguishable from the filler clay or the coating clay. The sodium or potassium salts of
bentonite exfoliate into very thin plates. Theoretically these plates can be as tiny as
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about 1 nm thick, yielding a vast surface area per unit mass (NCSU 2013). Figure 2.2
shows the ratio of length to thickness of a bentonite platelet.
Figure 2.2 – Approximate length to thickness ratio of bentonite platelet
Bentonite from a location 100 km west-northwest of Prague in the Czech Republic was
analysed using various techniques to have the following chemical makeup:
Figure 2.3 – Bentonite chemical composition, 1) raw olive grey bentonite,2) separated
<1µm size fraction (Konta 1986)
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3 Particle size and stability in suspension
3.1 Nano bentonite
4 Swelling and related information
Bentonite can be used as buffer for high level waste (HLW) repository due to its swelling
ability on contact with free water. The swelling causes pressures to build within the
bentonite layer and thus forming a hydraulic barrier with a self-sealing capacity.
Experimental results by (Lee et al. 2012) using compacted calcium (Ca) bentonite (from
Korean), show that swelling increases with an increase in dry density, and its
dependence on dry density increases at densities beyond 1.6 Mg/m3. The investigation
showed that the swelling behaviour of Ca bentonite subjected to NaCl (sodium chloride)
solution was different to that of Na bentonite. The swelling pressure of the Ca bentonite
was higher with 0.04M concentration of NaCl but decreased thereafter. This can be
explained by an ion exchange of Ca2+ cations for Na+ cations from the NaCl solution.
Once the Na+ ions transfer to the bentonite a concentration differential occurs in which
osmotic forces draw more water molecules to the bentonite (Lee et al. 2012). Osmotic
swelling is the second phase of the process as the bulk water has a less concentration of
ions than that between the particle layers.
The state of hydration (the amount of interlayer water within the montmorillonite
structure) is related to the extent and location of the 2:1 layer charge, interlayer cation
spices (Na of Ca), vapour pressure, temperature and salinity of the saturating or
permeating solution (Karnland 1998). When expansion of clays is prohibited the
hydration forces give rise to macroscopic swelling pressure, visible to the eye unaided.
When an equilibrium state is reached due to full water saturation of the clay particle
interlayer, the pressure is related to the water ratio mw/ms as mass of water over mass
of solid. Swelling pressure can be determined by the use of a triaxial cell.
Three stages of swelling: from ref;13-028 p22 to 28
1. Mainly driven by hydration of interlayer cations where the water forms a layer
midway between the platelet layers. No further swelling will occur if the
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predominat cations are divalent Ca or Mg, because of the strength of the
electrostatic forces between the cations and the overall negative charge coming
from within the octahedral layer due to isomorphus substitution.
2. Where monovalent caions such as Na are dominant in the interlayer region, these
have less electrostatic attraction due to their monovalent charge. This is where a
second stage of swelling occurs as an abrupt increase in interlayer spacing to
between from 3-4nm to tens of nms. This due to the formation of a diffuse
electrical double layers on the clay particle surface and osmotic forces push the
platelets apart and are only opposed by Van der Waals forces and the frictional
interaction of plate edge to face electrical charge attractions (due to the + edge
charges, resulting from exposed octahedral layer imperfections, being attracted to
the – face charge)
3. A final stage of swelling can be achieved by addition of more water and gentle
agitation which disrupts the frictional bonds described in 2. above. Platelets now
enter an exfoliated state where individual platelets are kept apart by thermal
motion, separated by large distances limited by the volume of water in the system.
Basal spacing is dependent on a number of factors including: the nature of the interlayer
cation, water molecules present in the interlayer, the ambient relative humidity. Where
complete dehydration and layer collapse the basal spacing is thought be in the region of
0.95nm (Theng 2012).
The extent in which osmotic swelling occurs is a function of the ionic strength of the
solution and the dominating exchange complex. When monovalent cations prevail as the
dominating exchange complex osmotic swelling will occur but when divalent cations are
involved in the exchange process then osmotic swelling is prohibited amounting to
several orders of magnitude jump in hydraulic conductivity values (k) (Scalia 2012).
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Swelling pressure and temperature
4.1 Barrier property enhancement
From ref: 13-028
Adding a few per cent in weight of platy nano filler such as nanoclay has been shown to
improve barrier properties once good dispersion is achieved. Montmorillonite is often
preferred for work with nanoparticles because of its high aspect ratio, relatively low
cost, low toxicity and its ability to be modified (Dunkerley 2011).
5 Particle orientation
Braggs law and Herman orientation parameter in relation to the direction of flow: -.5 for
parallel, 1 for perpendicular and 0 for random. This effects the permeability of a
clay/composite.
Thermally induced weakening of the electrostatic attraction between adjacent layers
6 The law of mass action
The law of mass action states the following principle: the rate of a chemical reaction is
directly proportional to the molecular concentrations of the reacting substances.
And
“The law of mass action; law stating that the rate of any chemical reaction is
proportional to the product of the masses of the reacting substances, with each mass
raised to a power equal to the coefficient that occurs in the chemical equation. This law
was formulated over the period 1864–79 by the Norwegian scientists Cato M. Guldberg
and Peter Waage but is now of only historical interest. This law was useful for obtaining
the correct equilibrium equation for a reaction, but the rate expressions it provides are
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now known to apply only to elementary reactions” (Encyclopedia Britanica, Kieth J.
Laidler)
7 Isomorphous substation
This is the process where lower charge cations within the clay particle lattice such as
Mg++ replace higher charged cations such as Al+++ which ultimately results in a net
negative charge on the clay particle. Figure 7.1 shows the 2-1 crystal lattice structure of a
layered clay particle.
Figure 7.1 – Alumina silicate clay particle structure
Figure 7.2 below shows the substitution of Mg cations for Al cations resulting in a net
negative charge on the particle in the octahedral layer. Montmorillonite smectite is
always negative due to isomorphous substitution which occurs during mineral
crystallization.
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Figure 7.2 – Isomorphpous substitution
The structural bonding of the oxygen-oxygen or the oxygen-cation leaves the layers
weakly held together which allows the adsorption of cations in the interlayer space. This
means the mineral is expandable and has a high cation exchange capcity (CEC). `
When dry the interlayer cations hold the layers together. The clays swell in water due to
the absorption of water to the interlayer space.
The resulting charge imbalance is equalised by hydrated cations like K, Na, Mg and/or
Ca. More than 80% of these are located in the interlayer region (Uskarci 2006).
Bentonite smectite formed in aqueous environments have hydrated ions which results in
the ions being only loosely held by the negatively charged clay layer thus making them
susceptible to cation exchange.
8 Cation exchange capacity
Cation exchange effects the mechanical and physical properties of clay. It is therefore an
important factor in the use of sodium bentonites, containing mainly montmorillonite
mineral, as hazardous waste barriers built deep in granite rock (Guggenhiem 2013).
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Ion exchange is the process where ions in an electrolyte solution exchange with the ions
in a solid phase material (Yen 2007). Montmorillonite in this case is the solid phase
material, making up the majority of the bentonite, which acts as the exchange
mechanism for the ions (cationic exchanger). Ions within the diffuse double layer, the
area where there are a combination of negatively charged mineral surfaces and
positively charged spaces around the mineral, may exchange between the ions on the
clay particle and the ions within this layer. The thickness of the absorbed water layer can
be affected as a result of ion exchange thus affecting the ability to swell. This could have
consequences for the integrity of bentonite in engineered barrier systems (EBS).
9 Van der Waals Attractive Forces
10 Particle associations in clay suspensions
11 Particle size and size distribution
Aggregate-size distribution should affect the rate of swell, and can affect the hydraulic
conductivity to non-standard liquids (Shackelford et al. 2000).
This next paragraph is in relation to ref: 13-116. Specific surface of a soil is the
relationship between the surface area and its mass, denoted Ss.
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12 Permeability,
Form 13-028 page 19;
In barrier applications it has been long established that the addition of a relatively small
addition of well dispersed nanoclay results in a dramatic decrease in permeability due to
the creation of torturous flow paths.
13 Polymer/clay nano-composites ref: 13-028
Nano composites are comprised of the same types of materials as conventional
composites but with fillers that are generally less than 100nm in at least one direction. In
this case it could be the thickness of a 2:1 phyllosilicate platelet like montmorillonite at
around 1-2 nm. Nano composite formation by solvent blending.
Page 22:
Of all the phyllosilicates montmorillonite clay is the most commonly used layered silicate
for nancomposite work. The aspect ratio being 70-150
13.1 Other
Montmorillonite has a high specific surface area, extremely low hydraulic conductivity
(approx. 1 x 10-9 cm/s) and high cation exchange capacity. Na bentonite is primarily used
in geosynthetic clay liners (GCLs) which mean the exchange complex of montmorillonite
is dominated by Na+ ions. Resulting problems occur in the gradual replacement of Na+
ions which exist on the surface of the montmorillonite particle surface by multivalent
Ca++ ions which may exist in the surrounding permanent liquid in which the bentonite
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GCL is in permanent contact with. This can result in an increase in hydraulic conductivity
in the order of a magnitude or more. This process continues very slowly until the
exchange of Ca++ ions for Na+ is complete (Ho Young et al. 2006) and the possible
increase of hydraulic conductivity due to compatibility problems with contaminant if not
prehydrated with a compatible water source (Bouazza 2002). In some cases several years
can be required to reach equilibrium (Egloffstein 2001).
The primary differences between GCLs are the mineralogy and form of bentonite such
as: powder versus granular, sodium versus calcium, etc. (Bouazza 2002). A compatibility
test is usually conducted prior to selection.
Results from test carried out on GCLs by (Shackelford et al. 2000) of the hydraulic
condiuctivity of swelling clays of nonstandard liquids (0.05 N CaSO4) containing both
monovalent cations (Na) as well as low concentrations of divalent cations (Ca) can cause
significant increases in hydraulic conductivity. This holds true if the test is significantly
long in duration to allow full exchange of adsorbed cations, equilibrium established.
They also show that the control of average effective stress is of more importance than
controlling the hydraulic gradient while evaluating the hydraulic gradient.
ASTM D 5084: standard test method for the measurement of the hydraulic conductivity
of saturated porous materials using a fexible wall permeameter.
Proff. Kerry Rowe
Daniels, permeability
Hydraulic conductivity is related to the mineralogy of bentonite.
The hydraulic conductivity of montmorillonite to water typically is very low (10-8 cm/s).
Also, the large affinity of montmorillonite for water molecules and hydrated cations
results in significant swelling of montmorillonite (5-10 times the dry volume) when
hydrated under low effective stress. The montmorillonite content in bentonite also is
reflected indirectly by the cation exchange capacity (CEC) of the bentonite. The CEC is a
measure of the total adsorption capacity of a soil for cations, and increases with greater
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surface charge deficiency and greater specific surface of the clay mineral portion of the
soil (Shackelford et al. 2000).
Figure 13.1 – Mineralogy of bentonite portion of 3 GCLs (Shackelford et al. 2000)
Table 13.1 shows the CEC of two different geometric forms of Wyoming (Na) bentonite.
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Table 13.1 – Chemical properties of two Wyoming bentonites (Shackelford et al. 2000)
Pure montmorillonite has a higher CEC than bentonite because of the impurities in the
bentonite such as quartz.
Since the low hydraulic conductivity of bentonite is primarily due to adsorbed molecules
associated with the montmorillonite restricting the pore spaced active in flow,
bentonites are particularly sensitive to changes in the composition of the pore fluid that
influence the thickness of the adsorbed layer. In particular, liquids that cause the
adsorbed layer to collapse also causes the hydraulic conductivity to increase, thus
bentonites with greater montmorillonite content are potentially more vulnerable to
chemical attack and incompatibility based on the permeating liquid being held.
Determination of the level of cations in the permeant, level of cations in the surrounding
host soil/clay/minerals/ and the cation exchange capacity of the bentonite liner.
13.2 Diffusion
In liquids, molecular diffusion occurs by jumps of the molecules from one position to
another; this arises when the energy of the molecule is high enough to rupture the
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bonds with the neighbouring molecules allowing the molecule to move. On average, the
jump does not exceed an intermolecular spacing, and since in a liquid this is much less
than in a gas, the diffusion is substantially lower. Since a liquid is virtually
incompressible, the diffusion rate is independent of pressure. Elevation of temperature
increases intermolecular spacing’s and the velocity of vibrations and jumps of molecules,
which enhances diffusion.
In general, only about 8% -10% solids slurries of good quality swelling smectite can be
produced in water. Indeed, at solids contents greater than about 8%, the viscosities of
the slurries can become so high that they cannot readily be pumped by conventional
equipment and gelling upon standing becomes a problem. At higher solids it becomes
virtually impossible to form a uniform paste without special equipment. Thus, there is a
need for slurries containing substantially greater than 8% by weight of smectite clay,
which have viscosities low enough to allow pumping (Uskarci 2006). Since bentonite ore
mined from bentonite deposit usually has a water content of 15 to 35%, it is primarily
broken and dried in the sun or hot air to obtain bentonite ore having a water content of 5 to
10%.
Ficks Law, differential equations,
13.3 Long term permeation
The effects of long term permeation of high cation solution on the swelling ability of the
bentonite layer, and maintenance of a low hydraulic conductivity need to addressed.
14 Bentonite products
1. MX-80
2. TRISOPLAST – The Netherlands
3.
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15 Bentonite polymer nanocomposites BPN
16 Radox potential
The clay liners were exposed to cyclic redox potentials (- 200 mV, 0 mV and + 300 mV)
and the experimental results indicated that pore size distribution and hydraulic
conductivity were changed for bentonite and Amite soil clay liners. However, kaolinite
clay liner showed little changes either in porosity or hydraulic conductivity. This from ref
013-030 look up the electrical conduction phenomena of soil and how it effects the soil
particle orientation
Web explanation
The redox potential is a measure (in volts) of the affinity of a substance for electrons —
its electronegativity — compared with hydrogen (which is set at 0).
Substances more strongly electronegative than (i.e., capable of oxidizing) hydrogen have
positive redox potentials. Substances less electronegative than (i.e., capable of reducing)
hydrogen have negative redox potentials.
Oxidations and reductions always go together. They are called redox reactions. When
electrons flow "downhill" in a redox reaction, they release free energy.
Discussion
We indicate this with the symbol ΔG (delta G) preceded by a minus sign. It requires an
input of free energy to force electrons to move "uphill" in a redox reaction. We show this
with ΔG preceded by a plus sign.
The electronegativity of a substance can also be expressed as a redox potential
(designated E)
The standard is hydrogen, so its redox potential is expressed as E = 0.
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Any substance — atom, ion, or molecule — that is more electronegative than hydrogen
is assigned a positive (+) redox potential; those less electronegative a negative (−) redox
potential.
The greater the difference between the redox potentials of two substances (ΔE), the
greater the vigor with which electrons will flow spontaneously from the less positive to
the more positive (more electronegative) substance.
The difference in potential (ΔE) is, in a sense, a measure of the pressure between the
two. ΔE is expressed in volts.
If we bring two substances of differing E together with a potential path for electron flow
between them, we have created a battery. Although it may be in a mitochondrion, it is
just as much a battery as a the lead-acid storage battery in an automobile.
The greater the voltage, ΔE, between the two components of a battery, the greater the
energy available when electron flow occurs. It is, in fact, possible to quantify the amount
of free energy available. The relationship is:
ΔG = − n (23.062 kcal) (ΔE)
where •n is the number of moles of electrons transferred and
•23.062 is the amount of energy (in kcal) released when one mole of electrons passes
through a potential drop of 1 volt.
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17 References
Bouazza, A. (2002) Geosynthetic clay liners. Geotextiles and Geomembranes, 20 (1) pp.
3-17.
Carlson, L. (2004) Bentonite Mineralogy. Geological Survey of Finland.
Cernica, J.N. (1995) Geotechnical Engineering: Soil Mechanics. Wiley
Dunkerley, E.J. (2011) A Study of High Clay Content Polymer/Organically Modified
Montmorillonite Hybrids.
Thesis (Doctor of Philosophy). University of
Massachusetts, Massachusetts.
Egloffstein, T.A. (2001) Natural bentonites—influence of the ion exchange and partial
desiccation on permeability and self-healing capacity of bentonites used in GCLs.
Geotextiles and Geomembranes, 19 (7) pp. 427-444.
Guggenhiem, S. (2013) Introduction to the Properties of Clay Minerals. [Online]. Available
at:
http://www.minsocam.org/msa/Monographs/Mngrph_03/MG003_371388.pdf [Accessed: 23/07/2013].
Ho Young, J., Benson, C.H. & Edil, T.B. (2006) Rate-limited cation exchange in thin
bentonitic barrier layers. Canadian Geotechnical Journal, 43 (4) pp. 370-391.
Karnland, O. (1998) Bentonite Swelling Pressures in Strong NaCl Solutions
Posiva: Clay Technology Lund Sweeden.
Konta, J. (1986) Textural Variation and Composition of Bentonite Derived from Basaltic
Ash. Clays and Clay Minerals, 34 (3) pp. 257-265.
Lee, J.O., Lim, J.G., Kang, I.M. & Kwon, S. (2012) Swelling pressures of compacted Cabentonite. Engineering Geology, 129–130 (0) pp. 20-26.
Ncsu
(2013)
Bentonite
(montmorillonite).
[Online].
Available
at:
http://www4.ncsu.edu/~hubbe/BENT.htm [Accessed: 03/07/2013].
Poerpressure (2013) Pore Pressure - Clay Diagenesis. [Online]. Available at:
http://www.porepressure.info/Clay-Diagenesis.html [Accessed: 03/07/2013].
Scalia, J. (2012) Bentonite Polymer Composites for Containment Applications. Thesis
(Doctor of Philosophy). University of Wisconsin-Madison, Wisconsin.
Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B. & Lin, L. (2000) Evaluating the
hydraulic conductivity of GCLs permeated with non-standard liquids. Geotextiles
and Geomembranes, 18 (2–4) pp. 133-161.
Theng, B.K.G. (2012) Formation and Properties of Clay-Polymer Complexes. Elsevier
Science Limited
Uskarci, T. (2006) Behaviour of Bentonite Suspensions in Non-Aqueous Media. Thesis
(Masters). Middle East Technical University.
Yen, T.F. (2007) Chemical Processes for Environmental Engineering. Imperial College
Press
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Book request:
lmolloy@research.ait.ie
25/07/13
Title: Formation and Properties of Clay-Polymer Complexes
ISBN: 9780444533548, ISSN: 1572-4352
Author: B.K.G Thang
Edition: Second
Publisher: Elsevier 2012
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Lab schedule for week starting 26/08/2013
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