bentonite in geotechnics

7/25/2019 Bentonite in Geotechnics

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I

Properties and Use of Bentonite in Geotechnics

1. Smectites

1.1 Structure and composition

2. Review of the principal applications of bentonites

3. The origin of bentonites and smectites

4. Basic principles concerning the use of bentonite in geotechnics

4.1 Swelling behaviour

4.2 Colloidal clay mineral dispersions

4.3 Flow behaviour of bentonite suspensions

5. References


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I. Bentonite

Composition - Properties

The term bentonite was first used to describe a plastic clay in upper cretaceoustoffs near Fort Benton/Wyoming (KNIGHT, 1898). ROSS & SHANNON (1926) de-fined bentonite as a clay mineral composed mainly of montmorillonite or beidel-lite, whose origin can be attributed to devitrification of tuffs or volcanic ash.

In a technically oriented definition, WRIGHT (1968) describes bentonite as a clayconsisting mainly of clay minerals of the smectite group, whose physical proper-ties are determined by these clay minerals. Montmorillonite is the most wide-

spread form of smectite.

1 Smectites

1.1 Structure and composition

OHO Si, Al Al, Fe, Mg

exchangeable cationsneutral molecules(H2O)

tetrahedron layer

octahedron layer

tetrahedron layer

interlayer

tetrahedron layer

d001

water molecule

counter ion

Fig. 1:: Crystal structure of montmorillonite; adapted from GRIM(1962)

The structure of smectites with two tetrahedron layers and one octahedron layeris shown in Fig. 1. Due to an ion exchange process whereby higher valency ionsare replaced by lower charge ions, the silicate layers of montmorillonite have aweakly negative charge. This is compensated for by the adsorption ofcounter-ions in the inter-layers, as a result of which individualTOT-layer groupsare elec-

Properties and Use of Bentonite in Geotechnics I-3


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trostatically held together. But the layer charge is such that the inter-layer cations(in the natural state mainly Ca2+- ,Mg2+- or Na+- ions) are present in exchange-able form. The minimal charge permits an expansion of the interlayer spaces todeposit the counter ions with their hydrate load (inner crystalline swelling).

Compared with other clay minerals, montmorillonites display a considerably finergranulometric structure. Kaolinite e.g. is often composed of penetrating silicatelayers, stapled in the form of books. In contrast, montmorillonite minerals aremostly finer than 1 m. Only a few idiomorphic crystals have a larger diameter(Fig. 2).

1 m1 m

Fig. 2:Smectite and kaolinite under the electronic microscope

Following dispersion, individual silicate layers may detach themselves from the

montmorillonite layer groups depending on their cation content. The silicate lay-ers are frequently distinguished by a flexible and pliable character, which is clear-ly identificable under the transmission electronic microscope (Fig. 3).

I-4 Properties and Use of Bentonite in Geotechnics


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0,2 m

a)

Fig. 3:Montmorillonite minerals under the trans-mission electronic microscope.

0,05 m

0,05 m

b)

c)

2 Overview of the principal applications of bentonites

The properties and potential applications of bentonite are largely dependent onthe swelling capacity of smectitic clay minerals and on exchange of cations

bound in the interlayer spaces. These may vary considerably from deposit todeposit depending on differences in the structural set-up, the distribution of char-ges, the form of cations in the intermediate layers, the grain size and shape etc.In order to optimise their properties for the intended application, bentonites aresubjected to mechanical and chemical treatments which range from classificationdrying and grinding to activation with acids, alkalis and various organic sub-stances.

A large portion of the annual bentonite production is used in the foundry industryas a mold sand binder. In the drilling industry, bentonite suspensions are used

as supporting fluid which, when the borer stops, keeps the drilling fines in sus-pension and can exert a supporting effect on the borehole wall. The plastic andthixotropic flow behaviour of bentonite suspensions is used in special under-ground projects as a supporting fluid in the construction of slot, sealing and slimwalls as well as in hydroshield drives operations.

In horizontal and vertical barrier systems around landfills and contaminated soils,bentonites are used for sealing and sorption. Several plans for final disposal ofradioactive waste involve the use of highly compacted bentonites to secure thecontainers with used fuel.



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Large quantities of bentonite are used for pelletising of animal feeds and miningproducts, especially iron ores. An important field of application is the use of ben-tonite as adsorbent. Bentonites and earths are used in the processing of mineraloils and fats, to discolour edible oils, and to clarify wine, beer and fruit juices.

Another important market for bentonite is its application as cat litter.

3 The origin of bentonites and smectites

Bentonites originate in most cases as products of transformation of volcanicrocks through hydrothermal solutions or, when there is abundant water supply,during early diagenesis. The formation of smectitic clay minerals from volcanitesis based mainly on the decomposition of minerals and volcanic glasses throughhydration and hydrolysis. The alteration process is a dynamic one, controlled

largely by three factors (FISHER &SCHMINCKE, 1984): 1. Composition of the initialrock, 2. Physical conditions (temperature, grain size, porosity, permeability), 3.Composition of the alteration -causing solution in the pore space.

4 Basic principles of bentonite application in geotechnics

4.1 Swelling behaviour

The behaviour of smectite to the water is of substantial importance for the rangesof application of bentonite in the geotechnical. When clay minerals are hydrated,

three types of water collection are distinguished: Hydration of the smectite interlayers (inner crystal swelling) by the adsorption

of water onto the interlayer cations and the clay mineral surfaces.

Osmotic swelling which results from the concentration gradient of cations atthe clay mineral surface and in the pores solution. What occurs here is anelectrostatic repulsion of particles through the formation of diffuse ion layers(see Ch.4. 2).

Uptake of free water into the micropores of the bentonite structure.

With the increasing offer of water, the hydration of the smectite interm- layers re-

sults in a gradual widening of the interlayer space (KRAEHENBOHL ET AL., 1987)and, depending on the amount of water available, two, three or four molecularwater layers may be deposited between the silicate layers. The factors influenc-ing most considerably the hydration of smectites depend on the characteristicproperties of the intermediate layer cations, on the silicate layers of the smectitesand of water.



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N+

N+

N+

N+

N+

N+

N+

N+

N+

N+

CH2 OH

air-dry after adding water

= Ca2+-Ionen d001: 1,5 nm

= Na+-Ionen d001: 1,2 nm

= cationic tensid d001: 1,8 - 3,0 nm

d001: 2,0 nm

d001:

d001: 1,8 - 3,0 nm

hydrophobic

N+

Calcium-bentonite

Sodium-bentonite

Organoclay

Fig. 4: Interval between layers of montmorillonite

a) Na-bentonite (2g) in water: 28 ml

b) Ca-bentonite (2g) in Water: 5 mlc) Na-bentonite (2g) in xylol: 4 ml

d) Organoclay (2g) in xylol: 25 ml

a) b) c) d)

Fig. 5:Swelling volumes of bentonites and organoclays in water and xylol



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If bivalent cations are present in the interlayers, a quasi-crystalline order is main-tained even in the case of sufficient water supply (HOFMANN ET AL.1957) and theinterval between layers does not exceed 20 A (Fig. 4). In the presence of Na+orLi+, a complete split into isolated silicate layers or thin packages with few layers

can occur in water or electrolyte-poor solutions (swelling > 4), resulting in a col-loidal dispersion. If in addition to Li+or Na+ions, other counter ions also occur,several silicate layers may coalesce to form tactoids, whose size depends on thekind of cations (SCHRAMM &KWAK, 1982).

Na+-montmorillonites can store large amounts of water in the interlayers and onthe clay mineral surfaces. Accordingly, a large swelling volume is observed (Fig.5). The extensive delamination of the silicate layers in a homo-ionic sodium ben-tonite is reflected in the small particle sizes recorded on the granulometric distri-bution curve (Fig. 6). Because of the higher charges of earth alkali ions, mont-

morillonites with Ca

2+

or Mg

2+

ions in the inter layers have less swelling potential.The silicate layers remain ordered in fairly large layer bunches even after suffi-cient water has been added.

0

10

20

30

40

50

6070

80

90

100

0,01m 0,1m 1m 0,01 mm

Grain size d

Masspercentage

s 50 ml

0

250

500

750

0 10 20 30 40 50

sheargradient[s-]

push tension [Pa]

Homo-ionic makeup with Na+- or Li+-ions

flow limit (Kugelharfe): 18,1 N/m 2marsh-time: 49 sec

filtrate water: 6,5 ml

0

250

500

750

0 10 20 30 40 50

sheargradien

t[s-]

push tension [Pa]

Increase in electrolyte concentration (pH > 8)

flow limit (Kugelharfe): > 70 N/m 2marsh-time: 120 sec

filtrate water: 7 ml

0

250

500

750

0 10 20 30 40 50

sheargradient[s-]

push tension [Pa]

Further increase in electrolyte concentration

flow limit (Kugelharfe): 13,8 N/m 2marsh time: 37 sec

filtrate water: 18,5 ml

Fig. 10:Arrangement of smectites at different electrolyte contents

(after LAGALY,1993; PERMIEN& LAGALY, 1994a)



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During a shearing, the viscosity declines to the point where the connection be-tween clay minerals is broken. After the shearing process comes to an end, thenetwork rebuilds itself and the viscosity of the suspension increases. This iso-thermal and reversible gel/fluid conversion through mechanical stress is known

as thixotropy. In bentonite suspensions, thixotropy is identifiable in the -D-diagram as a hysteresis in the flow curves. At the end of the flow process, theflow limit shows its minimal value (dynamic flow limit) and then approaches as-ymptotically a maximum value (static flow limit).

The velocity and the flocculation mechanism is significantly influenced by the pH-value and the electrolyte content of the suspension. At a pH-value of


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0

50

00

750

0 10 20 30 40 50

[Pa]

(1)(5)(3)

(4)

(2)

2

5

Schergeflle

[s-1]

sheargradient[s-]

Schubspannung push tension [Pa]

Fig. 11:Thixotropy of bentonite suspensions: the montmorillonite particles arrange them-selves after the dispersion (1) into a network. As time passes, the viscosity of the sus-pension increases (2, 3, 4). The flow limit approaches asymptotically a maximum limitvalue (4). Mechanical shearing causes destruction of the clay mineral network and thesuspension again becomes fluid. The viscosity declines to the point that the connectionbetween the clay minerals is completely broken (5).

5 References

CALLAGHAN, I.C.&OTTEWILL, R.H. (1974): Interparticle forces in montmorillonitegels.- Disc. Faraday Soc., 57: 110-118

FISHER,R.V.&SCHMINCKE, H.U. (1984): PyroclasticRocks.- Springer, 409 p.GRIM,R.E.&GVEN, N. (1978): Bentonites - Geology, Mineralogy, Properties andUses.- Developments in Sedimentology, Elsevier, 24: 256 p.

GRIM, R.E. (1962): Applied Clay Mineralogy.- McGraw-Hill Book Co., New York



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GUEVEN, N. (1988): Smectite.- In: S.W. BAILEY[ed.]: Hydrous Phyllosilicates (Ex-clusive of Micas). Min. Soc. Amer., Rev. in Min., 19: 497-559

GUEVEN, N. (1992a): Molecular aspects of clay/water interactions.- In: GUEVEN,N., POLLASTRO, R.M. [eds.]: Clay Water Interface and its Rheological Implica-

tions. Clay Min. Soc. Workshop Lectures Vol. 4, The Clay Minerals Society, 1-79

GVEN, N. (1992b): Rheological aspects of aqueous smectite suspensions.- In:GVEN,N.,POLLASTRO, R.M. [eds.]: Clay Water Interface and its Rheological Im-plications. Clay Min. Soc. Workshop Lectures Vol. 4, The Clay Minerals Society,81-126

HOFMANN,U.,FAHN,R.&WEISS, A. (1957): Thixotropie bei Kaolinit und innerkri-stalline Quellung bei Montmorillonit.- Kolloid Z., 151: 97-115

ISRAELACHVILI, T.N. (1994): Intermolecular and surface forces.- Academic Press

JASMUND,K.&LAGALY, L. (1993): Tonminerale und Tone.- Steinkopff Verlag, 490S., Darmstadt

KJELLANDER,R.,MARCELJA,S.&QUIRK, J.P. (1988): Attractive double layer inter-actions between calcium clay particles.- J. Colloid Interf. Sci., 126: 194-211

KNIGHT, W.C. (1898): Bentonite.- Eng. Min. J., 66: 491

KRAEHENBUEHL,F.,STOECKLI,H.F.,BRUNNER,F.,KAHR,G.&MUELLER-VONMOOS,M. (1987): Study of the water-bentonite-system by vapour adsorption, immersioncalorimetry and X-ray techniques: I. Micropore volumes and internal surface ar-eas, following Dubinin's theory.- Clay Min., 22: 1-9

LAGALY, G. (1988): Grundzuege des rheologischen Verhaltens waessriger Ton-mineraldispersionen.- Mitt. Inst. fr Grundbau und Bodenmechanik, ETH Zuerich,133: 1-27

LAGALY, G. (1993): Reaktionen der Tonminerale.- In: JASMUND, K., LAGALY, G.[eds.]: Tonminerale und Tone, Steinkopff, 89-167, Darmstadt

LOW, P.F. (1987): Structural component of the swelling pressure of clays.- Lang-muir, 3: 18

ODOM, I.E. (1984): Smectite clay minerals; properties and uses.- Phil. Trans.Roy. Soc. London, A311: 391-432

OSS van,C.J.,GIESE,R.F.&COSTANZO, PM. (1990): DLVO and non-DLVO inter-actions in hectorite.- Clays Clay Min., 38: 151-159

PERMIEN, T. & LAGALY, G. (1994a): The rheological and colloidal properties ofbentonite dispersions in the presence of organic compounds: I. flow behaviour ofsodium-bentonite in water-alcohol.- Clay Min., 29: 751-760ERMIEN,T.&LAGALY,G. (1994b): The rheological and colloidal properties of bentonite dispersions inthe presence of organic compounds: I. flow behaviour of wyoming-bentonite inwater-alcohol.- Clay Min., 29: 761-766



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RAND,B.,PEKENC,E.,GOODWIN,J.W.&SMITH, R.W. (1980): Investigation into theexistence of edge-face coagulated structures in Na-montmorillonite suspen-sions.- J. Chem. Soc, I 76: 225-235

ROSS,C.S.&SHANNON, E.V. (1926): Minerals of bentonites and related clays andtheir physical properties.- J. Am. Ceram. Sci., 9: 77-96

SCHRAMM,L.L.&KWAK, J.C. (1982): Influence of exchangeable cation composi-tion on the size and shape of montmorillonite particles in dilute suspensions.-Clays Clay Min., 30: 40-48

VALI, H. & BACHMANN, L. (1988): Ultrastructure and flow behaviour of colloidalclay dispersions.- J. Colloid Interf. Sci., 126: 278-291

WEISS,A.&FRANK, R. (1961): Ueber den Bau der Gerueste in thixotropen Ge-len.- Z. Naturforsch., 16 (b): 141-142

WRIGHT, P.C. (1968): Meandu creek bentonite - a reply.- J. Geol. Soc. Aust., 15:347-350

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