.Applied Clay Science 15 1999 447462

Influence of pore water chemistry on the swellingbehavior of compacted clays

Waddah S. Abdullah a,1, Khalid A. Alshibli b,),Mohammed S. Al-Zoubi c,2

a Department of Ciil Engineering, Jordan Uniersity of Science and Technology, PO Box 3030,Irbid, Jordan

b Department of Ciil and Enironmental Engineering, Uniersity of Alabama in Huntsille,Huntsille, AL 35899, USA

c Department of Ciil Engineering, Uniersity of Illinois at Urbana-Champaign, Urbana, IL61801, USA

Received 16 June 1998; received in revised form 7 May 1999; accepted 11 May 1999

Abstract

The influences of the exchange complex and pH of the solution used for cation saturation onAtterberg limits, compaction, and swelling potential of a compacted clay were investigated. Thestudy involved transforming the exchange complex from a heterogeneous to a homogeneous oneso that a frame of reference can be set for the clay behavior under such an ideal condition. Theemployed method for altering the exchange complex successfully yielded homo-ionic clay. Theintroduction of different species of cations gave rise to different particles associations. Whenintroduced to the tested clay, potassium cations bond its particles with a rather strong bond . K-linkage , causing a drastic decrease in the specific area of the clay about one-fourth of its

.untreated specific area , a decrease in the CEC, as well as a drastic decrease in the swell potential.For example, the swell pressure decreased from 1.87 kgrcm2 for the untreated samples to 0.4

2 .kgrcm for the K-treated samples under the same conditions . Also, the swell potential vs. timerelationships can be modeled accurately using a rectangular hyperbola. q 1999 Elsevier ScienceB.V. All rights reserved.

Keywords: expansive clays; swelling; cation exchange; compaction; Atterberg limits

) Corresponding author. Tel.: q1-256-544-3051; Fax: q1-256-890-6724; E-mail:khalid.alshibli@msfc.nasa.gov

1 E-mail: waddah@just.edu.jo2 Tel.: q1-217-333-7311; Fax: q1-217-333-9464; E-mail: al-zoubi@students.uiuc.edu

0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .PII: S0169-1317 99 00034-4

( )W.S. Abdullah et al.rApplied Clay Science 15 1999 447462448

1. Introduction

It is well known that the exchange complex has an important role incontrolling properties and engineering behavior of clays. The magnitude of theinfluence varies according to the properties under consideration, the type of clayinvolved, and other environmental conditions. Atterberg limits are fundamentalproperties that are extensively used as a measure for soil classification and asparameters for correlation to predict the soils engineering behavior such asswelling and compressibility.

Swell phenomenon is known to cause serious damage to low rise buildings,dams, and highways. Ultimate heave and rate of heave are the two factors thatcause structures built on expansive soils to experience damages due to differen-tial settlements or expansion. In clayey soils, the physico-chemical swell is themajor and the important part of the soils heave. The claywaterelectrolytesystem is the main factor affecting the physico-chemical swell. The clay mineralparticles together with the adsorbed exchangeable cations constitute the so-called

.exchange complex Marshall, 1977; Hillel, 1980 . An exchange reaction maytake place which results in altering the fractions of the exchangeable cationswithin a heterogeneous or a homogeneous exchange complex. A typical ex-change reaction is given according to the following equation:

In this paper, a physico-chemical study was conducted using an expansiveclay known as Azraq Green clay. The investigation involves altering the

q 2q 2qexchange complex from a heterogeneous untreated having Na , Mg , Ca ,q . and K exchangeable cations to a homogeneous exchange complex having

.mainly one species of cations present on the clay . This is considered as afundamental step for establishing a frame of reference, for future studies, forassessing real conditions of heterogeneous exchange complex. The alteration

.process was conducted three times, to transform the exchange complex to i . .K-dominated ii Na-dominated or iii Ca-dominated cation exchange complex,

so that the swell behavior under such an ideal condition can be established firstand then used as a basis for establishing soil behavior under real conditions ofheterogeneous exchange complex.

The influence of clay structure on the swell behavior was also investigated.The structure of compacted clays can be made to fit certain research needs

ranging from flocculated to oriented structure Lambe, 1953; Seed and Chan,.1959 . Because of that, compacted clays can offer an excellent opportunity for

studying the influence of clay structure on its behavior. In this investigation, the

( )W.S. Abdullah et al.rApplied Clay Science 15 1999 447462 449

Fig.

1.X

-ray

diffr

acto

gram

soft

hete

sted

soil.

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.structure has been manipulated via the use of i the molding moisture content, . .ii exchangeable cations, and iii the pH value of the solution used for cationsaturation.

2. Materials and methods

2.1. Clay compositional properties

The clay used in this investigation is natural clay known as Azraq green clay,obtained from Azraq basins, Jordan. This clay spreads over large area in manyparts of Jordan.

The clay minerals present in the Azraq green clay are: mixed layerillitersmectite, kaolinite, palygorskite, montmorillonite, and discrete illite.Kaolinite presence was evident as can be seen from the X-ray diffractograms of

Fig. 2. Transmission electron microscope photo of palygorskite.

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. .the tested soil Fig. 1 . The strongest peaks of the kaolinite d spacing in A are7.12 and 3.58. These peaks disappeared and became amorphous to X-rays afterheating to 5508C for 1 h. As for the mixed layer IrS, since the diffractionpatterns of the ethylene glycol solvated and the air-dried are different then weshould be having a certain mineralrsmectite. Since the EG has a peak at

16.8 A and the air-dried has a peak at 14.1 A, then the mixed layer must be an .IrS Moore and Reynolds, 1997 . The peak at 5.01 also an indication of the

.presence of IrS. The peaks d spacing in A at 10.56, 4.46, 4.26 and 3.18 are .indicative of the presence of palygorskite Moore and Reynolds, 1997 . Illite is

unaffected by ethylene glycol solvation and heating to 5508C. The strongest .peaks d spacing in A are 10.1, 5.01, and 3.34. The peaks at 16.8 A and 5.65 A

are indicative of the presence of montmorillonite.Fig. 2 is a Transmission Electron Microscope of a clay specimen showing a

concentration of palygorskite laths with other clay minerals particles this photo.does not reflect actual percentages of the present clay minerals . The approxi-

mate percentages of the clay minerals present in the soil, palygorskite, illite,illitersmectite, kaolinite, and montmorillonite, are about 16%, 39%, 9%, and10%, respectively. The specific surface area of the tested clay was determined

from different locations in Azraq basin, using the 2-ethoxyethanol Ethylene ..Glycol Monoethel Ether EGME surface area method. Cation Exchange Ca-

. .pacity CEC was determined according to Polemio and Rhoades 1977 for anumber of clay specimens obtained from different locations in the Azraq basin.The CEC vs. specific surface relationship is shown in Fig. 3.

Fig. 3. Cation exchange capacity vs. surface area of Azraq green clay.

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Table 1Influences of exchange complex and pH on Atterberg limitsType pH value LL PL PI SLUntreated clay 6.2 107 52 55 22Na-clay 2 58 37 21 21

7 57 33 24 2212 58 40 18 22

Ca-clay 2 82 45 37 217 86 51 35 21

12 87 55 32 21

K-clay 2 56 45 11 237 63 48 15 23

12 55 45 10 24

2.2. Alteration of the exchange complex

In order to make a specific species of cations dominant in the exchange .complex, the clay was treated with concentrated solutions 1yN concentration

of sodium, calcium, and potassium chlorides. In each case, the solution was . .buffered for three pH values; acidic pHs2 , neutral pHs7 , and basic

.pHs12 . The clay was washed three times for each case. After three washes,the clay became nearly homo-ionic more details about the cation saturation

.procedure and results are referred to Abdullah et al., 1997 .

2.3. Specimen preparation and testing procedures

.Liquid limit, plastic limit, and shrinkage limit Atterberg Limits for theuntreated and treated samples were measured following the standard proceduresD-4318, D-4318, and D-427 of the American Society for Testing and Materials .ASTM , respectively. Table 1 shows this highly plastic clay has a Liquid Limit . . .LL of 107, a Plastic Limit PL of 52, and Plasticity Index PI of 55. Theeffects of exchangeable cation type and pH-value on Atterberg Limits wereinvestigated.

Table 2States of compaction considered in the investigation

3 . .Compaction state Molding moisture content % Dry density grcmState No. 1 25.3 1.25State No. 2 31.5 1.25State No. 3 40.5 1.25State No. 4 31.5 1.30

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.Fig. 4. Moisture content vs. dry density relations for the tested clays standard proctor test .

The percent free swell and swelling pressure experiments were performed oncompacted specimens using Oedometer Apparatus with a diameter of 76 mm

.and a height of 20 mm following ASTM Test Methods for One-Dimensional .SwellrSettlement Potential of Cohesive Soils Methods D 4546 . The specimens

were prepared for four dry densities vs. molding moisture content compactionstates as defined in Table 2 and shown in Fig. 4. To prepare a specimen for apercent swell test, dry clay was mixed with deionized water to give the requiredwater content, placed in the consolidation cell, and compacted to give therequired density. A surcharge of 0.07 kgrcm2 was applied, then the specimenwas saturated with deionized water and the values of swelling with time wererecorded. The measurements continue until the swelling increment reach negligi-ble values. Also, all swelling pressure tests were performed using the samepreparation procedure and apparatus. Swelling pressure is defined as the valueof pressure required to keep the sample at zero swelling after saturating it withdeionized water.

3. Results and discussion

3.1. Moisturedensity relationships

Different species of cations present in the exchange complex influences theinteraction between clay particles. The kind of interaction is generated through

( )W.S. Abdullah et al.rApplied Clay Science 15 1999 447462454

.the repulsive forces due to the diffuse double layer DDL and the attractive vander Waals forces, leading to various types of particles associations. Accord-

.ingly, the moisture-dry density vr relationship assumes different locationsdin the vr domain. This behavior is depicted through the results of thed

.standard Proctor tests ASTM, D 698-78 on the untreated and treated samples .Fig. 4 . The K-clay and the Ca-clay produced rather close vr relationships,dhaving higher maximum dry densities and smaller Optimum Moisture Contentsas compared with that of the Na-clay and the untreated clay. This type of

.behavior is attributed for the Ca-clay , to the dominating effect of the van derWaals forces since the electric DDL is depressed drastically due to the presence

of a divalent Ca cations in the exchange complex Lambe, 1953; Olson, 1963;.Van Olphen, 1977 . The net attractive force leads to a face-to-face aggregation

of clay particles. Consequently, particles become bigger due to their stacking.K-clay behaves in a similar fashion as Ca-clay, due to face-to-face aggregationof particles. However, the mechanism responsible for the aggregation differsfrom that of the Ca-clay. As consequence of the isomorphous substitution a highelectric polarization exists near the clay particle surface especially within theStern layer. Such a high electric polarization coupled with a low hydration

.energy of the potassium cations 80 kcalrg ion, Lebedev, 1958 causes thehydrated K cations to shed all its water of hydration. As a result, K ions with an

. .unhydrated radius of 1.33 A enters the hexagonal holes of the siloxane silica .sheets having a radius of 1.32 A and provides a strong linkage between clay

.particles Grim, 1968; Greenland and Mott, 1978 . The K-linkage provided bythe Potassium ions causes the clay particles to aggregate and form domains .Mesri and Olson, 1970 . The K-linkage is stronger than the secondary valancevan der Waals attractive forces that are responsible for forming domains for thecase of Ca-clay. Thus, the domains formed by the K-linkage are more stable

.than that formed by the van der Waals forces Ca-clay . The size of eachdomain is much larger than the discrete clay particles. The K-clay can be easilydistinguished from the untreated clay by its coarse texture. Fig. 5 is a TEMpicture of a soil specimen treated with Kq which shows large domains formedby K-linkage. The larger the particles the less the colloidal activity, and the lessthe plasticity index of the K-clay. The plasticity index was reduced drasticallyfrom 55 at pHs6.2 for the untreated samples to 15 at pHs7 for the K-clay.

A percolation test with distilled de-ionized water was performed on theK-treated and the Na-treated samples. The percolation test was followed by a

hydrometer test with 15 min mechanical agitation and with adding dispersing.agent to determine whether changes to the soil particles association has taken

place and became permanent. The result of the hydrometer test is shown in Fig.6 which shows that the K-treated sample has become coarser indicating that theK-linkage in the K-treated samples is permanent. On the other hand, theNa-treated clay produces thick DDL; however, high concentration of Na reducesthe thickness of the DDL. Percolation with distilled and de-ionized water

( )W.S. Abdullah et al.rApplied Clay Science 15 1999 447462 455

Fig. 5. TEM of the K-clay soil showing lathes of polygorskite and large domains formed by theK-linkage.

reduces Na concentration; and with subsequent agitation and adding dispersingagent in the hydrometer test bring the Naq-treated soil to a state similar to thenatural soil as far as grain size is concerned.

For the Na-clay, the thick DDL leads to an important osmotic repulsive forcethat overwhelms the van der Waals attractive forces and causes a net repulsiveforce. This situation is conducive to a large volume occupied by water instead ofsolids resulting in low dry density and higher moisture content compared withthe other treated samples.

3.2. Atterberg limits

When the clay was treated with potassium, the specific surface area wasreduced from about 465 m2rg for the untreated samples to about 135 m2rg forK-clay. The CEC was also reduced from 33 meqr100 g to 10.3 meqr100 g .Fig. 3 . The reason for these changes is in the nature of the potassium ion and

.the surface nature of the clay minerals silica sheet . As mentioned earlier, theK-linkage causes face-to-face association of clay particles and the major part of

. .the K ions bonding the layer silicates stays in an unreplaceable fixed state,

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Fig. 6. Grain size distribution of the natural, Na-treated, and the K-treated soils.

hence causing the actual CEC to decrease. As a direct consequence, clayparticles are bigger and fewer, causing the specific surface area to decrease as

.well Fig. 3 .A change in the electric potential of the clay particles, brought about by

changes in the exchange complex, influences the state of attraction or repulsion .of the clay particles, and thus influences its properties. Atterberg limits Table 1

were greatly affected by changes in the exchange complex. The potassiumtreated samples were the most affected by these changes, since the K-linkagetransformed it into a coarser type, with less activity due to the reduction in themeasured CEC.

3.3. Clay structure and swelling behaior

.For a certain clay, the structure is influenced by: i the exchange complex, . . .ii molding moisture content, iii the pH value, and iv method of compaction.The exchange complex is mainly influenced by valence, concentration, size, andhydration characteristics of the cations present in the exchange complex. TheDDL is influenced by the exchange complex. There are a number of theoriesused to predict the DDL thickness as well as the distribution of ions adjacent tocharged surfaces in colloids. The GauyChapman theory is the most widelyused one. Predictions provided by most DDL theories have major limitation dueto the assumption that ions are point charges with no interactions between them,and the charge deficiency is uniformly distributed on a planner surface extend-ing large distance in the plane. Though quantitatively the DDL theories may not

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yield accurate predictions depending on the real condition compared to the.assumptions . However, they can be used qualitatively to interpret clay behavior

.Lambe, 1953; Seed and Chan, 1959 . The type of clay structure formed due tothe effect of the DDL and the molding moisture content was discussed earlier.The pH of the saturation cation solution does not influence particles surfacescharge. Such charge is mainly caused by isomorphous substitution. The pH

value mainly affects the hydroxyl group clay minerals at particle edges Green-.land and Mott, 1978 . The charges on particles edges arises from the associa-

q.tion of the hydroxyls with hydrogen ion H below Point of Zero Charge . qPZC giving rise to a positive charge, or loss of H above the PZC giving riseto a negative charge. Therefore a low pH value promotes a positive edge to

.negative surface interaction often if other mentioned factors are favorableleading to a flocculated structure. Oriented structure often occurs under condi-

.tions of high pH values i.e., pH)PZC .Clay structure is not an easy term to quantify nor a state that can be uniquely

described. For instance, a flocculated structure can occur at low molding . moisture content dry of optimum high in water deficiency Seed and Chan,

.1959 . Dry of optimum molding moisture content, low pH condition, high in .cation concentration having low percentage of mono-valent cations are condi-

tions conducive to a flocculated structure. Wet of optimum molding moisturecontent, high pH, low concentration of cations, and high percentage of mono-va-lent cations are conditions for a dispersed or oriented structure.

Swell potential and clay structure are intimately related. The more the .flocculated structure for a specific clay the more the swell potential. Thus, the

factors that give rise to a flocculated structure are conducive to a high swellingpotential and vice versa. Increase in pH value contributes to a dispersed

structure an oriented structure assembles due to the negative charge on clayparticles surfaces and the rise of negative edges charges, thus contributing to

.lower swelling potential swell pressure as well as percent free swell as shownin Figs. 7 and 8. Potassium dominated exchange complex produced the lowestvalues of swell as compared to untreated and other treated samples. TheK-linkage provided by the potassium cations contributed to strong face-to-faceassociations of clay particles preventing water molecules to penetrate through,thus causing drastic decrease in swell potential. Another aspect of the potassiumdominated exchange complex is its insensitivity to pH changes. The K-linkagecontributed, in fact, to decrease in specific area of the clay causing low surficial

.activity giving rise to larger size particles , and hence edge charges becameinsignificant.

3.4. Modeling swell potentialtime behaior

.Swell potential swell pressure or percent free swell is time-dependentprocess. Swell pressure or free swell constitute a single point on the swell

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Fig. 7. Influence of pH and compaction state on swell pressure.

potentialtime relationship. Knowing the full picture of the swelling behaviorrepresents an indispensable need for accurate analyses of swelling problems.Although, such relationships might be very difficult to incorporate in anyanalytical solution, they are easy to implement in finite element analysisprovided that such relationship can be modeled by a certain mathematicalformulation.

Moreover, measured swell potential in the laboratory requires saturation ofthe tested clay specimen. In the field, clay saturation is not necessary to takeplace. The amount of swell potential reached in the field is dependent on the

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Fig. 8. Influence of pH and compaction state on percent free swell.

amount of available moisture and time. Thus, swell potential prediction, basedon laboratory tests, represents an upper bound. The closeness of the predicted tothe actual is dependent on the amount of water absorbed by the clay which is, inturn, a time dependent process. The presence of a swell-time relationship modelprovides a powerful tool for accurate prediction of soils swell potential.

Fig. 9 shows typical results of percent free swell and swell pressure vs. squareroot of time relationships. These experimental results, very clearly, indicate that

.the behavior of the swelling potential vs. time or square of time can be

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Fig. 9. Measured and predicted swell potentials relations.

modeled accurately by a rectangular hyperbola. The mathematical form of therectangular hyperbola is given by:

tSs 2 .aqb t

where S is the percent free swell or swelling pressure, t is time; and a and b areconstants of the hyperbola to be determined from experimental results. Theconstants a and b are the slope and intercept of the straight line fit of 1rS vs.

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1r6t as given below and shown in Fig. 9. Let hs1rS and js1r6t: .substituting h and j in Eq. 2 and rearranging yields:

hsajqb 3 .

4. Summary and conclusions

An experimental study was conducted on influence of the exchange complexon Atterberg limits, compaction, and swell potential of an expansive clay. Thestudy involves transforming the heterogeneous exchange complex to a homoge-neous one. Swell potential tests were performed on compacted specimens.Compaction states were chosen so that clay structure influence on swellpotential can be included. CEC and specific surface area were determined so asto assist in the interpretation of the test results. The following conclusions arebased on data, analyses, and discussion presented in this paper.

.1 The K-dominated exchange complex caused fundamental changes to theinvestigated clay properties and behavior. These changes are caused by theK-linkage which bonds the silica sheets of the clay mineral particles in aface-to-face association. Consequently, the average size of the particles becamebigger, transforming it to a rather coarse grained clay.

. .2 The K-linkage caused drastic decrease in: a the plasticity index from 55 . . .pHs6.2 for the untreated clay to 15 pHs7 for the K-clay; b the surface

2 2 .area from 442 m rg for the untreated clay to 135 m rg for the K-clay; c theCEC from 31 meqr100 g for the untreated clay to 11.3 meqr100 g for the

. 2K-clay; d the swell pressure from 2.47 kgrcm for the untreated clay to 0.492 . .kgrcm for the K-clay under same conditions ; and e the percent free swell

from 16.9% for the untreated clay to 8.35% for the K-clay under same.conditions .

.3 Altering pH value influences orientation of clay particles. The higher thepH value the more the oriented clay particles.

.4 Swell potential is highly influenced by the exchange complex and to alesser degree by the pH value especially for the K-clay.

. .5 The swell potential swell pressure and percent free swell vs. timerelationships can be accurately modeled by a rectangular hyperbola.

5. Nomenclature

CEC Cation exchange capacityDDL Diffuse double layerKq, Naq, Ca2q Potassium, sodium, and calcium cations, respectivelyLL Liquid limit

( )W.S. Abdullah et al.rApplied Clay Science 15 1999 447462462

.PZC Point of Zero Charge the pH at which the net charge is zeroPI Plasticity indexPL Plastic limitS Swelling pressure or percent free swellSL Shrinkage limitt Timer Clay dry densitydv Moisture content

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