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Modelling physical properties of mixtures of clays:

example of a two-component mixture of kaolinite

and montmorillonite

Barbara Grabowska-Olszewska*

Institute of Hydrogeology and Engineering Geology, Faculty of Geology, Warsaw University,

Al. Z wirki i Wigury 93, 02-089 Warsaw, Poland

Received 24 May 2002; accepted 17 February 2003

Abstract

The paper presents the results of physical property tests expressing hydrophilic properties of a mixture of two model clays

(kaolinite and montmorillonite). It is shown that these properties are influenced by exchangeable cations (Ca, Na) and specific

surface (S). The Casagrande method for determination of the liquid limit (wL) in case of thixotropic montmorillonitic clays

produces higher values than those obtained using the cone method.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Model clays; Bentonite; Ca and Na montmorillonite; Kaolinite; Hydrophilic properties; Casagrande and cone penetrometer tests

1. Introduction

In geotechnical practice, soils are commonly met,

which do not always satisfy the requirements for a

given type of structure. The same is true for com-

pressed clays used for construction of multibarriers

around steel containers with radioactive wastes, as

well as clay liners used in landfill systems. Modellingof their properties is then attempted. The mineral

composition of clays that are responsible for their

mechanical and physical properties, including sorp-

tion, may be altered to influence the modelling of

those properties.

This paper presents the results of investigations of

physical properties of two model clays: a commonly

known kaolinite from Sedlec (Czech Republic) and a

bentonite from Chmielnik (Poland), which has been

thoroughly tested by mineralogists, and is described

as ideal calcium montmorillonite (Kulesza-Wie-

wiora, 1984) and its mono-ionic, modified form,

Na-montmorillonite. The paper also presents theresults of investigations of physical properties of

mixtures of the kaolinite and both the Ca- and Na-

montmorillonite.

It is noteworthy that natural Na-montmorillonite is

also know to occur in Poland. It occurs between coal-

carbon layers in Upper Silesia Radzionkow coal

mine (Baker et al., 1995; Kaczynski and Grabowska-

Olszewska, 1997). In terms of quality, it was found to

be comparable with Wyoming (USA) bentonite. How-

ever, it cannot be exploited any more because of coal

www.elsevier.com/locate/clay

* Tel.: +48-22-55-405-01; fax: +48-22-55-400-01.

E-mail address: [email protected]

(B. Grabowska-Olszewska).

0169-1317/03/$ - see front matterD 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0169-1317(03)00078-4

Applied Clay Science 22 (2003) 251259


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mine closure. The paper by Lang (1989) inspired this

presentation of the test results.

2. Experimental programme

Exchangeable cations have been replaced using

neutral normal ammonium acetate. Calcium and mag-

nesium estimated by titration with ethylenediaminete-

traacetic acid. A separate titration for calciumpermitted us to calculate of magnesium by difference

(Welcher, 1963). Sodium and potassium content have

been determined by flame photometry. Determination

of specific surface is based on calorimetric method

described by Grabowska-Olszewska (1968). This

method has been cited by van Olphen (1969, 1975).

2.1. Sorption complex analysis

To illustrate the variation in physical properties,

especially hydrophilic properties of kaolinitebenton-ite mixtures, their composition of exchangeable cations

has been examined. The results are given in Table 1 and

in Fig. 1. As could be expected, there is a linear relation

between the cation exchange capacity (CEC) values

and the content of a given mineral in the two-compo-

nent blend.

In the pure bentonite, the content of the exchange-

able cation Ca2 + was 98.00 cmol/kg and that of

exchangeable Mg2 + was 13.83 cmol/kg, while those

of exchangeable Na+ and K+ were insignificant.

Since the CEC for the examined bentonite was

112.66 cmol/kg, the previous characterization of the

Chmielnik bentonite as Ca-montmorillonite was con-

firmed.

2.2. Physical properties of two-component mix of

kaolinite Ca-montmorillonite: initial water content

(w0), consistency limits (wp, wL), plasticity index (Ip),

activity (A)

The values of the parameters listed above are given

in Table 2. They indicate that the values of all water

content indices (w0, wp, wL) rise from the 100%

kaolinite sample towards the 100% bentonite Ca-

montmorillonite sample.

Activity (A) values substantiate the basis for clas-

sifications of the investigated mixtures according to

different values of wL, and hence Ip. This observation

confirms the activation of thixotropic phenomena

under the influence of dynamic shocks transferred to

montmorillonitic clays in wL tests using the Casa-grande apparatus (Grabowska-Olszewska et al.,

1984).

A values for the same clays and their mixtures are

higher when calculated by the assumes relationship of

Ip to wL according to the Casagrande method, than by

this relationship according to the cone method.1

Table 1

Composition of exchangeable cations and cation exchange capacity (CEC) of model clays and of their mixtures

Clays (%) Ca2 + Mg2 + Na+ K+ CEC

Kaolinite Bentonite(Ca-montmorillonite)

(cmol/kg) (cmol/kg) (cmol/kg) (cmol/kg) (cmol/kg)

100 0 8.60 0.91 0.18 0.16 9.85

90 10 21.55 2.66 0.19 0.19 24.59

80 20 27.95 3.33 0.21 0.21 31.70

70 30 36.50 4.16 0.24 0.23 41.13

60 40 45.90 5.75 0.26 0.26 52.17

50 50 57.35 6.75 0.28 0.27 64.65

40 60 61.15 8.00 0.33 0.31 69.79

30 70 73.55 9.08 0.35 0.32 83.30

20 80 83.70 10.00 0.38 0.32 94.40

10 90 87.65 11.83 0.41 0.33 100.22

0 100 98.00 13.83 0.44 0.39 112.66

1 According to BS 1377 Part 2, 1990: 4.3.

B. Grabowska-Olszewska / Applied Clay Science 22 (2003) 251259252


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The classification of 100% kaolinite as inactive2

(A = 0.46 and 0.67), while 100% bentonite (Ca-mont-

morillonite) as very active (A =2.08 and 2.92) is

indisputable. All mixtures of kaolinite and bentonite

(Ca-montmorillonite) are located within the activity

range of normal to active.

2.3. Physical properties of two-component blend of

kaolinite and Na-montmorillonite: consistency limits

(wp, wL) plasticity index (Ip), activity (A)

In the case discussed, natural bentonite (Ca-mont-

morillonite) has been prepared so that its exchangeable

cation has been replaced with mono-ionic sodium. The

results of same tests performed for the kaoliniteCa-

montmorillonite are given in Table 3.

Fig. 1. Relationship between cation exchange capacity (CEC) and percentage of model clays and of their mixtures.

2 According to Head (1992): inactive A < 0.75; normal

A= 0.75 1.25; active A = 1.25 2.00; very active A>2.00.

B. Grabowska-Olszewska / Applied Clay Science 22 (2003) 251259 253


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The strong hydrophilicity of Na-montmorillonite

was confirmed. The values ofwp, wL andIp rose several

times more than in the case of kaoliniteCa-montmor-

illonite mixtures, but by few hundreds percent: the

liquid limit value wL according to the Casagrande

method increased by 240% (by 60% for Ca-montmor-

illonite), above that for kaolinite and it increased by

200% (by 65% for Ca-montmorillonite) according tothe cone method.

These relationships are illustrated in Fig. 2. The

results confirm Na-montmorillonite as the most highly

thixotropic mineral (Grabowska-Olszewska, 1968).

The blend was shown to be very active when it

contained 40% Na-montmorillonite with 60% kaolinite

(Ac2.00 and 2.31) and its activity increased to

A = 9.27 and 12.67 for 100% Na-montmorillonite.

2.4. Specific surface (S)

In order to test the hypothesis that there is arelation between the liquid limit values (wL),

determined according to various methods, and

the specific surface, a series of tests have been

performed to determine the specific surface (S) by

Table 2

Initial water content (w0), consistency limits (wp, wL), plasticity index (Ip), activity (A) of model clays and of their mixtures having natural

calcium sorption complex

Clays (%) w0(%)

wp(%)

wL (%) Ip (%) AaIp (%) calculated

using wL (%)

Kaolinite Bentonite

(Ca-montmorillonite)

According to

Casagrande

method

According

to cone

method

wL according

to Casagrande

method

wL according

to cone

method

According to

Casagrande

method

According

to cone

method

100 0 1.3 34.6 63.2 54.3 28.6 19.7 0.67 0.46

90 10 2.9 37.5 63.5 56.0 26.0 18.5 0.68 0.49

80 20 4.6 39.0 66.6 57.0 27.6 18.0 0.77 0.50

70 30 6.1 43.0 69.5 60.8 26.5 17.8 0.80 0.54

60 40 8.1 44.5 72.2 62.1 27.7 17.6 0.92 0.59

50 50 8.9 47.8 74.0 67.9 26.2 20.1 0.97 0.74

40 60 11.7 49.9 81.0 71.4 31.1 21.5 1.30 0.90

30 70 13.8 54.0 86.8 76.4 32.8 22.4 1.56 1.07

20 80 15.9 57.4 89.4 80.1 32.0 22.7 1.78 1.26

10 90 18.7 63.1 90.0 83.2 26.9 20.1 1.79 1.34

0 100 20.9 64.9 99.9 89.8 35.0 24.9 2.92 2.08

a A calculated according to the formula: A =Ip%/DV2 Am %.

Table 3

Consistency limits (wp, wL), plasticity index (Ip), activity (A) of model clays and of their mixtures secondarily saturated with sodium

Clays (%) wp(%)

wL (%) Ip (%) A Ip (%) calculated

using wL (%)

Kaolinite Bentonite

(Na-montmorillonite)

According to

Casagrande

method

According

to cone

method

wL according to

Casagrande

method

wL according

to cone

method

According to

Casagrande

method

According to

cone method

100 0 31.5 65.6 61.1 34.1 26.9 0.79 0.69

90 10 36.9 73.0 67.4 41.1 33.5 1.08 0.88

80 20 37.3 81.5 75.6 44.2 38.3 1.23 1.06

70 30 41.1 96.5 88.4 55.4 47.3 1.68 1.43

60 40 43.7 112.9 103.1 69.2 59.4 2.31 1.98

50 50 51.3 127.7 113.3 76.4 62.0 2.83 2.30

40 60 54.8 147.3 128.3 92.5 73.5 3.85 3.06

30 70 56.5 172.0 144.6 115.5 88.1 5.50 4.20

20 80 64.0 180.0 159.3 116.0 95.3 6.44 5.29

10 90 66.4 205.4 167.0 139.0 100.6 9.27 6.70

0 100 72.5 224.5 183.7 152.0 111.2 12.67 9.27



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the calorimetric method for 100% kaolinite, mixes

of 10% kaolinite to 90% bentonite (Ca-montmor-

illonite), and for 100% bentonite (Ca-montmoril-

lonite).

Relations shown in Fig. 3 confirm the hypothesis

that the larger the surface area of water-binding

minerals, the higher are values of parameters express-

ing their affinity to water.

2.5. Free swell (FS)

Table 4 and Fig. 4 comprise the results of free swell3

testing of powdered and compressed samples of kao-

linite Ca-montmorillonite that were saturated with

water of pH = 7 (the average value of ground waters

Fig. 2. Relationship between consistency limits (wp, wL) and percentage of model clays and their mixtures having natural calcium sorption

complex and secondarily saturated with sodium.

3 According to ASTM D-45466-90.



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in Poland). Samples so prepared showed that sample

height increase with bentonite (Ca-montmorillonite)

content.

These relations can be described with simple

empirical formulas:

P% 0:27b% 11:7

P% 0:27k% 38:8

R 0:96

It should be emphasised that the pH value of thesaturating solution is extremely important in swell

testing, regardless of the type of clay and its structure

(undisturbed, disturbed and other) (Grabowska-Ols-

zewska, 1994).

3. Discussion

The purpose was to show that there is a relation-

ship between physical properties expressing hy-

Fig. 3. Relationship between liquid limit (wL

) and specific surface (S

).

Table 4

Initial water content (w0), free swell (FS), final water content (wf) of

model clays and of their mixtures having natural calcium sorption

complex

Clays (%) w0 pH of FS wf

Kaolinite Bentonite

(Ca-montmorillonite)

(%) water (%) (%)

100 0 1.3 7 11.5 64.0

90 10 2.9 7 15.0 70.5

80 20 4.6 7 17.5 73.5

70 30 6.1 7 20.0 73.5

60 40 8.1 7 24.0 79.5

50 50 8.9 7 25.0 86.0

40 60 11.7 7 30.0 85.5

30 70 13.8 7 97.5

20 80 15.9 7 103.0

10 90 18.7 7 37.5 107.0

0 100 20.9 7 42.5 118.5



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drophilic properties and mineral composition of two-component mixture (kaoliniteCa- and Na-montmor-

illonite).

The double-layer theory provides a basis for an

explanation of these relationships, which are widely

discussed in classical monographs (Gouy, 1910,

1917; Chapman, 1913; Langmuir, 1938; Verwey

and Overbeek, 1948). Undoubtedly, these relation-

ships become increasingly complicated in the case of

multicomponent blends of clayey minerals.

The use of the well-known Casagrande chart is

controversial, with authors regarding it theoreticallyunquestionable for classification purposes.

As shown in Fig. 5, the values of plasticity index

Ip at given wL values, regardless of the exchange-

able cation, place tested blends mainly below the A

line, in the area of organic soils. It should also be

emphasised that Ip values calculated for wL values

according to the cone method are always below the

wL values according to the Casagrande method.

Sherwood and Ryley (1970) considered this prob-

lem too. Unfortunately, they did not fully confirm that

the Casagrande method gives higher values of theliquid limit than the fall cone method.

It seems then that the Casagrande chart, which is so

widely used, makes physical sense only in relation to

natural clays and organic soils, for which the liquid

limit may be determined only according to the Casa-

grande method.

Location of points representing examined mix-

tures of clays below the A line confirms only their

highly hydrophilic properties, which are similar to

those of organic soils. Hence, according to Head

(1992), they can be considered as soils of high(H), very high (VH) and extremely high (E)

plasticity.

The author wishes to emphasise that the inspira-

tion for taking up these studies (Grabowska-Ols-

zewska, 1968) came from the articles by Lambe

and Martin (19531957), Seed et al. (1964a,b) and

Grim (1968). Successive more recent publications

(Mitchell 1976; Sridharan et al. 1986; Gillott, 1987;

Pusch 1994 and many others) confirm the existing

relationship between the composition of clay min-

Fig. 4. Relationship between free swell (FS) and percentage of model clays and of their mixtures.



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erals with a different sorption complex and their

ability to bind water. Usually, however, these

authors do not study the relationships between the

percentage of the individual clay minerals, the

valence of exchangeable cations in such a multi-

component mixture as soil and values of such

parameters as consistency limits, activity, specific

surface, swelling, plasticity and others.

4. Conclusions

The experiments show that there is a relation

between the content of components of clayey mate-

rials in two-component mixtures of kaolinite and Ca-

or Na-montmorillonite, on the one hand, and the

values of the physical parameters that were examined

like initial moisture contents (w0), consistency limits

(wp and wL), plasticity index (Ip), activity (A) and

free swell (FS).

It has also been shown that there is a relationship

between the liquid limit (wL) and specific surface (S).

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