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V. Swelling of Calcium Clays

V. Swelling of Calcium Clays

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Table VI

Permeability and X-Ray Spacing for Na- and

Ca-Montmorillonite (Wyoming) in NaCl and CaCI, Solutions


0. I M CaCI,

< 10-5 M CaCI,

I .O M NaCl

0.5 M NaCl

0.3 M NaCl


X-ray spacing

(105 cm sec-1)


I .7


I .8








0.3 M NaCl (Quirk, 1952; see also Quirk, 1968). Mering (1946) considered that

the large macroscopic swelling of Na-montmorillonite was not due to the expansion of the crystals. However, Quirk (1952) concluded that the dramatic decrease

in permeability was due to the onset of extensive crystalline swelling as a result

of the development of diffuse double layers on all the opposing surfaces. This

interpretation was confirmed by X-ray measurements (Table VI) which showed

that the basal spacing jumped from 18.9 in 0.5 M NaCl to 43 A in 0.3 M NaCl.

Furthermore, Norrish and Quirk (1954) also reported that Ca-montmorillonite

maintained a basal spacing of 19 8, even in distilled water; the lamellae of the

montmorillonite remain in a primary potential minimum and are separated by 9 A

of water or about three molecular “layers.”

The swelling (solution content) of compressed cores of Na- and Camontmorillonite (Wyoming) and Ca-illite (Willalooka) in relation to electrolyte

concentration at 1 kPa suction (Aylmore and Quirk, 1962) is shown in Table VII.

Since the material is saturated, the volume of the solution taken up is a measure

of the swelling since the initial porosities were similar. It may be noted that the

solution content of Ca-montmorillonite changes to some extent over a wide

concentration range, whereas those of the Na-montmorillonite increase markedly

as a result of the development of diffuse double layers on opposing lamellae in

dilute solution. The characteristics of the swelling of Na-illite are essentially

similar to Na-montmorillonite in that the solution content attained at a given

suction is greater the more dilute the NaCl solution used to wet the clay (Fig. 5).

Although the swelling of Na-illite and Na-montmorillonite is qualitatively similar

the solution contents attained by the illite are less than those for montmorillonite

because the surface area of the illite is much less, being about 160 m2gg1

compared with 750 m2g-1 for the montmorillonite (Table VII, Fig. 5).

The Ca-illite solution contents (Table VII) are almost constant, differing little

between a concentration of 1 M CaCl, (K-I = 1.76

and distilled water.

Clearly diffuse double-layer theory cannot be used to describe the swelling





Table VII

Solution Content (g . g-1 Clay) at 1 kPa Suction

for Compressed Cores of Na- and Ca-Montmorillonite

(Wyoming) and Na- and Ca-Illite (Willalooka) in Relation

to the Concentration of NaCl and CaCI, Solutions





1 .o

0. I

















I .60








between crystals of Ca-illite and the intracrystalline swelling of Camontmorillonite. The similarity in behavior of the two systems led Aylmore and

Quirk (1960, 1962) to conclude that contiguous Ca-illite crystals at regions of

overlap reside in a potential minimum similar to that which gives rise to the fixed

19 8, basal spacing for Ca-montmorillonite.

These observations, together with electron microscopy and pore size distribution measurements, are the basis on which Aylmore and Quirk (1959, 1960,

1962, 1967), Quirk (1968), and Quirk and Aylmore (1971) have described the

assemblages of Ca-illite crystals and the assemblage of montmorillonite quasicrystals as clay domains. The principal basis for the stability of these compound

particles in water is the operation of ion-ion correlation forces at areas of overlap

between crystals of illite and quasicrystals of Ca-montmorillonite; as a result the

swelling of Ca clays is almost insensitive to electrolyte concentration. It is

noteworthy that Terzaghi (1956) referred to “clusters of clay particles” as consisting of a “great number of crystals.” He also drew attention to the need to

establish “the seat and nature of the molecular forces which prevent the clusters

or piles floating in a suspension from being dispersed by the osmotic pressure

acting on their elements.” Bolt and Koenigs (1972) have also discussed the

significance of potential barriers at regions of particle overlap.

Olson and Mitronovas (1962) studied the compression of Ca-Fithian illite and


Electrolyte concentration had almost no effect on the position of the rebound

swelling curves. The most important effect of electrolyte concentration

seemed to be its effect on the geometric arrangement of particles that were

sedimented from dilute suspensions. The geometric arrangement of particles

seems to be a more significant variable than the osmotic repulsion between the




Ben Rhaiem ef al. (1987) examined the structure of Ca-montmorillonite (Wyoming) in equilibrium with a range of suctions by means of high intensity, lowangle X-ray diffraction (see Table VIII). It may be seen that, over the pressure

range 100 MPa to 3 kPa on the rewetting cycle, the average number of lamellae

(Ei) in a quasicrystal changes from 1 1 to about 8, indicating that these entities are

relatively stable. The average number of lamellae in an assemblage of quasicrystals (M)vanes from 400 to 90, indicating that the clay domains break up to form

smaller domains on the wetting cycle; this is part of the swelling process. If it is

assumed, for the sake of illustration, that it is permissible to divide by E then

the number of quasicrystals per clay domain would vary from 36 to 11. The

average thickness of the clay domain, obtained by multiplying the 4001) value


varies from 0.76 to 0.17 pm.

Tessier ( 1990) concluded, from electron microscope observations, that the

pores between the quasicrystals within a clay domain of montmorillonite are 3040 A across and that during wetting the clay domains split at some of these sites

to give several domains as is indicated by the decrease in (Table VIII). Murray

et al. ( 1985) examined the microstructure of the natural aggregates of 12 Queensland vertisols containing smectite by applying the general B.E.T. equation to the

low temperature nitrogen adsorption isotherms. They concluded that their results

were consistent with the presence of pores with widths of 14 to 28 A in the dried

materials. The surface areas of these materials varied from 100 to 200 m2g-I

when expressed on a clay basis (<2 pm). Using 750 m 2 g 1as a reference area

for montmorillonite, then a value of ii of 11 (Table VIII) would indicate an area

of 68 m2g-1. Aylmore and Quirk (1962) reported a nitrogen surface area of 38

m2g- 1 for Ca-montmorillorite (Wyoming). The surface area obtained using E,

an average, would include areas of overlap, between quasicrystals, which would

not be accessible to nitrogen.




Table VIII

Characteristics of Quasicrystals and Domains of Ca-Montmorillonite(Wyoming)

Influenced by Applied Suctionu





















Low-angle X-ray diffraction results are from Ben Rhaiem ef al. (1987).

The average number of elementary layers in a clay domain of montmorillonite

The average number of elementary layers in a quasicrystal.

The average number of quasicrystals in a flay domain ( Z / E ) .

The average particle thickness [d(001) x MI.






As well as order in the c axis direction there is also a long-range order in the ab plane (Laffer et d.,

1969; Fitzsimmons et d., 1970; Greene e t a / ., 1973); these

authors prepared single lamellae of Ca-montmorillonite in a virtually electrolytefree solution. Optical birefringence developed after shaking a 0.3% suspension

for a period of I month, and electron microscope observation of the particles,

adsorbed onto a positively charged grid, indicated that single lamellae of about

0.3 pm across had condensed to give quasicrystals of more than 5 pm in the a-b

direction with a thickness of about 50 8,. The kinetics of the condensation was

hastened if the single lamellae were flocculated with 0.1 M CaCl,; birefringence

developed after a few days shaking and the quasicrystals were much thicker.

The stability of quasicrystals with respect to the degree of sodium saturation

has been investigated by Shainberg and colleagues (Shainberg and Otoh, 1968;

Shainberg and Kaiserman, 1969; Bar-On et al., 1970). They found that as the

percentage of exchangeable sodium increased up to 20% the electrokinetic velocity also increased but that the light scattering showed that the particle size was

unaffected. They therefore concluded that the Na+ ions were adsorbed on the

external surface of the quasicrystals. Quasicrystals progressively disintegrated as

increasing amounts of Na+ ions, between 20 and 60% exchangeable sodium,

occupied the interlamellar exchange sites.

Within a swollen mass of Ca-montmorillonite there are pores between the

individual lamellae within a quasicrystal, pores between the quasicrystals assembled in a clay domain, and pores between the clay domains themselves. The

definition of the clay domains and quasicrystal entities is primarily energetic in

that these entities represent relatively stable groups of particles connected by

overlapping areas in an adhesive potential minimum and have a near parallel

alignment because of their plate-shaped character. Within a clay soil matrix these

assemblages must be in random array to be consistent with the observed isotropic

behavior of the swelling of clay cores and clay soil aggregates.

Because two lamellae in contiguous quasicrystals of montmorillonite can conform more closely than two crystals of illite, perhaps 50 8, thick, the stability of a

quasicrystal of montmorillonite and an assemblage of quasicrystals would be

greater than an illite domain with the additional possibility that individual

lamellae could be shared between contiguous quasicrystals. Where there is a high

degree of conformity between illite surfaces the greater surface density of charge

would give rise to a strong attraction because of ion-ion correlation forces. The

distribution of particle thicknesses in illite may be an important feature contributing to the stability of a domain as the thin particles would interleave with coarser


The less than 0.2-pm fraction of Willalooka illite has a nitrogen surface area

of 227 m2g-' (Alymore and Quirk, 1967), indicating that there would be some

very thin particles since the average thickness would be less than four elementary

silicate sheets thick. Because of their flexibility these thin particles could act as



an aggregating agent and could play a significant role in determining long-range

order in the a and b directions in a clay domain. The stability of an illite domain

within a soil would be considerably enhanced by the large suctions developed

during drying. Aylmore and Quirk (1962) have shown that clay domains of

Willalooka illite, composed of individual crystals of about 50 8, thick and 700 8,

in lateral extent, have a lateral extent of about 5 km.

Lebron et al. ( 1993), using photon correlation spectroscopy, investigated the

nature of clay domains of Silver Hill illite which has 1.5 units of charge per unit

cell formula (Sposito and LeVesque, 1985); this is a similar surface density of

charge to that of Willalooka illite. They found that the Ca-saturated illite had a

much larger particle size than the Na-saturated illite. They also reported that the

Ca clay domains broke down to give smaller particles at SAR values in the range

of 10 to 15 and also that the particle diameter decreased by a factor of 2 to 3 ,

indicating a large decrease in particle size since this would vary as the cube of the

measured diameter.




1. Crystalline Swelling of Calcium Smectites

SIade and Quirk (1991) reported d(OO1) spacings in distilled water for the

following smectites for which the total charge and tetrahedral contribution to the

charge are shown in parentheses: hectorite, 20.7 8, (0.70,0.04);Wyoming, 19.1

8, (0.74, 0.08); Otay, 18.5 8, (1.04, 0.06); Nibost, 17.8 8, (1.14, 0.74); and

Drayton 16.7 A (1.06, 1.02). Thus the extent of the crystalline swelling of Casmectites is influenced by the magnitude of the charge and also by its structural


Figure 6 shows the effect of CaCl, concentration on the transition from the

spacing in water to about 15.5 8,. Posner and Quirk (1964a,b) reported that this

transition is reversible with respect to concentration and took place between 1

and 2 molal for Li, Na, Mg, and Ca chloride solutions and that the spacing did

not decrease appreciably at higher concentrations. Since the electrolyte did not

penetrate the interlamellar space until the transition had taken place, the transition is an osmotic phenomenon. The osmotic pressure of the solutions is indicated in Fig. 6. Figure 6 shows that the 15.5 8,spacing for Wyoming montmorillonite is attained at a pressure of I3 MPa, the transition for Otay montmorillonite

occurs in two stages with the first at 3.4 MPa and the second at about 13 MPa,

the transition for Nibost beidellite is from 17.8 8, to about 15 8, and also occurs

in two stages but at lower pressures. Each 3 8, increase in spacing contributes

about 0.10 g H,Og-l clay if 650 m2 g-I is taken as the internal area.

The different behavior of the smectites shown in Fig. 6 for pressures less than








Wyoming 0.74


0 may



A Nibost




u 16.5



5 4 1







Molality of CaCI,





Osmotic Pressure (MPa)



Figure 6 The d(001) spacings for Ca smectites as intluenced by the total charge, the structural

origin of the charge, and CaCI, concentration. Since salt does not enter the interlamellar spaces until

the 15.5 8, spacing is attained, the transition is in response to the osmotic pressure of the solution.

(After Slade and Quirk, 1991 .)

3.4 MPa (0.5 molal), the range of principal interest for agriculture, is such that

Wyoming montomorillonite cannot be regarded as representing the smectites

generally. It should be noted that Drayton, a subsoil clay is classed as subplastic

(Nomsh and Tiller, 1976).

2. Clay Swelling

The swelling of Ca clay cores in an overconsolidated state has been investigated with respect to applied suction and electrolyte concentration by Aylmore

(1960) and Aylmore and Quirk (1962, 1966). The swelling to equilibrium with 1

kPa suction and 1 M CaCI, for Ca-kaolinite (Rocky Gully), Ca-illite (Willalooka), and Ca-montmorillonite (Wyoming) expressed as a percentage increase

in the original volume of the core is, respectively, 3 1,45, and 85%. However, if

the solution uptake by montmorillonite is corrected for the interlamellar water

(0.30 cm3 g-'), then this last figure becomes 43%. The original porosity and

surface area of these clays are, respectively, 0.185, 0.188, and 0.135 cm3 g-'



and 36, 160, and 38 m* g-1. This information reveals that swelling cannot be

interpreted simply in terms of the nitrogen surface area involved. The work of

Ben Rhaiem et af. (1987) shows that quasicrystals separate to give a larger

number of quasicrystals and thereby increase the external surface area of Wyoming montmorillonite during swelling.

The swelling in a I M CaCl, solution ( K - ' = 1.76 A) and other evidence

indicates that diffuse double-layer forces make little contribution so that swelling

must result from a balance between attractive forces (ion-ion correlation and van

der Waals) and repulsive forces (water structural). The suction in the soil water

acts to restrain swelling, and as the suction is reduced the balance of the forces

favor swelling. A simple three plate model can be used to illustrate the interplay

of the forces; in this, two plates in parallel alignment are separated by a third

plate in such a way that a pore is formed with a surface separation similar to the

thickness of the middle plate and in the areas of overlap the surfaces are virtually

in contact. With the adsorption of water these contacting surfaces may be separated by distances up to 10 A. Water structural forces are operative in the slitshaped pore between the separated plates, and in the pores, generated by overlap,

ion-ion correlation forces, together with van der Waals forces, act to restrain

swelling. The available evidence indicates that the magnitude of the water structural forces is influenced by surface density of charge and that the ion-ion

correlation forces are a function of both the surface density of charge and the

electrolyte concentration. The surface density of charge of the kaolinite, illite,

and montmorillonite, referred to earlier, are, respectively, 0.106, 0.260, and

0.119 Cm-2; 1 unit of charge per 152, 62, and 135 A 2 of surface.

Figure 3 indicates the large repulsive pressures which can be generated by

water structural forces in relation to the distance of separation of surfaces for slitshaped pores. For surfaces with a smaller surface density of charge these pressures would be lower. To quantitatively appreciate the operation of these forces it

would, ideally, be appropriate to have pore size distribution measurements before

swelling takes place and at various stages of the swelling process. For a sample

of Urbrae loam B horizon (80% clay, surface area of 116 m2g-l), Murray and

Quirk (1980a) measured the pore size distribution of the dry clay material and in

equilibrium with a suction of 10 kPa; the modal pore sizes were 36 and 63 A. The

swollen porosity was preserved by the displacement of water by increasing the

concentration of dioxane in dioxane-water mixtures, followed by the displacement of dioxane by liquid C 0 2 which was then removed at its critical point.

Features of the pore size distributions could reasonably be interpreted as arising

from variable crystal thicknesses. The nitrogen desorption isotherm enabled pore

sizes less than 300 A to be measured. Pore sizes larger than this value could not

be measured because the pressure involved in Hg injection porosimetry caused

collapse of the pores and this, as well as the preservation of the swollen porosity,

indicates that the original swollen structure was, at least partially, maintained.



Although the initial porosity was preserved by freeze-drying techniques the pore

size in the < I 0 0 A range was identical with the oven-dry material because the

water migrated and was frozen elsewhere in the clay matrix.

In considering the application of information such as that given in Fig. 3 to the

swelling of a clay-water system, the range of surface separations (pore sizes) in

the system arising from the range of crystal thicknesses, needs to be taken into

account. The clay mass is thus more complex than the three plate model used

earlier, as can be seen from Fig. 8 in which the presence of wedge-shaped pores

is evident; however, the description of near parallel alignment is still apposite.

Another contribution to the swelling process is the interaction between the clay

domains. Since the domains are in random array and swell unidimensionally they

will compete for space and thereby enlarge the porosity as the suction is decreased.

The swelling results of Aylmore and Quirk (1962) given in Table VII show

that the effect of CaCI, concentration on swelling is greater for montmorillonite

than for illite; the surface charge of illite ( 1 unit of charge per 62 A) is in the

vermiculite range.

The variation of ion-ion correlation attractive pressure with CaCI, concentration has been considered by Kjellander et al. (1990) who for a charge density of 1

unit charge per I35 A 2 calculated that the pressure increased from I .7 to 5.5 MPa

over the concentration range from zero (counterions only) to 2 M CaCI,. For 1

unit of charge per 60 A2 the pressure increased from about 15 to 30 MPa over the

same concentration range.

The hysteresis between the wetting and drying curves for these materials is

accentuated with increasing CaCI, concentration; this would be consistent with

the operation of ion-ion correlation attractive forces on the wetting cycle. On the

drying cycle the solution content decreases only slowly to a suction of 0.1 MPa

and then falls more rapidly at greater suctions. This is thought to be associated

with the strength of the structure formed on the wetting cycle. On the drying

cycle the water structural forces between surfaces removed from the potential

minimum would be operative and, furthermore, considerable work would be

required to bring opposing surfaces into the potential minimum because the two

sets of ions associated with each surface have to be shared at the midplane in the

potential minimum.

Calcium ions at clay surfaces have a dual role; they are responsible for the

attractive ion-ion correlation pressures at regions of close approach of surfaces,

and in larger pores the Ca ions, because of their large concentration at the clay

solution interface, perturb the normal structure of water which is the basis for

the repulsive pressures described as the structural component of disjoining pressure or secondary hydration forces. It is also noteworthy that these countervailing forces each increase considerably in magnitude with increasing surface density of charge. The balance between the two sets of forces is obviously a delicate



one since 10 to 20% Na+ ions, or even less, can give rise to adverse physical

behavior at low electrolyte levels. If it were not for the presence of ion-ion

correlation forces within a clay matrix or soil aggregate it would be virtually

impossible to use soils since water structural pores would dominate.

Aylmore and Quirk (1966) have reported on the swelling of four Ca-kaolinites:

Mercks I1 ( 1 1 m2g-1, 0.18 Cm-z), Malone (17 m2g-1, 0.24 Cm-,), Rocky

Gully (36 m2g-', 0.11 Crn-,), and New Zealand (40m2g-', 0.08 Crn-,). The

initial porosity of these materials was similar and the percentage increases in

volume, from the dry state, for the clay cores in equilibrium with 1 M CaCI, and

I kPa were, respectively, 13, 22, 3 I , and 50; when these percentages are expressed in relation to the surface area of each clay the following results emerge:

1.18, 1.29, 0.86, and 1.25. These ratios may be compared with that for Willalooka illite of 0.3 which suggests that the kaolinites behave as single crystals.

The basic swelling mechanism must be water structural forces accompanied by a

general relaxation of the clay as the suction is reduced.


The B.E.T. equation and the Kelvin relationship [Eq. (2)] have been applied to

low temperature nitrogen adsorption-desorption isotherms to obtain, respectively, surface area and pore size distribution of clays and soils (Aylmore and

Quirk, 1967; Murray and Quirk, 1980a,b, 1990a,b; Murray et al., 1985). Murray et al. (1985) have critically assessed the assumptions used in the B.E.T. and

Kelvin equations as applied to clay systems. In applying the Kelvin equation,

allowance is made for the thinning of multilayers on the surface of pores already

emptied by capillary evaporation on the desorption isotherm (Aylmore and

Quirk, 1967). The procedure adopted is to obtain the volume of capillary evaporate removed over a series of intervals @lpo values) on the desorption isotherm.

Since the Kelvin equation provides the average slit width for each vapor pressure

interval, this value, together with the change in volume in the vapor pressure

interval, is used to yield a surface area. The sum of the surface areas for all of the

intervals provides the desorption isotherm surface area. The pore size distribution is also obtained and the procedures involved are set out by Aylmore and

Quirk (1967) and Murray el al. (1985).


Figure 7 shows the N, sorption isotherms for Ca-Willalooka illite (a separate

sample from that quoted earlier with an area of 160 m2g-') and the natural













0 .2

.4 .6


.8 1

0 . 2 . 4 .6 .8 1




6 810

pore width (nm)


4 6810

pore width (nm)

Figure 7 Nitrogen sorption isotherm and derived cumulative pore size distribution (a) natural

aggregates of a vertisol (total porosity, 0.13 cm3g-I) and (b) Ca-Willalooka illite-compressed cores

(total porosity, 0.23 cm3g-I). (After Murray er al.. 1985.)

aggregates of a Queensland vertisol (No. 10). It can be noted that the plot of

cumulative volume versus pore size indicates modal values of about 4 and 3 nm

respectively. The slope of the desorption isotherm for Willalooka illite indicates a

mixture of slit- and wedge-shaped pores. The isothern for the vertisol shows a

large hysteresis and its shape indicates a predominance of slit-shaped pores.

There is a considerable variation in the isotherms for the other vertisols studied

with some having a sloping desorption isotherm.

Figure 7 also shows that the pore size in these materials is predominantly < 10

nm. For the vertisol, 50% of the pore space is in pores less than < 3 nm and 95%

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V. Swelling of Calcium Clays

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