Clays and Clay Minerals, Vol, 46, No. 2, 117-131, 1998.

STATE A N D LOCATION OF WATER A D S O R B E D ON CLAY MINERALS: CONSEQUENCES OF THE HYDRATION A N D S W E L L I N G - S H R I N K A G E

P H E N O M E N A

R. PROST, T. KOUT]T, A. BENCHARA AND E. HUARD

Station de Science du Sol, INRA, Route de Saint-Cyr, 78000 Versailles, France

Abstract - -The application of the Frenkel-Halsey-Hiil (FHH) formalism to the water desorption iso- therms obtained for the whole range of the activity of water with the pressure membrane device (0.98 < aw < 1) and with the desiccator (0 < aw < 0.98) gives information concerning the nature and the relative importance of the 2 mechanisms involved in the dehydration-hydration processes: adsorption and capillary condensation. The state and location of water are described in each domain. An equation that gives the thickness t of the film of water adsorbed on the walls of pores versus the activity of water is developed. This t-curve is used to get, from the desorption isotherm, the pore size distribution curve of the studied hydrated materials. Then concepts of surface and fabric of clay pastes are discussed as a function of hydration and a mechanism is proposed to explain swelling and shrinkage of finely divided materials. Three kinds of surfaces, related to the aggregate fabric, are defined as a function of their capacity to adsorb water. Each kind of surface is determined by a specific technique: the total surface area (St) by ethylene glycol adsorption, the external surface area of particles (S~) by nitrogen adsorption and the external surface area of aggregates (Se) by hydraulic conductivity measurements. As a consequence it is only with completely dispersed clays that swelling is a function of St. With unwell-dispersed clays, water adsorption, which induces swelling, successively occurs on St, S s and Se surfaces.

Key W o r d s ~ l a y Paste Fabric, Hydration, State and Location of Water, Swelling and Shrinkage Mech- anisms, t-Curve.

I N T R O D U C T I O N

The unique propert ies of clays mixed with water have permi t ted their use s ince the ancient t imes to make domes t ic objects and beautiful pottery. Al though the phys icochemica l proper t ies of clays as a funct ion of water content have been known for a long t ime, it is a mat ter o f fact that the mechan i sms involved in their hydrat ion, swell ing and shrinkage are still not comple te ly unders tood. The aim of this paper is to analyze data obta ined by the senior author and his co- workers for a bet ter unders tanding o f the state and location o f water retained by clay materials. The states of water are character ized by the m e c h a n i s m (adsorp- tion or capi l lary condensat ion) involved in the water re tent ion phenomenon , and by the wa t e r - ca t i on -c l ay structure interact ions that de termine the ar rangement of water molecules with respect to solid surfaces, ex- changeable cat ions and other water molecules . The lo- cat ion of water refers to sites or surfaces where water molecules are adsorbed and pores where water is con- densed.

Determina t ion o f the affinity of clays for water will be d iscussed in the first part. At tent ion will be focused on the me thod deve loped to obtain a water desorpt ion i so therm for the whole range of the activity o f water (0 < aw < 1). In the second part, we will show how the applicat ion of the FHH formal i sm to the water de- sorpt ion i so therm gives informat ion concern ing the na- ture and the relative impor tance of the 2 mechan i sms

involved in the dehydra t ion-hydra t ion process: ad- sorpt ion and capi l lary condensat ion. In the third part, the state and location o f water will be descr ibed in each domain where 1 retent ion m e c h a n i s m is p redom- inant. In the fourth part, the concepts of surface and fabric o f clay will be d iscussed as a funct ion of hy- drat ion and a m e c h a n i s m will be p roposed to explain the swel l ing and shr inkage phenomena . Also, an over- v iew will be presen ted concern ing the effect o f the structure of adsorbed water on the hydrat ion process and, as a consequence , on the swel l ing p h e n o m e n o n of clay materials. As oxides are c o m m o n l y found in soils, they will be also taken into a c c o u n t .

C A R R Y I N G O U T OF W A T E R S O R P T I O N I S O T H E R M S F O R 0 < aw < 1

The water sorpt ion i so therms give, for a given tem- perature, the amount of water retained by clay mate- rials as a funct ion of the activity of wa te r (aw). It is not poss ible to obtain a water sorpt ion i so therm for 0 < aw < 1 per forming only 1 exper iment . Two me thods are needed according to the range of aw considered. For reasons o f e c o n o m y o f space only typical data will be repor ted here. The reader may refer for more details to another paper by Prost (1990).

Carry ing out Water Sorpt ion Iso therms for 0 < aw < 0.98

Water sorpt ion is general ly pe r fo rmed in a desic- cator where aw is control led by mixtures o f H2SO4 +

Copyright �9 1998, The Clay Minerals Society 117

1 18 Prost, Koutit, Benchara and Huard Clays and Clay Minerals

~ 3 ,

v i

E

§

I

0 n a ~ ~ g

0.0 0.2 0.4 0.6 0.8 1.0

Activity of Water

O o

_=

3 .0

2 . 0

1.0

b +

+ ~ + ~ + ~ + + _ + ~

0.0 t I t I t I t I

0992 0.994 0.996 0.998 1.000 Activity of water

Figure 1. a) Water desorption isotherm of S i O 2 3-6 ~m (data obtained with the pressure membrane device (X) and with the desiccator method (A)) and S i O 2 18-32 Ixm ((+) pressure membrane points, (�9 desiccator points), b) Water desorption isotherm of S i O 2 3-6 Ixm (• 18-32 p~m (+) for 0,99 < a,~ < I.

H20 or sa tura ted salt solut ions. The des icca tor is p laced in a ba th w h o s e t empera tu re is control led. Dif- ficulties arise due to condensa t ion on the coldes t poin t of the t empera tu re cont ro l l ing dev ice w h e n aw > 0.98; therefore , the "des i cca to r m e t h o d " a l lows accurate m e a s u r e m e n t s on ly w h e n aw < 0.98.

C a r r y i n g out Wate r So rp t ion I s o t h e r m s for 0 .98 < a ~ < 1

For sorpt ion in the r ange 0.98 < aw < 1, f i l tration unde r pressure is used. Indeed, as has been s h o w n by Sposi to (1972) and Bourr i6 and P6dro (1979), the gas pressure P, appl ied in the so-cal led "p res su re m e m - b rane d e v i c e " , is re la ted to the free energy AG of the c lay-wate r sys tem by the fo l lowing re la t ionship:

where:

R T

AG = --17P = R T L n a w

is the part ial mo la r vo lume of water, the gas constant , the tempera ture ,

[1]

5.

4

r - 3 O o

Figure 2.

0 .~ R--~-;--%':-"- ' ;~--- ' : -~176 : 0.0 0.2 0.4 0.6 0.8 1.0

Activity of water

Water desorption isotherm of AI203 (data obtained with the pressure membrane device (• and with the desic- cator method (�9

aw the act ivi ty of water.

The pressure m e m b r a n e dev ice a l lows the cont ro l of aw by the cont ro l of the appl ied pressure P. Pres- sures equal to 10 and 500 m b a r cor respond, accord ing to Equa t ion [1], to aw = 0 .999993 and aw = 0 .999637, respect ively. Because it is easy to cont ro l such pres- sures, the pressure m e m b r a n e dev ice is wel l adapted to car ry out wate r sorpt ion i so the rms for the h ighes t values of aw.

It is poss ib le to carry out wate r adsorp t ion i so the rms wi th these 2 me thods only i f the suspens ion is free of salt (Kout i t 1989). To avoid diff icul t ies due to hyster- esis, samples used in the des icca tor m e t h o d were pre- pared wi th the pressure m e m b r a n e dev ice under a pressure of 5 bars ( a , = 0 .996377) (Prost 1990).

F igure l a g ives water desorp t ion i so the rms carr ied out wi th porous grains of SiO2 used for ch romatogra - phy. A good over lap of the data ob ta ined wi th the pressure m e m b r a n e and the des icca tor dev ice is noted. The i so therms, (Figures l a and lb ) , p lot ted wi th 2 dif- ferent scales for aw, clear ly show the ex is tence of 2 steps that are re la ted to 2 d i f fe rent sets o f cons t r ic t ion radi i o f pores. The step that occurs on the wate r de- sorpt ion i so the rm of bo th samples at aw = 0.75 cor- responds to the empty ing of pores located ins ide the grains. The second step obse rved at aw = 0.9975 and aw = 0.9995 for 3 - 6 I~m and 18 -32 Ixm SiO2 grains, respect ively, co r responds to the emp ty ing of larger pores re la ted to the s tacking or the fabr ic of the grains. The empty ing of larger pores co r respond ing to larger grains occurs at a h igher va lue o f aw than for smal le r pores co r re spond ing to smal ler grains. F igure 2 gives the water desorp t ion i so the rm ob ta ined wi th nonpo- rous AI203 oxide grains whose specific surface area is

Vol. 46, No. 2, 1998 Hydration and swelling-shrinkage of clay materials 119

g = 3

g 0

-~ 2

4.0

~o) 3.5 o~ E ~ 3 .0

0 o 2.5

~ 2 . 0 o

1.5

1.0

0 .5

0.0

p, o

o

-Xx x--~--x x ~ - t ~

,\ \ \

, : ; : : : : : ; : ; : 1 -6 -5 -4 -3 -2 -1 1

l og ( l og ( 1 l a w ) )

--...

A

' Jl -6 -5 -4 -3 -2: I 0

; i I ~ 1 7 6 ' i I

q 0.971 0.807 0.253 aw

i i 4,64 5.46 6.3 p F

0 i

-5 .5

"------o ..~....o

~ ' ~ . ~ o

C

: ; : ; : ; i | t ! i I -4.5 -3.5 -2.5 -1.5 -0.5 0.5

Iog( log( l /aw))

100 m 2 g-L Only 1 step is o b s e r v e d that co r re sponds to the fabr ic of par t ic les in the pastes.

It is apparen t f rom this that ca r ry ing out wate r de- sorpt ion i so the rms in the whole r ange o f the act ivi ty o f wate r can give the size of the cons t r ic t ion radi i o f pores ( f rom the Ke lv in equat ion, for example ) and the porous v o l u m e of each set of pores i n v o l v e d in the re ten t ion m e c h a n i s m of water.

STATES O F W A T E R R E T A I N E D B Y C L A Y M A T E R I A L S

Clay minera l s and oxides are wel l k n o w n to re ta in large amount s o f wate r re la t ive to the i r mass ; there- fore, mul t i l ayer adsorp t ion is a s sumed to be an oper- at ive m e c h a n i s m in the wate r r e ten t ion process . Fo r this reason, we have appl ied a mul t i l ayer a d s o r p t i o n - desorp t ion fo rma l i sm to this process .

The F H H Equa t ion

Frenke l (1946), Ha l sey (1948) and Hil l (1952) showed, for in te rmedia te values o f aw, that sorp t ion i so the rms can be desc r ibed by the fo l lowing equat ion:

log p-- = ~ [2] Po wr

where : w is the wate r content , plpo the re la t ive pressure of wate r and k and r cons tan ts tha t charac te r ize the state of the sol id surface and solute and the size and shape of the par t ic les (Adk ins et al. 1986).

The F H H cu rves are ob ta ined p lo t t ing log w as a func t ion o f log ( logfpJp) ) . This p lo t is s imi lar to the plot o f log w versus pF, w h i c h is c o m m o n l y used in soil science. Here, we show that the usefu lness of the F H H equa t ion can be ex tended to the ent i re r ange of aw f rom 0 to 1, ob ta ined us ing the pressure m e m b r a n e and des icca tor methods .

F H H Plots o f F ine ly D iv ided Mater ia l s

F igures 3a and 3b give the F H H plot for SiO2 and A1203 pastes. The a i r -entry point , as d e t e r m i n e d b y the Ha ines ' s plot (Haines 1923) occurs at a wa te r con ten t

4--

Figure 3. a) FHH plots of SiOz 3-6 tzm (data obtained with the pressure membrane device (X) and with the desiccator method (A)) and SiO2 18-32 ixm ((+) pressure membrane points, (O) desiccator points), b) FHH plot of A1203 (data obtained with the pressure membrane device (X) and with the desiccator method (O)). A is the air-entry point. The numbers 1, 2, 3 and 4 correspond to domains where water retention is predominantly due to hydration of hydrophilic sites (l), mul- tilayer adsorption (2), capillary condensation (3) and both multilayer adsorption and capillary condensation (4). The amount of water retained at a given a , value by capillary condensation and adsorption are respectively represented by A - B and B - C. c) FHH plots of montmorillonite saturated by Na (O) and by Ca (A). A is the air-entry point.

120 Prost, Koutit, Benchara and Huard Clays and Clay Minerals

indicated on Figure 3b. The curve of Figure 3b is much simpler than the curves of Figure 3a obtained with SIO2. Therefore, it will be used to describe the different states and location of water and also the mechanisms involved in the water retention phenom- enon.

DOMAIN 1. It is observed for the lowest values o f aw and corresponds to water adsorbed on the hydrophil ic sites of the material. In the case of clays these hydro- philic sites are exchangeable cations, cations of the edges and surface OH groups. Capil lary condensation at contact points be tween particles may also occur at these low values of aw (Prost 1975b).

DOMAIN 2. It corresponds to the mult i layer adsorption process. It occurs on " f ree surfaces" (Pierce 1960), which are either walls o f unsaturated pores or surfaces that belong to layers or particles that can expand free- ly.

DOMAIN 3. It begins when the F H H curve diverges f rom linearity as aw increases. In this domain, capil lary condensat ion of water in pores must be involved (Car- ott et al. 1982; Benchara 1991). The capil lary conden- sation phenomenon occurs in pores that correspond to the fabric of hydrated particles.

DOMAIN 4. It corresponds to the part of the curve that is observed for the highest activities of water. This domain is related to the mult i layer adsorption process on " f ree surfaces" of grains or particles and to a cap- i l lary condensat ion mechan ism in saturated pores whose size increases or decreases as swell ing or shrinkage occurs.

Figure 3a gives the F H H plot of SiO2 3 - 6 ~ m and SiO2 18-32 pom samples. Several domains are ob- served that correspond to different states of water in these materials. The steps o f the F H H curves, ob- served for the highest water contents, occur at differ- ent values of aw for the 2 samples because the size difference leads to unique fabric structures for each particle size.

Figure 3c shows the F H H plots of Na- and Ca-mont- moril lonites. The air-entry point occurs at low water contents for Na-montmori l loni te (Tessier 1984). The S-shaped part of the curve, which occurs while the Na- montmori l loni te is saturated, is attributed to the con- traction of the interlamellar spaces f rom 4 to 2 nm (Norrish 1954).

In general, the F H H formal ism allows us to analyze water retention in terms of 2 different mechanisms:

1) adsorption, which occurs on hydrophil ic sites at the lowest value of aw or on " f ree surfaces" for higher values of aw;

2) capillary condensation, which occurs at contact points be tween particles at the low values of aw or in pores at the high values o f aw.

As a result, the state of water can be defined by the energy of hydration, which is related to aw by the mechanisms involved in the hydration process, and by the location of water in the paste. The energy of hy- dration is obtained f rom aw and each domain defined by the F H H plot is characterized by a range of aw. The mechanisms involved are adsorption and capi l lary condensation. The location and the amount of water adsorbed or condensed at each value of aw can be specified with the F H H plot.

Jurinak (1963) applied the F H H formal ism to the water adsorption isotherm obtained for kaolinite. He found a linear relationship for 0.2 < aw < 0.97, which is expected for the mult i layer adsorption process in this range of aw. Prost (1990), applying the F H H for- mal i sm to the water desorption isotherm of kaolinite, obtained in the whole range of the activity of water (0 < aw < 1), concluded that the mult i layer adsorption process is predominant in a larger range of aw for the < 5 0 Ixm fraction of kaolinite than for the < 1 p~m frac- tion.

In domain 3, where capil lary condensat ion occurs in pores resulting f rom the fabric of finely divided par- ticles (A1203 oxide, kaolinite . . . . ), both water reten- tion phenomena exist: capil lary condensat ion and mul- ti layer adsorption. Pierce (1953) has used the same mechanisms to describe the desaturation process of porous materials saturated by l iquid nitrogen. Our hy- pothesis is that the mult i layer adsorption process exists in domains 2, 3 and 4, fo l lowing the same linear re- lationship, even when finely divided materials are sat- urated by water rather than nitrogen.

So the F H H plot allows the determinat ion of the amount o f water retained by adsorption and by capil- lary condensation. Figure 3b shows how these deter- minations can be made for each value of aw. At the air-entry point 70% of the amount of water, repre- sented by A - B, is located in large pores whose size can be determined by the method described in the fol- lowing paragraph.

Structure of Gels and Pastes of Finely Divided Materials

Hydraulic conduct ivi ty measurements per formed on saturated pastes corresponding to domain 4 (Prost 1990) give indications of the mean hydraulic radii of pores as a function of water content. The application of an equation developed by Prost (1990) al lows the calculat ion of the specific surface area Se of grains and aggregates. The good agreement observed (Table 1) for wel l-dispersed materials (A1203, chrysoti le , kaolin- ite at pH = 9) be tween specific surface area deter- mined f rom the nitrogen adsorption isotherms, using the Brunauer, E m m e t and Teller formal ism (BET method), and f rom the hydraulic conduct ivi ty mea- surements, suggests that water flows between individ- ual particles. The lower specific surface area deter-

Vol. 46, No. 2, 1998 Hydration and swelling-shrinkage of clay materials 121

Table 1. Specific surface area S~ (BET method), S~ (Hydrau- lic conductivity method) and amount of water Wo located in the porosity of aggregates or flocculates.

Samples S~ (m 2 g-t) Sc (m 2 g 1) wo (g g t)

A1203 100 100 0 Chrysotile 34 27 0 Kaolinite pH = 9 19 20 0 Kaolinite pH = 5.6 19 6 0.32

m i n e d by hydrau l ic conduc t iv i ty for kaol in i te at pH = 5.6 (Table 1) arises f rom the f loccula t ion of the sus- pens ion and to the fact that wate r f lows be t w een floc- culates.

App l i ca t ion of the f o r m a l i s m deve loped by Pierce (1953) for n i t rogen desorp t ion i so the rms and adapted to wate r desorp t ion i so therms, a l lows the de te rmina- t ion of the pore size d is t r ibu t ion cu rves of we t mate - rials. Th i s fo rmal i sm, w h i c h impl ies a knowledge of the " t - c u r v e " , was appl ied to dom a i ns 3 and 2 of the F H H plot where desa tura t ion occurs.

Because finely d iv ided mater ia l s are no t r igid po- rous media , the pore size d is t r ibut ion cu rves f rom wa- ter desorp t ion are s l ight ly t r ans fo rmed re la t ive to those ob ta ined wi th mercu r y inject ion. Such pore size dis- t r ibu t ion cu rves (Figure 4) give values o f cons t r ic t ion radi i of we t sys tems wh ich are, o f course, s l ightly un- de res t ima ted c o m p a r e d to the or iginal paste. Never- theless , they c lear ly show that wa te r re ta ined in S i t 2 3 - 6 jxm gra ins b y cap i l la ry condens a t i on is loca ted in large pores , w h o s e cons t r ic t ion radi i are a round 350 nm, and in smal l pores, whose cons t r ic t ion radi i are c lose to 4 nm. The larger cons t r ic t ion radi i (450 n m ) o b s e r v e d on the pore size d is t r ibu t ion cu rve ob ta ined by mercu ry in jec t ion c o m p a r e d to the one deduced f rom the wate r desorp t ion i so the rm (350 nm) is due to the re laxa t ion of the s t ructure o f the solid phase w h e n the f i lm of wate r caus ing cohes ion be t w een par t ic les is r emoved .

In summary , the F H H plot appl ied to wate r desorp- t ion i so the rms ob ta ined in the who le r ange o f the ac- t ivi ty of wate r g ives qual i ta t ive and quant i ta t ive indi- ca t ions about the s tate and loca t ion of wate r (adsorbed on hydroph i l i c sites or on " f r ee su r faces" and con- densed in large pores) in pas tes m a d e of finely d iv ided mater ia ls . The next sect ion wil l be devoted to the s t ructure of wate r nea r sol id surfaces in order to un- ders tand the hydra t ion process of these finely d iv ided mater ia ls at the mo lecu l a r level .

S T R U C T U R E O F W A T E R A D S O R B E D O N C L A Y M A T E R I A L S

The s t ructure of adsorbed wate r on clay mater ia l s is def ined as the way wate r molecu les are a r ranged near c lay surfaces. To do so, we wil l first cons ider the struc- ture of bu lk wate r or ice.

-5 8 4

0 B

0 1 2 3 4 log rp (nm)

Pore size distribution curves of Si t2 3-6 p~m ob- Figure 4. tained from the water desorption isotherm (+), mercury in- jection (A) and from the nitrogen desorption isotherm (O).

E i senbe rg and K a u z m a n n (1969) deve loped an orig- inal concep t for the s t ructure of b u l k l iquid wa te r ba sed on the t ime scale tha t is specific to each tech- n ique used as a probe. On a t ime scale that is long re la t ive to the per iod of v ibra t ion o f a h y d r o g e n b o n d in l iquid wate r (about 10 13 s), bu t shor t re la t ive to the t ime requi red for a wate r mo lecu le to di f fuse a dis- tance equal to its o w n d iamete r (about 10 -1] s), a typ- ical mo lecu le in l iquid wate r " s e e s " a spat ia l ar range- m e n t o f its ne ighbors tha t is ca l led the v ib ra t iona l ly ave raged s t ructure (V-structure) . Th i s s t ructure wil l in- c lude on ly the effects o f v ibra t iona l mo t ions of the wate r molecu les and, accord ing to F igure 5, can be p robed by infrared (IR), neu t ron sca t ter ing and nuc lea r magne t i c r e sonance ( N M R ) spectroscopy. At the o ther ex t reme, on a t ime scale that is long c o m p a r e d wi th tha t du r ing wh ich a mo lecu le d i f fuses a n o m i n a l dis- tance in l iquid water, a typical mo lecu le " s e e s " a sur- r ound ing spatial a r r angemen t tha t is ca l led the diffu- s ional ly ave raged s t ructure (D-structure) . Th i s struc- ture inc ludes the effects of v ibra t iona l , ro ta t ional and t rans la t ional mo t ions o f the wate r mo lecu le s and wil l be more ordered than the V-st ructure because it com- prises on ly the mos t p robab le molecu la r conf igura- t ions. The D-s t ruc ture can be p robed by neu t ron and X- ray di f f rac t ion (XRD) exper iments , and it p rov ides a bas is in molecu la r s t ructure for t h e r m o d y n a m i c proper t ies , as sugges ted in F igure 5.

B e t w e e n the t ime doma ins o f the V- and D-s t ruc- tures, there is a t rans i t ion reg ion that can be invest i - ga ted by neu t ron scat ter ing, N M R and dielectr ic re lax- at ion spectroscopy. These t echn iques are expec ted to g ive in fo rmat ion abou t the ro ta t ional and t rans la t iona l mo t ions o f wate r molecu les tha t lead f rom the V-struc- ture to the D-s t ructure . Thus , it is c lear that the con- cept of " s t r u c t u r e " in l iquid wate r is a dynamic one,

122 Prost, Koutit, Benchara and Huard Clays and Clay Minerals

;TRUCTURE

TIME SCALE IN SECONDS ( log)

R SPECTRA

NEUTRON SCATTERING

ESR SPECTRA

N M R SPECTRA

DIELECTRIC RELAXATION

NEUTRON AND X-RAY DIFFRACTION

THERMODYNAMIC PROPERTIES

I I I ~ I I I I I I ~1/////I I I I 1 I 1 I -15 -14 -13 -12 -II - I0 -9 -S -7 -6 -5 -4 -3 -2 -I 0 ,*1 +2 +3 *4

_ _

l / Figure 5. Time scales for adsorbed water structures and the experimental methods used to measure the properties of adsorbed water (Sposito and Prost 1982).

and the same should be true for the water adsorbed by clays.

In the case of adsorbed water, an additional com- plexity is provided by the exchangeable cations, whose vibrational and translational motions take place in the transition region between the V- and D-structures. Sol- id state NMR of exchangeable cations such as Li, Na, Cs, Cd and K may contribute to a better understanding of the water-cation-clay structure interactions.

The review by Sposito and Prost (1982) gives the basic results concerning the structure of water ad- sorbed on clay surfaces. For reasons of economy of space, data and discussions developed in that review concerning IR spectroscopy, neutron scattering, NMR spectroscopy, dielectric relaxation, neutron and XRD and thermodynamic properties will not be reported here. Data obtained by solid state NMR and far-IR spectroscopy, which allow one to probe the surface through the exchangeable cations, will be presented. Taking into account all these data, a synthetic structure of adsorbed water will be given.

Solid-State NMR and Far-IR Spectroscopy of Compensating Cations

A high-resolution solid-state NMR study of e• changeable cations in the interlayer space of Llano vermiculite saturated by Na, Cd and Cs (Laperche et al. 1990) shows that each phase (dehydrated, l-layer hydrate and 2-layer hydrate) identified by X-ray is characterized by a unique chemical shift ~cc, (CG: cen- ter of gravity of the line) and by a typical bandwidth v~/2. On Figure 6 are gathered spectra of the different phases of Na-vermiculite. All spectra may be account- ed by 1 or 2 lines among 3 possible contributions: one corresponding to the 2-layer hydrate (~cc, -- 4,5 _+ 0,5

ppm), one to the l-layer hydrate (~cc = - 7 _+ 1 ppm) and one to the dehydrated phase (~cG = - 1 8 - 1 ppm). The analysis of the data leads to the conclusion that Na cations are located in 2 different sites in the l-layer hydrate and in consequence are solvated by 2 populations of water molecules. Such a conclusion was reached by the simultaneous analysis of IH and 23Na NMR spectra of the l-layer hydrate (Laperche et al. 1990).

A variable-temperature MAS NMR study of 133Cs- hectorite led Weiss et al. (1990a, 1990b) to assign the 2 peaks observed for clay-water slurries to Cs atoms in the Stern layer and in the Gouy diffuse layer. The peak ascribed to Cs in the Stern layer is not affected by Cs concentration, whereas the peak ascribed to Cs in the Gouy diffuse layer changes with increasing Cs concentration in the bulk solution with which the clay equilibrated. The 2 peaks observed for fully dehydrat- ed samples were assigned to 2 different interlayer sites.

Lambert et al. (1992) report data obtained with a K- montmorillonite submitted to wetting and drying (W- D) cycles. Figure 7 gives 39K solid-state NMR spectra of potassium in montmorillonite pastes prepared with samples submitted to 0, 11 and 100 W-D cycles and after a replacement of exchangeable K by Sr in the 100 W-D sample. Spectra were deconvoluted into a narrow and a wide peak. When the number of W-D cycles increases, the intensity of the wide peak in- creases. The narrow peak is assigned to K hydrated by more than 3 water molecules, and the wide peak to K which is hydrated by less than 3 water molecules. So the proportion of K hydrated by less than 3 water mol- ecules increases as the number of W-D cycles increas- es. The narrow peak disappears when exchangeable K

Vol. 46, No. 2, 1998 Hydration and swelling-shrinkage of clay materials 123

L

I 1

I

~ 3

d

C

b

,~- 18.5 i J

, ' 1 ',,(6.2)

T r

~5bis q 1

I l I I

80 60 40 20 Opprn-20 - 4 0 - 6 0 60 40 20 Oppm20 - 4 0 - 6 0

Figure 6. A) 23Na MAS NMR spectra at different hydration levels. The spinning rate (kHz) is indicated between parentheses. Circles show the spinning sidebands. B) 23Na MAS NMR spectra for biphasic samples at large scale (solid line). The dashed or interrupted lines show the deconvolution into 2 components. The areas under these lines are their relative contributions (Laperche et al. 1990).

I I I I I I I +2000 0 -2000 ppm

Figure 7. 39K solid state NMR powder spectra of K-mont- morillonite paste with 40% w/w clay. Key: a = 0 wetting and drying cycle (W-D) of the original montmorillonite, b = 11 W-D cycles, c = 100 W-D cycles and d = 100 W-D cycles + 4 Sr 2+ exchanges (Lambert et al. 1982).

is replaced by Sr (Figure 7). More work is needed to

ident ify K that is comple te ly dehydra ted and may be no longer exchangeable .

Cadmium-montmor i l lon i t e and Cd-hector i te were

also studied as a funct ion of water content by Bank and coworkers (Bank, Bank and Ellis 1989; Bank, Bank, Marchet t i et al. 1989) and Tinet et al. (1991).

The last authors show in particular that Cd in slurries

is located in 2 different sites: Cd be tween clay layers and Cd on external surfaces. The assessment of the

amount o f Cd in both sites may give indicat ions on

the average size of tactoids. The solid-state N M R study of exchangeable cat ions

may give complementa ry informat ion on water--cat- i on -c l ay structure interactions, particularly concern ing the distr ibution of exchangeable cat ions near clay sur-

faces. I f solid-state N M R spec t roscopy is well adapted to study the state and location of exchangeab le cat ions

in wet systems, far-IR spec t roscopy is more appropri-

ate to study clays at low water contents . Figure 8 gives the far-IR spectra recorded under vacuum of Llano

vermicul i te saturated by mono- and divalent cations.

The 2 peaks obse rved with Cd probably cor respond to 2 different states for that cation: Cd 2§ and C d O H +

(Laperche 1991).

124 Prost, Koutit, Benchara and Huard Clays and Clay Minerals

157 o o

~,_ 10694

" - - 157 105 160, ~

105

I I 200 150 100 50

WAVENUMBER CH-I

Figure 8. Far-IR spectra of vermiculite saturated by: NH4 § (1); K § (2); Sr 2§ (3); Cd 2* (4); Ba 2§ (5); Rb § (6); Cs § (7) (Laperche 1991).

Wate r -Ca t ion- -Clay Structure Interact ions: St ructure of A d s o r b e d Water

The F H H plot o f the wate r desorp t ion i so the rm data a l lows the ident i f ica t ion of 2 states of water: wate r re ta ined by adsorp t ion on hydroph i l i c sites or on " f r ee su r faces" and wate r re ta ined by condensa t ion at con- tact points be tween par t ic les or in pores (Prost 1990). So 2 m e c h a n i s m s are i nvo lved in the water re ten t ion p h e n o m e n o n : adsorp t ion and capi l la ry condensa t ion . Capi l la ry condensa t ion and mul t i l ayer adsorp t ion were a l ready d i scussed in the p reced ing pages. Par t icular a t ten t ion wil l be g iven now to the first s teps o f the hydra t ion p rocess wh ich are s t rongly re la ted to the wa- ter--cation--clay s t ructure in teract ions .

Table 2. Interlayer spacing and mean number of water mol- ecules per cation of Cs-, Na- and Ba-hectorite for water con- tent0.1 g g-l.

Mean number of Interlayer spacing water molecules

d(O01) (A) per cation % RH

Cs-hectorite 12.4 1.6 40 Na-hectorite 12.2 2.3 20 Ba-hectorite 12.5 8.2 15

At the lowes t va lues o f aw wate r molecu les re ta ined by clays are adsorbed on exchangeab le cat ions. The hydra ted ca t ions m a k e pillars, i nduc ing an increase of the spac ing be tween clay layers. Table 2 gives the ap- parent d(001) spac ing of Cs-, Na- and Ba-hec tor i t e layers w h e n the water con ten t of the c lay is 0.1 g g 1 (Prost 1975c) at the co r re spond ing re la t ive humidi ty . The d(001) spac ing is roughly the same for hec tor i te sa turated by Cs, Na and Ba. The m e a n n u m b e r of wa- ter molecu les pe r ca t ion ob ta ined by d iv id ing the a m o u n t of wate r loca ted in the in te r lamel la r spaces, as de t e rmined by IR spectroscopy, b y the n u m b e r of cat- ions (Prost 1990), is a func t ion of its ene rgy of hydra - tion: Cs ca t ions are less hydra ted than Na and Ba cat- ions. This resul t shows that the adsorp t ion o f wate r on clay surfaces does not occur by mono laye r s as is sug- ges ted by the apparen t d (001) spac ing d e t e r m i n e d by XRD. The adsorp t ion occurs a round each cat ion, mak- ing pil lars tha t fix the in te r lamel la r spacing. I f the hy- pothes is of m o n o l a y e r adsorp t ion on basa l surfaces is accepted, in the case p resen ted here where the acces- sibil i ty o f wate r to these surfaces is the same for each sample , the coverage of the in terna l surfaces should be grea ter wi th Cs-hec tor i te than wi th Ba-hec tor i t e be- cause aw is m u c h h igher in the Cs system. The con- t ra ry is obse rved , showing that the adsorp t ion in this range of the act ivi ty of wate r does not occur on oxy- gen a toms of basa l surfaces but on exchangeab le cat- ions (Prost 1990).

In the case of smect i tes tha t have the i r i somorph ic subst i tu t ions in the oc tahedra l shee t o f the layers and where water molecu les are no t i n v o l v e d in hyd rogen bonds wi th surface oxygen a toms (Prost 1975c), basa l surfaces have an hyd rophob i c character . The hydro- phi l ic i ty o f these clays is on ly due to the ex is tence in the in te r lamel la r spaces of exchangeab le cat ions. Tha t conc lus ion is suppor ted by wate r adsorp t ion i so the rms ob ta ined wi th reduced cha rge mon tmor i l l on i t e (Calvet and Prost 1971). Indeed the a m o u n t o f wate r adsorbed is a func t ion of the charge dens i ty o f the c lay and the nature o f the exchangeab le cat ions. This resul t is in good ag reemen t wi th the hyd rophob i c charac te r of talc and pyrophyl l i te wh ich have no defici t of charge.

In the case of smect i tes that have thei r i somorph ic subst i tu t ions in the te t rahedral sheet of the layer, wa te r molecu les adsorbed in the in te r lamel la r spaces are in- vo lved in hyd rogen bonds wi th oxygen a toms be long-

Vol. 46, No. 2, 1998 Hydration and swelling-shrinkage of clay materials 125

a Ma s

I

M S

I B

0 Li§ C) H20 0 Oxygen atom

Figure 9. Orientation of the water molecules with respect to Li § cation and oxygen atoms of the surface (Prost 1975c).

ing to tetrahedra where Si is replaced by A1 or Fe. Exchangeable cations are as close as possible to the isomorphic substitutions and only water molecules that hydrate cations can be invo lved in such hydrogen bonds. In minerals whose isomorphic substitutions are located in the tetrahedral sheet o f the layer, hydrogen bonds be tween water and oxygen atoms of the struc- ture may explain the general ly stronger water--clay structure interactions observed and the l imited expan- sion of the layers.

If domains without any charge exist on the surface there will be no adsorption of water molecules. Indeed, clays saturated by methyl- or t r imethyl ammon ium cat- ions do not adsorb water.

Structure of Adsorbed Water

Schemat ic representations of the arrangement of water molecules with respect to basal surfaces and to ex- changeable cations are now presented. Figure 9 shows the arrangement of water molecules in the case of Li- hectorite, whose isomorphic substitutions are located in the octahedral sheet of the layer. The exchangeable cations are hydrated by 3 water molecules when the sample is under vacuum (Prost 1975c). Water mole- cules are not involved in hydrogen bonds with the ox- ygen atoms of the surface, and their plane is perpen-

dicular to the plane o f the structure with 1 proton di- rected to the center of the hexagonal holes. F igure 10 shows the arrangement in the case o f Ca-saponite whose isomorphic substitutions are located in the tet- rahedral sheet o f the layer. Here water molecules have 1 OH group involved in a hydrogen bond with oxygen atoms of the surface belonging to tetrahedra where Si 4§ is replaced by A13§ (Suquet et al. 1977).

The quest ion of whether there is a long-range effect of clay basal surfaces and compensat ing cations on the structure of water still remains an object of controver- sy. Low (1979), analyzing thermodynamic data, ar- r ived at the conclusion that there is a long-range effect o f basal surfaces on the structure of adsorbed water. That conclusion is quest ionable because thermody- namic data should be related to both phases: clay lay- ers and water (Sposito and Prost 1982). Mul la and Low (1983) analyzed IR spectroscopic data of ad- sorbed water and concluded again that a long-range effect o f basal surfaces on the structure of water exists, based on an exponential change in the extinction co- efficient o f adsorbed water. The spectroscopic ap- proach has the advantage, compared to the thermo- dynamic one, of g iv ing information that is specific to adsorbed water; however, the exponential change o f the extinction coefficient of adsorbed water can also be explained by the existence of 2 kinds of water mol- ecules whose extinction coefficients are different.

The discontinuous mode l for adsorbed water, that is to say the existence of 2 kinds o f water molecules , one corresponding to water molecules which hydrate ex- changeable cations and the other adsorbed on the sur- face covered by hydrated cations, is supported by an exper iment per formed in the near-IR range, where the difference of the extinction coefficient o f both kinds of water molecule is much less important (Luck 1973). It was found (Prost 1982) that the spectrum of water adsorbed on Na-hectori te can be decomposed into the spectrum of adsorbed water at a water content o f 0.5 g g-I and the spectrum of bulk water. This exper iment and others per formed with different spectroscopies, for example N M R (Fripiat et al. 1982), lead to the con- clusion that there are no more than 2 or 3 monolayers o f water whose structure is perturbed by the topogra- phy of the surface electric field.

T H E S W E L L I N G A N D S H R I N K A G E P H E N O M E N A

The purpose of this section is to show how the hy- dration and dehydrat ion processes are related to the swell ing and shrinkage phenomena of clay systems.

The Hydrat ion and Dehydrat ion Mechanisms

It was shown in the preceding sections o f this paper that the water retention phenomenon has to be related to 2 mechanisms: adsorption on hydrophil ic sites and on " f ree surfaces" and capillary condensation in

126 Prost, Koutit, Benchara and Huard Clays and Clay Minerals

a

b

/

�9 �9 iiiiiii

. . . . . . .

iiii!i!i: ~c;

_ ~ - ,

( ~ oxygen upper sheet structure

,"~ oxygen lower sheet structure

Q H20 bound (ct) to the upper sheet structure (1~) to the lower sheet structure

0 calcium

Figure 10. Spatial arrangement of water molecules solvating exchangeable Ca 2+ on saponite as determined by XRD and IR spectroscopy, a) section view; b) plan view showing water molecules hydrogen-bonded to oxygen atoms in an upper silicate surface (0 0 and to oxygen atoms in a lower silicate surface (O.) bounding the interlamellar space (Suquet et al. 1977).

pores, wh ich are a consequence of the fabr ic of par- t icles or grains that m a k e the paste. A t the lowes t val- ues of aw adsorp t ion occurs on hydrophi l i c sites (ex- changeab le ca t ions in smect i tes , surface OH groups in oxides . . . . ). In smect i tes , hydra ted ca t ions make pil- lars wh ich expand the layers. This is s t rongly sup- por ted by the increase by steps of the d(001) spac ings of the layers wh ich is specific, for each va lue of aw, to the na ture of the exchangeab le cat ion. Except in a few cases, such as Na, the d (001) spac ing is l imi ted to a round 2 n m in clays that have thei r i somorph ic subs t i tu t ion in the oc tahedra l sheet of the layer and to a round 1.5 n m in clays that have thei r i somorph ic sub- s t i tut ion in the te t rahedra l sheet of the layer, even for the h ighes t water contents . The explana t ion lies in the na ture of specific water--cation--clay s t ructure interac- t ions.

Wi th Ca-clays , pi l lars are m a d e of the cat ions hy- dra ted by 6 or more wate r molecu les (Suquet et al. 1977). In the case o f clays that have thei r i somorph ic subs t i tu t ion in the te t rahedra l sheet of the layers, wate r

molecu les are i nvo lved in hyd rogen bonds wi th oxy- gen a toms be long ing to t e t rahedra where Si is rep laced

by A1 or Fe (Prost 1975a, 1975c; Sposi to and Prost 1982). Hydra ted Ca ions act as snaps b e t w e e n layers, avo id ing a larger expans ion . In the case of clays that have thei r i somorph ic subs t i tu t ion in the oc tahedra l

sheet of the layer, water molecu les tha t hydra te Ca are

no t i nvo lved in hyd rogen bonds and m a y have a dif- ferent or ien ta t ion wi th respect to the lattice, mak ing the comple te hydra t ion of Ca ca t ions easier. As a con-

sequence, pil lars are b igger and induce a larger bu t

a lways l imi ted expans ion . Wi th clays tha t have thei r i somorph ic subst i tu t ions in the oc tahedra l sheet of the

layers and ca t ions that may have more than 2 shel ls

of hydra t ion , such as Na, pi l lars are so big that they

induce an un l imi ted expans ion . Mac roscop ic data con- ce rn ing swel l ing are repor ted and ana lyzed by Low

(1979, 1980). Data ob ta ined at the molecu la r level are needed for a be t te r exp lana t ion of the X- ray and heat

of immers ion resul ts d i scussed by Low.

Vol. 46, No. 2, 1998 Hydration and swelling-shrinkage of clay materials 127

The water molecu les that hydra te cat ions and m a k e pil lars be tween the layers m a y j o i n toge ther and, de- pend ing on the dens i ty of charge, m a y cover the sur- face comple te ly or partial ly. Areas of the surface that are not covered by hydra ted ca t ions are hydrophobic . As aw increases to r each d o m a i n 2 of the F H H plot, adsorp t ion m a y be desc r ibed as a mul t i l ayer adsorp t ion process; that is to say, the adsorp t ion of success ive layers of water on a f i lm of wate r m a d e of the hydra ted hydrophi l i c sites. Th i s mul t i l ayer adsorp t ion process occurs on " f ree su r faces" (walls of unsa tura ted pores or surfaces of s tacked layers, part icles, aggregates that are free to expand) . The mul t i l ayer adsorp t ion process occurs unt i l aw = 1. It does not m e a n necessar i ly tha t the s t ructure o f wate r adsorbed accord ing to this pro- cess is affected by the surface. This pic ture is in agree- m e n t wi th the idea tha t on ly a l imi ted n u m b e r (a round 4) o f layers of wate r adsorbed on clay surfaces are pe r tu rbed by clay surfaces. These first layers of wate r whose s t ructures are pe r tu rbed by the surface corre- spond to wate r that hydra tes hydroph i l i c sites.

The wate r re ten t ion p h e n o m e n o n is due to adsorp- t ion and to capi l lary condensa t ion . Th i s last p h e n o m - enon m a y occur for ve ry low values o f aw at contac t points b e t w e e n par t ic les or aggregates , and for h igher values o f aw in pores that are the resul t o f the fabr ic o f wet par t ic les or aggregates . I f the mater ia l is m a d e of homod i spe r s e grains, the cons t r ic t ion radius of the pores wil l co r re spond to na r row peaks on the pore size d is t r ibut ion cu rve and the emp ty ing of pores wil l oc- cur in a smal l r ange of aw (Figure 4).

Bo th mechan i sms , adsorp t ion and capi l la ry conden- sat ion, may occur in the whole range of the act ivi ty of wate r but each state of water, adsorbed or con- densed, m a y be p r e d o m i n a n t in some par t icular r ange of aw. The F H H plot can be used to assess the amoun t of wate r re ta ined by finely d iv ided mater ia ls , and be- long ing to bo th states. The amoun t o f water re ta ined by adsorp t ion is a func t ion of the surface area where the adsorp t ion m e c h a n i s m occurs, bu t the a m o u n t of wate r re ta ined by capi l la ry condens a t i on is a func t ion o f the shape of the part icles. For example , wel l -dis- pe rsed smect i tes (Na-montmor i l lon i t e s ) have a ve ry low a m o u n t of wate r re ta ined by capi l la ry condensa- t ion. As a consequence , the states and loca t ion of wa- ter are s t rongly re la ted to the concep ts of surface and fabr ic o f par t ic les or aggregates wh ich m a k e the paste.

The Concep t of Sur face

Taking the case o f Ca -mon tmor i l l on i t e as an ex- ample , F igure 1 l a g ives a schemat ic represen ta t ion of the d i f ferent surfaces i nvo lved in water re tent ion. The b e g i n n i n g of the hydra t ion process occurs on Ca cat- ions. The a m o u n t of wate r re ta ined here is a func t ion of the surface charge density. T he surface invo lved is the total surface area St of the clay wh ich can be as-

sessed b y e thy lene glycol adsorp t ion expe r imen t s or by calculat ion.

The ca t ion-water - -c lay structure in te rac t ions in the over lapp ing area of ad jacent par t ic les of C a - m o n t m o - r i l loni te are the same as for in te r lamel la r spaces. Hy- dra ted Ca ca t ions located be tween these ove r l app ing areas act, as they do in the in te r lamel la r spaces, as snaps tha t l imi t expans ion . As a consequence , hydra t - ed Ca-aggrega tes have a r igid porous structure. The l imi ted expans ion of the layers as aw increases impl ies tha t the mul t i l ayer adsorp t ion of wate r occurs for h igh- er values of aw on " f r ee su r faces" . The surface area S~, wh ich has to be cons idered at these low va lues of aw, can then be de t e rmined by the appl ica t ion of the B E T fo rma l i sm to the n i t rogen adsorp t ion i so therm.

As the capi l la ry condensa t ion p h e n o m e n o n has filled these in t raaggrega te pores, adsorp t ion can on ly occur on " f r ee su r faces" w h i c h are the externa l sur- faces of aggregates . This surface area Se is d e t e r m i n e d by hydraul ic conduc t iv i ty m e a s u r e m e n t s (Prost 1990).

Thus 3 k inds of sur face- -S t , S s and Se - - a re i nvo lved to expla in the adsorp t ion o f water. The area of each k ind of surface is de t e rmined b y a specific technique .

The Concep t of Fabr ic

The fabr ic is the way par t ic les and aggregates are a r ranged in the paste. The hydrau l ic conduc t iv i ty mea- su rement s p e r f o r m e d on saturated mater ia l s ( doma in 4 of the F H H plot) give the m e a n hydrau l ic radius of the pores, that is to say, ha l f the m e a n d is tance be- tween " f r e e " par t ic les or aggregates (Figure 1 lb) . The appl ica t ion o f the Pierce fo rma l i sm (Pierce 1953) to the d o m a i n o f the wate r desorp t ion i so the rm that cor- r e sponds to desa tura t ion of the paste (domain 3 of the F H H plot) g ives the pore size d is t r ibut ion c u r v e of wet mater ials . The appl ica t ion of such a fo rma l i sm impl ies the knowledge of the th ickness t of the fi lm of wate r adsorbed on " f ree su r faces" as a func t ion o f aw. The ve ry good cons i s tency of the resul ts ob ta ined wi th A1203 grains us ing bo th t echn iques is o b s e r v e d in Fig- ure 12. It indicates tha t the m e a n d is tance be tween par t ic les decreases l inear ly as a func t ion of wate r con- tent. Our hypo thes i s is tha t the d is tance be tween gra ins at contac t points is equal to twice the th ickness t of the f i lm of wate r adsorbed on externa l surfaces o f par- t icles or aggrega tes (Figure 1 lb) . Thus the th ickness t of the fi lm of water adsorbed on externa l surfaces of par t ic les or aggregates fixes the d i s tance b e t w e e n par- t icles and aggrega tes as a func t ion of a w.

The t -Curve

The t -curve is the plot of the th ickness t of the fi lm of water adsorbed as a func t ion of aw. H a g y m a s s y et al. (1969) have de t e rmined the t -curve for d i f ferent finely d iv ided oxides us ing wate r adsorp t ion i so the rms ob ta ined by the des icca tor m e t h o d (aw < 0.98). Da ta p lo t ted on F igure 13 co r re spond to d o m a i n 2 of the

128 Prost, Koutit, Benchara and Huard

qd e � 9 �9 c o o �9 o e �9 e o o e e � 9 Q o o � 9 o e o o o �9

�9 �9 �9 ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: o o o o o o o o o ======================================================================================= :::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : o :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: o..~

o ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: �9

�9 ::::::::::::::::::::::::::::::::::::::::::::::::: o o o o o 9 0 ~ 1 1 ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

================================================================================================================================================================== �9

Clays and Clay Minerals

,~i �84 !Z:i i~ i : i i /

b

W�9

Figure 11. Schematic representation of the fabric of Ca-smectite pastes, a) Individual aggregate of Ca-smectite showing the different surfaces involved in the water retention phenomenon: the total surface area S t corresponds to surfaces labeled by 0 0 0 , OOO, and -.; the specific surface area S s, determined by nitrogen adsorption, corresponds to surfaces labeled by 0 0 0 and OOO; the external surface area S e of aggregates, determined by hydraulic conductivity, corresponds to surfaces labeled by O e O . b) Arrangement of individual aggregates showing how swelling is induced by the increase of the thickness t of the layer of water adsorbed on the external surfaces Se of aggregates; w�9 corresponds to water located in the interlamellar spaces and in the intraaggregate pores. Vp corresponds to water located in interaggregate pores.

F H H plot where the mult i layer adsorpt ion process is p redominant (Benchara 1991). The mean curve cal- culated f rom these data leads to the fo l lowing equa- tion:

0.24 t (nm) = [3]

(log(1/aw)) ~

The t values calculated for the highest values of aw

with this equat ion are in good agreement with exper- imental data obtained by Derjaguin et al. (1987) using e l l ipsometry for 0.97 < aw < 1. It is not iceable that t- curves obta ined are quite c lose to each other for ma- terials whose densi ty o f surface hydrophi l ic sites is sufficient to cover the surface with a layer of water when these sites are hydrated. It seems that the effect o f the substrate on the mult i layer adsorpt ion process

Vol. 46, No. 2, 1998 Hydration and swelling-shrinkage of clay materials 129

lOO E c

.~ 80 " 0

._~ 6 0

~6

e- 40 0 o

20

/§

§

f 0 t I t I I I I I ! I

0 .0 1.0 2 .0 3.0 4 .0 5.0

Water content ( g / g )

Figure 12. Constriction radii of pores of AI203 samples as a function of water content. Points (+) were obtained by hy- draulic conductivity measurements; (• by the application of the Pierce formalism to the water desorption isotherm; (O) by mercury injection and by the application of the Pierce formalism to the nitrogen desorption isotherm.

is insignificant beyond this first shell of hydration of particles which corresponds to water molecules that hydrate hydrophi l ic sites.

The Swel l ing and Shrinkage Mechanisms

Water is retained by finely divided materials ac- cording to 2 mechanisms: adsorption and capil lary condensation. Water adsorbed at the lowest values o f aw (domain 1) on hydrophil ic sites induces the creation o f pillars be tween clay layers that expand the layers and, eventually, results in the creation of a film of water on surfaces. Swel l ing should be, in that case, related to the total surface area of the clay.

For higher values of aw the mult i layer adsorption process occurs on the film of water that corresponds to the hydration o f hydrophil ic sites. The thickness t of the film of water adsorbed can only increase as a function of aw i f the mult i layer adsorption phenome- non occurs on what Pierce (1960) cal led " f ree sur- faces" (walls o f unsaturated pores or surfaces that may m o v e with respect to each other).

Thus, in the case of Ca-montmori l loni te , the mul- t i layer adsorption process can occur, at the beginning of domain 2, only on surfaces that are accessible to ni trogen (Figure 11). Because the expansion of Ca- smecti te layers with water is l imited to around 1 nm, the surface involved in the swell ing o f such a clay is the external surface area Se of particles or aggregates, which can be determined by hydraulic conduct ivi ty measurements (Prost 1990). So swell ing is due to the mult i layer adsorption process that occurs on the ex- ternal surfaces Se o f particles or aggregates and in- creases the thickness t o f the film of adsorbed water. This mechan ism exists even at the highest values of

a w .

1 . 6 ,

1 , 4 ,

E 1.2.

t~ O9

"~ 1.0

(1.8

0 . 6

0.4

x �9

0 . 2 % ~"

o

x

+

6 §

4 ~ § o z "

~ e . ~

0.0 , : : : : : ', : : : :

0.0 0.2 0.4 0.6 0.8 1.0

A c t i v i t y o f w a t e r

Figure 13. The t-curves obtained with different materials: (A) kaolinite; (+) chrysotile; (*) TiO2; ((>) A1203; (O) silica A200; ( - ) ; silica 280; (0) mean values of t taken from Ha- gymassy et al. (1969). The symbol w is the water content and S s the specific surface area.

During the mult i layer adsorption process, capi l lary condensat ion can occur at the lowest values of a , at contact points of particles or aggregates and at h igher values o f a , in pores whose radii satisfy the Kelvin equation. Capil lary condensat ion develops forces that bring particles or aggregates together (Parker 1986). Capi l lary condensat ion forces are opposi te to adsorp- tion forces. The equi l ibr ium is reached for 1 value o f a , when these forces are equal.

The proposed mechan i sm to explain swell ing or shrinkage was checked in the case of Na-montmor i l - lonites. It was assumed in the case of ve ry well-dis- persed Na-montmori l loni tes that the greatest part o f water fixed by the clay is adsorbed. Indeed it is not possible in these clays to identify domain 3 on the F H H plot (Figure 3c). The contribution of capi l lary condensat ion to water retention is weak, but strong enough to create opposite forces to adsorption forces. The mult i layer adsorption process in wel l -dispersed Na-montmori l loni te occurs on the total surface area of the clay. The amount of water that can be fixed as a function o f a , is calculated by mult iplying the total surface area St by t. It is notable (Figure 14) that the agreement with the exper imental data is quite good. It is not so in the case o f Ca-montmori l loni te , where the external surface area Se o f particles or aggregates is smaller and has to be taken into account for the high values of aw.

C O N C L U S I O N

The retention o f water by clays and finely divided materials is due to adsorption and capi l lary conden-

130 Prost, Koutit, Benchara and Huard Clays and Clay Minerals

0 i

0

�9 . i ~ I . �9 . , .

2o

tO1

18 v

~: 16

o o 14

10

1 2 3 4 5 6 7

pF

Figure 14. Calculated ( - ~ - ) and experimental (Low 1982; Tessier 1984) water contents as a function of pF in the cases of several Na-montmori l loni tes ([S~, + , • *, O) and 1 Ca- montmori l loni te (0 ) .

s a t ion . T h e a d s o r p t i o n c r e a t e s p i l l a r s b e t w e e n c l a y l ay -

e r s a n d a w a t e r f i lm o n e x t e r n a l s u r f a c e s o f p a r t i c l e s

o r a g g r e g a t e s w h i c h i n d u c e t h e i n c r e a s e o f t he s p a c i n g

b e t w e e n c l a y l a y e r s a n d o f t h e d i s t a n c e b e t w e e n par-

t i c l e s o r a g g r e g a t e s . T h i s is t he s w e l l i n g m e c h a n i s m .

A d s o r p t i o n f o r c e s tha t i n d u c e s w e l l i n g a re e q u i l i b r a t e d

b y c a p i l l a r y c o n d e n s a t i o n fo rces .

T h e t - c u r v e s o b t a i n e d fo r w a t e r s e e m to be v a l i d fo r

" h y d r o p h i l i c " m a t e r i a l s a n d c a n b e u s e d to a s s e s s

the i r s w e l l i n g i f t he e x t e r n a l s u r f a c e s Se o f pa r t i c l e s o r

a g g r e g a t e s c a n be d e t e r m i n e d . T h e p r o p o s e d h y p o t h -

e s i s to e x p l a i n s w e l l i n g a n d s h r i n k a g e is d i r ec t l y re-

l a t ed to t h e c o n c e p t o f e x t e r n a l s u r f a c e St o f p a r t i c l e s

o r a g g r e g a t e s . T h i s c o n c e p t u n d e r s c o r e s t he i m p o r -

t a n c e o f all p a r a m e t e r s t ha t a re i n v o l v e d in t he s i ze o f

t h e s e p a r t i c l e s o r a g g r e g a t e s . T h e m o r e i m p o r t a n t o f

t h e s e p a r a m e t e r s is p r o b a b l y , in t h e c a s e o f c l ay s , t h e

c a t i o n - w a t e r - c l a y s t r u c t u r e i n t e r a c t i o n s and , in pa r t i c -

ular, t h e w a y c a t i o n s a n d w a t e r m o l e c u l e s a re a r r a n g e d

b e t w e e n l aye r s , t ha t is to s a y t he s t r u c t u r e o f a d s o r b e d

water . T h e i on i c c o m p o s i t i o n o f t he s o l u t i o n m a y a l so

h a v e a t r e m e n d o u s e f f e c t t h r o u g h t he d i s p e r s i o n o r f l o c c u l a t i o n p r o c e s s it i n d u c e s .

A C K N O W L E D G M E N T S

We thank J. Driard for finalizing the manuscr ipt and the reviewers for their helpful suggestions.

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(Received 24 January 1997; accepted 31 March 1997; Ms. 97-010)