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Coupling analysis of macroscopic and microscopic behaviour in highly consolidated Na-laponite clays
ELSEVIER
Applied Clay Science 11 (1996) 185- 197
Coupling analysis of macroscopic and microscopic behaviour in highly consolidated Na-laponite clays 1 Y. Qi, M. A1-Mukhtar, J.-F. Alcover, F. Bergaya * CNRS, Centre de Recherche sur la Mati~re Divis~e, Universit~ d'Orl~ans, IB, Rue de la F~rollerie, 45071 Orleans, Cedex 2, France
Received 20 November 1995; revised 1 August 1996; accepted 1 August 1996
Abstract The great importance of problems related to the storage of chemical and radioactive wastes requires better understanding of the behaviour of the host media. In the case of clay soils, their behaviour is controlled by their microtexture, their mineral composition and their physico-chemical properties (interaction between interstitial fluid and solid matrix). In this paper, the analysis of the behaviour of consolidated synthetic model clay Na-laponite is presented. Tests are conducted on samples submitted to axial stress for the determination of the distribution of pore space and the type of interstitial water. Analysis is based on data given by different techniques: BET, mercury intrusion, X-ray diffraction and thermogravimetric analysis. The basal spacing d0m and the water content decrease respectively from 2.5 nm to 1.6 nm and from 140% to 63% when the axial mechanical stress increases from 1 to l0 MPa. Results obtained would allow to take into account the relationships between the applied stress on the samples and the basal spacing, the number of water layers and the water content in the formulation of numerical modelling of the macroscopic behaviour. Keywords: Na-laponite; clay; axial stress; BET; XRD; TGA; microstructural parameters
1. Introduction The i m p o r t a n c e o f p r o b l e m s related to the burial o f h i g h - l e v e l radioactive wastes has m o t i v a t e d m a n y scientists to study the b e h a v i o u r o f soils (Pusch, 1982; Baldi et al., 1988; Pusch et al., 1991; H u e c k e l , 1991; G r a h a m et al., 1992). As storage is supposed to
* Corresponding author. Fax: + 33-38633796. J Presented at the Euroclay 95 conference. 0169-1317/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH SO 169- 1317(96)00016-6
Y. Qi et al. / Applied Clay Science 11 (1996) 185 197
186
be more than 500 m below the surface, very high stresses may be encountered. The behaviour of the buffer materials used around the waste packages during the process of saturation also requires attention (Coulon et al., 1987). Clay soils are studied because of their interesting physico-chemical properties which include: permeability, swelling, ions exchange, plasticity, long-term chemical and mineralogical stability and thermal conductivity (Mitchell, 1993). It is recognised that the mechanical and hydraulic properties of clays are mainly controlled by their microstructure, their mineral composition and their physico-chemical properties such as interaction between interstitial fluid and solid matrix (Tessier, 1984). Therefore, two aspects requires attention at the microscopic level: (i) soil fabric studies of saturated material under high stress, and (ii) soil fabric studies of highly consolidated material during saturation and desaturation processes. This study examined the behaviour and the microstructural properties of a synthetic expansive clay (Na-laponite). This material was selected to overcome the current deficiency of in situ samples and the difficulties encountered in obtaining good quality natural samples to carry out tests with an acceptable reproducibility in results. Also, having a material with only one type of clay layer reduces the number of microscopic parameters in the analysis of the behaviour. Studies were carried out on Na-laponite in both saturated and unsaturated hydraulic states. Only the results of tests conducted on saturated material are presented in this paper.
2. Principal properties of the Na-laponite The Na-laponite is a synthetic clay of the smectite family. Its structure and chemical formula are similar to the natural clay hectorite which both are triooctahedral 2 / 1 clay. The chemical composition of the Na-laponite is presented in the Table 1. Its chemical formula is: (Si7.95A10.05) (Mgs.48Lio.36Tio.ol)020(0H)4, Nao.67 K 0.01 •
Table 1 Chemical compositionof the Na-laponite Components SiO 2 MgO Li20
(%) 51.5 23.8 0.58
Na20
2.25
K20 AI20 ~
0.08 0.28
F%O3 TiO2
< 0.05 0.06
Loss of ignition
21.2
187
Y. Qi et aL /Applied Clay Science 11 (1996) 185-197
Table 2 Principal physico-chemical properties of the Na-laponite Unit cell size Clay layer size Specific surface Cation exchange capacity
0.53 nm X 0.92 nrn X 1 nrn 30 nm X 30 nm x 1 nm 373 m 2/g 94 meq/100 g of ignited material
T a b l e 2 presents the principal properties o f Na-Laponite. It forms rapidly at v e r y low c o n c e n t r a t i o n ( > 3 % ) transparent gel. T h e Na-laponite p o w d e r has a white colour.
3. Method and devices 3.1. O e d o m e t e r tests
T h e o e d o m e t e r d e v i c e for the tests is presented in the Fig. 1. O e d o m e t e r tests are c o n d u c t e d at r o o m temperature at about 20°C. S a m p l e d i m e n s i o n s are 65 m m d i a m e t e r
Water pressure placement ~sducer
Fig. 1. Oedometer device for the saturated tests and schematic diagram of the saturation apparatus.
188
Y. Qi et al./Applied Clay Science 11 (1996) 185-197
and 65 m m of maximal initial height. The sample can be loaded with an axial stress of up to 40 M P a using this oedometer device. During the oedometer tests the applied stress and the change in height of the sample due to water extraction (consolidation) are measured. To obtain oedometer curves, two parameters are calculated: (i) the water content which is the mass of water divided by the mass of solid matter of the sample and (ii) the void ratio which is the void volume (equal to the water volume in the saturated samples) divided by the solid volume in the sample. 3.1.1. Saturation and loading procedure The preparation of the saturated samples is conducted in the oedometer device (Fig. 1). Distilled water is used for the saturation of the samples. The principal steps of saturation procedure are the following: - 50 g of Na-laponite powder is introduced in the oedometer. Two ceramic discs previously saturated (approximate pore diameter = 0.16 /xm, coefficient of permeability with respect to water = 2.59- 10-11 m / s and air entry value = 1500 kPa) are used in the upper and the bottom part of the sample with two filter paper Whatman No. 42. These ceramics and filter papers provide an homogeneous distribution of water to the edges of the sample. They are also used to ensure that sample particles do not leak during loading. Water is added to the sample to fill the remaining height of the oedometer (initial water content >_ 250%). The drainage circuit is connected to the upper part of the sample. - Vacuum pressure is applied on the lower part of the sample to increase the velocity of saturation and to eliminate air bubbles in the sample. An axial stress of 0.1 MPa is applied on the sample to assure the saturation procedure. - Water is only allowed to come out of the sample through the bottom of the sample. - The sample is considered to be ready for the test when water starts to flow through the bottom of the sample and its displacement is considered to be negligible as a function of time. The vacuum is then stopped and the bottom of the sample connected to the saturation circuit. - Loads are applied to the sample. The change in height of the sample is recorded as a function of time. More loads are added when this variation becomes negligible. - A water pressure of 0.2 MPa is applied to the sample to assure the saturation during loading and unloading. -
-
-
3.1.2. Samples Four samples were prepared in the oedometer at 0.1, l, 5 and 10 MPa using the saturation procedure described in Section 3.1.1. Another saturated sample was prepared using 3% of Na-laponite in distilled water (dilute suspension which becomes gel, with time). The initial water content of this sample was 3690%. It was consolidated at 1 MPa in the oedometer. Lyophilisation technique (Tessier and Berrier, 1978; Delage et al., 1982) was applied on all the
Y. Qi et al. / Applied Clay Science 11 (1996) 185-197
189
oedometer samples in order to dry the sample and to maintain their texture with minimum changes for the analysed tests 3.2. BET and mercury intrusion tests
Nitrogen adsorption at - 1 9 6 ° C is performed in a Sorptomatic Carlo Erba 1900 apparatus. Theoretically, pore size as low as 1.0 nm can be investigated by this gas. BET tests were conducted with three saturated samples consolidated at 1, 5 and 10 MPa, and with the uncompacted Na-laponite dry powder. Samples have been lyophilised and degassed at 140°C during 16 h before nitrogen adsorption. Mercury intrusion tests were performed using a Poresizer 9320 porosimeter. This apparatus has a maximum loading capacity of 210 MPa on the mercury. Theoretically, pore diameters between 350 /xm and 3.6 nm can be investigated. Practically, only pore spaces having diameters more than 50 nm will be considered with this method. 3.3. TGA ~ s ~
In thermogravimetric analysis (TGA) the physical properties of a substance a n d / o r their reaction products are measured as a function of temperature, while the substance is submitted to a controlled temperature program. Sample mass changes, due to the applied controlled temperature, are recorded automatically using an electrobalance. TGA using a controlled increasing temperature of 2.5°C/min were conducted on the Na-laponite powder and on the three saturated samples consolidated at 1, 5 and 10 MPa. 3.4. XRD tests
X-ray diffraction (XRD) is generally used for the identification and characterisation of clay minerals. It provides knowledge of the type of clay and its degree of crystalline order. XRD tests were conducted on the Na-laponite powder and on oriented samples of the three saturated samples consolidated at 1, 5 and 10 MPa. The basal spacements are measured directly during tests and the number of water layers is estimated from these measurements.
4. Results and discussion 4.1. Oedometer results: macroscopic behaviour of the saturated Na-laponite
Fig. 2 shows the oedometric curves of the saturated Na-laponite consolidated up to 10 MPa during loading and unloading. Loads applied are 0.2, 0.5, 1, 2, 5 and 10 MPa. As the axial stress increases, the void ratio decreases in the sample. Loads applied during unloading are 5, 2, 1, 0.5, 0.4 MPa and atmospheric pressure. Increasing void ratio during unloading displays the swelling behaviour of the Na-laponite.
Y. Qi et al. / Applied Clay Science 11 (1996) 185-197
190
6 5 e
4
~3 2
unloading 1
. 0,1
.
.
.
.
.
1
10
Axial stress (MPa)
Fig. 2. Oedometer curves of the saturated Na-laponite during loading and unloading, void ratio versus axial stress.
The slopes of the curves, void ratio versus logarithm of the axial stress, during loading and unloading are called respectively compression index (C c) and swelling index (Cs). The activity (CJC~) of the Na-laponite is then 0.35. This value demonstrates the high swelling capacity of this laponite in comparison to natural clay soils which generally have an activity less than 0.30 (Bolt, 1956). The comparison between the gel (3%) and the saturated Na-laponite both consolidated at the same axial stress of 1 MPa, shows that water content is identical for the two samples (Fig. 3). Moreover, changes in void ratio due to axial stress in the oedometer is linearly related to the water content of the sample (Fig. 3). These results demonstrate that the saturation procedure for the Na-laponite used in this study is correct as it produces an homogeneous sample. Usually an homogeneous sample is easy to obtain up to 5%, but after this percentage, air bubbles can not be avoided in the prepared samples.
10 --43--SaluralcdNa-Laponltc ---tr--Gelof Na-Laponite ~
8
~
2
MPa"
.~ 6 "el
~4
1MPa
2 10MPa 0 0
50
IO0
150
200
250
300
Water content (%)
Fig. 3. Water content versus void ratio for different axial stresses in saturated and gel of Na-laponite.
191
Y. Qi et al. / A p p l i e d Clay Science 11 (1996) 185-197
Another method which could be used to obtain homogeneous samples at high concentration or with low water content is the imposition of osmotic suction using Dextran (Morvan, 1993). The oedometric method used in this study leads to the same results in a shorter time and is less expensive. 4.2. B E T and mercury intrusion results
BET results show that the pore size distribution is similar for all consolidated samples (Fig. 4). However, at low loading (1 MPa), bimodal distribution occurred at 0.9 and 1.1 nm. As loading increases, a homogeneous distribution is observed at about 1.0 nm. The comparison of BET results in the consolidated samples with respect to the uncompacted sample (powder) of Na-laponite shows that the mechanical loading: reduces the extent of pore size distribution, increases the BET specific surfaces (about 500 mZ/g for the consolidated samples and 373 mZ/g for the laponite powder) and • increases the total pore volume (about 0.42 cm3/g for the consolidated samples and 0.36 cm3/g for the laponite powder). Meanwhile, it is observed that in the consolidated samples irrespective of the applied oedometric loading both BET specific surface and total pore volume are identical. These
l!~x
"
j'/ II
10 MPa
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iI
",%
"~
t
"',.
" ,,
.
.
.
.
.
.
I1
1 lUn
"'- - ~
'
J
Unconsolidated
. . . .
I
10
1
nm
.
.
.
.
100 nm
Fig. 4. Pore size distribution from BET tests in the uncompacted (powder) sample and in samples consolidated at 1, 5 and 10 MPa.
192
Y.
Qi et al. / Applied Clay Science 11 (1996) 185-197
0,035 --
0,03
Appliedstress = 1 MPa
---o--Appliedstress= 10 MPa 0,025 =~ __=
0,02
o,o15 0,01 0,005 0 . O,OOl
.
.
0,01
.
.
.
O,1
.
.
1
. 10
.
100
1000
Entrance pore diameter (lure) Fig. 5. Pore size distribution from mercury intrusion tests in two samples consolidated at 1 and 10 M P a .
results reveal insignificant modification of the microstructure characteristics as given by this technique between 1 MPa and 10 MPa. Fig. 5 shows the entrance pore diameter as a function of incremental pore volume during mercury intrusion tests for two samples consolidated at 1 and 10 MPa. Two distinguished entrance macropore diameters at 4 and 10 /xm are viewed in the sample consolidated at 1 MPa. This bimodal distribution is eliminated when loading reached 10 MPa. Table 3 summarises BET and porosimetric results obtained for two samples consolidated at 1 and 10 MPa. The effect of applied loading reduces particularly macropores measured by mercury intrusion tests ( > 50 nm) whereas the micropores ( < 2 nm) and the mesopores (between 2 and 50 nm) both measured by BET are unchanged. 4.3. XRD results
X-ray diffraction diagrams of Na-laponite samples consolidated at 10 MPa cut in two directions, perpendicular and parallel to the axial stress, are shown in Fig. 6. XRD results show that
Table 3 Effect of applied loading on the pore size of Na-laponite Pore space
Pore diameter
Measuring technique for sample consolidated at 1 M P a (cm 3/g)
Pore volume for sample consolidated at 10 M P a (cm 3/g)
Pore volume
Micropore Mesopore Macropore
< 2.0 n m b e t w e e n 2.0 a n d 50 n m > 50 nm
BET BET mercury intrusion
0.20 0.22 0.27
0.19 0.23 0.02
193
Y. Qi et al./Applied Clay Science 11 (1996) 185-197 F
001
005 ~,~
(~ A
~003
. ~A / ] \ k~
3N,,.~" N, (02,11) ~ V
_
8
~)31.,02( ~
~
l
,
(a) J_
24,2 15)6 (060) ~ ~
- (40~
2-Theta - Scale
Fig. 6.
X-ray diffraction diagrams: (a) sample perpendicular to the axial stress and (b) sample parallel to the axial stress.
• particles are oriented, as expected, in the perpendicular direction of the applied stress, • wide peaks and a nearly rational order indicate an interstratified material, composed by the packing of clay layers and various water layers. Fig. 7 shows the basal spacing (d00 ~) and the evaluated numbers of water layers between adjacent clay layers in samples consolidated at 1, 5 and 10 MPa. The number of water layers is calculated from d0o 1. In this calculation the thickness of the Na-laponite layer is considered to be about 1.0 nm and that of water is 0.30 nm. The average values of water layers are 5, 3 and 2 under the applied axial stress of 1, 5 and 10 MPa respectively. Linear logarithmic relationships can be found between the axial stress applied ( ~ ) to 2,5 ~
5 -4
2
~
&
m "-
2
.¢t 1
1
0 Axial stress (MPa)
Fig. 7. Basal
Z
10
spacing d001 and number of water layers in samples consolidated at
1, 5 and 10 MPa.
194
Qi et al. /Applied Clay Science 11 (1996) 185-197
Y.
70 60
t,,i,
! ....
I
50 -
I ........
:' -i . . . . . ~ .... II i '' ...... ,f ' t i i
, , i J i
i
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::
,
i ...........
~ ..... ~ !
i - i .......
i
i
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! ....
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!'~-~
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- ..... i ~]
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--
i
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i
i :
I0 M P a
i
o.~
i
0.5
0.1MPa !
0.4
!
0.2 2O
0
"'
,N-.,-,..-_ ....
0
100
200
' .... 300
J ....
~.
400
500
~
,~AT"~..~...u_~_ 600
700
0 800
Temperature(%) F i g . 8. C o m p a r i s o n
of TGA
results for t w o s a m p l e s c o n s o l i d a t e d at 0.1 and 1 0 M P a .
the sample with the basal spacing (d00 ~) and with the number of water layers. These approximate relations are: Basal spacing ( d00 , ) in nm = - 0.35 In(~ra) + 2.5, Number of water layers = - 1.1 ln(~ra) + 5.
4.4. TGA results Fig. 8 shows the comparison of thermogravimetric analysis (TGA) results for two samples consolidated at 0.1 and 10 MPa. Data show an important peak which is related to the departure of both interparticle water and interlayer water. This peak is located at about 60°C. Another (smaller) peak at 650°C related to the dehydroxylation of the Na-laponite (structural water) can also be observed in this figure. Table 4 presents the results obtained for the samples consolidated at 1, 5 and 10 MPa.
Table 4 R e s u l t s o f T G A t e s t s o n N a - l a p o n i t e s a m p l e s c o n s o l i d a t e d at 1, 5, 1 0 M P a Consolidated sample (MPa)
Total w a t e r content a (%)
Linked water b (%)
Structural water c (%)
1
140
5
5
5
88
4
5
10
64
3
5
W a t e r loss b e t w e e n 2 0 ° C and 8 0 0 ° C 150°C and 6 0 0 ° C c W a t e r loss b e t w e e n 6 0 0 ° C and 8 0 0 ° C b W a t e r loss b e t w e e n
Y. Qi et al./Applied Clay Science 11 (1996) 185-197
195
25O •" 4 200
-~
~
~
-- Saturated •
150
~
-~.
i
i ill
Na-Laponite
Gel of Na-Laponite
~...
100
50
0
i
,
i
i
0,1
i
i
i
1 Axial
stress
i
i ii 10
(MPa)
Fig. 9. Water loss from TGA in samples consolidated at 0.1, 1, 5, 10 MPa and a gel sample consolidated at 1 MPa.
TGA results show that the extraction of water at more than 150°C (8 to 10%) is practically not affected by the applied stresses. Water between 150°C and 600°C is strongly linked to the interlayer cation or trapped in the hexagonal cavities of the silicate sheet. Fig. 9 presents the water loss in these samples as a function of the logarithm of the applied axial stress. A linear relationship can be defined to represent these data o~% = -25.61n(o-,) + 119, where w is the water content of the sample. 4.5. Evaluation o f water ratio as a function o f applied stress
Mechanical stress acts on interparticle and interlamellar spaces. As the applied stress increases from 1 to 10 MPa, the basal spacing (d001) decreases from 2.5 to 1.6 nm and water content decreases from 140 to 63%. Considering the water in the interlamellar spaces as an internal water having a density equal to that of liquid water, two types of water can be evaluated, internal and external water (Qi, 1996). Table 5 presents the results obtained for the samples consolidated at 1, 5 and 10 MPa. These results show that the external water is always higher than that of internal water, in all consolidated samples. The same observations were shown in previous papers with alcohols adsorbed by smectites (Annabi-Bergaya et al., 1979) and with water adsorbed by hectorite (Annabi-Bergaya et al., 1996). Approximately, the same proportion of internal and external water is simultaneously extracted at each applied stress.
Table 5 Water ratio in Na-Laponite consolidated at 1, 5, 10 MPa Consolidated sample (MPa)
Total water content (%)
Internal water (%)
External water (%)
1 5 10
135 78 61
59 36 24
76 42 37
196
Y. Qi et al. / Applied Clay Science 11 (1996) 185 197
5. Conclusion
The determination of microstructural parameters of clays is a fundamental key to the understanding of mechanisms controlling their macroscopic behaviour. Numerous techniques are used in this study to achieve this goal. Results obtained demonstrate clearly the dependence between macroscopic behaviour and the microstructural parameters of Na-laponite. As the applied loading increases, the basal spacing d00 ~ decreases and the macropores reduce particularly while the micropores and the mesopores remain unchanged. In the consolidated samples, internal and external water amounts seem to be extruded in the same proportion as the applied stresses increased. Relationships between the applied stress on the samples and microscopic parameters measured by XRD (basal spacing and number of water layers) or TGA (water content) can be defined. Results obtained appear promising because it (i) leads to a better understanding of the effect of applied stress on consolidated clays and (ii) improves the expression of the numerical modelling of such systems by the introduction of the relationships between macroscopic and microscopic variables.
References Annabi-Bergaya, F., Cruz, A.M., Gatineau, L. and Fripiat, J.J., 1979. Adsorption of alcohols by smectites. I. Distinction between internal and external surfaces. Clay Miner., 14: 249-258. Annabi-Bergaya, F., Estrade-Swarckopf, H. and Van Damme, H , 1996. Dehydration of Cu-hectorite: Water isotherm, XRD and EPR studies. J. Phys. Chem., 100: 4120-4126. Baldi, G., Hueckel, T. and Pellegrini, R., 1988. Thermal volume changes of mineral-water system in low porosity clay soils. Can. Geo. J., 5, 25(4): 807-825. Bolt, G., 1956. Physico-chemical analysis of the compressibility of pure clays. Geotechnique, 6(2): 86-93. Coulon, H., Lajudie, A., Debrabrant, P., Atabek, R., Jorda, M. et Andr~-Jehan, R., 1987. Choice of french clays as engineered barrier components for waste disposal. Mater. Res. Symp. Proc, 84: 813-824. Delage, P., Tessier, D. and Marcel-Audiguier, M., 1982. Use of the Cryoscan apparatus for observation of freeze-fractured planes of a sensitive Quebec clay in scanning electron microscopy. Can. Geo. J., 19: 111-114. Graham, J., Oswell, J.M. and Gray, M.N., 1992. The effective stress concept in saturated sand-clay buffer. Can. Geo. J., 29: 1033-1043. Hueckel, T., 1991. On the nature of water-mineral interactions in saturated clays. Int. Workshop Stress Partitioning in Engineering Clay Barriers, Duke Univ. Mitchell, J.K., 1993. Fundamentals of Soil Mechanics. Wiley, New York, NY, 2nd ed., 422 pp. Morvan, M., 1993. Macrostructure des Syst~mes Smectite-Eau. Influence de Poly~lectrolytes Anioniques sur l'Organisation de Suspensions de Montmorillonite. Th~se de Doctorat Univ. Paris VI. Pusch, R., 1982. Mineral-water interactions and their influence on the physical behavior of highly compacted Na-Bentonite. Can. Geotechn. J., 19: 381-387, Pusch, R., Karnland, O. and H~kmark, H., 1991. The nature of expanding clays as exemplified by the multifaced smectite mineral montmorillonite. Int. Workshop Stress Partitioning in Engineering Clay Barriers, Duke Univ. Qi, Y., 1996. Comportement Hydro-m~canique des Argiles: Couplage des Propri~t~s Micro-macroscopiques de la Laponite et de l'Hectorite. Th~se de Doctorat, Univ. Orleans.
Y. Qi et al. /Applied Clay Science 11 (1996) 185-197
197
Tessier, D., 1984. Etude Exp6rimentale de l'Organisation des Mat6riaux Argileux. Th~se de Doctorat d'Etat, Univ. Paris VII. Tessier, D. and Berrier, J., 1978. Observation d'argiles hydrat6es en microscopie 61ectronique h balayage: importance et choix de la technique de pr6paration. Proc. 5th Int. Working-meet. Soil Micromorphology, Granada, pp. 117-135.
Applied Clay Science 11 (1996) 185- 197
Coupling analysis of macroscopic and microscopic behaviour in highly consolidated Na-laponite clays 1 Y. Qi, M. A1-Mukhtar, J.-F. Alcover, F. Bergaya * CNRS, Centre de Recherche sur la Mati~re Divis~e, Universit~ d'Orl~ans, IB, Rue de la F~rollerie, 45071 Orleans, Cedex 2, France
Received 20 November 1995; revised 1 August 1996; accepted 1 August 1996
Abstract The great importance of problems related to the storage of chemical and radioactive wastes requires better understanding of the behaviour of the host media. In the case of clay soils, their behaviour is controlled by their microtexture, their mineral composition and their physico-chemical properties (interaction between interstitial fluid and solid matrix). In this paper, the analysis of the behaviour of consolidated synthetic model clay Na-laponite is presented. Tests are conducted on samples submitted to axial stress for the determination of the distribution of pore space and the type of interstitial water. Analysis is based on data given by different techniques: BET, mercury intrusion, X-ray diffraction and thermogravimetric analysis. The basal spacing d0m and the water content decrease respectively from 2.5 nm to 1.6 nm and from 140% to 63% when the axial mechanical stress increases from 1 to l0 MPa. Results obtained would allow to take into account the relationships between the applied stress on the samples and the basal spacing, the number of water layers and the water content in the formulation of numerical modelling of the macroscopic behaviour. Keywords: Na-laponite; clay; axial stress; BET; XRD; TGA; microstructural parameters
1. Introduction The i m p o r t a n c e o f p r o b l e m s related to the burial o f h i g h - l e v e l radioactive wastes has m o t i v a t e d m a n y scientists to study the b e h a v i o u r o f soils (Pusch, 1982; Baldi et al., 1988; Pusch et al., 1991; H u e c k e l , 1991; G r a h a m et al., 1992). As storage is supposed to
* Corresponding author. Fax: + 33-38633796. J Presented at the Euroclay 95 conference. 0169-1317/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH SO 169- 1317(96)00016-6
Y. Qi et al. / Applied Clay Science 11 (1996) 185 197
186
be more than 500 m below the surface, very high stresses may be encountered. The behaviour of the buffer materials used around the waste packages during the process of saturation also requires attention (Coulon et al., 1987). Clay soils are studied because of their interesting physico-chemical properties which include: permeability, swelling, ions exchange, plasticity, long-term chemical and mineralogical stability and thermal conductivity (Mitchell, 1993). It is recognised that the mechanical and hydraulic properties of clays are mainly controlled by their microstructure, their mineral composition and their physico-chemical properties such as interaction between interstitial fluid and solid matrix (Tessier, 1984). Therefore, two aspects requires attention at the microscopic level: (i) soil fabric studies of saturated material under high stress, and (ii) soil fabric studies of highly consolidated material during saturation and desaturation processes. This study examined the behaviour and the microstructural properties of a synthetic expansive clay (Na-laponite). This material was selected to overcome the current deficiency of in situ samples and the difficulties encountered in obtaining good quality natural samples to carry out tests with an acceptable reproducibility in results. Also, having a material with only one type of clay layer reduces the number of microscopic parameters in the analysis of the behaviour. Studies were carried out on Na-laponite in both saturated and unsaturated hydraulic states. Only the results of tests conducted on saturated material are presented in this paper.
2. Principal properties of the Na-laponite The Na-laponite is a synthetic clay of the smectite family. Its structure and chemical formula are similar to the natural clay hectorite which both are triooctahedral 2 / 1 clay. The chemical composition of the Na-laponite is presented in the Table 1. Its chemical formula is: (Si7.95A10.05) (Mgs.48Lio.36Tio.ol)020(0H)4, Nao.67 K 0.01 •
Table 1 Chemical compositionof the Na-laponite Components SiO 2 MgO Li20
(%) 51.5 23.8 0.58
Na20
2.25
K20 AI20 ~
0.08 0.28
F%O3 TiO2
< 0.05 0.06
Loss of ignition
21.2
187
Y. Qi et aL /Applied Clay Science 11 (1996) 185-197
Table 2 Principal physico-chemical properties of the Na-laponite Unit cell size Clay layer size Specific surface Cation exchange capacity
0.53 nm X 0.92 nrn X 1 nrn 30 nm X 30 nm x 1 nm 373 m 2/g 94 meq/100 g of ignited material
T a b l e 2 presents the principal properties o f Na-Laponite. It forms rapidly at v e r y low c o n c e n t r a t i o n ( > 3 % ) transparent gel. T h e Na-laponite p o w d e r has a white colour.
3. Method and devices 3.1. O e d o m e t e r tests
T h e o e d o m e t e r d e v i c e for the tests is presented in the Fig. 1. O e d o m e t e r tests are c o n d u c t e d at r o o m temperature at about 20°C. S a m p l e d i m e n s i o n s are 65 m m d i a m e t e r
Water pressure placement ~sducer
Fig. 1. Oedometer device for the saturated tests and schematic diagram of the saturation apparatus.
188
Y. Qi et al./Applied Clay Science 11 (1996) 185-197
and 65 m m of maximal initial height. The sample can be loaded with an axial stress of up to 40 M P a using this oedometer device. During the oedometer tests the applied stress and the change in height of the sample due to water extraction (consolidation) are measured. To obtain oedometer curves, two parameters are calculated: (i) the water content which is the mass of water divided by the mass of solid matter of the sample and (ii) the void ratio which is the void volume (equal to the water volume in the saturated samples) divided by the solid volume in the sample. 3.1.1. Saturation and loading procedure The preparation of the saturated samples is conducted in the oedometer device (Fig. 1). Distilled water is used for the saturation of the samples. The principal steps of saturation procedure are the following: - 50 g of Na-laponite powder is introduced in the oedometer. Two ceramic discs previously saturated (approximate pore diameter = 0.16 /xm, coefficient of permeability with respect to water = 2.59- 10-11 m / s and air entry value = 1500 kPa) are used in the upper and the bottom part of the sample with two filter paper Whatman No. 42. These ceramics and filter papers provide an homogeneous distribution of water to the edges of the sample. They are also used to ensure that sample particles do not leak during loading. Water is added to the sample to fill the remaining height of the oedometer (initial water content >_ 250%). The drainage circuit is connected to the upper part of the sample. - Vacuum pressure is applied on the lower part of the sample to increase the velocity of saturation and to eliminate air bubbles in the sample. An axial stress of 0.1 MPa is applied on the sample to assure the saturation procedure. - Water is only allowed to come out of the sample through the bottom of the sample. - The sample is considered to be ready for the test when water starts to flow through the bottom of the sample and its displacement is considered to be negligible as a function of time. The vacuum is then stopped and the bottom of the sample connected to the saturation circuit. - Loads are applied to the sample. The change in height of the sample is recorded as a function of time. More loads are added when this variation becomes negligible. - A water pressure of 0.2 MPa is applied to the sample to assure the saturation during loading and unloading. -
-
-
3.1.2. Samples Four samples were prepared in the oedometer at 0.1, l, 5 and 10 MPa using the saturation procedure described in Section 3.1.1. Another saturated sample was prepared using 3% of Na-laponite in distilled water (dilute suspension which becomes gel, with time). The initial water content of this sample was 3690%. It was consolidated at 1 MPa in the oedometer. Lyophilisation technique (Tessier and Berrier, 1978; Delage et al., 1982) was applied on all the
Y. Qi et al. / Applied Clay Science 11 (1996) 185-197
189
oedometer samples in order to dry the sample and to maintain their texture with minimum changes for the analysed tests 3.2. BET and mercury intrusion tests
Nitrogen adsorption at - 1 9 6 ° C is performed in a Sorptomatic Carlo Erba 1900 apparatus. Theoretically, pore size as low as 1.0 nm can be investigated by this gas. BET tests were conducted with three saturated samples consolidated at 1, 5 and 10 MPa, and with the uncompacted Na-laponite dry powder. Samples have been lyophilised and degassed at 140°C during 16 h before nitrogen adsorption. Mercury intrusion tests were performed using a Poresizer 9320 porosimeter. This apparatus has a maximum loading capacity of 210 MPa on the mercury. Theoretically, pore diameters between 350 /xm and 3.6 nm can be investigated. Practically, only pore spaces having diameters more than 50 nm will be considered with this method. 3.3. TGA ~ s ~
In thermogravimetric analysis (TGA) the physical properties of a substance a n d / o r their reaction products are measured as a function of temperature, while the substance is submitted to a controlled temperature program. Sample mass changes, due to the applied controlled temperature, are recorded automatically using an electrobalance. TGA using a controlled increasing temperature of 2.5°C/min were conducted on the Na-laponite powder and on the three saturated samples consolidated at 1, 5 and 10 MPa. 3.4. XRD tests
X-ray diffraction (XRD) is generally used for the identification and characterisation of clay minerals. It provides knowledge of the type of clay and its degree of crystalline order. XRD tests were conducted on the Na-laponite powder and on oriented samples of the three saturated samples consolidated at 1, 5 and 10 MPa. The basal spacements are measured directly during tests and the number of water layers is estimated from these measurements.
4. Results and discussion 4.1. Oedometer results: macroscopic behaviour of the saturated Na-laponite
Fig. 2 shows the oedometric curves of the saturated Na-laponite consolidated up to 10 MPa during loading and unloading. Loads applied are 0.2, 0.5, 1, 2, 5 and 10 MPa. As the axial stress increases, the void ratio decreases in the sample. Loads applied during unloading are 5, 2, 1, 0.5, 0.4 MPa and atmospheric pressure. Increasing void ratio during unloading displays the swelling behaviour of the Na-laponite.
Y. Qi et al. / Applied Clay Science 11 (1996) 185-197
190
6 5 e
4
~3 2
unloading 1
. 0,1
.
.
.
.
.
1
10
Axial stress (MPa)
Fig. 2. Oedometer curves of the saturated Na-laponite during loading and unloading, void ratio versus axial stress.
The slopes of the curves, void ratio versus logarithm of the axial stress, during loading and unloading are called respectively compression index (C c) and swelling index (Cs). The activity (CJC~) of the Na-laponite is then 0.35. This value demonstrates the high swelling capacity of this laponite in comparison to natural clay soils which generally have an activity less than 0.30 (Bolt, 1956). The comparison between the gel (3%) and the saturated Na-laponite both consolidated at the same axial stress of 1 MPa, shows that water content is identical for the two samples (Fig. 3). Moreover, changes in void ratio due to axial stress in the oedometer is linearly related to the water content of the sample (Fig. 3). These results demonstrate that the saturation procedure for the Na-laponite used in this study is correct as it produces an homogeneous sample. Usually an homogeneous sample is easy to obtain up to 5%, but after this percentage, air bubbles can not be avoided in the prepared samples.
10 --43--SaluralcdNa-Laponltc ---tr--Gelof Na-Laponite ~
8
~
2
MPa"
.~ 6 "el
~4
1MPa
2 10MPa 0 0
50
IO0
150
200
250
300
Water content (%)
Fig. 3. Water content versus void ratio for different axial stresses in saturated and gel of Na-laponite.
191
Y. Qi et al. / A p p l i e d Clay Science 11 (1996) 185-197
Another method which could be used to obtain homogeneous samples at high concentration or with low water content is the imposition of osmotic suction using Dextran (Morvan, 1993). The oedometric method used in this study leads to the same results in a shorter time and is less expensive. 4.2. B E T and mercury intrusion results
BET results show that the pore size distribution is similar for all consolidated samples (Fig. 4). However, at low loading (1 MPa), bimodal distribution occurred at 0.9 and 1.1 nm. As loading increases, a homogeneous distribution is observed at about 1.0 nm. The comparison of BET results in the consolidated samples with respect to the uncompacted sample (powder) of Na-laponite shows that the mechanical loading: reduces the extent of pore size distribution, increases the BET specific surfaces (about 500 mZ/g for the consolidated samples and 373 mZ/g for the laponite powder) and • increases the total pore volume (about 0.42 cm3/g for the consolidated samples and 0.36 cm3/g for the laponite powder). Meanwhile, it is observed that in the consolidated samples irrespective of the applied oedometric loading both BET specific surface and total pore volume are identical. These
l!~x
"
j'/ II
10 MPa
\ " "\
iI
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t
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.
.
.
.
.
.
I1
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'
J
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. . . .
I
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1
nm
.
.
.
.
100 nm
Fig. 4. Pore size distribution from BET tests in the uncompacted (powder) sample and in samples consolidated at 1, 5 and 10 MPa.
192
Y.
Qi et al. / Applied Clay Science 11 (1996) 185-197
0,035 --
0,03
Appliedstress = 1 MPa
---o--Appliedstress= 10 MPa 0,025 =~ __=
0,02
o,o15 0,01 0,005 0 . O,OOl
.
.
0,01
.
.
.
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.
.
1
. 10
.
100
1000
Entrance pore diameter (lure) Fig. 5. Pore size distribution from mercury intrusion tests in two samples consolidated at 1 and 10 M P a .
results reveal insignificant modification of the microstructure characteristics as given by this technique between 1 MPa and 10 MPa. Fig. 5 shows the entrance pore diameter as a function of incremental pore volume during mercury intrusion tests for two samples consolidated at 1 and 10 MPa. Two distinguished entrance macropore diameters at 4 and 10 /xm are viewed in the sample consolidated at 1 MPa. This bimodal distribution is eliminated when loading reached 10 MPa. Table 3 summarises BET and porosimetric results obtained for two samples consolidated at 1 and 10 MPa. The effect of applied loading reduces particularly macropores measured by mercury intrusion tests ( > 50 nm) whereas the micropores ( < 2 nm) and the mesopores (between 2 and 50 nm) both measured by BET are unchanged. 4.3. XRD results
X-ray diffraction diagrams of Na-laponite samples consolidated at 10 MPa cut in two directions, perpendicular and parallel to the axial stress, are shown in Fig. 6. XRD results show that
Table 3 Effect of applied loading on the pore size of Na-laponite Pore space
Pore diameter
Measuring technique for sample consolidated at 1 M P a (cm 3/g)
Pore volume for sample consolidated at 10 M P a (cm 3/g)
Pore volume
Micropore Mesopore Macropore
< 2.0 n m b e t w e e n 2.0 a n d 50 n m > 50 nm
BET BET mercury intrusion
0.20 0.22 0.27
0.19 0.23 0.02
193
Y. Qi et al./Applied Clay Science 11 (1996) 185-197 F
001
005 ~,~
(~ A
~003
. ~A / ] \ k~
3N,,.~" N, (02,11) ~ V
_
8
~)31.,02( ~
~
l
,
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24,2 15)6 (060) ~ ~
- (40~
2-Theta - Scale
Fig. 6.
X-ray diffraction diagrams: (a) sample perpendicular to the axial stress and (b) sample parallel to the axial stress.
• particles are oriented, as expected, in the perpendicular direction of the applied stress, • wide peaks and a nearly rational order indicate an interstratified material, composed by the packing of clay layers and various water layers. Fig. 7 shows the basal spacing (d00 ~) and the evaluated numbers of water layers between adjacent clay layers in samples consolidated at 1, 5 and 10 MPa. The number of water layers is calculated from d0o 1. In this calculation the thickness of the Na-laponite layer is considered to be about 1.0 nm and that of water is 0.30 nm. The average values of water layers are 5, 3 and 2 under the applied axial stress of 1, 5 and 10 MPa respectively. Linear logarithmic relationships can be found between the axial stress applied ( ~ ) to 2,5 ~
5 -4
2
~
&
m "-
2
.¢t 1
1
0 Axial stress (MPa)
Fig. 7. Basal
Z
10
spacing d001 and number of water layers in samples consolidated at
1, 5 and 10 MPa.
194
Qi et al. /Applied Clay Science 11 (1996) 185-197
Y.
70 60
t,,i,
! ....
I
50 -
I ........
:' -i . . . . . ~ .... II i '' ...... ,f ' t i i
, , i J i
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,
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i
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--
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i
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0
100
200
' .... 300
J ....
~.
400
500
~
,~AT"~..~...u_~_ 600
700
0 800
Temperature(%) F i g . 8. C o m p a r i s o n
of TGA
results for t w o s a m p l e s c o n s o l i d a t e d at 0.1 and 1 0 M P a .
the sample with the basal spacing (d00 ~) and with the number of water layers. These approximate relations are: Basal spacing ( d00 , ) in nm = - 0.35 In(~ra) + 2.5, Number of water layers = - 1.1 ln(~ra) + 5.
4.4. TGA results Fig. 8 shows the comparison of thermogravimetric analysis (TGA) results for two samples consolidated at 0.1 and 10 MPa. Data show an important peak which is related to the departure of both interparticle water and interlayer water. This peak is located at about 60°C. Another (smaller) peak at 650°C related to the dehydroxylation of the Na-laponite (structural water) can also be observed in this figure. Table 4 presents the results obtained for the samples consolidated at 1, 5 and 10 MPa.
Table 4 R e s u l t s o f T G A t e s t s o n N a - l a p o n i t e s a m p l e s c o n s o l i d a t e d at 1, 5, 1 0 M P a Consolidated sample (MPa)
Total w a t e r content a (%)
Linked water b (%)
Structural water c (%)
1
140
5
5
5
88
4
5
10
64
3
5
W a t e r loss b e t w e e n 2 0 ° C and 8 0 0 ° C 150°C and 6 0 0 ° C c W a t e r loss b e t w e e n 6 0 0 ° C and 8 0 0 ° C b W a t e r loss b e t w e e n
Y. Qi et al./Applied Clay Science 11 (1996) 185-197
195
25O •" 4 200
-~
~
~
-- Saturated •
150
~
-~.
i
i ill
Na-Laponite
Gel of Na-Laponite
~...
100
50
0
i
,
i
i
0,1
i
i
i
1 Axial
stress
i
i ii 10
(MPa)
Fig. 9. Water loss from TGA in samples consolidated at 0.1, 1, 5, 10 MPa and a gel sample consolidated at 1 MPa.
TGA results show that the extraction of water at more than 150°C (8 to 10%) is practically not affected by the applied stresses. Water between 150°C and 600°C is strongly linked to the interlayer cation or trapped in the hexagonal cavities of the silicate sheet. Fig. 9 presents the water loss in these samples as a function of the logarithm of the applied axial stress. A linear relationship can be defined to represent these data o~% = -25.61n(o-,) + 119, where w is the water content of the sample. 4.5. Evaluation o f water ratio as a function o f applied stress
Mechanical stress acts on interparticle and interlamellar spaces. As the applied stress increases from 1 to 10 MPa, the basal spacing (d001) decreases from 2.5 to 1.6 nm and water content decreases from 140 to 63%. Considering the water in the interlamellar spaces as an internal water having a density equal to that of liquid water, two types of water can be evaluated, internal and external water (Qi, 1996). Table 5 presents the results obtained for the samples consolidated at 1, 5 and 10 MPa. These results show that the external water is always higher than that of internal water, in all consolidated samples. The same observations were shown in previous papers with alcohols adsorbed by smectites (Annabi-Bergaya et al., 1979) and with water adsorbed by hectorite (Annabi-Bergaya et al., 1996). Approximately, the same proportion of internal and external water is simultaneously extracted at each applied stress.
Table 5 Water ratio in Na-Laponite consolidated at 1, 5, 10 MPa Consolidated sample (MPa)
Total water content (%)
Internal water (%)
External water (%)
1 5 10
135 78 61
59 36 24
76 42 37
196
Y. Qi et al. / Applied Clay Science 11 (1996) 185 197
5. Conclusion
The determination of microstructural parameters of clays is a fundamental key to the understanding of mechanisms controlling their macroscopic behaviour. Numerous techniques are used in this study to achieve this goal. Results obtained demonstrate clearly the dependence between macroscopic behaviour and the microstructural parameters of Na-laponite. As the applied loading increases, the basal spacing d00 ~ decreases and the macropores reduce particularly while the micropores and the mesopores remain unchanged. In the consolidated samples, internal and external water amounts seem to be extruded in the same proportion as the applied stresses increased. Relationships between the applied stress on the samples and microscopic parameters measured by XRD (basal spacing and number of water layers) or TGA (water content) can be defined. Results obtained appear promising because it (i) leads to a better understanding of the effect of applied stress on consolidated clays and (ii) improves the expression of the numerical modelling of such systems by the introduction of the relationships between macroscopic and microscopic variables.
References Annabi-Bergaya, F., Cruz, A.M., Gatineau, L. and Fripiat, J.J., 1979. Adsorption of alcohols by smectites. I. Distinction between internal and external surfaces. Clay Miner., 14: 249-258. Annabi-Bergaya, F., Estrade-Swarckopf, H. and Van Damme, H , 1996. Dehydration of Cu-hectorite: Water isotherm, XRD and EPR studies. J. Phys. Chem., 100: 4120-4126. Baldi, G., Hueckel, T. and Pellegrini, R., 1988. Thermal volume changes of mineral-water system in low porosity clay soils. Can. Geo. J., 5, 25(4): 807-825. Bolt, G., 1956. Physico-chemical analysis of the compressibility of pure clays. Geotechnique, 6(2): 86-93. Coulon, H., Lajudie, A., Debrabrant, P., Atabek, R., Jorda, M. et Andr~-Jehan, R., 1987. Choice of french clays as engineered barrier components for waste disposal. Mater. Res. Symp. Proc, 84: 813-824. Delage, P., Tessier, D. and Marcel-Audiguier, M., 1982. Use of the Cryoscan apparatus for observation of freeze-fractured planes of a sensitive Quebec clay in scanning electron microscopy. Can. Geo. J., 19: 111-114. Graham, J., Oswell, J.M. and Gray, M.N., 1992. The effective stress concept in saturated sand-clay buffer. Can. Geo. J., 29: 1033-1043. Hueckel, T., 1991. On the nature of water-mineral interactions in saturated clays. Int. Workshop Stress Partitioning in Engineering Clay Barriers, Duke Univ. Mitchell, J.K., 1993. Fundamentals of Soil Mechanics. Wiley, New York, NY, 2nd ed., 422 pp. Morvan, M., 1993. Macrostructure des Syst~mes Smectite-Eau. Influence de Poly~lectrolytes Anioniques sur l'Organisation de Suspensions de Montmorillonite. Th~se de Doctorat Univ. Paris VI. Pusch, R., 1982. Mineral-water interactions and their influence on the physical behavior of highly compacted Na-Bentonite. Can. Geotechn. J., 19: 381-387, Pusch, R., Karnland, O. and H~kmark, H., 1991. The nature of expanding clays as exemplified by the multifaced smectite mineral montmorillonite. Int. Workshop Stress Partitioning in Engineering Clay Barriers, Duke Univ. Qi, Y., 1996. Comportement Hydro-m~canique des Argiles: Couplage des Propri~t~s Micro-macroscopiques de la Laponite et de l'Hectorite. Th~se de Doctorat, Univ. Orleans.
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197
Tessier, D., 1984. Etude Exp6rimentale de l'Organisation des Mat6riaux Argileux. Th~se de Doctorat d'Etat, Univ. Paris VII. Tessier, D. and Berrier, J., 1978. Observation d'argiles hydrat6es en microscopie 61ectronique h balayage: importance et choix de la technique de pr6paration. Proc. 5th Int. Working-meet. Soil Micromorphology, Granada, pp. 117-135.