Swelling and compaction phenomena in some bentonites from Tagus Basin (Madrid, Spain)

Cristina de Santiago Buey, María Santana Ruiz de Arbulo Laboratorio de Geotecnia. Centro de Estudios y Experimentación de Obras Públicas (CEDEX).

ABSTRACT Two expansive clays from Tagus Basin (Madrid, Spain) were studied in order to observe, analyse and meas-ure how their original fabric changes when they are compacted or moistened. For purposes of this, the mate-rials were first studied by means of X-ray diffraction (XRD) to identify their mineralogical composition. Then, clay microfabric was observed with a scanning electron microscope (SEM) and their main physico-chemical properties were measured: the specific surface area (BET method) was determined by isotherms of N2 adsorption, Cation Exchange Capacity (CEC) was measured and porosimetry parameters were defined by Mercury Intrusion Porosimetry (MIP). Keywords: bentonite, clays, swelling, compaction, SEM, porosimetry, BET surface area

1 INTRODUCTION

It has been known for a long time that geotechnical properties of bentonites depend mainly on their mineralogical composition, physico-chemical properties (cation exchange capacity, specific surface area, charge density) and their microstructure (particles and pores size distribution curves, preferred orientations, cementation between particles, etc.).

At the same time, changes of volume produced in a clayed expansive material due to consolida-tion, compaction or swelling phenomena lead to variations in their original microfabric and, con-sequently, in their geotechnical behaviour (Mitchel (1956, 1976), Lambe (1958), van Olphen (1966), Gillot (1968), de Santiago (2000b)). Within the past century, specific techniques and models have been developed to study the micro-scopic features of clay materials. These offer new approaches to the study of soils and rocks.The purpose of this work is to observe and analyse two well known bentonites from Tagus basin (Madrid, Spain) and measure the properties de-scribed above relating them with the compaction and swelling phenomena observed in these mate-rials.

2 MATERIALS AND METHODS

The two bentonites studied from Tagus basin (Madrid, Spain) are commonly known as “Green Clays” and “Pink Clays”, respectively, because of their characteristic colors. They are both com-posed mainly of trioctahedral (magnesian) smec-tites with small amounts of illite, quartz and feld-spars. The mineralogical identification was carried out by X-ray diffraction (XRD), using a Philips 1130/90 diffractometer with Cu anticath-ode and a graphite monochromator.

Both materials were compacted following the UNE 103-500-94 and UNE 103-501-94 Standard Techniques. On the other hand, free swelling and swelling pressure tests were conducted following the UNE 103-601/96 Standard Technique. After compaction and swelling tests, the resulting probes were studied by means of different tech-niques to detect possible changes and establish a comparative analysis.

Mercury intrusion porosimetry (MIP) analysis was performed to evaluate the pore-size distribu-tion of bulk materials, and its dependency on the compaction or swelling phenomena in clays. A Micromeritics Autopore IV porosimeter was used with this purpose.

Calculation and evolution of the BET surface and of the internal and external surfaces was per-formed using a computer program supplied with a Micromeritics ASAP 2010 analyser. Cation-exchange capacity (CEC) was calculated using distillation and measurement of NH4

+ by the Kjeldaljh method. Particle morphologies and tex-tural relationships were examined by means of scanning electron microscopy (SEM), using a JEOL JSM6400, operated at 40 kV and equipped with a Link eXL X-ray energy dispersive detec-tor.

3 RESULTS

3.1 Mineralogical and physico-chemical charaterization

3.1.1 “Green Clays” Green clays are mainly composed of trictahe-

dral smectite called saponite, with impurities of il-lite, quartz and alkali and plagioclase feldspars (de Santiago et al. (1998)). The electrical charge in saponite derives from Al-for-Si substitution in the tetrahedral sites of the structure.

Their specific surface area (calculated by appli-cation of the BET equation) is 114 m2/g. This moderately high value together with the low CEC (42 meq/g) results in a low electrical charge den-sity (0.358 Coulombs/m2).

In general, their fabric (studied by SEM) con-sists of open structures with pseudo-polygonal voids (“cornflake” and “honeycomb” microstruc-ture) (Figure 1)

Figure 1: Random array of smectite particles (honeycomb microstructure)

formed of small and irregularly shaped smectite

flakes, which keep edge-to-face and edge-to-edge

contacts (the latter being more abundant) (Figure 2).

Figure 2: Edge-to-edge and edge-to-face contacts be-tween particles

3.1.2 “Pink Clays” Pink clays are mainly composed of stevensite

(de Santiago et al. (2000)) showing the following properties: a variable interlayer > 10Å spacing, the layer charge arising from a deficiency in octa-hedral cations rather than from cation substitu-tions. This leads to an extremely low CEC and charge density. The specific-surface (N2-BET) is high (392 m2/g) owing to external surfaces of the particles (314 m2) and the remaining area (78 m2) corresponds to internal (interlayer) surfaces. Thus, an extremely small crystallite size is deduced, in agreement with incipient crystal growth. SEM micrographs permit the observation of a nearly isotropic fabric formed by a random (non-oriented) array of very small smectite particles (Figure 3).

Figure 3: Honeycomb microstructure in pink bentonites

This microstructure is typical of high-swelling

soils with high salt concentrations that reduce in-

terparticle repulsion. As in green clays, most samples have open structures with pseudo-polygonal voids (“cornflake” and “honeycomb” microstructure) formed of smectite laminar parti-cles, which keep edge-to-face and edge-to-edge contacts (Figure 3). This texture explains the very low density of the rock (0.7-0.9 g/cm3). Else-where, smectite particles show a tendency to be connected through the formation of flocs or ag-gregate structures with spherical morphologies (cellular texture) separated by voids of varying sizes and shapes (Figure 4).

Figure 4: Spherical aggregates of particles The mineralogical and physico-chemical fea-

tures of the two bentonites are listed in Table 1.

Table 1: Mineralogical and physico-chemical properties of the two bentonites Green

Clays Pink Clays

Main Clay Mineralogy Saponite Stevensite Origin of electrical charge Tetrahedral

substitu-tions Si/Al

Deficiency in octahedral cations

CEC (meq/100g ) 42 23 Total BET surface (m2/g) 114 392 Internal surface 38 78 External surface 76 314 Electrical charge density (Coulombs/m2) 0.358 0.057

Total porosity (%) 53.8 64.0 Efficient porosity 53.5 52.1 Median pore diameter (?m) 2.3360 0.0327

3.2 Geotechnical behaviour

First of all, a geotechnical characterization was done by determining fundamental parameters like dry bulk density, natural water content, specific gravity, and Atterberg limits. Resulting values are shown in the Table 2.

Table 2: Geotechnical characterization of the two ben-tonites Green Clays Pink Clays Dry bulk density (g/cm3) 1.21 0.90 Natural water content (%) 11-13 8-17 Specific gravity 2.62 2.50 Shrinkage limit (%) 25-30 45-52 Plastic limit (%) 42-51 84-94 Liquid Limit (%) 70-104 100-106 Plasticity index 22-31 12-16

Then, Proctor tests, were performed to estab-

lish the relationship between density of the ben-tonite, water content and force of compaction. In theory, if a soil material is compacted at succes-sively higher moisture content, assuming that the compactive effort remains constant, soil density increases until the optimum moisture content is reached. After that, density decreases with in-crease in moisture content. The green bentonite followed perfectly the theorical behaviour ex-plained before (Figure 5) but the pink clayed ma-terial showed a completely different proctor curve.

Green Clays

Pink Clays

Initial water content (%)

Dry

de

nsit

y (g

/cm

)3

Figure 5: Proctor curves The cause of the opposite curvature in the case

of pink bentonites is that the compaction energy applied in the Proctor test (0.583 J/cm3) is not

enough to neutralize the swelling pressure gener-ated in the pink clays when moistened.

The resulting probes after the Proctor compac-

tion tests were observed by SEM and analysed by MIP to study how the original fabric changed (Figure 6).

0.01 0.1 1 10 100

0

Vol

ume

(mL/

g)

Pore diameter ( m)?

Variation of the porous net after Proctor compaction testGreen Clays

0.005

0.01

0.015

0.02

0.025Green Clays - natural sampleGreen Clays - after Proctor test

4.8

2.5

98

207

0.01 0.1 1 10 100

0.05

0.04

0.03

0.02

0.01

0

Volu

me

(mL/

g)

Pore diameter ( m)?

Variation of the porous net after Proctor compaction testPink clays

Pink Clays - natural samplePink Clays - after Proctor test 4.00.023

Figure 6: Pore size distribution curves of green clays and pink clays measured in natural samples (solid lines) and in probes resulting after Proctor compaction tests (dotted lines).

As illustrated by the curves in Figure 6, the

variation of the porous net after Proctor compac-tion test is quite different in both materials, de-pending on their swelling behaviour during the probe preparation. In the case of Green clays, the compaction process to prepare the probe gives rise to a decrease in the porosity from 47% to 42% (Table 3). The median pore diameter falls down from 2.34 to 1.38 ? m and the maximum of the curve located over 100 ? m pore size disap-pears (Figure 6).

In contrast, Pink clays expand when they are moistened to construct the probe, and the energy applied during the Proctor test is not enough to prevent this expansion, the result being the dis-placement of the maximum of the porosimetry curve from 0.023 to 4 ? m, that led to an increase in the total porosity from 50.2% to 52% (Table 3) and in the median pore diameter from 0.03 to 1.60 ? m.

Table 3: Variations in porosimetry in both clay materials. M.P.D.: Median Pore Diameter (?m) Green

Clays natural

Green Clays compacted

Pink Clays natural

Pink Clays compacted

Porosity (%)

47.0 42.0 50.2 52.0

M.P.D. 2.34 1.38 0.03 1.60

To obtain more information about the swell ca-

pacity of both materials, free swell (expressed as a percentage of the initial volume of the soil) and swelling pressure tests were performed in samples with the optimum conditions of the Proctor com-paction test. In the case of pink clays, as there is no maximum density point, swelling studies were carried out with medium values of moisture and density. Results are listed below:

Table 4: Swelling conditions (moisture and density) and results.

Green Clays

Pink Clays

Optimum moisture (%) 42.6 60 Maximum density (g/cm3) 1.12 0.8 Free swelling (%) 4.7 9.1 Swelling pressure (Kp/cm2) 1.4 2.2

SEM micrographs show how the original fabric changed to a more opened one, with bigger pores, whose walls are made of swollen particles, show-ing sometimes fiber morphologies (Figures 7 a-d). These changes in the fabric took place during the Proctor compaction test (Figure 7 a-b) but they can be observed much more noticeable in the probe resulting from the free swelling test (Figure 7 c-d).

Figure 7: Microfabric of the expanded pink clays after the Proctor compaction test (a-b) and after the free swelling test (c-d).

Furthermore, BET specific surface area was

calculated in natural samples, as well as in the probes after the Proctor compaction tests and after the free swelling tests, in order to measure the variations in the external to internal (interlayer) surface ratio (Table 5).

The results shown in the Table 5 suggest that

Green clays are slightly compacted during the Proctor test (noticed by the little decrease in the total surface, while the external to internal surface ratio remains constant).

Table 5: Variations in BET specific surface area: Total, internal and external surface, as well as external to inter-nal surface ratio in both materials.

“Green Clays”

Total surface (m2/g)

External surface (m2/g)

Internal surface (m2/g)

Ext/Int Ratio

Natural 114 76 38 2.0 After proctor 108 73 35 2.1 After swelling 94 76 15 5.1

“Pink Clays” Total

surface (m2/g)

External surface (m2/g)

Internal surface (m2/g)

Ext/Int Ratio

Natural 392 314 78 4.03 After proctor 294 258 36 7.16 After swelling 332 308 24 12.8

The study of the probe resulting from the free

swelling test shows that, although the total surface decreases owing to the compaction needed to con-struct the probe, the swelling phenomena can be deduced from the rise of the external to internal ratio (from 2.0 to 5.1). This variation is thought to stem from the opening of the clay particles when moistened. Water molecules get inside the inter-layer space separating the clay layers and as a re-sult of this, the interlayer surface is measured as external surface.

Conversely, Pink clays expand in both tests, as evidenced by the increasing external to internal ratio values. During the Proctor compaction test, the compaction energy contributes to decrease the total surface but the expansion due to moisturiz-ing leads to a higher external to internal surface ratio (almost the double than the original value); this rises dramatically in the swelling test up to overtreble the initial value (from 4.03 to 12.8). These results are consistent with the anomalous Proctor compaction curves and the SEM observa-tions described above.

4 CONCLUSIONS

The swelling and compaction phenomena of two Spanish bentonites (Green clays and Pink clays) were studied by means of Mercury Intrusion Po-rosimetry, Scanning Electron Microscopy and Specific Surface Area measurements. Although both materials are mainly made of smectite min-

a

b

dc

erals, as it corresponds to bentonites, their clay mineralogy, electrical charge, porosity, BET spe-cific surface area, particle size and microfabric are quite different.

Differences between them generate different ways of response to physical processes like com-paction or wetting. The techniques used in this work have demonstrated to be powerful tools to observe, study, model, and forecast these phe-nomena.

Many gaps remain without answer but our re-sults suggest that these techniques could be cru-cial to understand clay materials and how their physico-chemical features can have an influence in their geotechnical behaviour.

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J.K. Mitchell, The fabric of natural clays and its relation to engineering properties, Proceedings HRB, Vol. 35, 1956

J.K. Mitchell, Fundamentals of soil behaviour, John Wiley and Sons, New York, 1976

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C. de Santiago, Las arcillas magnésicas de la cuenca del Tajo: Caracterización y propiedades, CEDEX, Ministerio de Fomento, 2000b

H. van Olphen, An introduction to clay colloid chemistry: for clay technologists, geologists and soil scientists, Interscience Publishers, 1966