Engineering Geology 139-140 (2012) 17–27

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Engineering Geology

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Microstructure and geotechnical properties of lime-treated expansive clayey soil

Muzahim Al-Mukhtar a,b,⁎, Suhail Khattab c, Jean-Francois Alcover a

a CNRS – Université d'Orléans: CRMD UMR 6619, 1b, rue de la Férollerie, 45071 Orléans Cedex 2, Franceb Université d'Orléans, Polytech' Orléans, 8 rue Léonard de Vinci, 45072 Orléans Cedex 02, Francec Mosul University, College of Engineering, Civil Engineering Department, Al-Majmooah street, Mosul, Iraq

⁎ Corresponding author at: CNRS – Université d'Orléade la Férollerie, 45071 Orléans Cedex 2, France. Tel.: +38 25 53 76.

E-mail address: muzahim@cnrs-orleans.fr (M. Al-Mu

0013-7952/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.enggeo.2012.04.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 September 2011Received in revised form 3 April 2012Accepted 5 April 2012Available online 12 April 2012

Keywords:Expansive clayLime stabilizationGeotechnical propertiesMicrostructureTextureCalcium hydrates

The changes in the main geotechnical properties (plasticity, unconfined compressive strength, swelling pres-sure and permeability) associated with the micro level texture and structure of untreated and lime-treatedcompacted FoCa clay samples were studied using X-Ray Diffraction, Thermogravimetric Analysis, ScanningElectron Microscopy (SEM) and Transmission Electron Microscopy (TEM) methods. The TEM was found tobe a useful tool in the identification of microscopic changes that arise in the lime-treated FoCa clay in com-parison to untreated soil. Based on these studies it was identified that the treated FoCa clay has a greaternumber of packed layers in comparison to untreated or natural FoCa clay. The TEM technique was also usefulto observe the appearance of a cementitious phase or pozzolanic phase that is constituted of a hydrate ofcalcium silicate aluminate (CSAH). SEM results show changes in the morphology of the clay particles andthe concentration of calcium is higher on the border of the particles. The focus of this paper is to providean explanation of improvements of geotechnical properties that arise in the expansive soil behaviour dueto the lime-clay reactions, mainly a pozzolanic reaction, using microscopic analysis.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Expansive clays are considered by civil engineers to be a problem-atic soil because they cause severe distress to the structures con-structed on them or with them. Several additives are commonlyused for the treatment of expansive clays which include salt, poly-mers, surfactants, cement, lime, fly ash and mixtures of some ofthese additives. Lime has, however, been more commonly used forthe treatment of expansive clays and other clay soils because it im-proves the mechanical properties of expansive soils. In addition, it iseconomical and is abundantly available in many parts of the world.

Several studies are undertaken and reported in the literature toexplain the improvements in themechanical and hydraulic propertiesof expansive soil due to the addition of lime and other stabilizers,mainly from a macroscopic point of view. The focus of most ofthese researches was on studying the changes associated with theAtterberg limit values (plastic and liquid limits) and the soil proper-ties, such as the volume change (i.e., swell pressure, swell potential,heave), shear strength and the coefficient of permeability of the stabi-lized expansive soils (Diamond and Kinter, 1965; Brandl, 1981; Locatet al., 1996; Sivapullaiah et al., 2000, 2003, Sivapullaiah and LakshmiKantha, 2005).

ns : CRMD UMR 6619, 1b, rue33 2 38 25 78 81; fax: +33 2

khtar).

rights reserved.

Other studies focused on understanding the fundamental reac-tions that arise between the clay and lime during the stabilization.Based on such studies, researchers attributed the improvements inthe geotechnical properties of lime-stabilised soils to four basicreactions: cation exchange, flocculation and agglomeration, limecarbonation and finally the pozzolanic reaction (Eades and Grim,1960; Ingles and Metcalf, 1972; Little, 1996; Boardman et al., 2001;Rao and Shivananda, 2005; Al-Rawas and Goosen, 2006; Al-Mukhtaret al., 2010a, 2010b; Cuisinier et al., 2011). There is a generalconsensus in the literature about the first three types of reactions asinstantaneous reactions. However, the pozzolanic reaction, which isconsidered as the key parameter in lime-soil stabilization, is a time-dependent reaction. The pozzolanic reaction arises in a highly alka-line environment (with a pH value≥12) produced by the additionof a minimum quantity of lime to the expansive soil. The highly alka-line environment is believed to be responsible for the slow dissolu-tion of the alumino-silicate constituents of clay (octahedral andtetrahedral sheets). These constituents react with Ca2+ of lime andprecipitate hydrated cementitious products and bond the adjacentsoil particles together (Ingles and Metcalf, 1972; Bell, 1996; Little,1996; James et al., 2008).

The main objective of this paper is to obtain coupled geotechnicalproperties with microscopic data of the lime-stabilized highly plasticmontmorillonitic clay from France, which is referred to in the litera-ture as FoCa clay. This soil supports many applications such aswaste storage and road construction in France. The lime-soil reactionis studied using X-Ray Diffraction, Thermogravimetric Analysis,Scanning ElectronMicroscopy and Transmission ElectronMicroscopy.

Table 2Chemical composition (weight proportion, %) of FoCa clayey soil.

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO K2O P2O5 Loss of ignition

48.2 1.1 21.9 6.0 0.0 0.8 2.8 0.1 0.0 19.8

18 M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

An attempt is made to explain the widely debated pozzolanic reactionand to demonstrate and analyse its effect on the microstructure ofclays and on the properties at the macroscopic level (geotechnicalproperties) of lime-treated expansive soils.

2. Lime-clay mechanism reactions

With the introduction of lime into the soil environment, Ca (OH)2is dissolved in water and Ca2+ and (OH)−1 are freed. Cation ex-change occurs between cations linked to the clay layers and Ca2+;Ca2+ becomes the only interlamellar cation. Clay particles are sur-rounded by a diffuse hydrous layer which is modified by the ion ex-change of calcium. This modifies the density of the electrical chargearound the clay particles and attracts them closer to each other toform flocs (flocculation). This implies stronger attraction forces be-tween layers and a stacking of a greater number of layers (Grim,1962). These reactions change the clay texture, giving thicker parti-cles, reducing plasticity and increasing the soil strength (Basma andTuncer, 1991; Khattab, 2002). Lime, Ca (OH)2, and atmospheric car-bon dioxide, CO2, react to produce crystals of calcite CaCO3, lime car-bonation. Calcite crystals could link clay particles, but weakly. Cationexchange and flocculation occur with typical time of day and changethe geotechnical soil properties.

The addition of a sufficient amount of lime induces a highly alka-line environment (pH≥12), as shown in Table 1, which helps pro-mote slow clay dissolution and the formation of aluminate andsilicate anions. Ca2+ cations can link aluminate and silicate anionsand induce the formation of calcium aluminosilicate hydrates, a ce-mentitious product bonding the adjacent soil particles together.These reactions, called pozzolanic reactions, are very slow at roomtemperature with typical time of year and yield various amorphousphases (gels).

XRD was often used to follow the evolution of the clay-lime reac-tion; researchers (Arabi and Wild, 1989; Bell, 1996) attribute the ap-pearance of new reflections to the formation of CSAH (CSH, CAH,….).With TEM, Arabi and Wild (1989) show, for a high temperature reac-tion and 2 years curing time, the appearance of non-crystallized mat-ter, CSAH, between clay particles.

3. Material: clay soil and lime

A natural clay soil (FoCa), with Ca2+ ions as predominant ex-changeable cations was chosen in this study. Table 2 gives the chem-ical composition of this clayey soil (Khattab, 2002). The meanmineralogical analysis (Atabek et al., 1991; Guillot, 2002) showsthat it contains predominantly interlayered dioctahedric Ca-smectite/kaolinite (percentage>80%, with about equal proportionsof smectite and kaolinite), a small amount of free kaolinite (4%) andnon-clay minerals: quartz (6%), goethite (6%) and others in verysmall quantities like anatase, calcite, etc.

Hydrated lime Ca (OH)2, manufactured by Balthazard & Cotte inFrance using very pure limestone rocks with more than 95% CaCO3,was used for the treatment of the studied clay. This lime is very fineand contains more than 90% of materials passing through a sievemesh of 80 μm.

4. Sample preparation and experimental procedures

The required amounts of air dried clay and lime were initiallymixed thoroughly by hand and distilled water was sprayed to reach

Table 1pH values of the soil specimens for different lime concentrations.

Percentage of lime added 0 1 2 3 4 7

pH 8.5 11.7 12.3 12.4 12.5 12.5

the desired constituent proportions, then they were mixed againand placed in closed containers for a period of 1.5 hours. FoCa claysamples, untreated and treated with 1%, 4% and 10% of hydratedlime Ca (OH)2, were compacted using the standard Optimum Proctorenergy of compaction of the untreated FoCa (Maximum Dry Density— MDD=1.32 and Optimum Moisture Content — OMC=32% ). Sam-ples were statically compacted in the mould to which the load wasapplied at a rate of 1 mm/min to reach the required density. The con-tinuous curing with the optimum moisture content for the testedsamples was ensured by sealing them in wax papers and then by im-mersion in liquid wax. Measurements were carried out on sampleswith two conditions (Khattab, 2002):

- Samples compacted at Standard Optimum Proctor (MDD=1.32and OMC=32%) called SOP

- Samples compacted at Standard Optimum Proctor and then satu-rated without (by preventing) any volume change (MDD=1.32and saturated moisture content SMC=38%) called SOP-S.

4.1. Geotechnical tests

All samples were prepared and cured at room temperature (20 °C)and for 7 and 90 days.

Unconfined compressive strength tests were conducted with FrenchStandard (NF P94-420) on SOP untreated and treated FoCa samples of100 mm length and 50 mm diameter. The compressive strength wasdetermined using a Universal Testing Machine (INSTRON 4485) atthe rate of 0.1 mm/min.

Swelling pressure tests were conducted with French Standard (NFP94-091) on SOP untreated and treated FoCa samples of 10 mmheightand 65 mm diameter. Swelling pressures were measured using thevolume change method with a compaction rate of 0.001 mm/min toachieve the initial sample height before wetting (Khattab, 2002).

Permeability tests were conducted with French Standard (NF P94-512) on SOP-S untreated and treated samples of 10 mm height and65 mm diameter. After stabilization of the swelling pressure mea-surements, distilled water was permitted to percolate through thespecimen from the bottom towards the top in order to facilitate theremoval of air bubbles. Water heads of 100 cm were applied with aconstant level water reservoir, which kept the head constant at anytime during the test, corresponding to a hydraulic gradient of 10.Water passing through the soil was collected in a container througha pipe connected to an outlet and maintained as far as possible with-out evaporation.

4.2. X-Ray Diffraction (XRD) and Thermogravimetric Analysis (TGA)

XRD study was conducted using the Siemens D5000 diffractometerwith Cu-Kα radiation (λ=0.15406 nm) filtered by a Ni foil. XRD mea-surements were carried out on SOP and SOP-S untreated or treated(lime addition of 4 or 10%; curing time: 1.5 h to 10 days; curing tem-perature: 20 °C and 50 °C) to study the influence of quantity of limeand water, periods of fast exchange (1.5 h) and slow pozzolanic reac-tions (10 days), accelerating the slow reaction rate with an increaseof temperature. Flat samples, studied in the θ–2θ diffractometer, werecut perpendicular to the compression axis. In order to compare theobtained diffractograms, the main quartz reflection was used to scalethe intensities of the X-ray diffractograms for all tested samples.

Thermogravimetric Analysis curves, mass loss (TG) and its deriva-tive (DTG), were recorded with a Setaram TGA 92 apparatus at a

Table 3Geotechnical properties of untreated and lime-treated FoCa clayey soil.

19M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

heating rate of 1.7 °C/min for a mass sample of about 100 mg. Themass loss (TG) is defined by TG (T)=[M (T)−M (T0)]/M (T0),with M (T0=800 °C) the mass of the sample at the end of the test(calcined mass) taken as the reference mass. The (TG) and (DTG)curves were obtained for samples compacted with SOP and SOP-Suntreated and treated with 4% of lime, after 7 days of curing at 20 °C.

4.3. Transmission Electronic Microscopy (TEM)

Electron microscopy is well suited to observe materials with scalesranging from nanometer to micrometer. It provides access to theirmicrotexture (shape, size, porosity) and their microstructure (atomicarrangement, chemical composition).

The Electronic Microscope PHILIPS CM20 (accelerating voltage200 kV; exposure time of 2 s; all optical corrections made) providesa magnified image of the tested sample (about x105). On a selectedarea of this sample (a circle of about 50–100 nm) an electron diffrac-tion pattern is conducted and a chemical analysis is carried out. Aspectrometer measures the energy of X-rays emitted by the sampleover electron bombardment (EDX: energy dispersive X-rays spec-trometer). The energy of X-rays is characteristic of elements presentin the sample. This technique allows us to assess quantitatively theelements with atomic numbers greater than that of sodium; therecorded signal (acquisition time of 100 s) is then proportional tothe number of atoms, and the results for each element are expressedas a percentage of the total number of atoms of all the elementsmeasured.

4.3.1. Samples preparation for Transmission Electronic MicroscopyPieces of compacted samples (SOP and SOP-S untreated and lime-

treated) of small thickness, low enough to be transparent to electrons(thickness between 40 and 100 nm) were observed using TEM. Ob-servations were carried out parallel and perpendicular to clay layerson untreated and 10% lime-treated samples (curing time of21 days), a higher lime concentration was chosen in order to moreeasily observe the treatment effect.

- Observations perpendicular to clay layers were performed on flatsamples obtained by sedimentation of large lamellar clay (abface or 001 plane) of an aqueous solution of untreated and treatedFoCa. TEM of these samples gives bright field images which allowmorphological observations.

- Observations parallel to clay layers (ab faces section) were carriedout on compacted samples where these ab faces are perpendicularto the compaction direction. To observe a section of ab faces, dueto the softness of the hydrated clay layers, a method of solvent ex-change, derived from that of Tessier (1984), was used to keep theoriginal structure of clay sections unchanged. After polymerizationof the solvent, oriented samples were cut by ultramicrotomy par-allel to the compaction direction and laid flat on a grid (Khattab,2002). High-resolution images of these sections show the sampletexture and the stacking of layers.

Properties of FoCa soil Untreated lime treated

lime added 0% 1% 4% 10%

Plasticity Index, PI (%,±2%) 70After 7 days 50 10 4After 90 days 46 6 2Swelling pressure (kPa,±5kPa) 150After 7 days 120 20 15After 90 days 110 10 10Unconfined Compression Strength, (MPa,±0,1 MPa) 0.3After 7 days 0.4 1.2 1.6After 90 days 0.4 1.8 2.4Permeability (10−8 m/s ,±50.10−8 m/s) 4After 7 days 70 600 90After 90 days 10 300 65

4.3.2. Electron diffraction diagramsElectron diffraction diagrams of selected areas of the sample

show: rings (powder diagram which means a lot of small partiallyor totally an-oriented crystals), reflections (single-crystal diagram)and central diffusion (very large particles, gel or pores). Distance(hkl) measurements were performed on reflections and rings to iden-tify the mineralogical structure.

4.4. Scanning Electron Microscopy (SEM)

A comparative study was conducted on untreated and 10% lime-treated compacted samples. To keep the structural characteristics of

the hydrated samples a special method, derived from that of Tessier(1984) and usually used in our laboratory, was employed.

SEM apparatus (Hitachi S4500 and Philips 525 M FEI) were usedto observe the morphology of the samples with two magnifications:×5000 (large) and ×500 (small).

Series of chemical analyses of calcium, aluminium and siliciumwere carried out on a line of 10 μm length with points having10 nm in diameter and regularly spaced every 60 to 100 nm. Linesof chemical analysis were chosen parallel to particle edges or towardthe inside of particles, to identify the localisation of calcium.

5. Results and discussion

5.1. Lime treatment effect on geotechnical properties of FoCa soil

The plasticity index (PI) determined with French Standard (NFP94-051) was 70% for the untreated FoCa soil. Table 3 shows how PIvalues decrease substantially with lime addition and slightly withcuring time. Further increases in lime beyond 4% provide little or noadditional increase of the plasticity index. The reduction in plasticitycan be attributed to the increasingly granular nature of the lime-treated clay soil induced by the short-term reactions (cation ex-change and flocculation–agglomeration). According to changes inthe plasticity of the FoCa soil (criteria of Hilt and Davidson, 1960and Eades and Grim, 1966), the 4% lime addition can be designatedby the appellation “initial lime consumption”.

Table 3 indicates the swelling pressure of 150 kPa for the SOPuntreated FoCa. Changes in the swelling pressure are limited with1% of lime added whatever the curing time. However, the swellingpressure decreases greatly with 4% lime from 150 kPa to 20 kPa and15 kPa after 7 days and 90 days respectively. Swelling pressure valuesfor 10% lime are similar to those with 4% lime. The swelling values de-crease considerably in the treated samples with the addition of limebut slightly with curing time from 7 to 90 days. This behaviour canbe attributed to the limited amount of water absorption of calciumclay owing to the short-term reaction. This result confirms the verypositive effect of the treatment on this plastic clay using only 4% oflime.

Table 3 shows that the Unconfined Compression Strength (UCS) ofthe treated FoCa increases with the lime added and curing time. How-ever, limited changes are observed in the samples tested with 1% oflime within 7 and 90 days. This amount of lime is completely usedfor the short time reaction. UCS increases in comparison to that ofthe untreated sample (0.30 MPa) more than 4 times in the samplestreated with 4% and 10% lime after 7 curing days and more than 6times after 90 days. The pozzolanic reaction is activated by the pres-ence of lime consecutively to the short time reactions. UCS increaseswith curing time are due to the development of cementitious bondsproduced by the pozzolanic reaction.

20 M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

The permeability (Table 3) increases from 4.10−8 m/s to600.10−8 m/s with 4% of lime added and then decreases to90.10−8 m/s with 10% lime. This behaviour can be attributed to theincreasingly granular nature and connected pores of the treated soilinduced by the immediate reactions of lime with clay. Adding morelime (>4%) seems to fill the porosity of the sample and reducesits permeability. With curing time, permeability decreases for the

0

400

800

1200

1600

2000

0 5 10 15

2θ°

Before saturation

After saturation

S00

1

K00

1

S00

S00

3

S00

1

Inte

nsit

y, a

rbit

rary

uni

ts

0 10 20 30

S 001

S 002 S 004 S 006

S & K {02-11}

G

Q Q C C L

INT

EN

SIT

Y

Smectite S

Kaolinite K

Goethite G

Quartz Q

Calcite C

Lime L

S 001 K 001 K 002

CAH

a

b

Fig. 1. XRD results of compacted untreated and tread samples. a: XRD of compacted, unsaturcompacted, unsaturated and saturated FoCa.

treated sample as the continuous lime/clay reaction modifies thepore size distribution by cementation and filling some pores. Changesin the texture, pore size and pore accessibility increase the tortuositywhich leads to a reduction in permeability. During the test of perme-ability, no variation of the swelling pressure was detected. Moreover,results prove again the important change induced in the soil treatedwith 4% of lime.

20 25 30 35 402θ°

S &

K 0

2-11 Q

G GS &

K 2

0-13

K00

2 &

AQ

S00

6

G

Smectite SKaolinite KGeothite GQuartz QAnatase A

40 50 60

4% lime After water saturation (SOP-S)

10% lime Before water saturation (SOP)

S & K {13-20}

S & K {15-24-31}

G G

Q L

Q C

4% Lime Before water saturation (SOP)

ated (SOP) and saturated (SOP-S) untreated FoCa clay. b: XRD of treated (4%, 10% lime),

21M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

5.2. Microstructure analysed by X Rays Diffraction (XRD)

The characteristics of the XRD diagrams (Fig. 1a and b) show thatall these compacted untreated and treated samples (SOP and SOP-S)contain mainly poorly organized smectite-kaolinite (broad and non-regular 00l reflections of smectite and kaolinite), with some quartzand goethite, (Khattab et al., 2000). The 00l smectite and kaolinite re-flections suggest that clay layers are predominantly oriented alongthe axis of compaction. Nevertheless the appearance and intensityof {hk} bands: {02–11}, {13–20} indicate a partial disorientation ofthe clay particles (not all the clay layers are perpendicular to theaxis of compaction).

When samples are saturated under constant volume, the smectitebasal spacing peak d001 shifts from about 1.6 nm to 1.9 nm (Fig. 1aand b). Thismeans an increase of the pseudo-water layers of the inter-lamellar space, from 2 to 3. The d001 reflection is also sharper withwater saturation; this means an increase in the number of clay layersin each particle, and so thicker particles. An estimation of the particlethickness using the Debye-Scherrer formula gives 2–3 nm (1–2 claylayers) and 5–10 nm (3–5 clay layers) before and after saturation.

Fig. 1a and b show that for unsaturated samples the intensities of{hk} bands are higher and the intensities of the (00l} reflections aresmaller than in saturated samples. This corresponds to a greater dis-orientation of clay layers in unsaturated samples or a better orienta-tion of clay layers in saturated samples. Saturation of the samplewhile maintaining its volume constant induces a decrease in the cap-illary forces. This increases the smectite interlayer spaces and inducesa better orientation of the clay layers. Pore spaces and clay particlesreorganize during the compaction and water saturation processes.

Very few differences concerning the particle size and arrangementcould be observed between the XRD of treated and untreated samplesfor the same saturation conditions (Fig. 1a, b):

- nearly the same d001 basal spacing- no displacement of the {hk} bands, which indicates that no defor-mation in the ab plane of clay layers occurred. The {hk} bands areobserved to be relatively more intense in the treated clay, whichcorresponds to a greater number of clay layers (ab face). Thiscan be explained by smaller clay layers or a greater disorientationof the initial clay layers due to the lime treatment.

TG

wei

ght

loss

/Cal

cine

d m

ass,

%

Fig. 2. TG and DTG curves of compacted untreated and 4% lime treated FoCa clay b

X-Ray Diffraction diagrams of treated samples, 4% and 10% of lime(Fig. 1b) show calcite crystals formation (CaCO3 peak at 29.4°). This isattributed to the carbonation reaction between lime, Ca (OH)2, andcarbon dioxide, CO2, existing in the air. With a lime content of 10%,due to the excessive amount of lime added, carbonation is more im-portant and lime is not completely consumed after a few hours of cur-ing time at 20 °C (lime peaks at 18° and 34° in Fig. 1b). The lime peaksdisappear with water saturation or with a longer curing time; waterand a large curing time are necessary for a complete lime treatment.Finally, the effect of saturation is dominant over the effect of treat-ment in the studied soil.

With the experimental conditions applied, the possible andexpected formation of a cement link (CSAH) between particles dueto the pozzolanic reaction is observed only with 10% of lime added.Fig. 1b shows that only the first peak corresponding to the hydrateCSAH can be observed.

As is well known, the XRD can barely distinguish phases that arebadly crystallised or/and with a small extension (nanometer order).However, with a very long curing time and a high percentage oflime, Bell (1996), Rao and Shivananda (2005) and Al-Mukhtar et al.(2010a, 2010b) confirm the presence of CSAH on lime-treated clayeysoil. The formation of these links in the treated soil explains the im-provement of geotechnical properties. In order to investigate thesenew microphases, electron microscopy is used.

5.3. Microstructure study by Thermogravimetric Analysis (TGA)

The TG and DTG curves obtained for untreated and lime-treated(4%) samples, compacted to SOP and SOP-S, after 7 days of curing,are presented in Fig. 2. For FoCa, the TG and DTG curves obtainedshow that, like the smectites, the FoCa absorbs water easily, thiswater content being able to reach 35–45% of the calcined mass. Dehy-dration is carried out in five stages (Table 4): below 140 °C, free water(interlamellar water not linked to the exchangeable cation and waterbetween clay particles); between 140 and 260 °C, water linked to theexchangeable cation of the smectite interlamellar space; towards300 °C the dehydration of goethite (transformation of goethite FeO(OH) in hematite Fe2O3); between 380 and 650 °C, the dehydroxyla-tion of the clay minerals: kaolinite (380–500 °C) and smectite (500–650 °C); and between 650 and 800 °C, decarbonation of calcite.

DT

G, a

rbit

rary

uni

ts

0

0.15

efore (SOP) and after water saturation (SOP-S) after 7 days of curing at 20 °C.

a: Untreated FoCa clay

b: Treated FoCa clay

Fig. 3. High resolution TEM images of untreated and lime treated FoCa clay (a, b) facessection. a: Untreated FoCa clay. b: Treated FoCa clay.

Table 4Thermo gravimetric analyses of tested samples: water loss, dehydration of goethite, dehydroxylation of clay minerals, dehydration of lime and decarbonation of calcite.

% weight loss/calcined mass (800 °C)±0,5% 20–140 °C 140–260 °C 20–260 °C 260–360 °C 360–800 °C

Free water Linked water Total water Goethite Lime, kaolinite smectite and calcite

Untreated FoCa before water saturation 31.0 4.2 35.2 1.3 7.54% lime treated FoCa before water saturation 34.6 3.6 38.2 1.8 8.84% lime treated FoCa after water saturation 43.0 2.2 45.2 1.6 8.8

22 M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

The heating rate used in these tests was 1.7 °C/min which does notallow equilibrium of weight loss at 105 °C (standard temperature fordetermining free water), as shown on the curve of DTG. Analysis ofthe obtained results shows that all free water was lost at about140 °C and total water loss occurred at about 260 °C. The water satu-ration does not modify the thermograms, except for the quantity offree water which varies from 35% to 43% before and after saturation.

For lime-treated samples and before water saturation, two addi-tional peaks are observed towards 390 and 680 °C. These are attribut-ed to the dehydration of lime (very small peak towards 390 °C), towhere lime is not completely consumed, and to the decarbonationof the calcite formed (towards 680 °C). This interpretation is in agree-ment with the quasi disappearance of these 2 peaks on the thermo-grams of SOP-S treated samples and with an increase of theintensities of these 2 peaks for samples treated with higher limequantities. Moreover, the lime treatment strongly modifies the largedehydroxilation band (400–700 °C). This band is composed of twocontributions coming from OH− of the kaolinite and OH− of thesmectite. Changes in the position of the peak from 550 °C to 480 °Cdue to lime treatment are attributed to the vanishing of the OH− ofthe smectite. It seems that treatment affects mainly the smectite hy-droxyl environment and not the kaolinite hydroxyl environment.This may be due to a preferential interaction between lime and smec-tite or a structural modification of the clay.

Tested samples treated with 4% lime show that all the added limewas consumed by the reaction of the expected formation of poorlycrystallized cement links (amorphous CSAH), but which was not vis-ible in the analysis. TGA results obtained after 7 and 110 days of cur-ing were similar. This can be explained by the very slow pozzolanicreaction.

In the tested soil, the two main minerals smectite and kaolinitereact differently with lime. The smectite reacts quickly with lime,probably inducing the formation of the new hydrate. The kaolinitewas not modified during the time of our experiment.

The treated soil behaves as a kaolinite soil: the plasticity index andswelling pressure reduce, while the permeability and unconfinedcompression strength increase. Previous geotechnical properties im-prove with curing time due to the slow growth and crystallizationof new hydrates (CSAH).

5.4. Microstructure observed by Transmission ElectronicMicroscopy (TEM)

Numerous observations were conducted with various samples; thepresented results give themean characteristics of all these observations.

5.4.1. Observations perpendicular to the direction of compaction (ab facessection)

The high-resolution images in Fig. 3a for untreated FoCa andFig. 3b for lime-treated FoCa show the stacking of clay layers. Thissuggests that FoCa is an interlayered smectite / kaolinite with an im-portant segregation of kaolinite, as observed by XRD on FoCa (Guillot,2002). Fig. 3b and the enlarged square show the heterogeneous orga-nisation characterised by:

− compact blocks of 5 to 20 kaolinite layers with regular interlamel-lar space of 0.7 nm and a weak lateral extension (about 50 nm).

− stackings of smectite layers of a few curved layers with irregularinterlamellar space between 1.2 and 1.5 nm and a greater lateralextension (about 100 nm or more)

A comparative study of Fig. 3a and b shows that flocculation is ob-served on the treated samples; the number of clay layers per particledoubled and individual layers disappear (rings on Fig. 3a and b). Thisbehaviour is explained by the calcium cation exchange in smectitewhich reduces the swelling of the inter-lamellar space and facilitatesthe aggregation of layers. However, no other well crystallised phasesare observed on these TEM high resolution images.

These observations demonstrate and can explain the changes inthe geotechnical properties: the flocculation (higher number of layersper particle) induces reduction in the plasticity and an increase in themechanical resistance of the treated FoCa. Moreover, the behaviour ofstacked layers is less flexible (plastic) than a single layer.

In the tested samples, the total volume was maintained constant,so increasing the dimension (thicknesses) of particles induces higherpore spaces, which explain the higher permeability in the treated FoCa.

23M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

5.4.2. Observations parallel to the direction of compaction of a lime-treatedsample (ab faces)

Fig. 4 (TEM images and electron diffractograms) and Table 5(chemical analyses) of various samples of lime-treated FoCa showthat it contains:

− the initial products of the mixture: FoCa clay (Fig. 4a) and limeadded (Fig. 4b)

− calcite formed by the reaction of atmospheric carbon dioxide onlime (Fig. 4c)

− but also new phases with the composition of a calcium silicate al-uminium hydrate (CSAH) formed by the action of lime on clay(Fig. 4d, e, f, g and h).

Fig. 4a shows the original FoCa clay:

− The TEM image show crystals with the 120° angle typical of kao-linite ab faces.

− The electron diffraction diagram corresponds to a phyllosilicate,such as smectite and kaolinite of the FoCa. The indexation of re-flections (JCPDS 80–0886 for the kaolinite, 13–0135 for the mont-morillonite) shows that only (hk0) reflections are observed,which means that the layers are perpendicular to the electronbeam.

− Chemical analysis results (Table 5) may be explained by a mix ofdioctahedric calcic smectite and kaolinite. These results agree

a: TEM image and electron diffraction of FoCa clay in the selected area

b

c: TEM image and electron diffraction of calcite (CaCO3) in the selected area

dC

Fig. 4. TEM images and electron diffraction of typical samples of a lime treated FoCa clay. a:electron diffraction of lime, Ca (OH)2 in the selected area. c: TEM image and electron diffracti9% in the selected area. e: TEM image and electron diffraction of CSAH 18% in the selectedimage and electron diffraction of CSAH 28% in the selected area. h: TEM image and electron

with the FoCa mineralogical composition (Atabek et al., 1991;Guillot, 2002) characterised by about half kaolinite Si 4 Al 4O10

(OH) 8 and by half Ca-smectite of mean composition (Si 8–X, Al X )(Al 4–Z–Y, Fe Y, Mg Z ) O 20 (OH) 4 (x+z)/2 Ca+nH 2O with x≈0,8;y≈0,4 and z≈0,2.

Fig. 4b shows the original lime added:

− The TEM image shows many small crystals: elongated cylinders ofa hundred nm, randomly oriented. Chemical analysis correspondsto a calcium hydroxide or calcite.

− But the diffraction diagram indexation agrees only with calciumhydroxide (JCPDS 84–1271 for calcium hydroxide)

Fig. 4c shows calcite:

− The TEM image shows one crystal with a rhombohedric symmetry:− The diffraction diagram corresponds to the calcite diffraction pat-

tern (JCPDS no.05-0586).− The chemical analysis results (71% of calcium, 23% silicium and 5%

aluminium) suggest that the sample is not only calcite but withprobably some FoCa clay and quartz.

Fig. 4d, e, f, g and h show a new component at different stages offormation:

− TEM images of this new phase show an evolution from the claylayer morphology to the appearance of a less compacted structure,

: TEM image and electron diffraction of lime, Ca(OH)2 in the selected area

: TEM image and electron diffraction ofSAH 9% in the selected area

TEM image and electron diffraction of FoCa clay in the selected area. b: TEM image andon of calcite (CaCO3) in the selected area. d: TEM image and electron diffraction of CSAHarea. f: TEM image and electron diffraction of CSAH 21.5% in the selected area. g: TEGdiffraction of CSAH 35% in the selected area.

e: TEM image and electron diffraction of CSAH 18% in the selected area

f: TEM image and electron diffraction of CSAH 21.5% in the selected area

g: TEG image and electron diffraction of CSAH 28% in the selected area

h: TEM image and electron diffraction of CSAH 35% in the selected area

Fig. 4 (continued)

24 M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

looking like a crumpled sheet of paper. This new phase growsmainly on the edges of clay particles.

− This new phase has a different composition as revealed by chemicalanalyses (Table 5). In these figures, the calcium proportion is higher(from 9% to 35%) than in the non-treated FoCa (only 2%). This phasehas the composition of a calciumsilicate aluminiumhydrate (CSAH).

− As calcium increases from 9% to 35%, the morphology of the newphase changes and seems less and less compacted. A decrease in al-uminium follows the calcium increase, silica staying approximatelyconstant.

− The electronic diffraction diagrams show, as the calcium percent-age increases, a progressive elimination of clay reflections alongwith a new ring at 0.29–0.30 nm of increasing intensity. Moreover,central diffusion develops, which can be attributed to a gel forma-tion, as the calcium percentage increases.

Table 5Micro chemical analyses (Energy Dispersif X-rays) of tested samples.

Elements Atomic percentage of the 5 elements in the samples

4aclay

4blime

4ccalcite

4dCSAH

4eCSAH

4fCSAH

4 gCSAH

4 hCSAH

Ca 2 94.5 71 9 18 21.5 28 35Al 37 0 5 34 21.5 22.5 17 10Si 56.5 2.5 23 53 57 52 52 54Fe, Mg 4.5 3 1 4 3.5 4 3 1

Changes in the properties at macroscopic level and the observednew morphology of the treated FoCa can be explained by the forma-tion of the new hydrate composed of Ca, Si and Al. With curingtime, the growth and crystallization of CSAH and their connectionswith clay particles (pozzolanic reaction) solidify the structure and im-prove the main geotechnical properties of soil.

5.5. Microstructure observations by Scanning Electron Microscopy (SEM)

Scanning electron microscopy allows us to observe the effect oflime treatment on morphological structure (observations at μmscale) and, by chemical analysis, the calcium localisation on clay par-ticles. SEM images of untreated (Figs. 5a and 6a) and 10% lime treated(Figs. 5b and 6b) clay at two scales (5 μm and 50 μm) show very dif-ferent faces (size, homogeneity) and holes (size, pores open orclosed). The lime treatment strongly modifies the clay texture at themicrometer scale.

Figs. 5a and b (small magnification) show that the flat and largeparticles in the untreated sample are highly divided, inducing a highnumber of small connected particles making a structure more com-pact in the treated sample. These images agreed with the decreaseof the plasticity index and the increase of unconfined compressivestrength.

Fig. 6a and b (large magnification) show pores are connected onlime-treated samples, making a structure more permeable. These ob-servations corroborate the increase in the permeability of the treatedsamples.

a: SEM image of untreated FoCa clay

b: SEM image of 10% lime treated FoCa clay

Fig. 5. SEM images of untreated and lime treated FoCa clay. a: SEM image of untreatedFoCa clay. b: SEM image of 10% lime treated FoCa clay.

a: SEM image of untreated FoCa clay

b: SEM image of 10% lime treated FoCa clay

Fig. 6. SEM images of untreated and lime treated FoCa clay. a: SEM image of untreatedFoCa clay. b: SEM image of FoCa clay treated with 10% lime.

25M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

Fig. 7a shows two lines of analysis: LP1 parallel to the particleedges and LP2 towards the inside of the particle on which Ca, Si andAl concentrations are analysed. Fig. 7b shows on line LP1 that the cal-cium concentration is constant, and greater than the normal clay cal-cium concentration, on the border of the particle. Fig. 7c shows online LP2 that the Ca concentration decreases towards the inside ofthe particle as Si and Al concentrations increase.

The Si/Al concentration ratio remains equal to 1.5 on lines LP1 andLP2. These analyses show that calcium is mainly present on the outerpart of the particle and moves inside from the pores. The results con-firm TEM analysis concerning the different chemical composition ofCa within soil.

6. Conclusion

The geotechnical properties of the clayey soil change immediatelyafter the treatment with lime. These treated soils lose their cohesiveproperties and behave as a granular material: the plasticity indexand swelling pressure reduce, while unconfined compressive stressand permeability increase. In fact, all these property changes havetheir origin in the modification induced by the lime-clay reactionsin the texture and the structure of the treated clayey soil. These prop-erties change also with curing time, as modification in the microscop-ic scale continues with the progression of the slow pozzolanic lime-clay reaction.

SEM shows morphological changes when FoCa is treated withlime: a more compact structure, and thicker but less large particleswith interconnected pores. TEM reveals the different stages of thelime-clay reaction:

1) the flocculation of the particles and the increase of stacked layerswith high resolution images of lime-treated and untreated FoCasoil

2) the formation of calcite crystals (lime carbonation)3) the creation of a new phase in the lime-treated FoCa. This new

phase is not detected by XRD due, probably, to the small extensionand very low crystallinity of this phase. This hydrate has thechemical composition of a Calcium Silicium Aluminium Hydrate,CSAH. These micro-phases are observed on the borders of clayparticles; their aspects progress from compact phillosilicate layersto porous, membranous sheet and their extension increases withthe quantity of calcium. The morphology of this CSAH looks likea waved membrane forming a porous lamellar structure (like acrumpled paper sheet).

With the measurements and observations obtained, the mecha-nism of the lime-clay pozzolanic reaction can be described by pro-gressive dissolution of large clay crystals (starting from the edge ofthe particle) followed by synthesis of CSAH (formed with Ca++ oflime and the products of clay dissolution). The lime reaction is abasic attack which promotes a progressive destruction of the aluminosilicates; octahedric and tetrahedric sheets are slowly taken into solu-tion. On electron diffractograms, the clay reflections slowly disappearas the calcium concentration increases and the morphology changesfrom compact to porous, which means samples with a more completelime-clay reaction. The solution precipitates and forms with SiO4 tet-rahedra of the clay, a hydrated cementitious product. Electron diffrac-tograms show new diffraction rings (different from clay and lime)and central diffusion (porous structure) which correspond to thenew phase. The greatest CSAH extension is obtained with the highestcalcium concentration in the lime-treated FoCa.

a: SEM image of 10% lime treated FoCa (Chemical analyses lines LP1 and LP2)

2µm

c: Calcium, aluminum and silica concentrations of line LP2, line towards inside of particle

b: Calcium, aluminum and silica concentrations of line LP1, line parallel to a particle edge

Fig. 7. SEM images of untreated and lime treated FoCa clay and chemical analyses. a: SEM image of 10% lime treated FoCa (Chemical analyses lines LP1 and LP2). b: Calcium,aluminum and silica concentrations of line LP1, line parallel to a particle edge. c: Calcium, aluminum and silica concentrations of line LP2, line towards inside of particle.

26 M. Al-Mukhtar et al. / Engineering Geology 139-140 (2012) 17–27

The small amount of newly formed “calcium hydrates” due to thepozzolanic reaction was not always detected by all the techniquesused, as measurements were realised within a short time of curing.However, coupling the macro and microscopic techniques allows usto analyse the lime-clay reactions and to explain the improvementof geotechnical properties.

Acknowledgements

The authors acknowledge the assistance of Mssrs Alain Pineau,C. Clinard and T. Cacciaguerra for the TEM images and S.K.Vanapalli for his valuable comments and for providing languagehelp.

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