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Literature review on the study ofchanges in pore size distributionof clay upon loading

Jun Kang ChowSupervisor: Prof. YH Wang

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Interpret the changes in poresize distribution of clay

during the loading processes.

Preconsolidation

SEM MIP

Loading

Freeze drying

Washing

Adjust properties

Direct observation(from SEM)

Measurement (from MIP)+

Current stage of research(Wang, Y.H. & Xu, D., 2007; Wang, Y.H. & Yu, C.Y.,

(unpublished)) Sample : Speswhite

kaolin

Mechanical responses of kaolinite claydepend on cluster-to-cluster interactions

rather than particle-to-particle interactions

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Samples obtained before andafter loading for MIP test arenot identical .Freeze-drying process mayalter the pore sizedistribution of clayspecimens.

Preconsolidation

SEM MIP

Loading

Freeze drying

Washing

Adjust properties

Results of MIP test show thatlarger pores reduce in size after

loading.

This does not guarantee thepores involved are inter-aggregated pores .

PROBLEMS ?

Current stage of research(Wang, Y.H. & Xu, D., 2007)

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~ 0.42 m (pore C of pH 4 spec.)~ 0.18 m (pore B of spec. withsalt)

~ 1.22 m (pore D of spec. withsalt)

~ 0.42 m (pore B of pH 7.8spec.)

Existence of smaller/larger Types B, C and D pores than thesizes indicated.SEM image shows that particles that form aggregatedassociations can be unevenly arranged, creating larger intra-

aggregates pores .

Result of MIP test(Wang, Y.H. & Xu, D., 2007)

Figure is taken from Wang, Y.H. and Xu, D.(2007)

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We are unable to observe

the loading process.

We are unable to visualize

how the arrangement of clayfabric is altered due toloading.

To search the suitable method thatallows the visualization of changes

in clay fabric arrangement upon

loading

HOW?

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Methods to study pore sizedistribution of clay

Directobservation

Measurementtechnique

Scanning ElectronMicroscope (SEM)EnvironmentalScanning ElectronMicroscope (ESEM)X-ray computedtopography (CTscanning)

Mercury IntrusionPorosimetry (MIP)Nitrogen sorptionmethod

Non-polar liquiddesorptionWater-desorption

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Systematic errors due to:Inappropriate pore model (assuming cylindrical pore model)Variation in surface tension and contact angle

Measurement of pore size distribution of clay (Lawrence, G.P., 1977; Westermack, S., 2000)

Mercury intrusionporosimetry

Nitrogen sorption Non-polar liquiddesorption

Water desorption

Measure poresize of ~ 3 nm 200 mMeasure thepressure requiredto force mercuryinto poresErrors due to:distribution ofpore volume andpore collapse

Carried out atlow temperature(~ -196 C)Able todetermine rangeof pore size ~ 0.3- 300 nm

Interactionsbetween non-polar liquidsand claysurfaces are

weakEquilibriumtimes are long

Most fine-textured soilsshrink andparticlerearrangement

takes placeStresses onpore wallsduring anincrease insuction cause a

reduction inpore size

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A pore system cannot be adequatelycharacterised by the use of only onemethod.Combine both methods to measure thepore size distribution of clay. (Sills, I.D. etal. 1973a, 1974)

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Technology available

At this T, only a modest gas pressure isrequired to stabilize water (achieve saturation)

Method

Focus

Type of samples

Sample Size

Preparation

Resolution

Workingtemperature

SEM

Morphology ofmaterials

Dry samples(Absence of water)

~ few mm indiameter

Freeze-drying &coating of samples

are required

~

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An electron beam is emittedtowards a conductive sample inhigh vacuum conditions .

Secondary electrons are releasedfrom the sample, collected bydetector and amplified to producean image.

The conductive coating of sample(gold) improves the image quality.

The vacuum ensures the effectiveoperation of the electron gun.

Working principle of SEM

(Absence ofwater)

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Working principle of ESEM When electrons are emitted, theyhave to travel through the ga s .

Secondary electrons (with lowenergies) have a high collisioncross-section with the gas

molecules.Ionizing collisions lead to thegeneration of addition daughterelectrons a cascadeamplification . At the same time, positive ions arealso produced.

These ions drift down towards thesample compensate the chargebuild-up at the surface of insulators.

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ESEM Quanta 250 FEI NanoCharacterization

Metals & alloys,magnetic &superconductingmaterials

Ceramics, composites,plasticsFilms/coatingsGeological materials

mineralsSoft materials polymers, gels,tissues, plant

Particle, porousmaterials fibers

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Application of ESEM Journal Description

Montes-H, G. et al.(2005) To study the swelling-shrinkage behavior of compactedbentonite submitted to hydration/dehydration conditions

Vaunat, J. et al. (2007) To study the residual shear strength of a plastic clay(Boom clay) under high suction

Zhang, G. et al. (2004) To study how Fe oxides alter the soil structure of a

highly weathered oil alluviumSelvam, A. &Barkdoll, B. (2005)

To observe changes in clay fabric using ESEM &correlate it to the permeability changes occurring in clay

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One Sample

Preparation Stage:WashingAdjusting

PropertiesPreconsolidation

Loading

ESEM

Varyloading

Many Samples

Preconsolidation

SEM MIP

Loading

Freeze drying

Washing

Adjust properties

Future work/Challenge:

Design of cellDigital imageanalysis