ENVIRONMENTAL GEOMECHANICS

CE-641

Lecture # 15

Geomaterial Characterization

Sub-topics

• Need for Geomaterial characterization

• Geotechnical

• Mineralogy

• Morphology

• Physical

• Chemical

Pore-solution sampling

Corrosion potential

Sorption-Desorption

• Thermal

• Electrical

Scanning Electron Microscopy (SEM)

For obtaining very detailed images at much higher magnifications ~100,000x than is possible with a light microscope. The SEM images the surface structure of bulk specimens (biological, medical, materials sciences and earth sciences) Image is created by using electrons instead of light waves. Images have a greater depth of field and resolution than optical Micrographs. Ideal for fracture surfaces & particulate materials. Energy Dispersive Spectrometer (EDS) allows elemental analysis (Sodium to Uranium, excluding Lanthanides, Actinides & gases down to levels of ~0.1 wt %) with the SEM. X-ray mapping is also possible, which shows the distribution of elements in the material. X-ray line-scans show the concentration variation of elements along a line in the material.

SEM- Working principle

• A beam of highly energetic electrons is focused on the sample

• Interaction of electrons is transformed into a 3-D image to obtain

topographical, morphological, compositional & crystallographic information.

Compacted sample Cubic specimen

Determination of fabric structure of fine-grained soils

Using SEM

Specimen preparation (Challenges):

• Removal of pore fluid from the specimen without disturbing its microstructure.

• Freeze-drying technique (for swelling/shrinking type of soils)

• Air-drying technique (for non swelling/shrinking type of soils)

• Specimen should be able to withstand the vacuum inside the microscope.

• As illumination is with electrons, specimen should be made to conduct electricity.

• Specimen are coated with a very thin layer of Gold or Carbon (a sputter coater).

• Gold coating film can absorb X-ray signal generated into the specimen.

• For obtaining X-ray spectrum of a non-conducting sample a coating material very

transparent to the X-ray (Carbon) must be utilized.

Kaolinite plate stacks

Face-Face interaction

Face-Edge & Edge-Edge

interactions

Geomaterials are composed of wide range of particle sizes and

shapes and are porous in nature.

A knowledge of pore structure of these materials is important as it can

give insight in to both the microstructure and the performance.

Rather than measuring the porosity, It becomes more informative if the

manner in which volume is distributed With respect to pore size.

Mercury Intrusion Porosimetry (MIP)

Dead end

Closed

Inter-connected

Passing

Non-porous solids

(Extremely low surface area)

Porous solids

medium high surface area, pore

volume and dimension

Particulates

particle size and surface area

Catalysts:

activated sites on porous

support or powder

Porosity

Conical

Slits Cylindrical

Spherical or

Ink Bottle Interstices

Shape of Pores

Micropores: 0 < d < 2 nm (zeolites, carbons, silica fumes)

Mesopores: 2 < d < 50 nm (alumina, polymers, catalysts)

Macropores: 50 < d < ...nm (rocks, cements, soils, ...)

Bulk, apparent and real density [g/cc]

Percentage porosity [%]

Pore volume/pore size distribution [pore volume vs pore size]

Total pore volume [cc/g]

Average pore size

Specific surface area [m2/g]

Particle size distribution [relative percentage vs particle size]

Pore size classification and parameters

Pore size distribution

Particle size distribution

Bulk density

Apparent density

Total porosity

Pore area distribution

Low/high specific surface

Micro/mesopores distribution

Micro/mesopores total

volume

Real density

Mercury porosimetry

Gas adsorption

Helium Pycnometry

Characterization schemes

Mercury Intrusion Porosimetry (MIP)

• Mercury intrusion Porosimetry is regarded as a standard

measure for macro and meso pore size distributions.

• Since this technique is Conceptually much simpler.

• Experimentally much faster .

• Unique in its ability to evaluate a much wider range of

pore sizes than the alternative methods (gas sorption ,

calorimetry, scanning electron microscopy,

thermoporometry).

• The technique of mercury Porosimetry is used not only

to determine the distribution of pores in various soils but

also how it changes for various loading conditions

Mercury Porosimetry concept

• Hg is a non-wetting liquid for many solids

• Hg must be forced to penetrate pores

• Penetration pressure is related to pore size

• Volume of Hg is related to pore volume

wetting non wetting

Hg cannot enter pores under

vacuum

An increasing pressure forces

Hg to penetrate all accessible pores

Working principle:

P = 2.(T.cosθ)/r ……Washburns Equation

Volu

me

of m

erc

ury

Pressure

Intrusion curve

Extrusion curve

A Information obtained

the pore size distribution

surface area

equivalent pore size

critical pore diameter

distribution of total porosity

free porosity and trapped porosity

Typical MIP characteristic curve

A: hysterisis

Two systems presenting similar mercury

intrusion test results

Different forms of pore size distribution curves for a concrete sample

dt : pore size at which

there is a sudden

increase in the number,

and therefore the

cumulative volume, of

pores

dm: mean pore diameter,

which corresponds to the

pore diameter at which

50% of the pore volume

gets intruded in the pore

size range considered

dc : continuous pore

diameter, the maxima of

the curve. Corresponds

to the group of the

largest fraction of

interconnected pores.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0

20

40

60

80

100

100 10 1 0.1 0.01 1E-30.00

0.01

0.02

0.03

0.04

0.05

V

Hg (

cc/g

)

Pore Diamater (m)

(a)

dt

% v

olu

me

in

tru

de

d

(b)

dm

(dV

Hg/d

(lo

g d

), c

c/g

)

(c)

dc

0 10 20 30 40 50 60 70 80 90 1000.0

0.1

0.2

0.3

0.4

0.5

0.0

0.2

0.4

0.6

0.8

1.0

0.00

0.05

0.10

0.15

0.20

0.25

dc (m

)

t (Days)

dt (m

)C1 C2 C3 F1 F2 F3

dm (m

)

Variation of pore diameters

of concrete with curing

time