environmental geomechanics ce-641 lecture # 15
TRANSCRIPT
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ENVIRONMENTAL GEOMECHANICS
CE-641
Lecture # 15
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Geomaterial Characterization
Sub-topics
• Need for Geomaterial characterization
• Geotechnical
• Mineralogy
• Morphology
• Physical
• Chemical
Pore-solution sampling
Corrosion potential
Sorption-Desorption
• Thermal
• Electrical
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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.
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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.
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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.
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Kaolinite plate stacks
Face-Face interaction
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Face-Edge & Edge-Edge
interactions
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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
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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
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Conical
Slits Cylindrical
Spherical or
Ink Bottle Interstices
Shape of Pores
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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
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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
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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
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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
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Hg cannot enter pores under
vacuum
An increasing pressure forces
Hg to penetrate all accessible pores
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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
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Two systems presenting similar mercury
intrusion test results
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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
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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