ENVIRONMENTAL GEOMECHANICS CE-641 Lecture No. 22 Prof. D N Singh - - PowerPoint PPT Presentation

ENVIRONMENTAL GEOMECHANICS

CE-641 Lecture No. 22

• Prof. D N Singh

Department of Civil Engineering

6.11.2019 Lecture No. 22 Lecture Name: Geomaterial Characterization

Sub-topics

• Pore-Structure Determination

– Scanning Electron Microscopy (SEM) – Mercury Intrusion Porosimetry (MIP)

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

Volume of mercury 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

• f

the largest fraction

• f

interconnected pores.

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 20 40 60 80 100 100 10 1 0.1 0.01 1E-3 0.00 0.01 0.02 0.03 0.04 0.05

VHg (cc/g)

Pore Diamater (m) (a)

dt % volume intruded

(b)

dm (dVHg/d (log d), cc/g)

(c)

dc

10 20 30 40 50 60 70 80 90 100 0.0 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

• f concrete with curing

time