Clays, clay minerals and soil shrinkswell behavior

Clays, Clay Minerals and Soil Shrink/Swell Behavior Hillel, pp... • Volume and pore space of swelling clayey soils vary with hydration state.. • Constitutive transport properties for s

Clays, Clay Minerals and Soil

Shrink/Swell Behavior

Hillel, pp 75-100

• Volume and pore space of swelling clayey soils vary with

hydration state.

• Shrink-swell phenomena affect many mechanical and

engineering properties of soils and clay liners.

• Constitutive transport properties for swelling soils are

complicated by hydration-dependent soil attributes (pore

space, strength, etc).

Clay shrink/swell damage to structures & roads

 Changes in soil water content or solution chemistry of clayey soils induce swelling pressures sufficiently large

to fracture and damage structures and roads.

 Estimated damage in excess of $7 billion/yr in the US.

● Distinguish between “clay” size <2 µ m and clay minerals

● Basic building blocks of clay minerals:

● Silica centered tetrahedra

● Al 3+ (+ other cations Mg 2+ ) centered octahedra

Clay Minerals – building blocks

● The tetrahedra are joined (share

oxygen) at their basal corners in a

hexagonal pattern forming flat sheets

~ 0.493 nm thick.

● The octahedra join along their edge to

form triangular array 0.505 nm thick

● Stacked sheets form lamellae

Formation of Silica and Alumina Sheets

1 0 m µ

Isomorphic substitution

● The space occupied by silica in a tetrahedron can

accommodate atoms <~0.4 times O 2 radius (Si 4+ & Al 3+ ).

● Octahedra - 0.732 times O 2 radius (accommodates iron,

magnesium, aluminum, manganese, titanium, sodium, calcium)

● Substitution of central atoms with valence < +4 (tetrahedron)

or < +3 (octahedron) during crystallization is known as

isomorphic substitution and results in net negative charge

that must be balanced externally by adsorption of cations.

● These cations are not permanent and can exchanged by other cations in soil solution

Cation Exchange Capacity

● The cation exchange capacity (CEC) describes the

amount of exchangeable cations per unit soil mass:

CEC = cmol of positive charge/kg

● CEC values range from 2-15 cmol + /kg for Kaolinite;

20-40 illite, and 60- 100 for montmorillonite.

Formation of a Diffuse Double Layer (DDL)

● Some of the exchangeable cations are

bounded to surfaces whereas others

may be dispersed in the aqueous

solution – hence a “double layer”…

● The distribution of cations (and

associated anions) in solution reflect a

balance between electrical and thermal

forces resulting a diffuse “cloud” of

cations with concentration diminishing

with distance from clay surface.

● The extent of this diffuse layer is not

constant and varies with solution

concentration, clay hydration, cation

valence and clay type.

Different Clay Minerals

● Distinguished by number and order of layering of basic tetra & octahedral sheets

● 2:1 - one octahedral sheet sandwiched

between two tetrahedral sheets

● Many isomorphic substitutions: Mg +2 ,

Fe +2 , & Fe +3 for Al +3 in octa

● High surface area (600-800 m 2 /g)

● Large CEC

● Very active shrink/swell behavior

1 0 m µ

Exchangeable cation

● 1:1 alternating octa/

tetrahedral sheets.

● Few isomorphic substitutions

● Thicker and stable stacks

● Relatively low surface area:

5-30 m 2 /g

● Not much swelling

Swelling and changes in lamellar Spacing

-

-+ +

+ +

Swelling and Lamellar Spacing

• Clay lamellar spacing increases

with increasing potential (less

negative/ wetter) resulting in

swelling.

• Interacting diffuse double layers

(DDL) dominate swelling behavior.

• Reasonable agreement exists

between measured lamellae

spacing and DLVO-theory:

approaching DDLs develop a repulsive

force proportional to excess ions

relative to bulk (giving rise to

osmotic pressure).

Low [1980]; Warkentin et al [ 1957]

Interacting DDL and swelling pressure

 When two DDLs approach each other they

develop a repulsive force that is

proportional to excess ions relative to bulk

(giving rise to osmotic pressure).

 A convenient point for evaluation is

midplane where d ψ /dx=0 (due to symmetry

for equal surfaces).

 Langmuir [1938] calculated the swelling

pressure as:

which simply van’t Hoff relations.

 For short separation distances Langmuir

obtained:

) 1 Y

(cosh RTc

2 )

c 0 = bulk electrolyte concentration [mol m -3 ]

e = electron elementary charge [1.60218x10 -19 C]

k = the Boltzmann constant [1.38066x10 -23 J K -1 ]

R = universal gas constant [8.3145 J mol -1 K -1 ]

ψ 1 = ψ (h/2) mid-plane electric potential [V]

z = signed ion valence.

kT n

) h

Π

The “trick” is how to determine the

mid-plane electric potential ψ 1 ?

Scale electric potential

Measurement of swelling pressure

• A very useful approximation for

swelling pressure at large spacing

and weak interactions was developed

by Derjaguin [1987]:

• Note that this expression is

dependent on surface potential ψ 0

(and not on mid-plane ψ 1 )

Large spacing weak interaction approximation

h

2 0

e ( h ) = 64 n kT γ e − κ Π

Low [1980]; Warkentin et al [ 1957]

ze tanh 4

Y

γ

 Consider two DDLs separated by a

distance of h=5 nm with bulk monovalent

electrolyte concentration of

[NaCl]=0.001 M; surface potential ψ 0 =55

mV Find the swelling potential.

 Using the approximation:

 Simplified as:

 Approximating κ :

Calculation of swelling pressure - Example

h 2 0

e ( h ) = 64 n kT γ e − κ

Π

] nm [ ] NaCl [

/ 304 0 /

1 κ =

 We find that Π e (5 nm)=22.5 kPa

Changing the concentration to 0.01 M, we obtain (5 nm)=73.2 kPa

h 0

2 8

e ( h ) = 1 59 x 10 [ NaCl ] tanh [ ψ ( mV ) / 103 ] e − κ

Π

Lamellar swelling – the disjoining pressure

• A more general treatment considers the various interactions

between charged clay surfaces and aqueous solutions using the

disjoining pressure formalism (Π), or the so-called DLVO theory.

• The equilibrium potential ( µ ) as function of water film thickness (h =half lamellar spacing) is comprised of three primary

components:

1

h e

Π

= ρ

µ

Where:

3

ssl m

h 6

A )

h (

π

= Π

λ

/ h h

h ( h ) = K exp −

Π

h 2 0

( ( h ) ( h ) ( h ) )

1

h e

h 6

A )

h ( h ) = K exp −

Π

h

2 0

e ( h ) = 64 n kT γ e − κ

Π

van der Waals forces (attractive)

hydration force (repulsive)

electrostatic force (repulsive)

The disjoining pressure at equilibrium

Mesopores and their role in volume change

network with micropores

separating tactoids

(quasi-crystalline stacks of lamellas).

 Important for modeling clay

fabric response.

 Lamellar swelling alone cannot

explain volume changes and water

retention in clay fabric

Hydration effects on clay fabric geometry

network with micropores between

evolution of microstructure and bulk

volume of Greek Na+ montmorillonite

during first drying [Tessier, 1990]

structure and micropores (1-2 mm)

0.03 bar

1.0 bar

10 bars

Interacting DDL different electrolytes

• Ion distribution between two clay surfaces – different electrolytes.

Electrolyte effects on microstructure

microstructure prepared with dilute solutions [Tessier, 1990]

concentration affects:

 Arrangement and spacing

between layers (smaller

for Ca 2+ ), between

ordered stacks, and

between tactoids.

 Number of layers and

apparent length of

quasi-crystals (tactoids) -

more lamellae for Ca 2+

Evolution of clay fabric - mesopore formation

skeletal pore space by jell-like clay fabric.

formed between glass beads.

among other soil textural components remains

Clay barriers for waste isolation

Clay Liners

Clay layers (Bentonite) to prevent leaching

Geotextile layers for mechanical stability

Clay Liners

Geotextiles are permeable fabrics

(polypropylene, polyesters, etc.)

which, when used in association

with soil, have the ability to

separate, filter, reinforce, protect

or drain.

Geomembranes are impermeable

membranes used widely as

cut-offs and liners

Clay Liners

Effect of shrink-swell on soil pore volume

Shrink-swell affects soil pores at all scales

Microscale

(clay fabric)

Mesoscale (texture)

Macroscale (cracks)

Effect of clay content on porosity & permeability

porosity is about 35-40% (and minimum overall porosity).

Modeling clay fabric geometry

(a) SEM of montmorillonite; (b) approximated clay fabric structure; and (c) idealized clay fabric representation applied in the model

used to derive and constrain parameter values

for the idealized clay fabric.