Transcript Research Tutorial - Georgia Institute of Technology

Lithum Additives:
The Cure of Alkali-Silica Reaction?
Courtney Collins . Jason Ideker . Gayle Willis . Jessica Hurst
Outline
• What is ASR?
• How does ASR gel expand?
• How could lithium prevent ASR gel expansion?
What is ASR?
Alkali Silica Reaction (ASR)
Alkalis
+
Reactive
Silica
+
Moisture
ASR Gel
which
expands
Concrete
expansion
and
cracking
Images of ASR damage
Images of ASR damage
Images of ASR damage
Why does the gel expand?
Why does the gel expand?
Theories of expansion:
• Double-layer theory
(Prezzi, 1997)
• Osmotic pressure theory
(Hansen, 1944)
• CSH-shell theory
(Chatterji et al. [1986, 1988])
• CSH/alkali exchange theory
(Rotter [1995)
Why does the gel expand?
Double-Layer Theory
(Prezzi, 1997)
Solid particles in the alkali-silica gel are surrounded by a sea
of cations and anions
Two layers form:
1. “rigid layer” of cations near surface of particle
2. “diffuse layer” of cations and anions further
from particle surface
Expansion occurs when:
1. water is taken in
2. water increases layer size and lowers ionic strength
3. particle-distance increases
4. electrostatic repulsive forces push particles apart
Why does the gel expand?
Double-Layer Theory
(Prezzi, 1997)
Initial state
Why does the gel expand?
Double-Layer Theory
(Prezzi, 1997)
Water is taken in,
increasing layer size
around particle
Electrostatic
forces predominate,
particles push
apart and gel
increases
Why does the gel expand?
Double-Layer Theory
(Prezzi, 1997)
Water is taken in,
increasing layer size
around particle
Electrostatic
forces predominate,
particles push
apart and gel
increases
Why does the gel expand?
Osmotic Pressure Theory
(Hansen, 1944)
• Cement paste acts as semi-permeable membrane, allowing
water and cations in, while keeping silica from escaping.
• Gel expands, exerting pressure on surrounding matrix until
cracking results
Why does the gel expand?
CSH-Shell Theory
(Chatterji et al. [1986, 1988])
• Calcium reacts with grain to make calcium silicate hydrate
(CSH), forming a shell around the grain.
• Silica reacts to form gel, grain size increases
Why does the gel expand?
Calcium/Alkali Exchange Theory (Rotter 1995)
• Gel takes in Ca+2 ions, releasing alkali ions to react and
create more alkali-silica gel
Why does the gel expand?
Why the theories may be inaccurate:
• Role of short-range forces were not
accounted for in any theory
• Role of calcium was not considered for the
double-layer and osmotic pressure theories
How does lithium help?
How does lithium help?
Theory 1: Silica loves lithium
Lithium (Li+) is believed to be preferentially
bound to silica over Na+ and K+, making lithium
silicates that are non-expansive. (Stark, 1992)
Theory 2: ASR hates ions with large radii
Rate of silica dissolution decreases as cation
radius near silica surface increases. Therefore,
as Li+ has a larger hydrated radius than Na+ and
K+, concentrations of lithium near the silica would
decrease alkali-silica gel creation. (Wijnen, 1989)
How does lithium help?
Theory 3: Lithium and the double layer
The addition of lithium (Li+) causes the
concentration of ions in the double layer to
increase. Higher ionic concentration results in a
smaller double layer because attractive Van der
Waals forces predominate and particles are held
closer together. (Prezzi)
How does lithium help?
• Research shows a threshold concentration of
lithium exists, below which gel continues to
expand: [Li]/[Na+K]  0.74 (Blackwell, 1997)
• LiNO3 is preferred over other compounds
because it has no “pessimum” effect (increased
gel expansion at lithium doses below threshold
concentration). (Stark, 1992; Diamond and Ong,
1992)
Conclusion
What we know:
Lithium does help prevent ASR damage. Here is
one graph of expansion in mortar bars, each with a
different lithium additive. (LiOH, LiNO3, or LiCl)
Conclusion
What we don’t know:
• How does ASR gel expand?
• How does lithium help?
We at Georgia Tech, along with many other
researchers in concrete science, are looking to
answer those questions!
Conclusion
Any questions?