Courtney Collins . Jason Ideker . Gayle Willis . Jessica Hurst
Lithum Additives:The Cure of Alkali-Silica Reaction?
• What is ASR?
• How does ASR gel expand?
• How could lithium prevent ASR gel expansion?
Outline
Alkali Silica Reaction (ASR)
Alkalis+
Reactive Silica
+Moisture
ASR Gel which
expands
Concrete expansion
andcracking
What is ASR?
Images of ASR damage
Images of ASR damage
Images of ASR damage
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)
Why does the gel expand?
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 particle2. “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 increases4. electrostatic repulsive forces push particles apart
Why does the gel expand?
Initial state
Double-Layer Theory (Prezzi, 1997)
Why does the gel expand?
Water is taken in,increasing layer sizearound particle
Electrostatic forces predominate,particles push apart and gel increases
Double-Layer Theory (Prezzi, 1997)
Why does the gel expand?
Water is taken in,increasing layer sizearound particle
Electrostatic forces predominate,particles push apart and gel increases
Double-Layer Theory (Prezzi, 1997)
Osmotic Pressure Theory (Hansen, 1944)
Why does the gel expand?
• 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
CSH-Shell Theory (Chatterji et al. [1986, 1988])
Why does the gel expand?
• Calcium reacts with grain to make calcium silicate hydrate (CSH), forming a shell around the grain.• Silica reacts to form gel, grain size increases
Calcium/Alkali Exchange Theory (Rotter 1995)
Why does the gel expand?
• Gel takes in Ca+2 ions, releasing alkali ions to react and create more alkali-silica gel
Why does the gel expand?
• 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
Why the theories may be inaccurate:
How does lithium help?
How does lithium help?
Theory 1: Silica loves lithiumLithium (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 radiiRate 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 layerThe 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?