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?