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 and cracking What is ASR?

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 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? Initial state Double-Layer Theory (Prezzi, 1997)

Why does the gel expand? Water is taken in, increasing layer size around 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 size around 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?

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) LiNO 3 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, LiNO 3, 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?