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Lithum Additives: The Cure of Alkali-Silica Reaction?

Lithum Additives: The Cure of Alkali-Silica Reaction?

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Lithum Additives: The Cure of Alkali-Silica Reaction?

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  1. Lithum Additives:The Cure of Alkali-Silica Reaction? Courtney Collins . Jason Ideker . Gayle Willis . Jessica Hurst

  2. Outline • What is ASR? • How does ASR gel expand? • How could lithium prevent ASR gel expansion?

  3. What is ASR? Alkali Silica Reaction (ASR) Alkalis + Reactive Silica + Moisture Concrete expansion and cracking ASR Gel which expands

  4. Images of ASR damage

  5. Images of ASR damage

  6. Images of ASR damage

  7. Why does the gel expand?

  8. 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)

  9. 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

  10. Why does the gel expand? Double-Layer Theory (Prezzi, 1997) Initial state

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. How does lithium help?

  18. 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)

  19. 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)

  20. 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)

  21. 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)

  22. 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!

  23. Conclusion Any questions?