Role of Potassium Acetate Deicer in ASR

1. Role of Potassium Acetate Deicer in ASR Francis Nelson, Jamilla Beale, Li Ai, Leslie Struble January 11, 2007

2. Scope of Report • Review status of previous work • Present current study: effects of potassium acetate deicing solution • Mortar expansion in potassium acetate solution • pH of potassium acetate solution

3. Status of Previous Work • TN27, Alkali Silica Reaction in Concrete • Background on ASR • TN29, Alkali Silica Reactivity of Selected Sands • All 5 natural sands were potentially reactive and should not be used without mitigation • TN 30, Assessment of Selected Mineral Admixtures for Mitigating Alkali Silica Expansion • Slag effective at 30%, Class F fly ash effective at 10%, Class C fly ash not effective at 15%, silica fume effective at 10%, metakaolin effective at 10% • TN31, Alkali Silica Reaction of Candidate Aggregates for Concrete at O-Hare International Airport • Francis Nelson’s MS thesis

4. Background on ASR • Chemical reaction between alkali and hydroxide ions in pore solution and reactive silica in aggregate • Alkali generally derived from cement • Concentration of hydroxide controls aggregate dissolution, reaction requires pH>13.6 • Concentration of hydroxide roughly balances concentration of alkali • Reaction exhibits pessimum response Figure showing pessimum Mindess, Young, and Darwin

5. Background on Potassium Acetate Deicer • Non-chloride based deicer used for airport runways • Very concentrated KAc solution (~6 M) • Lowers freezing temperature (-76o F) • P. Rangaraju (Clemson University) showed that deicer causes considerable ASR expansion

6. Effect of Deicer on ASR • All candidate sands showed little expansion when tested in KAc deicing solution • Fused silica showed extreme expansion (~20%) when tested in deicing solution • Reaction requires pH>13.6, pH of deicing solution not this high but increases in contact with mortar Rangaraju et al 2005

7. Key Issues Studied in Current Research • Why do candidate OMP sands not expand in deicer? • Why does deicer increase in pH?

8. Expansion Experiments • Hypothesis: OMP sands do not expand in KAc because reactive constituent (chert) much below its pessimum proportion • In KAc solution (~6 M), pessimum proportion of chert may be higher than in standard test solution (1 M) • Experiment: mixed various proportions of chert and quartz sand and measured expansion in standard solution and KAc deicer solution

9. Expansion Results • Chert pessimum about 30% in standard NaOH solution • Little expansion of chert in KAc deicer solution at all proportions • Pessimum effect does not explain lack of chert expansion • Needs further study to be confident that OMP sands are not expansive in deicer solution

10. pH Experiment 1 • Hypothesis: reaction drives up pH, most likely dissolution of Ca(OH)2 and precipitation of CaAc • Experiment: mixed deicing solution and Ca(OH)2, measured pH, analyzed solid

11. pH Results: pH of Solution Mixed with Ca(OH)2 • Measured pH of 50-mL solution mixed with 5-g Ca(OH)2 • pH increased from 10-11 to more than 14.2 • Increase due to combination of potassium acetate and calcium hydroxide

12. pH Results: Chemical Compositions • Measured composition of solution and solid before and after reaction with Ca(OH)2 • [Ca2+] in solution still low, indicates little dissolution • Acetate content in solid still low, indicates little acetate precipitation • Increase in pH not due to dissolution of Ca(OH)2

13. pH Results: Phase Composition of Solid • Analyzed solid after reaction using X-ray diffraction • Solid mainly Ca(OH)2 and KAc • Results indicate no reaction between Ca(OH)2 and KAc

14. pH Experiment 2 • Hypothesis: OH-activity coefficient high due to very high ionic strength • Experiment: titrated solution, measured pH while NaOH or water was added • If pH due to high activity coefficient, expect to see gradual and progressive change in pH on titration

15. pH Results: Titration • Measured pH of solution after reaction with Ca(OH)2 as function of added water • Measured values greater than calculated (g=1) • With addition of water, g 1 • No sharp drop in pH that would indicate chemical reaction (dissolution or precipitation)

16. KOH NaOH pH Results: Titration • Measured pH of deicer solution or DD water as function of concentration of added OH- • pH in deicing solution much higher than in DD water at the same concentration • No sharp rise in pH that would indicate chemical reaction (dissolution or precipitation)

17. Conclusions • Pessimum effect does not explain lack of chert expansion • Very high pH of deicing solution when mixed with calcium hydroxide due to very high ionic strength of the deicing solution and the resulting high activity coefficient, which amplifies the modest [OH-] provided by the calcium hydroxide

18. Work Proposed for Next Period • Additional experiments to verify lack of expansion observed with chert in deicing solution • Compare dissolution of chert in 1-M NaOH (quick chemical test, ASTM C 289) and in KAc solution • When we are satisfied with our expansion results, we will prepare TN on expansion in deicing solution • Calculate theoretical pH in mixtures containing deicing solution • When we are satisfied with our understanding of the reaction chemistry, we will prepare TN on reaction chemistry in deicing solution

19. strong base neutral strong acid pH Brown, LeMay, Bursten

20. Chemical Activity • Chemical potential (m) – tendency of substance to undergo chemical reaction • Chemical activity (a) – effective concentration in thermodynamics • Related to potential • Activity coefficient (g) • Relates activity to concentration • In ideal solution (m0) g=1 • Reflects intermolecular forces

21. Ionic Strength • Ionic strength (I) – concentration of all ions in solution ci is concentration, zi is charge of ion I • Activity coefficient depends on ionic strength • Example given by Debye-Hückel model