Role of Potassium Acetate Deicer in ASR
Francis Nelson, Jamilla Beale, Li Ai, Leslie Struble
January 11, 2007
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
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
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
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
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
Deicer Expansion (Modified ASTM C 1260)
Sand 1 Deicer
Sand 2 Deicer
Sand 3 Deicer
Sand 4 DeicerSand 5 Deicer
Sand 1
Sand 2
Sand 3
Sand 4
Sand 5
0.000%
0.050%
0.100%
0.150%
0.200%
0.250%
0.300%
0.350%
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Time (days)
Exp
ansi
on (
%)
Rangaraju et al 2005
Key Issues Studied in Current Research
• Why do candidate OMP sands not expand in deicer?
• Why does deicer increase in pH?
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
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
0.00%
0.05%
0.10%
0.15%
0.20%
0.25%
0.30%
0.35%
0.40%
0.45%
0.50%
0.55%
0.60%
0% 10% 20% 30% 40% 50% 60%
Chert in Aggregate (%)
Exp
ansi
on
(%)
5-M KAc standard 1-M NaOH
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
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
0
2
4
6
8
10
12
14
16
0 10 20 30 40 50 60
Time, hours
pH KAC
Deicer
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
Solution IonsBefore reaction,
mol/LAfter reaction,
mol/L
Deicer + Ca(OH)2
Ac- ~6.20 7.12
K+ ~6.20 6.28
Ca2+ 0 0.013
KAc+ Ca(OH)2
Ac- ~6.10 6.78
K+ ~6.10 5.46
Ca2+ 0 0.02
Solid (per 1g)
IonsBefore reaction,
mg/gAfter reaction,
mg/g
Deicer + Ca(OH)2
Ac- 0 32
K+ 0 145
Ca2+ 541 314
KAc + Ca(OH)2
Ac- 0 33
K+ 0 136
Ca2+ 541 297
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
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
7
8
9
10
11
12
13
14
15
0 100 200 300 400 500 600
Added DD water
pH
pH Results: Titration
• Measured pH of solution after reaction with Ca(OH)2 as function of added water
• Measured values greater than calculated (=1)
• With addition of water, 1• No sharp drop in pH that would
indicate chemical reaction (dissolution or precipitation)
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)
11
12
13
14
15
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Concentration, M
pH
12
12.5
13
13.5
14
14.5
15
15.5
16
0.0 0.5 1.0 1.5 2.0 2.5
Concentration, M
pH
NaOH KOH
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
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
Chemical Activity
• Chemical potential () – tendency of substance to undergo chemical reaction
• Chemical activity (a) – effective concentration in thermodynamics– Related to potential
• Activity coefficient () – Relates activity to concentration
– In ideal solution (0) =1– Reflects intermolecular forces
)ln(0 aRT
ca
Top Related