chemical composition of fly ash
DESCRIPTION
Fly Ash Chmeical composition and usesTRANSCRIPT
Some Thoughts on The Chemical Compositions of Fly AshICAR’s 17th Annual Symposium
The University of Texas At Austin
Lianxiang Du, Ph.D., PEDepartment of Civil, Construction & Environmental Engineering
The University of Alabama At Birmingham
Elizabeth Lukefahr, PETexas Department of Transportation
Presentation Outline
• Introduction
• Fly ash classifications
• TxDOT database on fly ash
• Variations of chemical composition of fly ash
• Some thoughts on fly ash classification based on chemical compositions
• Summary
Introduction• Fly ash origin
• Challenges for sustainable development and green construction
• Brief history of use of fly ash in concrete
http://geoserver.cee.wisc.edu/fauga/_borders/right.4.jpg
Origin of Fly Ash
• Power plants
• Burning of feed coal particles
• Light residue captured from the gaseous effluent
• Heavy residue forms bottom ash
http://showmethehoney.files.wordpress.com/2008/08/coal‐power‐plant.jpg
Challenges/Headaches• 50% of electricity in US generated is from coal (http://www.pewclimate.org/global‐warming‐basics/coalfacts.cfm)– 1,128.0 million short tons in 2007 (DOE report)– 32 million short tons of fly ash in 2007 (only 44% used)– Remaining fly ash stored (ponded)
• Challenges/headaches– Fly ash contains heavy metals– Coal ash spill highlights concerns about storage and handling
– Future Clean Air measures mayfurther complicate fly as chemistry
• Our understanding on fly ash is limited
Kingston Fossil Plant in Roane County, Tennessee, 12/22/2008
Brief History of Use of Fly Ash in Concrete
• Earliest proponents ‐ power companies– Dated back prior to 1932
• Raymond E. Davis et al at UC Berkeley should be credited (1937)– 15 fly ashes, 81 cements, and about 5,000 specimens
– Important conclusions still valid today• Finer particle size than portland cement
• Longer set time with fly ash inclusion (Class F)
• Fly ash replacement up to 50%
• Limiting LOI
• Lower hydration heat
• Better durability against sodium sulfate
Chemical Composition Information
• Similar to what I have today for fly ash
Davis et al 1937
Fly Ash Classifications• How to describe a fly ash?
– ASTM C 618• Class F (%): SiO2+Al2O3+Fe2O3 > 70
• Class C (%): SiO2+Al2O3+Fe2O3 < 70
• What else information used?
– Canadian specification CSA A 23.5• Type F ( ≤8 ± 1% CaO),
• Type CI (>8% and 20 ± 2%)
• Type CH (>20% CaO)
• Category approach: behave similarly when in the same category
• Is this approach reasonable?
TxDOT Database on Fly Ash
• Reliable chemical compositions of a total of 5,599 fly ashes (85 < Sum of oxide < 105)
• From 1992 to 2008
• Fly ashes from 50 power plants inside and outside Texas
• Class F fly ashes: 1,608 & Class C :3,991
• Alkali contents recorded from 2002 with 2,087 fly ashes
Presentation Outline
• Introduction
• Fly ash classifications
• TxDOT database on fly ash
• Variations of chemical composition of fly ash
• Some thoughts on fly ash classification based on chemical compositions
• Summary
Variations of Chemical Compositions of Fly Ash with Time
• Composition changes of fly ashes in one single power plant (Class C)
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100 120 140 160 180 200 220
Oxide
Con
tents (%)
Time Lapse (months)
SiO2+Al2O3+Fe2O3
CaO
SO3
Simple Statistics(not all information)• Class C fly ash
• Class F fly ash
Variable Number Mean (%) Standard Deviation (%)
Coefficient of Variation (%)
Median (%)
Minimum (%)
Maximum (%)
SiO2 4016 33.74 2.84 8.42 33.49 21.89 46.16Al2O3 4016 18.88 1.52 8.05 18.93 13.23 24.11Fe2O3 4016 5.85 0.94 16.07 5.91 0.70 11.55Total 4016 58.47 3.90 6.67 58.48 43.07 69.66CaO 4016 26.44 2.13 8.06 26.59 13.96 35.08
Variable Number Mean (%)
Standard Deviation
(%)
Coefficient of Variation
(%)
Median (%)
Minimum (%)
Maximum (%)
SiO2 1608 55.28 5.51 9.97 55.05 37.95 68.37Al2O3 1608 22.09 3.03 13.72 21.79 14.34 31.11Fe2O3 1608 6.34 3.39 53.47 5.25 0.51 36.70Total 1608 83.70 5.93 7.08 83.20 70.13 99.48CaO 1608 8.76 4.22 48.17 9.03 0.39 24.96
Ternary Diagram on CaO and Oxide Contents
Type CH <-> Class C; Type F + Type CI <-> Class F
70% oxide sum
20% CaO
Variations of CaO of Fly Ashes with Time
Class F fly ashes
Class C fly ashes
Cumulative Percent of Oxide Contents
Correlations among Oxides
Linear Models on Oxide Contents
• Central question– How many oxide parameters are adequate to describe the fly ash?
• Other than Class C and Class F
– Minerals and glass contents not considered
• SAS® proc GLM (general linear model) was used to build linear models
Selected Models
• SiO2=68.9593‐0.1829*Al2O3‐1.0955*CaO‐1.2285*SO3, R2=0.9295
• Fe2O3=43.9908‐0.4625*SiO2‐0.5322*CaO‐0.4925*Al2O3+ 2.8485*K2O, R2=0.6883
• CaO=40.8001‐0.5724*SiO2‐0.4964*Fe2O3+ 1.4133*MgO, R2=0.9607
• Na2O no fitting model found
• K2O=‐1.4304+0.03520*SiO2+0.08825*Fe2O3
, R2=0.7381
Disappointing Result• Only CaO or SiO2 can be reliably predicted by other oxides
• We need at least sevenoxide contents to describe the chemical composition
Presentation Outline
• Introduction
• Fly ash classifications
• TxDOT database on fly ash
• Variations of chemical composition of fly ash
• Some thoughts on fly ash classification based on chemical compositions
Element Origins in Fly Ash
• About 2.8% element matters in feed coal ended in fly ash (based on data listed above)
• Most inorganic matters ended in fly ash and bottom ash
• Will the possible tracing of element sources in fly ash helpful for our understanding?– Using correlation models
Some Preliminary Observations
• Na2O no fitting model found– Possibly not much contribution from minerals with regular compositions
– Indicate the major source is clay minerals
(Na+)
http://serc.carleton.edu/images/NAGTWorkshops/mineralogy/montmorillonite_structure.jpg
Potassium Oxide Model
• K2O=‐1.4304+0.03520*SiO2+0.08825*Fe2O3
R2=0.7381
• SiO2 term indicates K element from clay minerals
• Fe term indicates the replace of Si in clay minerals
• A surprise of the lack of Al2O3 term– K‐feldspar (KAlSi3O8) is possibly not a major source of K element
Iron Oxide Model
• Fe2O3=43.9908‐0.4625*SiO2‐0.5322*CaO‐0.4925*Al2O3+ 2.8485*K2O, R2=0.6883– If pyrite (FeS2) and gypsum (CaSO4) are considered as the main sulfur sources in feed coals, the element matter from pyrite must be lumped in bottom ash instead of fly ash.
– Or pyrite is not a main source for sulfate in fly ash.
K2O vs SiO2/Al2O3 Ratios
Na2O vs SiO2/Al2O3 Ratios
Summary
• Class F fly ashes have larger variations than Class C fly ashes. Support a finer system than ASTM C 618
• At least seven oxide contents needed to fully describe fly ash
• Differences of Class C and Class F fly ashes are mainly in CaO and SiO2 contents
• Al2O3 and Fe2O3 contents are very much similar
• Careful about empirical correlations because of the variety of data
Further Research
• A better way to evaluate/rank/describle fly ash for use in concrete?
