Characterization and Use of CFBC Fly Ash in Concrete€¦ · •Reactive phases in CFBC fly ash include glass, anhydrite, free lime, and likely clay phases (talc, muscovite): •

Rajabipour and Zahedi, ARIPPA Convention (August 2018) Slide: 1/25

Farshad Rajabipour, Mahboubeh ZahediPennsylvania State University

Characterization and Use of CFBC Fly Ash in Concrete

PCARG


Motivation

• PC fly ash is an important ingredient of concrete:

− Lowers cost− Improves workability,

durability, sustainability

• Recently, there has been widespread shortage of good quality fly ash

• Currently, 14.3M tons of FBC ash is produced annually, in par with the tonnage of PC fly ash used in concrete

Data from ACAA


Task 1: Does it meet ASTM C618 requirements?

Task 2: Can it lead to producing high-quality concrete?

Objective: Can we use CFBC fly ash as concrete pozzolan?

CFBC fly ash was collected from two PA sources

Anthracite CFBC fly ash Bituminous CFBC fly ash


What is a Pozzolan?

• A siliceous or alumino-siliceous material that by itself does not react with water, but in the presence of water, reacts with calcium hydroxide and forms C-S-H

• 2(CaO)3(SiO)2 + 11H2O (CaO)3(SiO2)2(H2O)8 + 3Ca(OH)2

cement C-S-H

• 5Ca(OH)2 + 6(SiO)2 + 3H2O (CaO)5(SiO2)6(H2O)8

pozzolan

• Pozzolan improves the long-term strength and durability of concrete


Task 1: Does CFBC fly ash meet ASTM C618 requirements for concrete

pozzolan?




Bulk chemistry of fly ashes was measured using XRF

Oxide Anthracite CFBC Bituminous CFBCSiO2 50.10 37.64

Al2O3 22.54 16.88Fe2O3 7.66 9.91S+A+F 80.30 64.43CaO 5.06 15.39SO3 2.39 9.83

Na2Oeq 2.26 1.62TiO2 1.34 0.91MgO 0.75 1.29P2O5 0.14 0.13


Physical properties of fly ashes

Property Method Anthracite CFBC Bituminous CFBC

Moisture content (%wt) ASTM C311 0.12 0.18

Density (g/cm3) He pycnometry 2.61 2.63

Fineness (% >45µm) ASTM C430 401 29.5

LOI (%wt) ASTM C311 6.65 5.21

Carbon (%wt) Leco IR 6.26 3.80

Sulfur (%wt) Leco IR 1.01 4.64

Particle size distribution

D10 (μm)

Laser diffraction

2.95 3.42

D50 (μm) 28.7 26.1

D90 (μm) 128 102.5

Soundness (AC exp. %) ASTM C151 -0.02 -0.031 Fineness of Anthracite fly ash passed thru #140 sieve is 27%


QXRD was used to determine fly ash mineralogy

Anthracite CFBC fly ash


QXRD was used to determine fly ash mineralogy

Bituminous CFBC fly ash


• The glass content of CFBC fly ash (~50%) is smaller than that of typical PC fly ash (>75%)

• Reactive phases in CFBC fly ash include glass, anhydrite, free lime, and likely clay phases (talc, muscovite):• Sum of reactive phases for anthracite CFBC = 74.5%• Sum of reactive phases for anthracite CFBC = 78.5%

• Quartz and hematite are non-reactive, making up 25.5% and 20.7% of the mass of CFBC fly ashes

Observations from QXRD


SEM images show that CFBC fly ash particles have irregular shape and internal porosity, resulting in

higher water demand

Anthracite CFBC fly ash Bituminous CFBC fly ash


Strength activity index (SAI) in combination with portland cement - ASTM C311

Mortar mixturecement

(g)fly ash

(g)sand (g)

water (g)

SP (g) w/cmflow (%)

Control 750 - 2062.5 363 - 0.484 88Anthracite CFBC 600 150 2062.5 397.5 - 0.53 86Bituminous CFBC 600 150 2062.5 397.5 - 0.53 93Anthracite CFBC w/ SP 600 150 2062.5 363 2.3 0.484 85Bituminous CFBC w/ SP 600 150 2062.5 363 1.7 0.484 88

• The water requirement of both fly ashes are 109.5%.


Strength of mortars containing 20% fly ash is comparable with control, but plateaus after 28 days

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Com

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ssiv

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trength

(psi)

Age (days)

Control Anthracite w/o SP Bituminous w/o SP

Anthracite w/ SP Bituminous w/ SP


Both fly ashes satisfy the ASTM SAI limit of 75%

• SAI is ratio of strength of mortar with 20% fly ash to that of control (100% cement) mortar

• Mixtures containing superplasticizer (SP) show higher SAI, due to their lower w/cm.

• Anthracite CFBC show a higher SAI

75

85

95

105

115

0 10 20 30 40 50 60

SA

I (%

)

Age (days)

Anthracite w/o SP Bituminous w/o SP

Anthracite w/ SP Bituminous w/ SP


• Both CFBC fly ashes meet ASTM C618 requirements, except for:• High LOI in both ashes• High SO3 in bituminous CFBC

• They are both highly reactive Good strength development in mortar

• Reactive phases are Al-Si glass, calcined clays, anhydrite, and free lime

• Irregular particle shape and porosity leads to high water demand

Summary of Task 1 observations


Task 2: Can we use CFBC fly ash to produce high-quality concrete?


Typical paving concrete mixtures were prepared with w/cm=0.47 and 20% fly ash

Material Control mix (kg/m3)Anthracite CFBC

mix (kg/m3)Bituminous CFBC

mix (kg/m3)Cement 362 290 290CFBC fly ash - 72 72Water 170 170 170Coarse aggregate 1098 1098 1098Sand 643 639 640Air-entraining admixture 0.51

4”

8”


Properties Control mix Ant CFBC mix Bit CFBC mixSlump (in) 3.25 0.75 1.75Fresh air content (vol %) 6.0 1.3 1.5Hardened air content (vol %) 7.2 2.2 3.1Air spacing factor (in) 0.007 0.020 0.015

Mixtures containing CFBC fly ashes show lower slump and air content but better strength vs. control

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trength

(psi)

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trength

(M

Pa)

Age (Days)

Control mix Bit CFBC mix Ant CFBC mix


-300

-200

-100

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0 2 4 6 8 10 12 14

Expansio

n (

μm

/m)

Age (days)

Control Anthracite Bituminous

Autogenous expansion results show that the higher SO3 in bituminous CFBC is not deleterious

420 mm

29 mm

• The initial expansion is likely due to ettringite formation • Bituminous CFBC fly ash acts as a shrinkage compensating

additive


What is Alkali-Silica Reaction (ASR)?

Deleterious reaction b/w OH- and alkali ions (from cement)and silicate minerals in aggregates formation of expansive silica gel

cracking of concrete


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0.05

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0.30

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Expansio

n (

%)

Days exposed to NaOH

Control Anthracite 20%

Bituminous 20% Anthracite 30%

Bituminous 30%

failure threshold

Both CFBC fly ashes can mitigate ASR according to ASTM C1567

11 ¼”

1”

• Anthracite fly ash is more effective due to its lower Ca and higher Al


• CFBC fly ash improves the strength and durability of concrete

• High LOI remains a problem, causing reduced slump and air content

Task 2 observations


Conclusions and Future Work

• There’s high market demand for new fly ash resource

• CFBC fly ashes showed very good performance as concrete pozzolan⁃ met ASTM C618 requirements (except for LOI, SO3)⁃ improved the strength and durability of concrete

• High SO3 was not deleterious, but high LOI remains a problem⁃ Must explore strategies to manage LOI

• Further testing is needed to prove long-term durability, allow writing CFBC fly ash in concrete specs.

Thank You


Appendix


A batch leaching experiment was used to monitor dissolution of CFBC fly ash into cement pore solution

• Synthetic pore solution of 0.2M Na(OH), 0.5M KOH in saturated lime water with pH of 13.85 was prepared.

• Fly ash or cement was added to solution at ratio of 1:20 by mass

• The solution was tumbled for 100 hr• Pore solution was collected at 1, 3, and 7 days of exposure,

filtered, and measured by ICP-AES


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140

0 2 4 6 8

Ca

(μ

g/m

L)

Age (days)

Anthracite Bituminous Cement

15000

20000

25000

30000

35000

40000

0 2 4 6 8

Na

+K

(μ

g/m

L)

Age (days)


Both fly ashes remove a considerable quantity of alkalis and Ca from the solution

• This suggests a strong pozzolanic reaction and potential for ASR mitigation:

5Ca(OH)2 + 6(SiO)2 + 3H2O (CaO)5(SiO2)6(H2O)8


0

2000

4000

6000

8000

10000

12000

14000

16000

0 2 4 6 8

S (

μg

/mL

)

Age (days)


0

50

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0 2 4 6 8

Al (μ

g/m

L)

Age (days)


Both fly ashes are a significant source of Al and Si;Bituminous fly ash releases S (from anhydrite)

• Dissolved Al (from glass and clay phases) can provide ASR mitigation benefits.

• High concentration of S can lead to volume expansion.


Fineness is measured according to ASTM C430

• 1 g of sample is wet sieved using a No. 325 (45-μm) sieve at pressure of 10psi for 1 min.

• The remaining sample is dried in the oven• 𝑓𝑖𝑛𝑒𝑛𝑒𝑠 = 𝑚𝑎𝑠𝑠 𝑟𝑒𝑡𝑎𝑖𝑛𝑒𝑑 𝑜𝑛 𝑁𝑜. 325 𝑠𝑖𝑒𝑣𝑒 × 100


Loss on ignition (LOI) is measured according to ASTM C311 to provide an estimate of the unburned carbon

content

• Sample is dried overnight in oven at 110 C• 1 g of dried sample is heated at 750 C for 2h, to achieve constant mass• 𝐿𝑂𝐼 = (𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑖𝑛 𝑚𝑎𝑠𝑠 𝑓𝑟𝑜𝑚 110 𝐶 𝑡𝑜 750 𝐶) × 100


Particle shape and agglomeration is obtained using Scanning Electron Microscopy (SEM)

• Mixture of epoxy and fly ash (mass ratio of 1.0) is poured into 0.25” holes• Specimen is dried overnight in an oven at 60C• Sample is polished in the order of #120, #240, #400, #600, #800 and #1200

grit. Next 6, 3, 1, 0.25 μm diamond paste is used to polish the samples.• Samples are coated with carbon thread (5.37 nm)• Interactions between the electron beam and atoms produce high-

resolution images of the sample surface

1”

0.25”


The soundness of paste specimens was measured according to ASTM C151

Specimen w/c* water cement fly ashControl 0.265 172.25 650 0

Anthracite FA 0.31 201.5 520 130Bituminous FA 0.285 185.25 520 130

• After 24h of moist curing, initial comparator reading was obtained.• Expansion is reported as the percent length change after placing the

specimens in the autoclave at 2 ±0.07 MPa and 220oC for 3 h, compared tothe initial comparator reading.

* To obtain normal consistency (ASTM C187)

10 ± 1 mm

11 ¼”

1”


Autogenous shrinkage (ASTM C1698) was measured at final setting time (ASTM C403) and 1, 3, 5, 7, 14,

and 28 days from casting

34

Material Control (lb/yd3) Anthracite FA (lb/yd3) Bituminous FA (lb/yd3)

cement 2200 1760 1760fly ash - 440 440Water 1200 1200 1200sand 5950 5950 5950

• Mortar mixtures are prepared with w/cm=0.50 and 20% cementreplacement with anthracite/bituminous fly ash.

420 mm

29 mm


ASR expansion (ASTM C1567) was measured at 1, 3, 5, 7, 10, 12 and 14 days from zero reading time

35

Sample Cement (g) Fly ash (g) Sand (g) Water (g)control 600 - 1329.9 302.1

Test 480 120 1329.9 302.1

• Mortar prisms were prepared with s/b=2.25 , w/b=0.47, and and 20%cement replacement with anthracite/bituminous fly ash.

• Specimens were stored in a water bath and placed in an oven at 80°C for 24hours, after which zero comparator and mass reading was taken

• Specimens were stored in 1M NaOH bath and placed in an oven at 80°Cuntil time of testing

11 ¼”

1”


The mineral composition was determined using Quantitative X-ray Diffraction (QXRD)

• Samples were ground using micronizer to less than No. 400 sieve• Fly ash was ground with 15% internal standard (ZnO)• Samples were placed on spinner stage and recordings were made over the

range of 5-70o 2θ with step size of 0.02o.• The crystalline structure causes the beam of incident X-rays to diffract at

specific angles and intensities. The diffraction angle is unique to eachmineral and the Rietveld refinement method is used to determine theintensity of various minerals in a material.


Pozzolanic reactivity of lime-fly ash paste samples is measured according to RILEM TC TRM267 at the age

of 1, 7, and 28 days using TGA analysis

• Specimens are cured at 40°C for 1, 7, and 28 days from mixing

• TGA is used to determine the amount of Ca(OH)2 by heating the samplesfrom 30°C to 950°C, at the rate of 10°C /min, while the chamber is purgedwith N2.

Components Fly ash Ca(OH)2 Deionized water KOH K2SO4 CaCO3

Weight (g) 11.11 33.33 60 0.24 1.2 5.56


Reactivity of lime-fly ash paste samples is measured according to RILEM TC TRM267 at the age of 7 days

using the “oven method” • Ground samples are oven dried at 105oC until reaching constant weight.

• Samples are calcined at 350oC for 2 hours

𝐻2𝑂𝑏𝑜𝑢𝑛𝑑,𝑑𝑟𝑖𝑒𝑑 =𝑤0 − 𝑤𝑡

𝑤0 − 𝑤𝑐

weight of empty crucible (wc)weight of crucible containing sample after 105oC (w0)weight of crucible containing sample after 350oC (wt)


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0 5 10 15 20 25 30

Calc

ium

Hydro

xid

e c

onte

nt

(%)

Age (days)

Anthracite Bituminous

Results for RILEM TC TRM267 show similar Ca(OH)2

uptake, yet higher bound water content for bituminous fly ash

• Both fly ashes show similar pozzolanic reactivity at 28 days, with slightlyhigher Ca(OH)2 uptake by anthracite fly ash at 1 and 7 days.

• The 7-day bound water content of bituminous fly ash (0.0336g per gram ofdried paste) is greater than anthracite fly ash (0.0234g).


Compressive strength of lime-fly ash mortar is measured according to ASTM C593 after 7 and 28

days from casting

Sample Fly ash (g) Hydrated lime (g) Standard Sand (g) w/b Water (g)1 Flow (%)2

Anthracite CFBC 360 180 1480 0.78 421.5 65

Bituminous CFBC 360 180 1480 0.72 388.8 67.5

1sufficient water is added to produce a flow of 65 to 75 mm2 10 drops in 6 seconds

• Mortar cubes were steam cured in oven at 40 C for 7 days, and latermoist cured for the remaining 21 days.

2” 2”


ASTM C593 results show higher reactivity for bituminous fly ash compared to anthracite fly ash

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trength

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Pa)

Age (days)

Anthracite Bituminous

• This observation is in accordance with SAI results.

Documents

Characterization and Use of CFBC Fly Ash in Concrete€¦ · •Reactive phases in CFBC fly ash include glass, anhydrite, free lime, and likely clay phases (talc, muscovite): •