Fly Ash Alternatives for Texas

Maria Juenger

Saif Al-Shmaisani, Dr. Ryan Kalina, Jae Kyeong Jang, Dr. Saamiya Seraj, Rachel Cano, Dr. Raissa Ferron and Michael Rung

Sponsored by the Texas DOT under projects: 0-6717, 5-6717, and 0-6966

Fly ash in Texas• Fly ash is important to TxDOT for

concrete for the following reasons (Andy Naranjo):- Control of expansion due to

alkali-silica reaction (ASR)- Reduction of permeability/protection against corrosion- To produce high performance concrete- To reduce risk of thermal cracking in mass concrete

(i.e. temperature control)

Wikipedia

Fly ash supply is threatened• Coal-fired power plants are:

- Closing• Gibbons Creek, Monticello, Big Brown, Sandow…

- Producing Class C fly ash- Producing off-spec fly ash

earthjustice.org

Solutions to fly ash supply problem1. Better storage and logistics

- Store fly ash during times of low demand

- Transport fly ash cost-effectively

mylittlesalesman.comlbindustrialsystems.com

Solutions to fly ash supply problem2. Beneficiate off-spec fly ashes

Solutions to fly ash supply problem3. Harvest ponded or

landfilled fly ash

4. Use alternative SCMs- Raw natural pozzolans- Calcined clays- Limestone-SCM blends- Ground glass…

Solutions to fly ash supply problem

Case Studies for Texas1. Beneficiated fly ashes

- Harvested from landfill, dried, and ground/classified- Off-spec fly ash blended with natural pozzolans- Chemically treated fly ash- Class C/Class F blended fly ash

2. Natural pozzolans

Harvested, Beneficiated, and Off-Spec Fly Ashes• Reclaimed from landfills and ponds, dried and ground

or classified.• Chemically or thermally treated to reduce LOI and/or

absorption of air-entraining admixtures.• Blended with other fly ashes or natural pozzolans to:

- Reduce SO3 content- Reduce CaO content

• Bottom ash• Economizer ash• Trona-impacted ash

http://charah.com

Harvested Fly Ash• Landfilled fly ash is an untapped resource• Low calcium, siliceous ashes can be unreacted and need

minimal processing

Kaladharan et al.

• Tested several fly ashes:- Non-compliant, blended or treated ashes- Reclaimed from landfills and treated by grinding and

drying• Some important observations

Type Designation Material Classification/Properties

Remediated (RM) Fly Ash

RM-C

Non-compliant Fly Ash; Blended or TreatedRM-L

RM-M

RM-S

Reclaimed (RC) Fly AshRC-G

Class F Fly Ash; Ground and DriedRC-M

http://library.ctr.utexas.edu/ctr-publications/5-6717-01-1.pdf

Example 1:Harvested (Reclaimed) and Beneficiated Fly Ash

They all (mostly) pass ASTM C618 Class FDesignation SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Na2O (%) K2O (%)

OPC 20.4 5.8 2.3 62.5 1.4 3.3 0.2 0.9

FA 50.9 22.8 5.0 10.6 2.5 0.5 0.1 0.9

Q 99.2 0.2 0.1 0.2 0.0 0.1 0.0 0.0

B-C 59.1 14.1 3.2 9.4 1.6 3.4 3.3 2.9

B-L 61.0 13.1 3.1 10.2 0.6 3.9 2.7 3.9

B-S 60.1 14.4 4.7 6.9 1.1 3.2 3.2 3.5

R-G 51.5 21.3 4.9 11.3 2.1 2.7 0.2 0.8

R-M 47.0 19.9 8.4 14.1 3.4 0.9 0.8 1.0

DesignationSiO2+Al2O+

Fe2O3(%)

SO3(%)

Moisture Content

(%)

Loss on Ignition

(%)

Passes ASTM C618?

FA 78.7 0.47 0.5 0.4 YES

B-C 76.3 3.41 2.9 1.8 YES

B-L 77.2 3.92 1.4 1.6 YES

B-S 79.2 3.22 2.1 5.9 YES

R-G 77.8 2.72 3.9 0.9 NO

R-M 75.2 0.86 0.9 0.7 YES

ESEM Images – Beneficiated Fly Ashes

B-S B-L

B-C

d50 = 9.8 µm d50 = 9.5 µm

d50 = 6.3 µm

ESEM Images – Reclaimed Fly Ashes

R-G R-Md50 = 22.2 µm d50 = 12.8 µm

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Paste Rheology

Angular particles of beneficiated fly ashes interlock and impede flow of paste. Round particles of reclaimed fly ashes have “ball-bearing” effect

R² = 0.8754

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Concrete Compressive Strength

Concrete compressive strengths strongly correlated with median particle size of reclaimed and remediated fly ashes

Concerns about ASR

1. For reclaimed materials that are ground or classified, if the ash is coarse (i.e. less reactive) will it still control ASR expansion?

2. For materials beneficiated though blending with natural pozzolans, does ASR performance meet expectations?

3. What is the role of alkalis in fly ash on ASR?

Fly Ashes Beneficiated by Blending

Type Material Description Designation CaO (%)

Beneficiated (B) Fly Ash Coal Combustion Residual; Blended to meet ASTM C618 Class F

B-C 9.4

B-L 10.2

B-S 6.9

B-B 11.64

B-H 16.97

B-P 12.69

B-S2 10.82

B-V 16.98

Control Production Class F Fly Ash FA 10.6

All of these materials pass ASTM C618 sum of oxides for Class F

1. Role of Fly Ash Fineness

Expansion Limit

• Mortar mixtures with 20% replacement of cement with fly ash• OPC control expansion = 0.36%

Diagram from J. Ideker

2. Impact of Blending

Expansion Limit

• Mortar mixtures with 25% replacement of cement with fly ash

• OPC control expansion = 0.36%

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C1293 0.04% Expansion Threshold

Na2Oe > 4.0%

Fly ashes at 25% replacement amount (Shehata and Thomas 2000)

3. Role of Alkalis

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(20/18/4.4)

(35/12/6.9)

(30/12/8.8)

• High-alkali fly ashes follow trend for low-CaO content• High-alkali fly ashes deviate from trend at CaO contents greater than 10%

Na2Oe > 4.0%

High alkali fly ash, still passing (Replacement/Age in Months/Na2Oe)

High alkali fly ash, failed (Replacement/Age in Months/Na2Oe)

Summary: Fly Ashes

1. You can beneficiate fly ash to meet ASTM C 618 Class F. - The fly ash performance is dependent on particle size

and shape, which can be controlled by the beneficiation process.

2. Beneficiated fly ashes can control ASR expansion.- Be careful of high CaO contents and high alkali

contents.

Natural Pozzolans• Volcanic pozzolans

• Pumice, Perlite, Volcanic Ashes, Zeolites• Sedimentary pozzolans

• Calcined clays & shales (metakaolin, impure, mixed)

• Availability of many of these is low and regional, but increasing

• Of particular interest are overburden materials (e.g. impure pumice and clays) for reduced cost

• Some concerns: workability, frauds, and performance optimization

Natural Pozzolans – ASTM C618

A variety of natural pozzolans from different sources can meet the Class N criteria in ASTM C618 and not cause high water demands.

Heat of Hydration

All reduce heat of hydration from cement, so can be used in thermal control plans.

Compressive Strength

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Control 20% Pumice-D20% Perlite-I 20% Vitric Ash-S20% Metakaolin-D 20% Shale-T20% Fly Ash

All increase long-term compressive strength, some better than fly ash.

Alkali-Silica Reaction – ASTM C1567

0.000.050.100.150.200.250.300.350.400.450.50

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Control 15% Pumice-D15% Perlite-I 25% Vitric Ash-S15% Metakaolin-D 25% Shale-T

14-day expansion limit

All can control expansion due to ASR in mortar.

ASR in Concrete – ASTM C1293

All can control expansion due to ASR in concrete.

Sulfate Resistance - ASTM C1012

Some can control damage due to sulfate attack.

Pass Class 3:• Pumice-D• Perlite-I

Pass Class 2:• Metakaolin-D 1

2 3

Caution: Zeolites

Zeolites, as well as metakaolin, can cause increases in viscosity. These can be reduced by treatments, like calcination, but at a cost to reactivity.

Zeolite pastes

Cement and calcined zeolite pastes

No Caution Needed: Alkalis & ASR

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Inert quartz, still passing (20% Replacement/18 Months/0.04% Na2Oe)

Inert natural minerals, failed (Replacement/Age in Months/Na2Oe)

Natural pozzolans, passing (15 and 25% Replacement/18 and 24 Months/6.5-7.8% Na2Oe)

C1293 0.04% Expansion Threshold

• High-alkali inert minerals deviate from trend• High-alkali natural pozzolans and inert quartz follow trend

(20/18/9.7)

(20/18/6.4)

(20/18/5.8)

• ASTM C618 does not assess pozzolanicity

• Quartz qualifies as a Class N natural pozzolan

Kalina et al. ACI Materials Journal, V. 116, No. 1, January 2019.

Caution: Beware of Imposters

What tests could we use?• Can test pozzolanic reaction

- Chapelle test- Frattini test- Calcium hydroxide consumption (using TGA)

• Can test reactivity- Lime reactivity test- R3 test

(None of these is standardized by ASTM yet, but R3 is close)

A Single Test is not Definitive• ASTM C618 screens for pozzolans using sum of oxides and

SAI- Quartz passes

• R3 tests reactivity using heat or bound water- Quartz “fails”- OPC “passes”

A blend of about 40% OPC + 60% quartz with sum of oxides > 70% could “pass” both ASTM C618 and R3

We know that pozzolans control ASR• Why not use ASTM C1567 to test for pozzolanic

reactivity?• Results in 2 weeks• TxDOT is doing this

Thomas “Optimizing the Use of Fly Ash in Concrete,” PCA

Cannot use ASR alone• For non-pozzolanic materials, ASR expansion

decreases due to dilution• Strength also decreases due to dilution• Need to test both strength and ASR to evaluate

reactivity of materials

Idea: Test both ASR & “SAI”• Step 1: Determine the amount of SCM needed to

pass ASTM C1567 for a highly reactive aggregate• Step 2: Determine if this amount passes a modified

SAI.- Modifications to SAI:

• SCM percentage can be greater than 20%• Fixed w/cm of 0.485• Trying out 2 in. x 4 in. cylinders instead of cubes

(greater precision)

Example: Quartz• While 30% quartz is projected to be sufficient to pass

ASR, 30% quartz does not pass “SAI”

Example: Fly Ash• Approximately 15% of this fly ash should control ASR

expansion• A mixture with 15% fly ash should pass “SAI”

Example: Pumice• Approximately 15% of this pumice should control

ASR expansion• A mixture with 15% pumice will pass “SAI”

Example: Inert mineral• Approximately 40% of this mineral is projected to be

sufficient to pass ASR, but 40% mineral won’t reliably pass “SAI” given the error inherent in the test method (~10% of the measured strength value)

Combined data• In order to be classified as reactive, a material must

be able to reliable fall into the “good zone” (accounting for measurement error)

Summary: Natural Pozzolans• Natural pozzolans are excellent substitutes for Class

F fly ash- Performance is generally excellent- May not be cost-competitive in some markets- Some pozzolans (e.g. zeolites and some metakaolins)

increase water demand- In spite of high alkali contents, natural pozzolans

control ASR well- We need better, more conclusive tests to screen out

unreactive materials

Emerging Materials: Limestone Calcined Clay Cement (LC3)• LC3 holds promise as a combined

cement-SCM material- High availability- Good CO2 reductions- Good performance