EVALUATING THE USE OF FLUIDIZED BED COMBUSTION FLY ASH AS CONCRETE POZZOLAN

EVALUATING THE USE OF FLUIDIZED BED COMBUSTION FLY ASH AS CONCRETE POZZOLAN

ABSTRACT

Supplementary cementitious materials (SCM) are a key ingredient of today’s concrete and can vastly improve the durability and sustainability of concrete mixtures. While the demand for fly ash and other suitable pozzolans continues to escalate, the supply of high-quality and economically available fly ash has been shrinking. To maintain and expand the market share of concrete products, it is critical that high-quality, long-lasting and cost-competitive concrete is widely available; this requires a stable and abundant supply of cheap fly ash. Fluidized bed combustion (FBC) fly ash is an alternative source of fly ash that is widely available, but currently not used in concrete. This is due to the lack of guidelines and protocols to evaluate the quality/performance of FBC fly ash and identify necessary beneficiation procedures before it can be incorporated into concrete mixtures. The FBC process is a cheaper and more efficient way of burning waste coal compared to conventional pulverized coal combustion (PCC). In this technology, sulfur-absorbing minerals (e.g., limestones) are added as kiln feed and turbulence is increased, to enable combustion at lower temperatures (750-900°C), which in turn reduces NOx emissions. Although FBC fly ash may be a great SCM source, its performance as concrete pozzolan is not yet well understood, and there is a significant need for research to develop guidelines that distinguish usable sources of FBC ash.

To address these knowledge gaps, the purpose of this research is to evaluate the feasibility, performance, hydration, and beneficiation of FBC fly ash and determine if and how this alternative fly ash can be used as a viable pozzolan for concrete. In this study, circulating fluidized bed combustion (CFBC) fly ashes were collected from two sources in Pennsylvania (products of anthracite and bituminous waste coal combustion) and characterized for their physical properties, unburned carbon content, bulk chemistry, mineralogy, and reactivity. Results were compared against the requirements of ASTM C618-19 and areas of non-compliance were identified. Further, the characteristics of CFBC fly ashes were linked to the fresh and hardened properties of concrete

 

and mortar mixtures. The fly ashes were used to substitute 20% of Portland cement in concrete mixtures, and their effect on the slump, fresh air content, hardened air properties, compressive strength, chloride ion permeability, and water absorption rate of concrete was evaluated. Equivalent mortar mixtures were also prepared and tested for their drying shrinkage, autogenous shrinkage, expansion in water, resistance to sulfate attack and alkali-silica reaction. The fly ashes met the chemical and physical requirements of ASTM C618-19, except for elevated LOI (in both fly ashes) and elevated SO₃ (in bituminous fly ash). Despite this, concrete with proper slump, air content, and strength development was produced by adequate dosing of chemical admixtures. The high SO₃ content in bituminous fly ash did not produce deleterious expansion during autogenous shrinkage testing and the value of 14-day expansion in water was close to the ASTM threshold. Use of CFBC fly ash was found to be most effective in mitigating chloride ion penetration and alkali-silica reaction. This was mainly due to the contribution of CFBC fly ash in lowering the alkalinity of the pore solution, increasing its aluminum and silicon ion concentration, as well as refining the pore structure. Both fly ashes did not have a significant effect on drying shrinkage. Samples containing anthracite fly ash were able to withstand severe sulfate attack, but the use of bituminous fly ash led to premature failure.

Understanding the pozzolanic mechanism of CFBC fly ashes in concrete was one of the main components of this research. For this purpose, X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), and thermogravimetric analysis (TGA) was performed on binary paste mixtures containing 20% CFBC fly ash. Hydration of reactive phases (calcined clay, aluminosilica glass, anhydrite, free lime) in CFBC fly ash which made up close to 75 % of its mass, resulted in the generation of ettringite, CO₃-AFm (i.e., hemicarboaluminate, monocarboaluminate), and C-A-S-H phases. Bituminous fly ash contributed to greater ettringite and secondary C-A-S-H gel formation, while anthracite fly ash was responsible for greater AFm phase production. Use of CFBC fly ash led to greater Si/Ca and Al/Ca values for the C-A-S-H phase, compared to the 100% OPC mixture.

Finally, carbon neutralization/reduction techniques were investigated for their efficiency in reducing the interference of CFBC fly ash with air-entraining admixtures (AEA) performance in concrete. Coating of unburned carbon with sacrificial surfactants improved the slump and compressive strength of concrete, but slightly reduced its fresh and hardened air content. Combustion of fly ash at 500_oC for 2h was very effective in reducing the AEA uptake by CFBC fly ashes, and increased combustion temperatures did not yield any significant improvements.

 

TABLE OF CONTENTS

List of Figures…………………………………………………………………………………………………….. viii

List of Tables………………………………………………………………………………………………………… xi

 

Acknowledgments ……………………………………………………………………………………………………. xiii

Chapter 1  Introduction ……………………………………………………………………………………………… 1

1.1 Motivation and Significance ………………………………………………………………………….. 1

1.2 Research Objectives …………………………………………………………………………………….. 2

1.3 Organization of Contents ………………………………………………………………………………. 4

Chapter 2  Review of Coal Combustion Processes and the Performance of Raw or

Beneficiated Fly Ash in Concrete ………………………………………………………………………… 8

2.1 Coal Combustion Technologies ……………………………………………………………………… 8

2.2 Suspension Firing of Pulverized Coal (PC) and Its Products ……………………………… 14

2.2.1 Pulverized Coal (PC) Fly Ash ………………………………………………………………. 15        2.2.2 Other PC Combustion Products …………………………………………………………….. 212.3 Fluidized Bed Combustion (FBC) and Its Products ………………………………………….. 24

2.3.1 Hydration Mechanism of FBC Fly Ash in Concrete ………………………………… 28        2.3.2 Performance of FBC Fly Ash in Concrete ………………………………………………. 30

2.3.3 Beneficiation of FBC Fly Ash ………………………………………………………………. 32

Chapter 3  Characterization of Circulating Fluidized Bed Combustion (CFBC) Fly Ash

and Its Performance in Concrete ………………………………………………………………………….. 39

3.1 Abstract ………………………………………………………………………………………………………. 393.2 Introduction …………………………………………………………………………………………………. 40

3.3 Experimental Procedures ………………………………………………………………………………. 40       3.3.1 Physical Properties and Unburned Carbon ………………………………………………. 41

3.3.2 Bulk Chemistry and Mineralogy…………………………………………………………….. 42       3.3.3 Pozzolanic Reactivity …………………………………………………………………………… 433.3.4 Performance of CFBC Fly Ash in Mortar and Concrete …………………………………. 45

3.4 Results and Discussion …………………………………………………………………………………. 47       3.4.1 Physical Properties and Unburned Carbon ………………………………………………. 47

3.4.2 Bulk Chemistry and Mineralogy…………………………………………………………….. 50

3.4.3 Pozzolanic Reactivity …………………………………………………………………………… 52

3.4.4 Performance of CFBC Fly Ash in Mortar and Concrete ……………………………. 55

3.5 Conclusions …………………………………………………………………………………………………. 59

3.6 References …………………………………………………………………………………………………… 60  Chapter 4  Understanding the Pozzolanic Reaction Mechanism of Fluidized Bed

Combustion Fly Ash in Concrete…………………………………………………………………………. 62

4.1 Abstract ………………………………………………………………………………………………………. 624.2 Introduction …………………………………………………………………………………………………. 63

4.3 Experimental Procedures ………………………………………………………………………………. 64       4.3.1 Batch Leaching of CFBC Fly Ash ………………………………………………………….. 65       4.3.2 XRD and SEM-EDS Analysis of Binary Paste Mixtures …………………………… 65

4.3.3 Thermodynamic Analysis of CFBC Fly Ash and Binary Paste Mixtures …….. 66

4.4 Results and Discussion …………………………………………………………………………………. 67

4.4.1 Batch Leaching of CFBC Fly Ash ………………………………………………………….. 67       4.4.2 XRD of Binary Paste Mixtures ………………………………………………………………. 68       4.4.3 SEM-EDS of Binary Paste Mixtures ………………………………………………………. 72

4.4.4 Thermodynamic Analysis of CFBC Fly Ash and Binary Paste Mixtures …….. 84

4.5 Conclusions …………………………………………………………………………………………………. 91

4.6 References …………………………………………………………………………………………………… 91

Chapter 5  Performance of FBC Fly Ash in Concrete ……………………………………………………. 94

5.1 Abstract ………………………………………………………………………………………………………. 94

5.2 Introduction …………………………………………………………………………………………………. 955.3 Experimental Procedures ………………………………………………………………………………. 95

5.3.1 Performance of CFBC Fly Ash in Mortar and Concrete ……………………………. 96

5.4 Results and Discussion …………………………………………………………………………………. 100       5.4.1 Performance of CFBC Fly Ash in Mortar and Concrete ……………………………. 100

5.5 Conclusions …………………………………………………………………………………………………. 108

5.6 References …………………………………………………………………………………………………… 109

Chapter 6  Performance of Beneficiated FBC Fly Ash in Concrete …………………………………. 111

6.1 Abstract ………………………………………………………………………………………………………. 111

6.2 Introduction …………………………………………………………………………………………………. 112 6.3 Experimental Procedures ………………………………………………………………………………. 112       6.3.1 Chemical Passivation ……………………………………………………………………………. 114

6.3.2 Carbon Burn Out (CBO) ……………………………………………………………………….. 114

6.4 Results and Discussion …………………………………………………………………………………. 115

6.4.1 Chemical Passivation ……………………………………………………………………………. 115       6.4.2 Carbon Burn Out (CBO) ……………………………………………………………………….. 1176.5 Conclusions …………………………………………………………………………………………………. 119

6.6 References …………………………………………………………………………………………………… 120

Chapter 7  Summary, Conclusions, and Future Approach ……………………………………………… 121

7.1 Conclusions …………………………………………………………………………………………………. 122

7.2 Future Directions …………………………………………………………………………………………. 124 7.3 References …………………………………………………………………………………………………… 127

Chapter 1

 

Introduction

1.1 Motivation and Significance

In recent years, reports of fly ash shortages for use in concrete have become more common in the United States [1] , [2] , [3] , [4] . A recent survey by the AASHTO subcommittee on materials [5] of all State Departments of Transportation (DOTs), the Federal Aviation Administration (FAA), and the Army Corps of Engineers showed that the concern with fly ash supply is not a minimal or regional issue, as over 80% of the respondents indicated having issues with fly ash availability over the past 4 years. These supply issues are in part due to a decline in the total quantity of fly ash produced [6] (Figure 1-1) as utilities have shifted fuel sources (e.g., coal to natural gas). They are also due to stricter air pollution and environmental regulations (e.g., [7] , [8] ), which affect the quality of fly ash by increasing its carbon content (e.g., due to the use of low NO_xburners and activated carbon), and calcium and sulfur contents [9] [10] . The economic and regulatory incentives have also encouraged the use of low sulfur western coals (yielding Class C fly ash), at the cost of

Class F fly ash, with the latter having a superior performance in mitigating alkali-silica reaction (ASR) in concrete [11] .

Based on today’s practices and standards, it is estimated that by year 2030, the supply of ASTM C618-complient fly ash in the United States will be approximately 14 million tons, while the demand will exceed 35 million tons [12] . The shrinking supply of high-quality and economically available fly ash threatens the competitiveness and market share of concrete against other materials such as asphalt (for pavements), steel (for buildings and bridges), and plastics (for pipes). The concrete industry relies on cheap and abundant fly ash for producing economical, durable, and sustainable (e.g., low CO₂) concrete [10] [13] . While fluidized bed combustion fly ash is widely available, it is not being used since its performance in concrete is not fully understood, and guidelines (e.g., ACI documents, ASTM specifications) to evaluate and beneficiate this alternative form of fly ash does not currently exist.

 

 

Figure 1-1. Statistics on production and utilization of pulverized coal (PC) fly ash and FBC ash (from ACAA [6] )

1.2 Research Objectives 

This Ph.D. research pursues the following main objectives:

Objective 1: Characterization of as-produced FBC fly ash and its performance in concrete

Literature on the use of FBC fly ash as concrete pozzolan is limited, and lacks a complete evaluation of the physical and chemical properties of FBC fly ash against ASTM C618 [14] – Standard

Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. Past studies have been mainly focused on evaluating the effects of FBC fly ash on the mechanical properties of concrete, and have reported contradictory results. To address these deficiencies, the goal of this study is to present a complete characterization of several CFBC fly ashes produced in Pennsylvania, and to link these properties with important characteristics of concrete. It is worth noting that not all FBC fly ashes are created equal and their composition and properties may vary considerably from one source to another, partly due to differences in their source coal. In this study, two CFBC fly ashes of distinct compositions (originating from anthracite and bituminous coal sources) are evaluated for their physical, chemical, and mineralogical properties and compared against ASTM C618-19 requirements. The fly ashes are then used as partial (20% by mass) replacement of Portland cement in DOT-compliant pavement concrete mixtures, and their impact on the short and long-term performance of concrete is investigated.

Objective 2: Understanding the hydration mechanism of FBC fly ash in binary cement-basedmixtures

FBC fly ashes possess pozzolanic properties and this research is aimed at understanding the reaction mechanism of FBC fly ash, in the presence of Portland cement and water. Few earlier studies have looked at the hydration of self-cementitious FBC fly ashes in no-cement mortar and paste mixtures. These fly ashes contain high contents of free lime and sulfur, which significantly surpass ASTM C618 limits, and will most likely cause performance complications such as volume instability, undesirable setting, and extreme heat of hydration. As such, use of self-cementitious FBC fly ashes in concrete may not be practical or require extensive beneficiation procedures. In this study, binary mixtures of Portland cement and CFBC fly ash (containing moderate levels of CaO and SO₃) are prepared and monitored over a duration of 90 days to evaluate their pozzolanic reactivity and reaction products. The main crystalline phases are identified using XRD testing, while SEM-EDS is used to determine the chemical composition of various crystalline/amorphous phases (e.g., C-A-S-H, AFt, and AFm phases) within the microstructure. Thermogravimetric analysis is performed on the control and binary mixtures, to quantify the pozzolanic reactivity of CFBC fly ashes, by measuring changes in portlandite content with time.

Objective 3: Beneficiation of FBC fly ash and its performance in concrete

The high unburned carbon content, measured as loss-on-ignition (LOI), is a major limiting factor for the use of FBC fly ash in concrete due to its negative effects on concrete air entrainment and workability. Although various beneficiation techniques (e.g., size separation, electrostatic separation, froth flotation, oil agglomeration, chemical passivation and thermal processing [15] ) have been successfully used to reduce/neutralize the unburned carbon in conventional PC fly ash, no treatment of this kind has been tested for FBC fly ash. The goal of this study is to evaluate the effectiveness of chemical passivation and Carbon Burn Out (CBO) methods in reducing the interference of FBC fly ash with the performance of air entraining admixture (AEA) in concrete. In the chemical passivation method, the surface of carbon is coated with sacrificial surfactants to prevent it from adsorbing AEA in concrete. As for the CBO approach, fly ash is combusted at temperatures above carbon ignition (460_oC) and below glass devitrification (700_oC), to reduce the unburned carbon content. The efficiency of the two methods in terms of improving the airentrainment property of concrete is evaluated using the Foam Index (FI) testing and fresh and hardened air analysis of concrete.

1.3. Organization of Contents

This document is organized in 6 chapters, with the above research objectives addressed in chapters 3-5 (Table 1-1). Chapter two provides a background and literature review on coal combustion processes and byproducts with a focus on PC and FBC fly ashes. The current state of understanding and practice with respect to FBC fly ash is summarized, and research needs to facilitate the use of FBC fly ash as a viable concrete pozzolan is highlighted. In Chapter three, two sources of CFBC fly ash are characterized for their bulk chemistry and mineralogy (using X-ray florescence, batch leaching, and quantitative XRD); unburned carbon (using LOI, and Leco infrared analyzer); physical properties (moisture content, particle size, fineness, particle shape/agglomeration (via SEM), density, soundness, and water requirement); and reactivity (strength activity index, compressive strength of lime-fly ash mortar according to ASTM C593-06, pozzolanic reactivity of lime-fly ash paste according to RILEM TC TRM267). The fresh (slump, fresh air content) and hardened (compressive strength, hardened air content) properties of concrete mixtures incorporating 20% CFBC fly ash are obtained, and mortar samples are tested for their autogenous shrinkage and effectiveness in mitigation of alkali-silica reaction (ASR). Chapter four addresses knowledge gaps regarding the hydration mechanism of FBC fly ash with moderate levels of CaO and SO₃. Binary mixtures incorporating 20% CFBC fly ash as a replacement of cement are prepared and tested for their mineralogy, chemical composition, and pozzolanic reactivity at 1, 7, 28, and 90 days, using XRD, SEM-EDS, and TGA methods. Chapter five investigates the durability of concrete (chloride ion penetration, water sorbtivity) and mortar (sulfate attack, drying shrinkage) mixtures containing 20% CFBC fly ash. Mercury intrusion porosimetry (MIP) and pore fluid extraction tests are used to relate concrete performance to microstructural features such as pore size distribution and pore conductivity. In Chapter six, the effectiveness of chemical passivation and CBO beneficiation techniques on the air entrainment properties of concrete containing CFBC fly ash is evaluated using fresh air and hardened air analysis of concrete and foam index testing.

Chapter seven provides a summary of the findings and conclusions of this research study.

 

 

 

 

Table 1-1. Desired outcomes related to each chapter

Chapter	Desired outcomes
Chapter 1: Fluidized Bed Combustion (FBC) Fly Ash and its Performance in Concrete	•  Characterizing CFBC fly ash for its bulk chemistry, mineralogy, physical properties, unburned carbon content, and reactivity   •  Determining the effect of CFBC fly ash on the fresh and hardened properties of concrete
Chapter 2: Understanding the pozzolanic reaction mechanism of FBC fly ash in concrete	•   Identifying the main hydration products of mixtures incorporating CFBC fly ash and Portland cement, and monitoring their change over time.   •   Determining the composition of C-A-S-H phase in such mixtures
Chapter 3: Performance of FBC fly ash in concrete	•  Determining the effects of CFBC fly ash on concrete durability   •  Relating concrete performance to microstructural features
Chapter 4: Performance of beneficiated FBC fly ash in concrete	• Evaluating the effectiveness of chemical passivation and CBO beneficiation techniques in reducing the interference of carbon in CFBC fly ash with the performance AEAs in concrete

1.4. References

• Engineering district notification regarding fly ash availability for cement concrete, Pennsylvania Dep. Transp. Strike-off Lett. Available at: http://www.paconstructors.org/wpcontent/uploads/2014/09/481–14–pdf. Accessed: July 2019.
• Where has the fly ash gone?, Texas Dep. Transp. Constr. Mater. Tips. Available at:

https://ftp.dot.state.tx.us/pub/txdot–info/cst/tips/fly_ash_0412.pdf. Accessed: July 2019.

• Fly ash supply, Texas Dep. Transp. Tehcnical Advis. Constr. Bridg. Div. (2010).
• L. Robl, C.J. McCormick, We are running out of fly ash: The nature of regional supply problems, in: DOE/FETC 3_rdConf. Unburned Carbon Util. Fly Ash, Pittsburgh, PA, 1997. [5] AASHTO Subcommittee on Materials (SOM) fly ash task force report, (2016).
• American Coal Ash Association (ACAA), coal combustion products production and use survey report, (2017). Available at: https://www.acaa–org/Portals/9/Files/PDFs/2017Survey–Results.pdf. Accessed: July 2019.
• US-EPA Clean Air Interstate Rule (CAIR), (2005). Available at: https://archive.epa.gov/airmarkets/programs/cair/web/html/index.html. Accessed: July 2019.
• US-EPA Cross-State Air Pollution Rule (CSAPR), (2011). Available at: https://www.epa.gov/csapr/overview–cross–state–air–pollution–rule–csapr. Accessed: July 2019.
• Seraj, R. Cano, S. Liu, D. Whitney, D. Fowler, R. Ferron, J. Zhu, M. Juenger, Evaluating the performance of alternative supplementary cementing material in concrete, FHWA Report No. FHWA/TX-14/0-6717-1, (2014).
• ACI 232.2R, Report on the use of fly ash in concrete, Draft Rep. under Revis. Am. Concr. Inst. (2016).
• Thomas, The effect of supplementary cementing materials on alkali-silica reaction: A review, Cem. Concr. Res. 41 (2011) 1224–1231.
• Production and use of coal combustion products in the U.S., Market Forecast through 2033, Am. Road Transp. Build. Assoc. (2015).
• AASHTO PP-65, Standard Practice for Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction, Am. Assoc. State Highw. Transp. Off. (2013).
• ASTM C618-19, Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete. ASTM International.
• C. Hower, J.G. Groppo, U.M. Graham, C.R. Ward, I.J. Kostova, M.M. Maroto-Valer, S. Dai, Coal-derived unburned carbons in fly ash: A review, Int. J. Coal Geol. 179 (2017) 11– 27.

EVALUATING THE USE OF FLUIDIZED BED COMBUSTION FLY ASH AS CONCRETE POZZOLAN

Sharing is caring!