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2015 World of Coal Ash (WOCA) Conference, May 5-7, 2015, Nashville, TN http://www.worldofcoalash.org/

Effects of Micromorphology and Chemical Composition on Densification of CCPs

David J. White1, Pavana K. R. Vennapusa2, Eric Hageman3, Barry Christopher4, Nick McClung5 1Geotechnical Consultant and Richard L. Handy Professor of Civil Engineering at Iowa State University; 2Geotechnical Consultant and Research Assistant Professor at Iowa State University; 3Project Engineer at HDR Engineering, Inc.; 4Geotechnical Consultant, 5Geotechnical Engineering Manager at Tennessee Valley Authority KEYWORDS: Coal Combustion Products, Fly Ash, Micro-Analysis, SEM, TGA, XRD, XRF, Chemical Analysis ABSTRACT Densification of coal combustion products (CCPs) during the stacking process is an important aspect to minimize landfill space. It is also important that the CCPs are stacked to provide sufficient shear strength and to minimize volume change during the service life. Target limits for compaction control are currently based on standard Proctor laboratory testing, which was originally developed for dam construction using soils in the 1930s. This study shows that the morphological and chemical composition properties of fly ash play a critical role in the moisture-density-strength-volume change relationships and that the current “soil-based” Proctor testing protocols are not sufficient to characterize this material for field control purposes. XRD/XRF results show that the elemental/mineral compositions vary with time and that the constituents influence the compaction characteristics. SEM analysis showed that fly ash is composed of various sizes and types of spheroids, needle-like minerals, highly angular particles, and agglomerations of particles. Particle sizes range from < 1 µm to about 50 µm. This complex morphology makes routine testing like moisture content by oven drying and setting Proctor mellowing periods complex due to time and temperature dependent chemical reactions for different mineral phases. TGA results showed that the fly ash materials liberate water at different temperatures and typically have a strong transition near 60oC, due to ettringite (a mineral with high affinity for water). The results of this study were used to provide guidelines to improve the compaction characterization of CCPs. (225 words).

2015 World of Coal Ash (WOCA) Conference in Nasvhille, TN - May 5-7, 2015http://www.flyash.info/

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INTRODUCTION Tennessee Valley Authority (TVA) builds numerous coal combustion product (CCP) impoundments and stacking facilities at its fossil-power generation plants. Densification of these CCP stacks is critical to provide sufficient shear strength and volume change characteristics. Currently, the quality control (QC) and quality assurance (QA) specifications for construction of CCP stacks include placing the material within a specified moisture range relative to the optimum moisture content and compacting the material to a target dry density based on laboratory standard Proctor compaction tests. These compaction QC/QA specifications were originally developed for earthen dam construction and the fly ash materials used in CCPs are not soil-like materials, and has complex composition. Further, preliminary testing conducted in 2012 at two TVA fossil power plants (White et al. 2012a, White et al. 2012b, Christopher et al. 2013) indicated that the properties and chemical compositions of the fly ash material produced at a plant can vary significantly. This study was undertaken to monitor fly ash materials produced at the TVA Shawnee fossil power plant over a three month period and assess how the material morphological and chemical compositions of the materials vary and influence the soil moisture-density relationships. Testing was conducted on materials produced from the TVA Shawnee fossil-power generation facility, over a three-month period. Scanning electron microscopy (SEM) tests were conducted to obtain micrographs at various magnifications to identify particle shapes and sizes at the micro-level. Thermo-gravimetric analysis (TGA) analysis was conducted to determine the appropriate temperature required in conducting moisture content testing. X-Ray diffraction (XRD) and X-Ray fluorescence (XRF) tests were conducted on the materials to assess the morphological and chemical compositions of the materials. Standard Proctor testing was conducted on the materials to determine the moisture-density relationship. TESTING METHODS Scanning Electron Microscopy SEM micrographs were obtained at different magnifications (100x, 500x, and 1500x) using FEI Quanta-250 SEM equipped with a field-emission gun providing a resolution of 1.0 nm. Tests were conducted at the Material Analysis Research Laboratory (MARL) at Iowa State University. X-Ray Diffraction XRD tests were conducted for phase identification of crystalline materials in the fly ash. A Siemens D500 X-Ray diffractometer equipped with a diffracted beam monochromater and sample spinner was used for XRF testing. Tests were conducted at the Material Analysis Research Laboratory (MARL) at Iowa State University.

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X-Ray Fluorescence XRF tests were conducted for quantitative elemental analysis using PANalytical PW2404 XRF spectrometer. The spectrometer is equipped with a sample changer for automated operation and utilizes a rhodium target X-ray tube and a 4kW generator to provide the primary X-ray beam. Tests were conducted at the Material Analysis Research Laboratory (MARL) at Iowa State University. Thermo-gravimetric Analysis TGA tests were conducted to record percent mass loss, indicating moisture loss, with increasing temperature. Sample heating was carried out in an air atmosphere at 5°C/min. Readings were obtained every 0.5 second until the percent loss with time became relatively constant. Proctor Compaction Testing Standard Proctor compaction testing was conducted in accordance with ASTM D698-10. Samples were prepared at 6 to 8 desired target moisture contents and compacted within 4 hours of fill placement. Optimum moisture content and maximum dry density values were determined from each test. RESULTS AND ANALYSIS Micrographs from SEM analysis on one dry fly ash sample and a wet fly ash sample at three different magnifications are presented in Figure 1. Both dry and wet ash materials consisted of various sizes and types of spheroids. Wet ash material showed agglomerations of particles because of calcium hydroxide bonding between amorphous silicates and alumina, due to pozzalonic reactions. Needle-like and highly angular crystalline minerals representing formation of ettringite. Ettringite formation is confirmed in the XRD results presented in Figure 2 in the wet ash material. XRD results indicated that the ettringite consisted of chemically bound water with 26 water molecules (Ca6Al2(SO4)3(OH)12).26H2O). These observations were similar for SEM analysis of all 24 samples collected in this study. TGA results of a wet ash sample are presented in Figure 3, which shows that the material liberated water at different temperatures. A strong transition near 60oC was observed, which is attributed to drying of the chemically bound water to ettringite mineral. Most free water is dried before 60oC. An illustration of a four-phase system for CCPs representing air, free water, chemically bound mineral water, and solids is provided in Figure 4.

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500 x

1500 x

5000 x

500 x

1500 x

5000 x

DRY FLY ASH WET FLY ASH (MAP 24)

Figure 1. SEM images of dry and wet ash materials at 500x, 1500x, and 5000x magnification

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Figure 2. X-Ray diffractograms for wet fly ash (top) and dry fly ash (bottom)

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XRF results provided a quantitative summary of elemental composition of the materials. Results of sum of oxides (silicon dioxide, aluminum oxide, and iron oxide), sulphur and calcium oxide contents, and loss of ignition (LOI) for the 24 samples collected over the 3 month period are presented in Figure 5. The results of standard Proctor maximum dry density and optimum moisture contents over the 3 month period are shown in Figure 6. These results indicated that the chemical compositions of the material and the compaction characteristics of the materials changed significantly over the monitoring period. Statistical multivariate analysis was performed to explore strength of the linear relationships between the Proctor optimum moisture content and maximum dry density values and the elemental composition values determined from the XRF testing. The correlation matrix results are presented in Tables 1 and 2 for optimum moisture content and maximum dry density, respectively. The values presented in the matrix represent the Pearson product moment correlation coefficient r value, which provides a measure of the degree or strength of a linear relationship among the different variables. The values vary from -1 to +1, with negative values representing a negative relationship (i.e., if a variable increases the correlating variable decreases) and positive values representing a positive relationship. A value of 1.0 represents a perfect relationship, 0.8 to 1.0 represents a very strong relationship, 0.60 to 0.80 represents a strong relationship, 0.40 to 0.60 represents a moderate relationship, 0.20 to 0.40 represents a weak relationship, and 0.0 to 0.20 represents none to extremely weak relationship. In Tables 1 and 2, stronger relationships are shown in darker colors with blue color representing positive relationships and brown color representing negative relationships. The multivariate analysis indicated that LOI value and most of the elemental compositions, expect silica dioxide, sulphur, potassium oxide, and phosphorous pentoxide, have a strong correlation with both optimum moisture content and maximum dry density of the material. With material morphological and chemical compositions of the fly ash material influencing the soil compaction characteristics, it can challenging to implement the target limits established from Proctor testing to properly characterize the material for field QC/QA purposes.

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MAP 24

TEMPERATURE, oC

0 20 40 60 80 100 120 140 160 180 200 220

MA

SS

LO

SS

, %

90

91

92

93

94

95

96

97

98

99

100

Figure 3. TGA analysis results for fly ash material

Temperature (degrees C)

0 200 400 600 800

Pe

rce

nt M

ass (

ba

se

d o

n intiia

l we

t m

ass)

0.6

0.7

0.8

0.9

1.0

CUFP Fly Ash

CUFP Gypsum - Reject

CUFP Gypsum - Temple

WCFP Fly Ash

WCFP Gypsum

Temperature (degrees C)

20 40 60 80 100 120 140 160 180 200

Pe

rce

nt M

ass (

ba

se

d o

n intiia

l we

t m

ass)

0.75

0.80

0.85

0.90

0.95

1.00

60 C

110 C

Air

Free Water(Loss at 60 C)

Solid Phases

Ettringite: Ca6Al2(SO4)3(OH)1226H2O

Gypsum: CaSO42H2O

Quartz:SiO2Calcite:CaCO3Hematite: Fe2O3Dolomite: CaMg(CO3)2

CCP Phase Diagram

Mineral Phase (chemically

bond) water

CCPs are multi-phase materials

Figure 4. Representation of CCP’s as four-phase systems (Christopher et al. 2013)

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Sili

co

n D

ioxid

e [

SiO

2]

+ A

llum

inu

m

Oxid

e [

Al 2

O3]

+ I

ron

Oxid

e [

Fe

2O

3],

%

64

66

68

70

72

74

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Su

lph

ur

(SO

3)

Co

nte

nt,

%

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Ca

lciu

m O

xid

e (

Ca

O)

Co

nte

nt,

%

9

10

11

12

13

14

15

SO3

CaO

Date

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Lo

ss o

f Ig

nitio

n,

%

4

6

8

10

12

14

16

18

Figure 5. Chemical compositions of fly ash materials collected during the

monitoring period

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Std

. P

rocto

r M

axim

um

Density,

PC

F

60

65

70

75

80

85

90

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Std

. P

rocto

r O

ptim

um

M

ois

ture

Conte

nt,

%

20

25

30

35

40

45

Date

05/19/14 06/02/14 06/16/14 06/30/14 07/14/14 07/28/14 08/11/14

Spe

cific

Gra

vity

2.1

2.2

2.3

2.4

2.5

Figure 6. Standard Proctor and specific gravity test results on fly ash materials

collected over the monitoring period

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Table 1. Correlations table from multivariate analysis between standard Proctor optimum moisture content (w) and elemental/mineral compositions

Parameter w SiO2 Al2O3 Fe2O3 SUM SO3 CaO MgO Na20 K2O P2O5 TiO2 SrO BaOOxide

TotalLOI

w 1.00 -0.31 -0.94 -0.74 -0.77 -0.28 -0.76 -0.79 -0.64 0.23 -0.39 -0.81 -0.80 -0.78 -0.93 0.93

SiO2 -0.31 1.00 0.22 0.02 0.83 -0.32 -0.21 -0.06 -0.05 0.76 -0.66 -0.11 -0.18 -0.15 0.33 -0.32

Al2O3 -0.94 0.22 1.00 0.70 0.71 0.41 0.81 0.83 0.69 -0.29 0.43 0.90 0.84 0.84 0.93 -0.92

Fe2O3 -0.74 0.02 0.70 1.00 0.51 0.32 0.76 0.77 0.71 -0.46 0.48 0.77 0.78 0.75 0.78 -0.81

SUM -0.77 0.83 0.71 0.51 1.00 -0.01 0.32 0.45 0.38 0.34 -0.21 0.44 0.37 0.38 0.78 -0.77

SO3 -0.28 -0.32 0.41 0.32 -0.01 1.00 0.65 0.60 0.75 -0.40 0.57 0.63 0.62 0.59 0.45 -0.40

CaO -0.76 -0.21 0.81 0.76 0.32 0.65 1.00 0.98 0.89 -0.66 0.78 0.96 0.99 0.98 0.84 -0.83

MgO -0.79 -0.06 0.83 0.77 0.45 0.60 0.98 1.00 0.90 -0.57 0.68 0.95 0.97 0.98 0.90 -0.89

Na20 -0.64 -0.05 0.69 0.71 0.38 0.75 0.89 0.90 1.00 -0.39 0.60 0.87 0.88 0.87 0.82 -0.80

K2O 0.23 0.76 -0.29 -0.46 0.34 -0.40 -0.66 -0.57 -0.39 1.00 -0.84 -0.58 -0.63 -0.65 -0.23 0.23

P2O5 -0.39 -0.66 0.43 0.48 -0.21 0.57 0.78 0.68 0.60 -0.84 1.00 0.72 0.77 0.75 0.40 -0.40

TiO2 -0.81 -0.11 0.90 0.77 0.44 0.63 0.96 0.95 0.87 -0.58 0.72 1.00 0.97 0.97 0.89 -0.87

SrO -0.80 -0.18 0.84 0.78 0.37 0.62 0.99 0.97 0.88 -0.63 0.77 0.97 1.00 0.98 0.86 -0.86

BaO -0.78 -0.15 0.84 0.75 0.38 0.59 0.98 0.98 0.87 -0.65 0.75 0.97 0.98 1.00 0.86 -0.85

Oxide Total -0.93 0.33 0.93 0.78 0.78 0.45 0.84 0.90 0.82 -0.23 0.40 0.89 0.86 0.86 1.00 -0.99

LOI 0.93 -0.32 -0.92 -0.81 -0.77 -0.40 -0.83 -0.89 -0.80 0.23 -0.40 -0.87 -0.86 -0.85 -0.99 1.00

Table 2. Correlations table from multivariate analysis between standard Proctor maximum dry density (DD) and elemental/mineral compositions

Parameter DD SiO2 Al2O3 Fe2O3 SUM SO3 CaO MgO Na20 K2O P2O5 TiO2 SrO BaOOxide

TotalLOI

DD 1.00 0.23 0.95 0.78 0.73 0.34 0.80 0.82 0.68 -0.32 0.44 0.86 0.83 0.82 0.94 -0.92

SiO2 0.23 1.00 0.22 0.02 0.83 -0.32 -0.21 -0.06 -0.05 0.76 -0.66 -0.11 -0.18 -0.15 0.33 -0.32

Al2O3 0.95 0.22 1.00 0.70 0.71 0.41 0.81 0.83 0.69 -0.29 0.43 0.90 0.84 0.84 0.93 -0.92

Fe2O3 0.78 0.02 0.70 1.00 0.51 0.32 0.76 0.77 0.71 -0.46 0.48 0.77 0.78 0.75 0.78 -0.81

SUM 0.73 0.83 0.71 0.51 1.00 -0.01 0.32 0.45 0.38 0.34 -0.21 0.44 0.37 0.38 0.78 -0.77

SO3 0.34 -0.32 0.41 0.32 -0.01 1.00 0.65 0.60 0.75 -0.40 0.57 0.63 0.62 0.59 0.45 -0.40

CaO 0.80 -0.21 0.81 0.76 0.32 0.65 1.00 0.98 0.89 -0.66 0.78 0.96 0.99 0.98 0.84 -0.83

MgO 0.82 -0.06 0.83 0.77 0.45 0.60 0.98 1.00 0.90 -0.57 0.68 0.95 0.97 0.98 0.90 -0.89

Na20 0.68 -0.05 0.69 0.71 0.38 0.75 0.89 0.90 1.00 -0.39 0.60 0.87 0.88 0.87 0.82 -0.80

K2O -0.32 0.76 -0.29 -0.46 0.34 -0.40 -0.66 -0.57 -0.39 1.00 -0.84 -0.58 -0.63 -0.65 -0.23 0.23

P2O5 0.44 -0.66 0.43 0.48 -0.21 0.57 0.78 0.68 0.60 -0.84 1.00 0.72 0.77 0.75 0.40 -0.40

TiO2 0.86 -0.11 0.90 0.77 0.44 0.63 0.96 0.95 0.87 -0.58 0.72 1.00 0.97 0.97 0.89 -0.87

SrO 0.83 -0.18 0.84 0.78 0.37 0.62 0.99 0.97 0.88 -0.63 0.77 0.97 1.00 0.98 0.86 -0.86

BaO 0.82 -0.15 0.84 0.75 0.38 0.59 0.98 0.98 0.87 -0.65 0.75 0.97 0.98 1.00 0.86 -0.85

Oxide Total 0.94 0.33 0.93 0.78 0.78 0.45 0.84 0.90 0.82 -0.23 0.40 0.89 0.86 0.86 1.00 -0.99

LOI -0.92 -0.32 -0.92 -0.81 -0.77 -0.40 -0.83 -0.89 -0.80 0.23 -0.40 -0.87 -0.86 -0.85 -0.99 1.00

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SUMMARY OF KEY FINDINGS A summary of the key findings from this study are as follows:

SEM and XRD analysis on fly ash materials showed agglomerations of particles, and needle-like and highly angular crystalline minerals representing formation of ettringite.

TGA analysis showed that the fly ash material liberated water at different temperatures and a strong transition was present near 60oC due to drying of the chemically bound water associated with ettringite.

XRF results results indicated that the chemical compositions of the material and the compaction characteristics of the materials changed with time.

Statistical analysis indicated that LOI values and most of the elemental compositions (expect silica dioxide, sulphur, potassium oxide, and phosphorous pentoxide) have a strong correlation with both optimum moisture content and maximum dry density of the material.

With material morphological and chemical compositions of the fly ash material influencing the soil compaction characteristics, it can challenging to implement the target limits established from Proctor testing to properly characterize the material for field QC/QA purposes.

Strength or stiffness or volume change based performance related target values are recommended as discussed further in White et al. (2015).

REFERENCES Christopher, B.R., White, D.J., Sanchez, R.L. (2013). “Proposed Compaction QC/QA Specifications for TVA’s CCP Stacking Facilities: Part 1. Evaluation of Performance Requirements,” 2013 World of Coal Ash (WOCA) Conference, April 22-25, Lexington, KY. White, D.J., Christopher, B.R., Sanchez, R.L. (2013). “Performance Based QC/QA Specifications for TVA’s CCP Stacking Facilities: Part II. Proposed Specifications,” 2013 World of Coal Ash (WOCA) Conference, April 22-25, Lexington, KY. White, D.J., Vennapusa, P., Gieselman, H., Miller, K., Harland, J. (2012a). “Quality Compaction Field Research: TVA Widows Creek Fossil Plant,” Final Report Submitted to TVA, Center for Earthworks Engineering Research, Iowa State University, Ames, IA. White, D.J., Vennapusa, P., Gieselman, H., Miller, K., Harland, J. (2012b). “Quality Compaction Field Research: TVA Cumberland Creek Fossil Plant,” Final Report Submitted to TVA, Center for Earthworks Engineering Research, Iowa State University, Ames, IA. White, D.J., Vennapusa, P., Hageman, E., Christopher, B., McClung, N., Sanchez, R. (2015). “Assessment of New QC/QA Compaction Monitoring Program at TVA’s Coal

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Combustion Product Stacking Facilities: A Case Study,” 2015 World of Coal Ash (WOCA) Conference, May 5-7, Nashville, TN.