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American Journal of  
Chemistry and Pharmacy (AJCP)

A Review on the Utilization and Environmental Concerns of  Coal Fly Ash
Saidu Kamara1, Edward Hingha Foday Jr2, Wei Wang1*

Volume 2 Issue 2, Year 2023
ISSN: 2834-0116 (Online)

DOI: https://doi.org/10.54536/ajcp.v2i2.1609
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Article Information ABSTRACT

Received: April 26, 2023

Accepted: June 10, 2023

Published: July 02, 2023

Coal is a well-known source of  electric power generation all over the globe. Pulverized coal 
combustion and fluidized coal bed combustion are the two conventional methods involved 
in the combustion of  coal in thermal power plants. Pulverized coal combustion is operated 
at >14000C while the fluid bed is operated at a temperature between 850-9000C. The first 
two reactions are the main reactions in the coal combustion process and they are exothermic 
reactions. CO2 and CO are the first two combustion products. The inorganic minerals in 
coal are released as waste products in the combustion plants in which fly ash made a large 
part of  the waste materials. Fly ash is formed due to the incomplete combustion of  coal. 
The fly ash from both methods contains SiO2, Al, etc, which are significant constituents. 
High-grade coal has a higher SiO2 than the low grade. An experiment was conducted 
by preparing four samples of  fly ash heated at 5000C, 6000C, 7000C, and 8000C, and one 
unheated sample to investigate the chemical composition of  the fly ash obtained from 
Xi’an Linyuan Silica Limited and to prove its suitability as raw material for the fabrication 
of  refractory composites. SEM, EDS, XRD, and FTIR characterization were done on all 
five samples to determine the chemical parameters of  the fly ash. According to the result 
and analysis of  the four characterizations, it was discovered that the fly ash used in this 
research contains SiO2. SEM/EDS morphological analysis reveals the presence of  spherical 
and a few geometrical crystalline-shaped structures known as cenospheres. Cenospheres are 
important components for the synthesis of  refractory composites. The EDS micrographs 
show the percentages of  silicon and oxygen in the fly ash. The FTIR results show Si-O-Si 
stretching in all the fly ash prepared samples. In the XRD, it was discovered that the fly ash 
was purely SiO2. 

Keywords
Characterization, Silica, Coal, 
Combustion, Fly Ash

1 Department of  Chemical Engineering, School of  Water and Environment, Chang’an University, Xi’an, China
2 Key Laboratory of  Subsurface Hydrology and Ecology in Arid Areas, Ministry of  Education, Chang’an University, Xi’an, China
* Corresponding author’s e-mail: ksaidu2013@gmail.com

INTRODUCTION
The popular source of  electric power generation around the 
world today is coal utilized in pulverized coal combustion 
and fluid bed combustible conventional plants(S. H. Lee, 
Lee, Jeong, & Lee, 2019; Miller, 2010). The two different 
combustion methods operate at different temperatures 
and therefore each method produces different fly ash with 
different characteristics (Erol, Küçükbayrak, & Ersoy-
Mericboyu, 2008; L. Li, Wang, & Zhu, 2006; Molina & 
Poole, 2004). The temperature range in the coal fluid 
bed is 8500C-9000C while the temperature in pulverized 
combustion is between 14000C-15000C. At temperatures 
greater than 14000C (>14000C), minerals such as the 
aluminosilicate present in coal disintegrate or decompose 
(Mishra & Baliarsingh, 2008; Pavlish et al., 2003; Rios 
Reyes, 2008). The disintegrated mineral particles form 
hollow or spherical objects called cenospheres as a 
result of  rapid cooling that takes place during the post-
combustion phase (Kim, 2002; Pundienė, Korjakins, 
Pranckevičienė, & Kligys, 2018). High melting point 
minerals sometimes remain unaltered. The physical and 
chemical composition of  fly ash is significantly influenced 
by the type of  coal used and the variation in combustion 
temperatures (Adriano, Page, Elseewi, Chang, & 
Straughan, 1980; Pedersen, Jensen, Skjøth-Rasmussen, 
& Dam-Johansen, 2008; Ratafia-Brown, 1994). Both two 
types of  combustion processes contain Al, SiO2, Fe, and 
Ca which are the four most constituents in fly ash(Cheng 

& Chen, 2003; Criado, Fernández-Jiménez, De La Torre, 
Aranda, & Palomo, 2007). The fly ash produced from 
coal fluidized beds (CBD) has no mullite and glass balls. 
It contains high quantities of  unburnt carbon, calcium 
carbonate, and calcium sulfate and it forms irregular 
objects. Unburned carbon exists in both types of  fly 
ash(Batra, Urbonaite, & Svensson, 2008; Maroto-Valer, 
Taulbee, & Hower, 2001). Fly ash is among the popular 
global raw material and one of  its most important 
utilization is in the fabrication of  composites(Gollakota, 
Volli, & Shu, 2019). In addition to the current global use 
of  fly ash, further research and development are required 
to enhance the management of  fly ash and bring out new 
techniques to control the end products that result from 
burning coal.
Harmful elements and compounds such as aromatic 
compounds, carbon, silica, fine particulate matter, 
etc., are aggregated in high concentrations during 
combustion(Schwarze et al., 2006; Valavanidis, Fiotakis, 
& Vlachogianni, 2008). Thermal power plants produce 
enormous inorganic particles and emit combustible gases 
into the atmosphere which negatively affects the health 
of  residents around the thermal plants(Rahman, Farrok, 
& Haque, 2022). The thermal industries have no use 
for the ash generated and therefore regard it as a solid 
waste product with the potential to negatively impact 
the environment if  not properly regulated(Foday Jr et 
al., 2017). A small amount ( ≈20%) of  the global fly ash 

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produced annually, is used for many purposes and the 
rest reportedly contributes to the existing global pollution 
trend. 
Characterizations using SEM, EDS, XRD, FTIR, etc, to 
determine the composition of  fly ash have been studied. 
Several past works on the two types of  fly ash were 
centered on their properties, morphology, and chemical 
composition. In this study, the morphology and chemical 
information of  the fly ash samples is determined by 
SEM and EDS. The XRD reveals information about the 
crystalline size and amorphous phase of  the samples. The 
in-situ functional groups (Si-O-Si, C-O, Al-O, etc,) in fly 
ash are accounted for by the Fourier transform infrared 
spectroscopy (FTIR). These functional groups are the 
typical determinants for the use of  fly ash as raw material 
for the synthesis of  composites. This experiment, 
therefore, studies the characterization of  purified fly ash 
from the Xi’an Linyuan silica limited.
The major problem encountered in the characterization 
of  fly ash is the amorphous phase of  the sample which 
is normally not captured by the XRD. A more adequate 
instrument is therefore required for the characterization 
of  fly ash to identify its glass phase and interpret chemical 
composition. EDS is the most common effective tool used 
for this purpose but the contrasting characteristics of  fly 
ash present a large scattered result that makes the analysis 
very difficult.  In recent times, new EDS technology or 
detectors have been invented with the capability to collect 
the spectrums of  all pixels in not more than an hour. 
The spectra collected are further processed into maps 
that show the elemental intensities used to quantify the 
chemical compositions of  the elements present. 

LITERATURE REVIEW
Process and end products of  coal combustion
Coal constitutes inorganic minerals released as waste 
products (fly ash, bottom ash, and slag) during the 
burning of  coal. The greater part of  the waste products 
in the combustion plant is made of  fly ash. Fly ash is 
made of  fine light particles and has a density of  about 
2.5g/cm3, bottom ash is composed of  coarse particles 
while slag is the melted phase of  bottom ash(Bourtsalas, 
2015; Bunge, 2015). The ash generated in the combustion 
plant is made of  a large mass of  oxides (silica, alumina, 
titanium oxide, magnesium oxide, calcium oxide, 
sulfoxide, etc,) and heavy metals (zinc, lead, copper, 
nickel, cobalt, cadmium, etc,)(Foday Jr, Bo, & Xu, 2021; 
Halmann, 1995; Oves, Khan, Zaidi, & Ahmad, 2012). Fly 
ash contains some quantities of  spherical matter called 
cenospheres formed by the decomposed minerals in the 
condensation or cooling stage (Blissett & Rowson, 2012; 
Fenelonov, Mel’gunov, & Parmon, 2010; Oves et al., 2012; 
Vassilev, Baxter, Andersen, & Vassileva, 2013). 
Fly ash is formed due to the incomplete combustion 
of  the mineral components in coal which subsequently 
transform into some crystalline phases called quartz 
(silica), magnetite, aluminosilicate glass (mullite), spinel, 
etc. It is confirmed that fly ash is made of  over 80% of  
silica (SiO2)(Criado et al., 2007; Joshi & Lohita, 1997; 
Vassilev & Vassileva, 1996). The glass phase in fly ash 
is formed from low-grade bituminous coals which 
constitute lesser quantities of  silica and a higher amount 
of  magnetite compared to high-grade bituminous coal 
which composes high silica content. The process leading 
to the formation of  fly ash is explained in figure 1 below:

Figure 1: A modified schematic diagram for the formation of  fly ash

The process begins with vaporizable components which 
nucleate to produce new crystalline objects, form char 
and release heavy metals by vaporization. 

Reactions in coal combustion
The carbon in coal undergoes the following chemical 
reaction with oxygen. 

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C + O2 = CO2                   ∆H=-393.7 kj/mol      (1)
C + 0.5O2 = CO               ∆H=-110.1kj/mol       (2)
C + CO2 = 2CO               ∆H=+172.6kj/mol      (3)
C + H2O = CO + H2       ∆H=+131.4kj/mol      (4)
CO + 0.5O2 = CO2          ∆H=-283.6kj/mol       (5)
H2 + 0.5O2 = H2O          ∆H=-242.4kj/ml        (6)
CO + H2O = CO2 + H2  ∆H=-41.2kj/mol          (7)
The reaction is exothermic when the enthalpy ∆H change 
is negative (i.e. heat is given out during the reaction) while 
a positive enthalpy ∆H means the reaction is endothermic 
(i.e. heat is absorbed during the reaction). The first two 
reactions are the main combustion reactions according 
to their products. CO2 and CO are the first combustion 
products and their ratios increase with an increase in 
temperature and a decrease in pressure. The whole 
combustion process leading to the formation of  a coal 
boiler takes about two seconds.

Mathematical models for fly ash deposition in a 
boiler tube
Mathematical models are employed to determine the 
activity of  ash precipitation and to ameliorate the lapses 
in industrial design (Fan, Zha, Sun, & Cen, 2001).  The 
prediction of  ash deposition has been done by a good 
number of  researchers using experimental notations but 
it becomes misleading when taking into consideration 
other operating factors (Fan et al., 2001; Zeng, Zheng, 
Zhou, Fang, & Lou, 2016). The deposition of  ash in 
boilers has been adequately studied with the application 
of  numerical procedures. Fly ash data obtained from a 
computer control scanning electron microscope has been 
used to develop a fly ash prediction model (F. Lee & 
Lockwood, 1999). The temperature, porosity, and rate of  
deposition were simulated by Galen et al (Richards, Slater, 
& Harb, 1993).   

Particle Movement Expressions
Newton’s equation of  motion below is used to account 
for the particle movement in a pulverized boiler. 

The drag force Fd is given by
Fd= -3πdμ(v-u)f                                
where μg is the gas viscosity and f  is the drag force 
coefficient that is used to revise the Stokes expression 
and assuming that f=1. 
Waldmann equation below is used to determine the 
thermophoretic force (Waldmann & Schmitt, 1966)                          

Where 
Ktr is the thermal conductivity of  the gas and
∆T is the temperature gradient. 
The mean speed c of  gas molecules is given by                                              

Particle Collision Forces
The particle equation is used to compute the collision 
velocities of  fly ash deposition in a boiler. To ensure 
efficient and smooth velocity collision, the particle in 
motion must be spherically rigid, the interaction forces 
and infinite forces should be impulsive and negligible 
respectively, and the particle movement should be centered 
and two-dimensional. The mathematical expression for a 
particle is given below (Hoomans, Kuipers, Briels, & van 
Swaaij, 1996).              

where,
vi.0 and vi,1  is the pre-collisional and post-collisional 
velocities of  particle i, and 
vj,0 and vj,1 are the pre-collisional and post-collisional 
velocities of  particle j.
In the absence of  particle-particle collision, we can also 
find the sample position using the single-particle equation 
at a time. The method of  Monte Carlo is exclusively used 
to compute the particle-particle collision. If  particle i 
collides with particle j in the time step, the post-collision 
velocities of  particles are accounted for by taking into 
consideration the dynamics of  the collisions. The 
velocities after particle collision will now substitute the 
initial particle velocities but the particles remain fixed in 
their fixed positions. The particle i in a time step Δt is 
determined by the probability of  the collision below(Bird, 
1994; Tsuji, Tanaka, & Yonemura, 1998):

     
where 
N is the number of  simulated particles in the cell, 
n is the local particle number, 
d is the particle diameter, 
go is the radial distribution function, 
vij v is the relative velocity between particle i and j, and
∆t is the time step.
The collision pairs searching is the key problem in the 
calculation of
particle-particle collision. 

Ash Deposition Model              
During the process of  ash deposition, the large ash 
particle causes an impact in the boiler tube and rebounds 
while the smaller ones can stick onto the tube with 
high adhesive energies(G. Li, Li, Huang, & Yao, 2015). 
The formation of  the first layer improves the surface 
sticking capability in the tub. The critical velocity model 
is employed to ascertain if  a particle motion sticks or 
rebounds in the boiler tube. The critical velocity equation 
is written as:

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Where,
W is the adhesion energy and 
E is Young’s modulus (S. Li, Marshall, Liu, & Yao, 2011; 
Liu, Li, & Yao, 2011). 
According to the simulation of  the critical velocity ( 
Vcritical ) expression above, the impact of  the particle 
velocity is compared with the critical velocity. When the 
critical velocity of  a particle is larger than the velocity 
impact, the particle clings to the surface. 

MATERIALS AND METHOD
Five samples of  fly ash were measured on an electronic 
weighing scale. Each sample weighs 4g. Four of  these 
samples were heated at 5000C, 6000C, 7000C, and 8000C 
for 3 hours in a muffle furnace and one remain unheated. 
The samples were then taken for SEM, EDS, XRD, and 
FTIR characterization. The aim was to determine the 
elemental characteristics of  fly ash that make it a suitable 
raw material to synthesize refractory composites.

Figure 2: Illustration of  instrumental setup and sample preparation

RESULT AND DISCUSSION
SEM Result
This technique is utilized by material scientists to 
determine and interprete the microstructure of  materials. 
The sample electrons are bombarded to produce a wide 
range of  emissions (such as X-rays, visible photons, auger 
electrons, secondary electrons, backscattered electrons, 
etc). The SEM result of  the fly ash specimens is discussed 
as follows:

Heated fly ash
Figures A and B below present the SEM results of  four 
samples of  fly ash heated at 5000C, 6000C, 7000C, and 
8000C. The images in figure (A) above are the results 
of  sample sizes scanned at 100 um pixels while figure B 
scanned image sizes at 20 um. The morphology of  the 
images is based on the materials present in the fly ash 
samples.

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Figure 3: SEM images showing samples scanned at 100 um in (A) and at 20 um in (B).

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All four sample images show un-interconnected irregular 
oval structured layers in both figures A and B. Few 
geometrically shaped structures are observed in sample 
3. The agglomerated oval glassy and geometrically shaped 
structures are typical of  the presence of  silica crystalline 
structures in the sample materials. The balloon-shaped 
crystals are indicative of  the presence of  cenospheres 
in the fly ash samples. There are significant pore sizes 
in the samples and are observed to be highly visible in 
specimens 1, 2, and 4 but less in specimen 3. This shows 
that there might be other materials present in the samples 
but in less or insignificant quantities. All four samples 
heated at various temperatures show that quartz or silica 
is the dominant crystalline phase present in the fly ash 
sample. The samples were heated at temperatures that 
will lead to the elimination of  carbon contents present in 
the fly ash with no chemical reaction taking place. 

Figure 4: SEM images of  unheated raw fly ash at three 
different magnifications

According to the micrographs, the oval crystalline 
structures become more pronounced as the temperature 
increases from 5000C, 6000C, 7000C, and 8000C. 
Conversely, the dark fields observed in the micrographs 
according to the morphology of  the sample become 
lesser as the temperature is increased from sample one to 
four. This can be attributed to the carbon content which 
decreases as more temperature is applied to the fly ash. 

Unheated fly ash sample 
Figure 4 shows the SEM micrographs of  the unheated fly 
ash specimen with image sizes scanned at 200 um, 30 um, 
and 20 um pixels. The oval crystals are the same as those 
of  the heated samples shown in figure 3 but are visibly 
observed to be far apart from each other compared to 
those seen in figure 3A above. This probably implies the 
presence of  carbon content and other minor constituents 
that occupies the spaces between the crystals.

EDS Result
This is a powerful analytical detector or tool used to 
determine the chemical elements present in a sample 

and estimate their relative abundance. It relies on the 
interaction of  some source of  X-ray excitation and a 
sample. The capability of  this tool is due to the fact or 
principle that each element has its unique structure that 
allows several peaks to display on its electromagnetic 
emission spectrum (which is the main principle of  the 
electromagnetic spectrum). 
In the emission of  X-ray characteristics from a sample, 
a beam of  electrons is focused on the sample under 
investigation. At rest, an atom within the sample contains 
ground state (or unexcited) electrons in discrete energy 
levels or electron shells bound to the nucleus. The 
incident beam may excite an electron in an inner shell, 
ejecting it from the shell and forming a hole in the initial 
position of  the electron. An electron from an outer, 
higher-energy shell then fills the hole, and the difference 
in energy between the higher-energy shell and the lower-
energy shell may be released in the form of  an X-ray. The 
EDS is then used to measure the number and energy of  
the X-rays emitted from the sample. The detector or tool 
allows the elemental composition of  the sample to be 
measured since the energies of  the X-ray are characteristic 
of  the atomic structure of  the emitting element. 

Figure 5: EDS result of  heated fly ash specimens 1,2,3 
& 4

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The EDS shown in figures 5 and 6 confirmed the 
percentage weight and elemental composition of  both 
heated and unheated fly ash samples. According to the 
EDS result, oxygen and silicon are the predominating 
elements present in all five prepared samples. The 
percentage composition of  oxygen is greater than that 
of  silicon in all five samples. It is also observed from 
the elemental tables of  the EDS figures that the weight 
percentage of  silicon is higher in the first, third, and 
unheated samples than those of  oxygen atoms but lesser 
in the second and fourth samples. The results and analysis 

of  this work show that fly ash used is silica pure and 
therefore a recommendable source for the fabrication of  
refractory composite materials.

Cenospheres
According to the SEM and EDS results of  this research, 
the hollow and spherical-shaped objects observed in 
all five samples depict the presence of  cenospheres. 
Cenospheres are one of  the end products in coal 
combustion and are significant components for the 
synthesis of  refractory composites. They are formed as 
a result of  the decomposition of  the mineral matter in 
coal at high temperatures >14000C. The disintegrated or 
decomposed mineral matter form into spheres known as 
cenospheres during the post-combustion phase or cooling 
process. The decomposition of  calcium carbonate and 
other compounds below 10000C produces gases that blow 
up the minerals. The gases emitted during coal combustion 
inflate the decomposed minerals to form cenospheres.
These objects are largely made of  mullite, quartz 
(silica), aluminosilicate glass, calcite, sulfates, etc. Inside 
the balloon-shaped structure of  the cenospheres 
are gases while the outer membrane or layer is made 
of  aluminosilicate, cristobalite, quartz, mullite, etc. 
Cenospheres are discovered to be highly porous and can 
therefore effectively absorb water. 

Figure 6: EDS result of  unheated fly ash specimen

Figure 7: EDS and SEM micrographs showing cenospheres

The cenospheres discovered from the fly ash samples 
in this experimental research can be used for electronic 
and radar purposes due to their metallic and magnetic 
properties. Cenospheres are useful to refractory industrial 
engineers due to their low thermal conductivities and 
high heat resistance. They also have properties that are 
suitable to make lightweight composite materials used in 
aerospace engineering.

FTIR Result
This technique is employed to study the vibrations 
of  atoms in a molecule or compounds of  a particular 
sample. The fractions of  absorbed incident radiation at a 
particular energy are determined by passing Infrared (IR) 
radiation through the specimen. The FTIR results and 
analysis of  the prepared fly ash specimens are discussed 
below.

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Heated Samples
The figure below shows the FTIR spectra obtained from 
the characterization of  fly ash samples heated at 5000C, 
6000C, 7000C, and 8000C. According to the experimental 
result of  the sample heated at 5000C, a strong appearance 
of  O=C=O bending of  carbon dioxide compound is 
observed at a wavenumber of  2349cm-1. Further, the 
wavenumber 1083cm-1 depicts the appearance of  Si-
O-Si stretching vibrations and strong absorption peaks 
while the 799cm-1 wavenumber shows Si-O symmetric 
stretching of  quartz.
Analysis of  the FTIR experimental data for the sample 
heated at 6000C also depicts the appearance of  O=C=O 
at 2349cm-1 which indicates a strong vibrational stretching 
of  the carbon dioxide compound. The wavenumbers 

1090cm-1 and 807cm-1 indicate Si-O-Si and Si-O stretching 
vibrations respectively. 
In sample three, the wavenumber 2363cm-1 from FTIR 
results shows O=C=O bending but does not fall at 
2349cm-1 which is the strong absorption band for 
O=C=O. Therefore, the appearance of  carbon dioxide 
compound at 2363cm-1 can either be weak or medium. The 
frequency range for the absorption of  O=C=O is 2400cm-

1–2000cm-1. The wavenumber1090cm-1 and 803cm-1 show 
the stretching vibrations of  Si-O-Si and Si-O respectively. 
The FTIR of  the fourth sample heated at 8000C shows 
the absorption frequencies of  2369cm-1, 1123-1027cm-

1, and 798cm-1. The wavenumber 2369cm-1 shows weak 
or medium O=C=O stretching vibrations while 798cm-1 
depicts the Si-O symmetric of  quarts.  

Figure 8: FTIR results of  four heated fly ash samples

Unheated samples
Figure 9 shows the FTIR result of  the unheated fly ash. 
According to the analysis of  this result, the wavenumber 
2349 cm-1 shows the appearance of  O=C=O stretching 
vibrations, 1049 cm-1 indicates the strong appearance of  
S=O stretching vibration of  sulfoxide while 801 cm-1 
depicts the Si-O symmetric stretching vibration of  the 
quartz. The frequency range for the strong absorption of  
sulfoxide compounds is 1070-1030. 
It is observed that the unheated fly ash shows a strong 
appearance of  O=C=O and S=0 stretching. All five samples 
(both heated and unheated) show O=C=O bending with 
very strong stretching vibrations for the unheated sample 
and those heated at 5000C and 6000C while the third and 
fourth samples heated at 7000C and 8000C show a weak or 
medium appearance of  O=C=O stretching. This suggests 
that the more the temperature the greater the chances of  
eliminating the carbon content in the fly ash. Samples 1, 
2, and 3 heated at 5000C, 6000C, and 7000C respectively 
also show the appearance of  Si-O-Si but not observed for 
the unheated and the heated samples at 8000C. The Si-O 

quartz appears in all five samples according to the FTIR 
result. Finally, it is observed from the FTIR result of  this 
work that sulfoxide functional group (S=O) only appears 
in the unheated fly ash and is not seen in all the heated 
samples. This is an indication that sulfoxide in the raw fly 
ash easily disappears when subjected to a certain amount 
of  heat.

Figure 9: FTIR result of  unheated fly ash sample

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Table 1: Wave numbers of  different types of  functional groups
Wavenumber 
(cm-1) 

Compound Functional Reference

3454 Montmorillonite OH group of  absorbed water 
dust-stretching

(Summer, 1995)

2923 Organic carbon C-H stretching vibration (Hlavay, Jonas, Elek, & Inczedy, 1978)
1629 Organic matter C=O carboxylate group (Langford, Hodson, & Banwart, 2011; 

Matrajt et al., 2004)
1384 Nitrate species N-O stretching (Smidt, Böhm, & Schwanninger, 2011)
1097 Quartz Si-O-Si asymmetric stretching 

vibration
(Katara, Kabra, Sharma, Hada, & Rani, 
2013)

794 Quartz Si-O symmetric (Coates, 1977)
463 Feldspar Amorphous silica Si-O-Si band (Karr, 2013)
2517 Calcite Vibrational mode of  carbonate (Ramasamy, Rajkumar, & Ponnusamy, 2009)
1448 Calcite C-O stretching for carbonate (Ramasamy, Suresh, Meenakshisundaram, 

& Ponnusamy, 2011)
1032 Kaolinite Si-O vibration of  clay mineral (Ramasamy, Rajkumar, & Ponnusamy, 2006)
873 Calcite C-O bending for carbonate (Dahlan, Mei, Kamaruddin, Mohamed, & 

Lee, 2008)
795 Quartz Si-O symmetric (Coates, 1977)
712 Calcite Carbonate (Adler & Kerr, 1963)
466 Feldspar Si-O-Si bending (Dahlan et al., 2008)
1467 Calcite C-O stretching for carbonate (Ramasamy et al., 2011)
2520 Calcite Vibrational mode of  carbonate (Ramasamy et al., 2006)
796 Quartz Si-O symmetric (Coates, 1977)
800, 802 Quartz Si-O bending (Ramasamy et al., 2009)
1083, 1089 Si-O-Si stretching (Ramasamy et al., 2011)
1080, 1025 Si-O-Si stretching vibration (Ramasamy et al., 2011)
1020, 1090  Si-O-Si (Ramasamy et al., 2011)
910 - 830 Si-O stretching vibration (Ramasamy et al., 2006)
3550 - 3200 Alcohol O-H (Ramasamy et al., 2006)
1685 Conjugated ketone C=O stretching (Ramasamy et al., 2006)
1662-1626 Alkene C=C (Ramasamy et al., 2006)
1205 - 1124 Tertiary alcohol C-O stretching (Smidt et al., 2011)
1124 - 1087 Secondary alcohol C-O stretching (Smidt et al., 2011)
1070 - 1030 Sulfoxide S=O stretching (Ramasamy et al., 2006)
850 - 550 Halo compound C-Cl stretching (Ramasamy et al., 2006)
840 - 790 Alkene C=C (Ramasamy et al., 2006)
690 - 515 Halo compound C-Br stretching (Smidt et al., 2011)
600 - 500 Halo compound C-I stretching (Smidt et al., 2011)
1650 - 1556 Cyclic Alkene (Smidt et al., 2011)
2400 - 2000 Carbon dioxide O=C=O (Smidt et al., 2011)

XRD Result 
Material scientists and solid-state chemists use the powdered 
X-ray diffraction technique to determine the structure 
of  a crystalline solid specimen under investigation. The 
diffraction pattern is formed by the interaction of  the 
X-ray and atomic structure. Brag’s law below is used to 
explain the direction of  the scattered X-rays produced. 
nnλ =2dsinθ

Where
n = an integer
λ = wavelength of  the X-ray
d = space between two layers of  atoms
θ = angle between the incoming x-ray and the atom layer

The XRD results obtained from four heated and one 
unheated specimen of  fly ash samples are explained in the 
figures below

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Am. J. Chem. Pharm. 2(2) 41-52, 2023

Figure 10: XRD result of  heated fly ash

The XRD result in figures 10 and 11 shows the presence 
of  SiO2 in all the fly ash samples. This information is in 
line with the SEM/EDS and FTIR characterizations. The 
results and analysis of  all the characterization in this work 
confirmed that the purified fly ash obtained from Xi’an 
Linyuan Silica Limited is pure silica.

CONCLUSION
Critical analysis of  the experimental results from the four 
characterization techniques shows that fly ash purely 
constitutes SiO2 which is a significant constituent for the 
fabrication of  refractory composites. According to the 
SEM/EDS results, the cenospheres discovered in the 
specimens are useful to refractory industrial engineers due 
to their low thermal conductivities and high heat resistance.

Acknowledgments
This work was supported by the National Natural Science 
Foundation of  China. Thanks, and appreciation to Wang 
Wei of  the chemical engineering department, school of  
Water and Environment of  Chang’an University for the 
adequate supervision and for providing the platform for 
the successful conduct of  this work.

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