CHARACTERIZATION OF FLY ASH FOR THEIR EFFECTIVE MANAGEMENT AND UTILIZATION

CHARACTERIZATION OF FLY ASH FOR THEIR EFFECTIVE MANAGEMENT AND UTILIZATION
CHARACTERIZATION OF FLY ASH FOR THEIR
EFFECTIVE MANAGEMENT AND UTILIZATION
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
MINING ENGINEERING
By
RAKESH KUMAR BEHERA
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA, ORISSA - 769008
DEPARTMENT OF MINING ENGINEERING
2009-2010
CHARACTERIZATION OF FLY ASH FOR THEIR
EFFECTIVE MANAGEMENT AND UTILIZATION
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
MINING ENGINEERING
By
RAKESH KUMAR BEHERA
Under the Guidance of
Prof. H.K.NAIK
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA, ORISSA - 769008
DEPARTMENT OF MINING ENGINEERING
2009-2010
I
DEPARTMENT OF MINING ENGINEERING
National Institute of Technology,
Rourkela-769008
CERTIFICATE
This is to certify that the thesis entitled “characterisation of fly ash for their
effective management and utilization” submitted by Sri Rakesh Kumar
Behera in partial fulfillment of the requirements for the award of Bachelor of
Technology degree in Mining Engineering at National Institute of Technology,
Rourkela is an authentic work carried out by him under my supervision and
guidance.
To the best of my knowledge, the matter embodied in the thesis has not been
submitted to any other University/Institute for the award of any Degree or
Diploma.
Prof. H.K.NAIK
II
ACKNOWLEDGEMENT
I wish to express my deep sense of gratitude and indebtedness to Prof. H.K.NAIK,
Department of Mining Engineering, N.I.T Rourkela, for introducing the present topic and for
their inspiring guidance, constructive criticism and valuable suggestion throughout this
project work.
My sincere thank to all my friends who have patiently extended all sorts of help for
accomplishing this undertaking.
Date:
RAKESH KUMAR BEHERA
III
CONTENTS
PAGE NO
TOPICS
CHAPTER-1
2-3
1.1 INTRODUCTION
1.2 OBJECTIVES
1.2.1 SPECIFIC OBJECTIVES
3
3
CHAPTER-2
LITERATURE REVIEW
2.1 GENERATION
5-8
2.2 CLASSIFICATION
9-11
2.3 FEATURES
11-12
2.4 NATURE AND COMPOSITION
12-14
2.5 UTILIZATION
14
2.6 COLLECTION
15
2.7 TRANSPORTATION
15-17
2.8 CHARECTERISTICS OF FLY ASH SLURRY
17-18
2.9 FLY ASH MANAGEMENT
18-19
CHAPTER-3
MECHANISM AND MATERIAL
21
3.1 MATERIALS AND METHODS
3.1.1SAMPLE COLLECTION
21-23
3.2 SETTLING CHARACTERISTIC
23-30
3.3 PHYSICOCHEMICAL PROPERTIES
3.3.1 MOISTURE CONTENT
30
30-31
3.3.2 TRUE DENSITY
32
3.3.3 SPECIFIC GRAVITY
3.3.4 PARTICLE SIZE DISTRIBUTION
32-33
34-37
3.3.5 BET- SURFACE AREA METHOD
38-41
3.3.6 SEM (SCANNING ELECTRON MICROSCOPE)
3.4 SLUMP TEST
41-43
43-44
CHAPTER-4
4.1 RESULT AND DISCUSSION
46-48
4.2 CONCLUSION
48-49
IV
LIST OF TABLES
TOPICS
Table: 2.1 Fly ash generation and utilization in different countries
PAGE NO
8
Table: 2.2 Typical Chemistry of Coal Fly Ash (in Wt. %)
13
Table: 2.3 Chemical composition of fly ash
14
Table: 3.1 Sample name: F50W50 settling characteristic
23
Table: 3.2 Sample name: F25W75 settling characteristic
24-25
Table: 3.3 Sample name: F40W60 settling characteristic
26-27
Table: 3.4 Sample name: F35W65 settling characteristic
28-29
Table: 3.5 Moisture content of fly ash
31
Table: 3.6 True density of fly ash
32
Table: 3.7 Specific gravity of fly ash
33
V
LIST OF FIGURES
TOPICS
PAGE NO
Figure: 2.1 Method of fly ash transfer can be dry, wet or both
5
Figure: 2.2 Coal ash generations from a pulverized coal-fired boiler
6
Figure: 2.3 Section with Class C Fly Ash
10
Figure: 2.4 Section with Class F Fly Ash
11
Figure: 2.5 Fly Ash Handling Systems
17
Figure:
3.1 sampling location in the ESP
22
Figure:
3.2 Fly ash (class C) sample as taken from an ash impoundment
22
Figure:
3.3 Fly ash (class F) sample as taken from an ash impoundment
22
Figure:
3.4 settling characteristic graph of
F50W50
24
Figure:
3.5 settling characteristic graph of
F25W75
26
Figure:
3.6 settling characteristic graph of
F40W60
27
Figure:
3.7 settling characteristic graph of
F35W75
29
Figure:
3.8 Experimental set up for settling characteristic of fly ash
30
Figure: 3.9 Particle size distribution of fly ash
35
Figure:
36
3.10 Dynamic Light Scattering Zeta Sizer: Malvern Instrument
Figure: 3.11 BET plot
39
Figure: 3.12 Adsorption isotherm graph
40
Figure: 3.13 SEM microphotographs of fly ash under different magnification
VI
42-43
ABSTRACT
Large amount of fly ash discharged from coal-fired power stations is a major problem.
Amount of fly ash available can be recycled, mainly by adding fly ash to cement. However,
the addition of fly ash to cement is limited because the production rate of cement is leveled
off, and also the concentration of fly ash in cement is limited.
Researchers have studied the aspect of particle size distribution, permeability, slurry flow
characteristics, settling characteristics, slump characteristics and so on for mill tailings. Fly
ash in terms of particle size and mineralogical composition is similar to mill tailings. There is
however not much literature available on direct placement of fly ash as a fill material.
Investigations on the fly ash have been largely confined to determining physico-chemical
properties, strength properties (as cement substitute or with binders). The present study
therefore makes use of literature available in terms of fly ash, to design and conduct different
experiments on settling rate and hydraulic transportation aspects of fly ash.
Different experiments were conducted to find the pH at different levels of lime, cement and
gypsum. These were done to further utilize the cementing property of fly ash and its use for
support and fill the mine voids as well as construction of cement of different strength levels.
It was observed that the strength level increase with the increase in the percentage of lime and
it was observed to be maximum at 4% of lime and 8 % cement and 4% gypsum. Thus as pH
is directly related to strength so it indicated that the strength characteristics was further
enhanced using the aforesaid composition. The SEM of the samples were done to study the
characteristics of individual elements as the element having spherical shapes showed
maximum pozzolanic character.
VII
CHAPTER-1
INTRODUCTION
OBJECTIVES
1
1.1 INTRODUCTION
The combustion of pulverized coal at high temperatures and pressures in power stations
produces different types of ash.
The 'fine' ash fraction is carried upwards with the flue gases and captured before reaching the
atmosphere by highly efficient electro static precipitators. This material is known as
Pulverized Fuel Ash (PFA) or 'fly ash'. It is composed mainly of extremely fine, glassy
spheres and looks similar to cement. The 'coarse' ash fraction falls into the grates below the
boilers, where it is mixed with water and pumped to lagoons. This material, known as
Furnace Bottom Ash (FBA) has a gritty, sand-like texture.
Fly ash closely resembles volcanic ashes used in production of the earliest known hydraulic
cements about 2,300 years ago. Those cements were made near the small Italian town of
Pozzuoli - which later gave its name to the term "pozzolan." A pozzolan is a siliceous or
siliceous / aluminous material that, when mixed with lime and water, forms a cementitious
compound. Fly ash is the best known, and one of the most commonly used, pozzolans in the
world.
Instead of volcanoes, today's fly ash comes primarily from coal-fired electricity generating
power plants. These power plants grind coal to powder fineness before it is burned. Fly ash the mineral residue produced by burning coal - is captured from the power plant's exhaust
gases and collected for use.
Fly ash is a fine, glass powder recovered from the gases of burning coal during the
production of electricity. These micron-sized earth elements consist primarily of silica,
alumina and iron.
The difference between fly ash and portland cement becomes apparent under a microscope.
Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely
in mixtures. That capability is one of the properties making fly ash a desirable admixture for
concrete.
2
Fly ash is one of the residues generated in the combustion of coal. Fly ash is generally
captured from the chimneys of coal-fired power plants, whereas bottom ash is removed from
the bottom of the furnace. In the past, fly ash was generally released into the atmosphere, but
pollution control equipment mandated in recent decades now requires that it be captured prior
to release. Depending upon the source and makeup of the coal being burned, the components
of the fly ash produced vary considerably, but all fly ash includes substantial amounts of
silicon dioxide (SiO2) (both amorphous and crystalline) and calcium oxide (CaO). Fly ash is
commonly used to supplement Portland cement in concrete production, where it can bring
both technological and economic benefits, and is increasingly finding use in the synthesis of
geopolymers and zeolites.
1.2 OBJECTIVES
The object of this study is “characterization of fly ash for their effective management and
utilization”. This requires following specific objectives:
1.2.1 SPECIFIC OBJECTIVES
 Study engineering properties
 Settling characteristics of the fly ash samples collected.
 Particle size analysis of fly ash
 BET- Surface area of fly ash
3
CHAPTER-2
LITERATURE REVIEW
GENERATION
CLASSIFICATION
FEATURES
NATURE AND COMPOSITION
UTILIZATION
COLLECTION
TRANSPORTATION
CHARECTERISTICS OF FLY ASH SLURRY
FLY ASH MANAGEMENT
4
2.1
GENERATION OF FLY ASH
Fly ash is produced by coal-fired electric and steam generating plants. Typically, coal is
pulverized and blown with air into the boiler's combustion chamber where it immediately
ignites, generating heat and producing a molten mineral residue. Boiler tubes extract heat
from the boiler, cooling the flue gas and causing the molten mineral residue to harden and
form ash. Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the
combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in
the flue gas. Prior to exhausting the flue gas, fly ash is removed by particulate emission
control devices, such as electrostatic precipitators or filter fabric baghouses.
Figure: 2.1 Method of fly ash transfer can be dry, wet or both
Fly ash is one of the residues generated in the combustion of coal. Fly ash is generally
captured from the chimneys of coal-fired power plants, and is one of two types of ash that
jointly are known as coal ash; the other, bottom ash, is removed from the bottom of coal
furnaces. Depending upon the source and makeup of the coal being burned, the components
of fly ash vary considerably, but all fly ash includes substantial amounts of silicon dioxide
(SiO2) (both amorphous and crystalline) and calcium oxide (CaO), both being endemic
ingredients in many coal bearing rock strata.
5
Toxic constituents depend upon the specific coal bed makeup, but may include one or more
of the following elements or substances in quantities from trace amounts to several percent:
arsenic, beryllium, boron, cadmium, chromium, chromium VI, cobalt, lead, manganese,
mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with dioxins and
PAH compounds.
Figure: 2.2 Coal ash generations from a pulverized coal-fired boiler
(Source: Japan Fly Ash Association)
6
In the past, fly ash was generally released into the atmosphere, but pollution control
equipment mandated in recent decades now requires that it be captured prior to release. In the
US, fly ash is generally stored at coal power plants or placed in landfills. About 43 percent is
recycled, often used to supplement Portland cement in concrete production.
In India the annual production of flyash is nearly 45 million tones per year. This is likely to
increase to 70 million tons per year by 2010. At present most of the flyash is being dumped.
The disposal of the flyash is a serious hazard to the environment that consumes millions of
rupees towards the cost of its disposal. About 14000 Hect. Of precious land have already
been used for dumping it and another thousand of Hect. Would be required in future.
In India coal/lignite based thermal power stations account for more than 55% of the
electricity installed capacity and 65% of electricity generation. The ash content of the coal
used at the thermal stations ranges from 30-40%, with the average ash content around
35%.Since low ash, high grade coal is reserved for metallurgical industries; the thermal
power plants have to utilize high ash, low grade coal.
The thermal power plant ash generation has increased from about 40 million tones during
1993-1994 to 120million tonnes during 2005-06 and is expected to be in the range of 175
million tonnes per year by 2012, on account of the proposal to double the power generation.
Coupled with this, the deteriorating quality (increasing ash quantity) of coal is expected to
aggravate the situation.
7
Table: 2.1 Fly ash generation and utilization in different countries
SL NO
Country
Annual ash
Ash utilization %
production, MT
1
India
112
38
2
China
100
45
3
USA
75
65
4
Germany
40
85
5
UK
15
50
6
Australia
10
85
7
Canada
6
75
8
France
3
85
9
Denmark
2
100
10
Italy
2
100
11
Netherlands
2
100
As per estimates, the annual fly ash generation in the country in 2007-08 (data of 2008-09
under compilation, expected to be 150 million tonnes) was about 125 million tonnes; fly ash
recycled is about 30 per cent, i.e about 40 million tonnes. Out of this the cement industry
consumes around 28-30 million tonnes which is above 70 per cent of the recycled ash. Hence,
there is still a huge surplus of 85 million tonnes which is being disposed off as slurry in the
ponds.
Another approximately 78,000 mw of new power generation capacity is expected to come up
in the country within three-four years. Out of this major portion of around 60 per cent would
come in form of thermal power. Estimated generation of fly ash till 2012 would be 175
million tonnes, which again would pose a serious problem of disposal. The major consumer
of fly ash is the cement industry only, while some small quantities are used for making flyash
bricks,
landfill
etc.
As per the table, the utilization of fly ash in the cement industry in manufacturing PPC
cement is increasing on yearly basis. Further motivation and freight equalization to cement
industry can result into 100 per cent fly ash disposal in a most eco-friendly manner.
8
2.2
CLASSIFICATION
Fly ash material solidifies while suspended in the exhaust gases and is collected by
electrostatic precipitators or filter bags. Since the particles solidify while suspended in the
exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 µm
to 100 µm. They consist mostly of silicon dioxide (SiO2), which is present in two forms:
amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and
hazardous; aluminum oxide (Al2O3) and iron oxide (Fe2O3). Fly ashes are generally highly
heterogeneous, consisting of a mixture of glassy particles with various identifiable crystalline
phases such as quartz, mullite, and various iron oxides.
Fly ash also contains environmental toxins in significant amounts, including arsenic (43.4
ppm); barium (806 ppm); beryllium (5 ppm); boron (311 ppm); cadmium (3.4 ppm);
chromium (136 ppm); chromium VI (90 ppm); cobalt (35.9 ppm); copper (112 ppm); fluorine
(29 ppm); lead (56 ppm); manganese (250 ppm); nickel (77.6 ppm); selenium (7.7 ppm);
strontium (775 ppm); thallium (9 ppm); vanadium (252 ppm); and zinc (178 ppm).
Two classes of fly ash are defined by ASTM C618: Class F fly ash and Class C fly ash. The
chief difference between these classes is the amount of calcium, silica, alumina, and iron
content in the ash. The chemical properties of the fly ash are largely influenced by the
chemical content of the coal burned (i.e., anthracite, bituminous, and lignite).
Not all fly ashes meet ASTM C618 requirements, although depending on the application, this
may not be necessary. Ash used as a cement replacement must meet strict construction
standards, but no standard environmental standards have been established in the United
States. 75% of the ash must have a fineness of 45 µm or less, and have carbon content,
measured by the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be under
6%. The particle size distribution of raw fly ash is very often fluctuating constantly, due to
changing performance of the coal mills and the boiler performance. This makes it necessary
that fly ash used in concrete needs to be processed using separation equipment like
mechanical air classifiers.
2.2.1 Class C fly ash
Fly ash produced from the burning of younger lignite or sub bituminous coal, in addition to
having pozzolanic properties, also has some self-cementing properties. In the presence of
9
water, Class C fly ash will harden and gain strength over time. Class C fly ash generally
contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C fly ash does not
require an activator. Alkali and sulfate (SO4) contents are generally higher in Class C fly
ashes.
Figure: 2.3 Section with Class C Fly Ash
(Source: U.S department of transportation, Federal Highway Administration)
2.2.2 Class F fly ash
The burning of harder, older anthracite and bituminous coal typically produces Class F fly
ash. This fly ash is pozzolanic in nature, and contains less than 10% lime (CaO). Possessing
pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing
agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in
order to react and produce cementitious compounds. Alternatively, the addition of a chemical
activator such as sodium silicate (water glass) to a Class F ash can lead to the formation of a
geopolymer.
10
Figure: 2.4 Section with Class F Fly Ash
Source: U.S department of transportation, Federal Highway Administration
2.3
FEATURES
 Spherical shape: Fly ash particles are almost totally spherical in shape, allowing
them to flow and blend freely in mixtures.
 Ball bearing effect: The "ball-bearing" effect of fly ash particles creates a lubricating
action when concrete is in its plastic state.
 Higher Strength: Fly ash continues to combine with free lime, increasing structural
strength over time.
 Decreased Permeability : Increased density and long term pozzolanic action of fly
ash, which ties up free lime, results in fewer bleed channels and decreases
permeability
Increased Durability. Dense fly ash concrete helps keep aggressive compounds on the
surface, where destructive action is lessened. Fly ash concrete is also more resistant to
attack by sulfate, mild acid, soft (lime hungry) water, and seawater.
 Reduced Sulfate Attack: Fly ash ties up free lime that can combine with sulfate to
create destructive expansion.
 Reduced Efflorescence: Fly ash chemically binds free lime and salts that can create
efflorescence and dense concrete holds efflorescence producing compounds on the
inside.
 Reduced Shrinkage: The largest contributor to drying shrinkage is water content.
The lubricating action of fly ash reduces water content and drying shrinkage.
11
 Reduced Heat of Hydration: The pozzolanic reaction between fly ash and lime
generates less heat, resulting in reduced thermal cracking when fly ash is used to
replace portland cement.
 Reduced Alkali Silica Reactivity: Fly ash combines with alkalis from cement that
might otherwise combine with silica from aggregates, causing destructive expansion.
 Workability: Concrete is easier to place with less effort, responding better to
vibration to fill forms more completely.
Ease of Pumping. Pumping requires less energy and longer pumping distances are
possible.
 Improved Finishing: Sharp, clear architectural definition is easier to achieve, with
less worry about in-place integrity.
 Reduced Bleeding: Fewer bleed channels decreases porosity and chemical attack.
Bleed streaking is reduced for architectural finishes. Improved paste to aggregate
contact results in enhanced bond strengths.
 Reduced Segregation: Improved cohesiveness of fly ash concrete reduces
segregation that can lead to rock pockets and blemishes.
 Reduced Slump Loss: More dependable concrete allows for greater working time,
especially in hot weather.
2.4
NATURE AND COMPOSITION
Fly ash is a fine, glass powder recovered from the gases of burning coal during the
production of electricity. These micron-sized earth elements consist primarily of silica,
alumina and iron. When mixed with lime and water the fly ash forms a cementitious
compound with properties very similar to that of Portland cement.
12
Table: 2.2 Typical Chemistry of Coal Fly Ash (in Wt. %)
ClassF*
ClassF*
ClassC*
Class C*
low-Fe
high-Fe
high-Ca
low-Ca
SiO2
46-57
42-54
25-42
46-59
Al2O3
18-29
16.5-24
15-21
14-22
Fe2O3
6-16
16-24
5-10
5-13
CaO
1.8-5.5
1.3-3.8
17-32
8-16
MgO
0.7-2.1
0.3-1.2
4-12.5
3.2-4.9
K2O
1.9-2.8
2.1-2.7
0.3-1.6
0.6-1.1
Na2O
0.2-1.1
0.2-0.9
0.8-6.0
1.3-4.2
SO3
0.4-2.9
0.5-1.8
0.4-5.0
0.4-2.5
LOI
0.6-4.8
1.2-5.0
0.1-1.0
0.1-2.3
TiO2
1-2
1-1.5
<1
<1
There are substantial amount of non-combustible impurities present in coal in the form of
limestone, shale, dolomite, feldspar and quartz. As the fuel travels through the hightemperature zone in the furnace, the volatile matter and carbon are burnt off whereas of the
mineral impurities are carried of in the form of ash by the flue gas. The ash particles become
fused in the combustion zone of the furnace; however on leaving the combustion zone of the
furnace the molten ash is cooled rapidly and solidifies as spherical glassy particles. Some of
the fused matter agglomerates to form bottom ash, but most of it flies out with the flue gas
stream and is therefore called flyash.
The flyash is removed from the flue gas by means of a series of mechanical separators
followed by electrostatic precipitators or bag filters. Typically the ratio of flyash to bottom
ash is 70:30 in wet bottom boilers or 85:15 in dry bottom boilers
13
Table: 2.3 Chemical composition of fly ash
Component
Bituminous
Sub bituminous
Lignite
SiO2 (%)
20-60
40-60
15-45
Al2O3 (%)
5-35
20-30
20-25
Fe2O3 (%)
10-40
4-10
4-15
CaO (%)
1-12
5-30
15-40
LOI (%)
0-15
0-3
0-5
2.5
UTILIZATION
The reuse of fly ash as an engineering material primarily stems from its pozzolanic nature,
spherical shape, and relative uniformity. Fly ash recycling, in descending frequency, includes
usage in:

Portland cement and grout

Embankments and structural fill

Waste stabilization and solidification

Raw feed for cement clinkers

Mine reclamation

Stabilization of soft soils

Road sub base

Aggregate

Flow able fill

Mineral filler in asphaltic concrete
Other applications include cellular concrete, geopolymers, roofing tiles, paints, metal castings, and
filler in wood and plastic products
14
2.6 COLLECTION
2.6.1 Ash Collection
Ash can be collected in following categories
 Dry Fly Ash
Dry ash is collected from different rows of electrostatic precipitators. It is available in two
different grades of fineness in silos for use as resource material by different users.
 Bottom Ash
Bottom ash is collected from the bottom of the boiler and transported to hydro bins and then
ash mound for use in road embankment.
 Conditioned Fly Ash
Conditioned fly ash is also available in ash mound for use in landfills and ash building
products.
2.7 TRANSPORTATION
2.7.1 Fly Ash Transportation
Fly ash can be supplied in four forms:
 Dry: This is currently the most commonly used method of supplying fly ash. Dry fly
ash is handled in a similar manner to Portland cement. Storage is in sealed silos with
the associated filtration and desiccation equipment, or in bags.
 Conditioned: In this method, water is added to the fly ash to facilitate compaction and
handling. The amount of water added being determined by the end use of the fly ash.
Conditioned fly ash is widely used in aerated concrete blocks, grout and specialist fill
applications.
 Stockpiled: Conditioned fly ash not sold immediately is stockpiled and used at a later
date. The moisture content of stockpiled ash is typically 10 to 15%. This is used
mainly in large fill and bulk grouting applications.
 Lagoon: Some power stations pump fly ash as slurry to large lagoons. These are
drained and when the moisture content of deposited fly ash has reached a safe level
may be recovered. Because of the nature of the disposal technique, the moisture
content can vary from around 5% to over 30%. Lagoon fly ash can be used in similar
applications to stockpiled conditioned fly ash.
15
2.7.2 CHALLENGES IN HANDLING FLY ASH
Many challenges have been reported in the handling and utilization of fly ash. Some of these
difficulties include:
 The wet system of fly ash collection/disposal is the most common practice in
India. Fly ash is mixed with bottom ash in slurry form before transporting it to ash
ponds/lagoons. This process of fly ash dumping is largely unsuitable for all
purposes where pozzolonic properties are essential to its use. Fineness and lime
reactivity are seriously imparted and the ash from the ponds is unsuitable for use
in most applications needing strength.
 Variations in ash composition are unavoidable and it largely depends on the
quality of coal utilizes. Customer therefore can never therefore be sure of the
quality of ash available from a particular source.
 There is no system of testing, labeling and packing of coal ash. Most of the ash
producers are not equipped to certify the quality or specifications of an ash. This
undermines the confidence of the end users of the fly ash from a particular source
compelling them to set up such testing and other facilities at their own cost. This
obviously makes them somewhat reluctant to use it.
 Most thermal stations are located in remote areas and the user industry faces
difficulty in lifting and transporting the fly ash.
16
Figure: 2.5 Fly Ash Handling Systems
2.8 CHARECTERISTICS OF FLY ASH SLURRY
The ash is characterized by its physical (lightweight, small spherical particles, hardness) and
chemical (cement-like) properties that gives it with an economic value as a raw material in
many applications.
2.8.1 PHYSICAL PROPERTIES
Fly ash particles are very fine, light weight (density 1.97-2.89 g/cc) and spherical (specific
surface area 4000-10,000 cm2/g; diameter, 1-150µ), refractory and have pozzolanic ability.
Fly ash grey to blackish grey and is dependent on coal type and combustion process. Fly ash
has dielectric property (dielectric constant, 104) and can be used in electronic application.
17
2.8.2 CHEMICAL PROPERTIES
Chemical composition of fly ash is as follows: SiO2, 59.38; Fe2O3, 6.11; CaO, 1.94; MgO,
0.97; SO3, 0.76; alkalies, 1.41; and unburnt sulphur and moisture, 3.74%. According to
ASTM C618 fly ash is classified into two classes (C & F) based on the amount of lime
present. Class C lignite and sub-bituminous coal (>10% CaO) where as class F is bituminous
or anthracite coal (<10% CaO). Oxides of silicon, aluminum, calcium and iron in fly ash are
responsible for pozzolanic activity, which decreases by loss of ignition. Fly ash contain
following toxic metals Hg, 1; Cd, Ga, Sb, Se, Ti and V, 1-10; As, Cr, La, Mo, Ni, Pb, Th, U
and Zn, 10-100; and B, Ba, Cu, Mn and Sr, 100-1000 mg/kg. Heavy metals (As, Mo, Mn and
Fe) show leaching with concentration above permissible limits.
2.9 FLY ASH MANAGEMENT
2.9.1 Eliminating Waste and Abating CO2 Emissions
Different methods of processing coal, different coal washery systems, clean coal technologies
and particularly the development of ultra clean coal can, and do, have a dramatic effect on the
quality and quantity of fly ash, generated by the combustion of pulverised coal. This type of
solution, while reducing the problem of fly ash disposal, still leaves us with a problem of
disposal of coal washery refuse, or some other form of processed coal waste. There are other
potential possibilities for modifying the characteristics of fly ash to advantage, converting it
from waste into a value added product. Two examples are increasing pozzolanicity and
enhancing the cenosphere content of fly ash.
2.9.2 Increasing pozzolanicity
The pozzolanicity of fly ashes can vary widely. This reflects both the amount and nature of
the mineral matter in the pulverised coals being used as fuel and the combustion conditions
under which the fly ash is formed. It is believed that the production of fly ash of very high
pozzolanicity is possible, without compromising the operation of a power station, through the
judicious selection of appropriate coals or through slight modification of the chemical
composition of the mineral matter present in the pulverised coal, and also through controlling
the combustion process. Much higher Portland cement replacement rates with fly ash in
concretes can be achieved using highly reactive ash, and still comply with the relevant
Australian standards. Pozzolanic reactivity of fly ash is enhanced under steam-curing
18
conditions. If concrete block manufacturing plants could be located near electric power
stations, eliminating the need for transportation of ash and making full use of the available
steam, again there could be a significant increase in the use of the fly ash.
2.9.3 Cenospheres
Most fly ashes contain a small proportion of thin-walled hollow spheres which are called
'cenospheres'. Many of these have a specific gravity less than one and when the fly ash is
sluiced to the ash disposal dam they float and can be recovered as a value-added product. For
example, cenospheres have been used as a filler in plastic and paint manufacturing and in the
production of insulating refractories, which are known for their excellent strength to density
ratios and for the thermal shock resistance. In Australia cenospheres after appropriate
processing can fetch prices well when used in these applications.The cenosphere content of
fly ashes varies from coal to coal and with combustion conditions (furnace load). If we can
understand and control the factors responsible it should be possible to increase significantly
the cenosphere content of selected fly ashes, again without compromising the operation of a
power station for electricity production. If the amount of cenospheres in fly ash can be
increased significantly, and hence become available in sufficient quantities on a reliable longterm basis, at a reduced unit cost, a range of excellent lightweight building materials and
other products can be manufactured at a competitive market price.
Very large volumes of fly ash can be used in manufacturing special slurries for diaphragm
walls and land reclamation, in underground mining, in special grouts, and for the
encapsulation of hazardous wastes and for the decommissioning of underground fuel tanks.
19
Chapter 3
MECHANISM AND MATERIAL
MATERIALS AND METHODS
SAMPLE COLLECTION
SETTLING CHARACTERISTIC
PHYSICOCHEMICAL PROPERTIES
MOISTURE CONTENT
TRUE DENSITY
SPECIFIC GRAVITY
PARTICLE SIZE DISTRIBUTION
BET- SURFACE AREA METHOD
SLUMP TEST
SEM (SCANNING ELECTRON MICROSCOPE)
20
3.1 MATERIALS AND METHODS
The study of the physicochemical and engineering properties of fly ash is necessary to
understand the variation in the properties of fly ash In the Indian context, in order to utilize
the same as large volume backfill media. In addition to this the study is required to establish
properties such as permeability, particle size distribution, and morphological characteristics
of the fly ash which influence the settling behavior and flow properties during hydraulic
transportation.
3.1.1SAMPLE COLLECTION
Fly ash is a fine, powdery material that is produced by burning coal to produce electricity,
primarily in pulverized coal combustion (PCC) boilers. It is composed mainly of noncombustible inorganic material, but also contains some carbon that is leftover from partially
combusted coal. Although fly ash particles are generally largely spherical in shape, there are
usually irregularly-shaped particles present also, such as angular particles of quartz. The
spherical shape of fly ash results from the formation of tiny molten droplets as the ash travels
through the boiler. The droplets form spheres because this shape minimizes the surface area
relative to the volume. Since it is so fine, removal and collection of fly ash from combustion
gases (flue gas) requires specialized equipment such as electrostatic precipitators or fabric
filters.
The sample collection of different types of ashes such as fly ash, bottom ash and pond ash has
different procedures. The fly ash and the bottom ashes are generated at the power plant and
can be collected directly from the discharge points. In most of the power plants sampling
pipes are provided at places near the discharge point or near the storage point for collection of
ash samples. The sample can be directly collected into a bucket or any other container and
can be suitable packed for transportation.
21
Figure: 3.1 sampling location in the ESP
Figure: 3.2 Fly ash (class C) sample as taken from an ash impoundment
Figure: 3.3 Fly ash (class F) sample as taken from an ash impoundment
22
Sample from Jindal Steel Plant
The sample was collected from Jindal Steel Plant, from near the hopper of the power plant.
The fly ash sample collected was dark grey in color. The sample was collected in a sack.
3.2 SETTLING PROPERTIES
Tests on settling rates establish the ease with which solid-liquid separation takes place in
slurries during filling activity, and the tests also provide a means of determining the recycled
water quality.
Experimental procedure:
Take a measuring jar graduated in ml. clean it thoroughly with pure water. Take water into
the flask up to certain level (x ml) and then add fly ash (100-x ml) slowly into the jar. Mix the
water and fly ash thoroughly with a stirrer for some time. Note down upper and lower
meniscus of the mixture. Then note down time for each ml settling of fly ash in water. Repeat
the other sample and tabulate the results.
Table: 3.1 Sample name: F50W50 settling characteristic
Time (hrs)
Upper level reading (ml)
Lower level reading (ml)
2.45
2.50
3.02
3.14
3.25
3.37
3.47
3.56
4.07
4.16
4.24
4.32
4.40
4.49
4.59
5.12
5.28
6.00
74
74
74
74
74
74
74
74
74
74
74
74
74
74
74
74
74
74
72
71
70
69
68
67
66
65
64
63
62
61
60
59
58
57
56
55
23
F50W50
80
Settling in ml
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
time in hrs
Figure: 3.4 settling characteristic graph of F50W50
Table: 3.2 Sample name: F25W75 settling characteristic
Time (hrs)
2.06
2.06
2.07
2.07
2.08
2.09
2.10
2.10
2.11
2.12
2.12
2.13
2.14
2.15
2.15
2.16
2.17
2.17
2.18
2.19
2.20
2.21
Upper level reading (ml)
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
24
Lower level reading (ml)
84
83
82
81
80
79
78
77
76
75
74
73
72
71
70
69
68
67
66
65
64
63
2.22
2.23
2.23
2.24
2.25
2.26
2.27
2.28
2.29
2.30
2.31
2.32
2.33
2.34
2.36
2.37
2.39
2.40
2.42
2.44
2.46
2.49
2.51
2.54
2.57
3.01
3.05
3.11
3.18
3.25
3.35
3.54
7.00
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
87
25
62
61
60
59
58
57
56
55
54
53
52
51
50
49
48
47
46
45
44
43
42
41
40
39
38
37
36
35
34
33
32
31
30
90
F25W75
80
Settling in (ml)
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Time in (hrs)
Figure: 3.5 settling characteristic graph of F25W75
Table: 3.3 Sample name: F40W60 settling characteristic
Time (hrs)
Lower level reading (ml)
Upper level reading (ml)
2.04
78
79
2.06
77
79
2.11
76
79
2.14
75
79
2.18
74
79
2.22
73
79
2.25
72
79
2.28
71
79
2.32
70
79
2.35
69
79
2.38
68
79
2.42
67
79
2.45
66
79
2.48
65
79
2.51
64
79
2.55
63
79
2.59
62
79
3.02
61
79
26
3.05
60
79
3.09
59
79
3.12
58
79
3.15
57
79
3.19
56
79
3.22
55
79
3.27
54
79
3.30
53
79
3.35
52
79
3.40
51
79
3.45
50
79
3.51
49
79
3.58
48
79
4.08
47
79
4.24
46
79
4.51
45
79
90
F40W60
80
Settling in (ml)
70
60
50
40
30
20
10
0
0
1
2
3
Time in (hrs)
Figure: 3.6 settling characteristic graph of F40W60
27
4
5
Table: 3.4 Sample name: F35W65 settling characteristic
Time (hrs)
Upper level reading (ml)
Lower level reading (ml)
1.48
82
80
1.49
82
79
1.50
82
78
1.53
82
77
1.55
82
76
1.57
82
75
1.59
82
74
2.02
82
73
2.04
82
72
2.06
82
71
2.08
82
70
2.11
82
69
2.13
82
68
2.15
82
67
2.18
82
66
2.20
82
65
2.22
82
64
2.24
82
63
2.27
82
62
2.29
82
61
2.31
82
60
2.33
82
59
2.35
82
58
2.37
82
57
2.40
82
56
2.42
82
55
2.45
82
54
2.47
82
53
2.50
82
52
2.53
82
51
2.55
82
50
2.58
82
49
3.02
82
48
28
3.06
82
47
3.09
82
46
3.13
82
45
3.18
82
44
3.24
82
43
3.32
82
42
3.41
82
41
3.55
82
40
4.28
82
39
90
F35W65
80
Settling in (ml)
70
60
50
40
30
20
10
0
0
0.5
1
1.5
2
2.5
3
3.5
Time in (hrs)
Figure: 3.7 settling characteristic graph of F35W65
29
4
4.5
Figure: 3.8 Experimental set up for settling characteristic of fly ash
3.3 PHYSICOCHEMICAL PROPERTIES
Collected fly ash samples are examined under the different processes to know the major
physicochemical properties and settling properties of all the samples. The properties studies
are chemical composition, particle, size distribution, specific gravity, true density.
3.3.1 MOISTURE CONTENT
Coal due it its nature, origin and occurrence is always associated with same amount of
moisture, which is both physically and chemically bound. It is customary to differentiate
between external and inherent moisture. When a wet fly ash is exposed to atmosphere, the
external atmosphere evapourates, but the apparently dry fly ash still contains some moisture,
which can be removed only on heating above 100οC. External moisture is also called
accidental or free moisture where as inherent moisture is termed as equilibrium or air dried or
hygroscopic moisture. The quantity of external moisture depends mainly on the mode of
30
occurrence and handling of fly ash, but the air-dried moisture is related to the inherent
hygroscopic nature of the fly ash.
Test procedure
About 1 gm of finely powdered (-212 micron) air-dried fly ash sample is weighed in a silica
crucible and then placed inside an electric hot air oven, maintained at 1080±20C. The crucible
with the fly ash sample is allowed to remain in the oven for 1.5 hours and is then taken out
with a pair of tongs, cooled in desiccators for about 15 minutes and then weighed. The loss in
weight is reported as moisture (on % basis). Then calculation is done as per the following.
% moisture = Y−Z/Y−X
Where X= weight of empty crucible, gram
Y= weight of crucible + fly ash sample before heating, gram
Z= weight of crucible + fly ash sample after heating, gram
Y−X= weight of fly ash sample, gram
Y−Z= weight of moisture
Table: 3.5 MOISTURE CONTENT OF FLY ASH
Weight of
Weight of fly
Weight of
Weight of
Moisture
Average
empty
ash (gm)
crucible and
crucible and
content (%)
moisture
fly ash
fly ash after
before
heating (gm)
crucible(gm)
content (%)
heating (gm)
15.236
1.001
16.237
16.102
0.135
15.125
1.005
16.130
15.914
0.210
16.856
1.004
16.860
16.679
0.180
0.175
31
3.3.2 TRUE DENSITY
True density of fly ash is the weight per unit volume of very finely powdered sample.
Therefore, the volume of pores spaces and the interspaces is not included here. To determine
the true density, fly ash sample is dispersed in water. The amount of water dispersed per gram
of fly ash gives the true density of fly ash. The density of the fly ash, which ranges from 22.8, determines the volume it will occupy for a given mass. Density changes may indicate a
different coal source.
Test procedure
Take a measuring jar graduated in ml. clean it thoroughly with pure water. Take water into
the flask up to certain level and note down its level (initial reading). Drop slowly 20 grams of
the supplied fly ash sample into the jar. Shake the jar for some time. Now note down the level
of eater in the jar (final reading). Repeat this for 4 samples and tabulate the results. Divide the
difference of the final and initial reading by weight of the sample to obtain true density.
Table: 3.6 TRUE DENSITY OF FLY ASH
SL NO
Amount of Initial
Final
Difference
True
Average
fly
reading
in (ml)
Density
true
in (gm/cc)
density
ash reading
taken (gm)
in (ml)
in (ml)
1
20
80
89
9
2.23
2
20
80
88
8
2.5
3
20
81
90
9
2.23
4
20
81
90
9
2.23
2.29
3.3.3 SPECIFIC GRAVITY
Specific gravity is the ratio of the mass of unit volume of soil at a stated temperature to the
mass of the same volume of gas-free distilled water at a stated temperature. The specific
gravity of a fly ash is used in the phase relationship of air, water and solids in a given volume
of the fly ash.
32
Test procedure
Determine and record the weight of the empty clean and dry pycnometer, W P. Place 10g of a
dry fly ash sample (passed through the sieve No. 10) in the pycnometer. Determine and
record the weight of the pycnometer containing the dry fly ash, WPS. Add distilled water to
fill about half to three-fourth of the pycnometer. Soak the sample for 10 minutes. Apply a
partial vacuum to the contents for 10 minutes, to remove the entrapped air. Stop the vacuum
and carefully remove the vacuum line from pycnometer. Fill the pycnometer with distilled
(water to the mark), clean the exterior surface of the pycnometer with a clean, dry cloth.
Determine the weight of the pycnometer and contents, WB. Empty the pycnometer and clean
it. Then fill it with distilled water only (to the mark). Clean the exterior surface of the
pycnometer with a clean, dry cloth. Determine the weight of the pycnometer and distilled
water, WA. Empty the pycnometer and clean it.
Calculate the specific gravity of the fly ash using the following formula:
Specific Gravity, Gs = Wo/Wo+ (WA − WB)
Where:
W0 = weight of sample of oven-dry fly ash, g = WPS − WP
WA = weight of pycnometer filled with water
WB = weight of pycnometer filled with water and fly ash
Table: 3.7 SPECIFIC GRAVITY OF FLY ASH
Sl.no
.
Mass of empty,
Mass of
Mass of
Mass of
Specific
Averag
clean
empty
pycnometer +
pycnometer
gravity
e
pycnometeter
pycnometer
dry fly ash +
+ water
(GS)
Specifi
(WP), (grams)
+ dry fly ash
water
(WA),
c
(WPS),
(WB),(grams)
(grams)
gravity
(grams)
1
41.69
73.42
157.28
139.38
2.294
2
49.93
79.67
157.29
142.40
2.254
2.275
33
3.3.4 PARTICLE SIZEDISTRIBUTION
The particle size distribution (PSD) of a powder, or granular material, or particles dispersed
in fluid, is a list of values or a mathematical function that defines the relative amounts of
particles present, sorted according to size. PSD is also known as grain size distribution.
Particle Size: A better indication of the fineness is to determine the particle size distribution.
For example, one can determine the mass percentage below 10 um or determine the mean
particle diameter. The particle size of fly ash varies from below 1 um to 200 um or more.
Thus a fly ash might have the following distribution (on a mass basis): 0.3-2 % below 1 um,
30-70 % finer than 10 um, 0.5-7 % above 100 um and 0-2 % above 200 um. It should be
noted that to increase the Strength Activity Index (ASTM C 618) one can air-classify or grind
the fly ash to improve its fineness. On a numerical basis: 40-60% of total numbers of
particles are from 0-1 um. This is more significant with regards to greater surface area for
pozzolanic reactions and leaching potential of trace metals.
The particle size of the fly ashes was measured using a laser based particle size analyzer,
namely a Mastersizer 2000 of Malvern Instruments Ltd. It utilizes Fraunhofer diffraction of
light formed by particles with a diameter larger than the incident laser beam wavelength. A
combination of an optical filter, lens and photo detector coupled with a computer loaded with
Mastersizer software enables one to compute the particle size distribution from the diffraction
data and store it as volume percentage against the particle size.
34
Particle Size Distribution
Volume (%)
9
8
7
6
5
4
3
2
1
0
0.01
0.1
1
10
100
1000 3000
100
1000 3000
Particle Size (µm)
flyash-mining-100, Tuesday, April 13, 2010 3:15:15 PM
Volume (%)
Difference Graph - Ref: None
7
6
5
4
3
2
1
0
0.01
0.1
1
10
Particle Size (µm)
flyash-mining-100, Tuesday, April 13, 2010 3:15:15 PM
Figure: 3.9 Particle size distribution of fly ash
PARTICLE SIZE
 Minimum-3.44 um
 Mean-8.080 um
 Maximum-18.585 um
35
Figure: 3.10 Dynamic Light Scattering Zeta Sizer: Malvern Instrument
Definition
A suspension of powder in isopropanol is measured with a low angle laser beam, and
the particle size distribution is calculated.
Scope
This is a fast method for measuring particle size distribution of powders.
Principle
The method can be used on all powders containing less than 10% fat.
Apparatus
 Malvern Instrument, Mastersizer Basic, equipped with software version B.0 or
similar equipment.
 Malvern QS Small Volume Sample Dispersion Unit.
 Malvern in/out measuring cell, beam length 2.0 mm.
 Dispenser 0-50 ml with container.
 Filling knife.
 Waste container.
36
Reagents
 Isopropanol, IPA (technical quality).
Procedure
 Look at the particle size in a microscope and choose a lens capable of
measuring the largest particles.
 Prepare the instrument for measuring in wet mode using IPA as the liquid, as
described in the user manual.
The stirrer regulator should be set at 2000 rpm on the Malvern unit.
 Measure the background for IPA.
 Quickly add a sufficient amount of milk powder and measure as soon as the
powder is dispersed and not later than 20 seconds after addition of the powder.
For detailed instructions about measuring, see the Malvern user manual.
 Rinse twice with IPA.
All measurements are made in duplicate
Result
The following calculations are done automatically:
 The volume median diameter D (v, 0.5) is the diameter where 50% of the
distribution is above and 50% is below.
 Two determinations of mean particle size should not differ by more than 5%
relative. The shape of the curves in the two determinations should be the same.
 D (v, 0.9), 90% of the volume distribution is below this value.
 D (v, 0.1), 10% of the volume distribution is below this value.
 The span is the width of the distribution based on the 10%, 50% and 90%
quantile.
Span = D [v, 0.9]−D [v, 0.1]
D [v, 0.5]
37
3.3.5 BET- SURFACE AREA METHOD
Specific surface area is defined as the ratio A / m (unit: m2/g) between the absolute surface
area of a solid and its mass (sample weight). The surface area includes all parts of accessible
inner surfaces (mainly pore wall surfaces).
BET theory
BET theory is a rule for the physical adsorption of gas molecules on a solid surface and
serves as the basis for an important analysis technique for the measurement of the specific
surface area of a material.
The concept of the theory is an extension of the Langmuir theory, which is a theory for
monolayer molecular adsorption, to multilayer adsorption with the following hypotheses: (a)
gas molecules physically adsorb on a solid in layers infinitely; (b) there is no interaction
between each adsorption layer; and (c) the Langmuir theory can be applied to each layer. The
resulting BET equation is expressed by
v = vmcp/ (p0−p) {1+(c−1) (p/p0)}…….. (1)
A more convenient form is the following:
p/v (p0−p) = 1/vmc + c−p/vmc (p/p0)
p and p0 are the equilibrium and the saturation pressure of adsorbates at the temperature of
adsorption, v is the adsorbed gas quantity (for example, in volume units), and vm is the
monolayer adsorbed gas quantity. c is the BET constant, which is expressed by
c = exp (E1−EL)/ RT
E1 is the heat of adsorption for the first layer, and EL is that for the second and higher layers
and is equal to the heat of liquefaction.
Plot a straight line with 1 / v[(P0 / P) − 1] on the y-axis and φ = P / P0 on the x-axis according
to experimental results. This plot is called a BET plot. The linear relationship of this
equation is maintained only in the range of 0.05 < P / P0 < 0.35. The value of the slope A and
the y-intercept I of the line are used to calculate the monolayer adsorbed gas quantity vm and
the BET constant c. The following equations can be used:
vm = 1/ A+I
c = 1+A/I
38
Figure: 3.11 BET plot
The BET method is widely used in surface science for the calculation of surface areas of
solids by physical adsorption of gas molecules. A total surface area Stotal and a specific
surface area S are evaluated by the following equations:
SBET, total = vmNs/V
SBET = Stotal/a
N: Avogadro's number
s: adsorption cross section
V: molar volume of adsorbent gas
a: molar weight of adsorbed species
Principle of the method
The BET method involves the determination of the amount of the adsorbate or adsorptive
gas required to cover the external and the accessible internal pore surfaces of a solid with a
39
complete monolayer of adsorbate. This monolayer capacity can be calculated from the
adsorption isotherm by means of the BET equation.
The gases used as adsorptives have to be only physically adsorbed by weak bonds at the
surface of the solid (van der-Waals forces) and can be desorbed by a decrease of pressure
at the same temperature. The most common gas is nitrogen at its boiling temperature (77.3
K). In the case of a very small surface area (below 1 m2/g), the sensitivity of the
instruments using nitrogen is insufficient and krypton at 77.3 K should be used.
In order to determine the adsorption isotherm volumetrically, known amounts of adsorptive
are admitted stepwise into the sample cell containing the sample previously dried and
outgassed by heating under vacuum. The amount of gas adsorbed is the difference of gas
admitted and the amount of gas filling the dead volume (free space in the sample cell
including connections). The adsorption isotherm is the plot of the amount gas adsorbed (in
mol/g) as a function of the relative pressure p/p0.
Adsorption
Figure: 3.12 Adsorption isotherm graph
40
Physisorption, determined by:
 Temperature
 Gas pressure
 Interaction between surface and gas (e.g vapor pressure)
 Surface area
Monolayer adsorption: Langmuir isotherm
Multilayer adsorption: BET theory
The specific surface area of a fly ash is estimated from the amount of nitrogen adsorbed
in relationship with its pressure, at the boiling temperature of liquid nitrogen under
normal atmospheric pressure. The observations are interpreted following the model of
Brunauer, Emmett and Teller (BET Method).
The BET method is widely used in surface science for the calculation of surface areas
of flyash by physical adsorption of gas molecules.
3.3.6 SEM (Scanning Electron Microscope)
The scanning electron microscope (SEM) is a type of electron microscope that images the
sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern.
The electrons interact with the atoms that make up the sample producing signals that contain
information about the sample's surface topography, composition and other properties such
as electrical conductivity.
The types of signals produced by an SEM include secondary electrons, back-scattered
electrons (BSE), characteristic X-rays, light (cathodoluminescence), specimen current and
transmitted electrons. Secondary electron detectors are common in all SEMs, but it is rare
that a single machine would have detectors for all possible signals. The signals result from
interactions of the electron beam with atoms at or near the surface of the sample. In the most
common or standard detection mode, secondary electron imaging or SEI, the SEM can
produce very high-resolution images of a sample surface, revealing details about less than 1
to 5 nm in size. Due to the very narrow electron beam, SEM micrographs have a large depth
of field yielding a characteristic three-dimensional appearance useful for understanding the
surface structure of a sample. A wide range of magnifications is possible, from about 10
times (about equivalent to that of a powerful hand-lens) to more than 500,000 times, about
250 times the magnification limit of the best light microscopes. Back-scattered electrons
(BSE) are beam electrons that are reflected from the sample by elastic scattering. BSE are
41
often used in analytical SEM along with the spectra made from the characteristic X-rays.
Because the intensity of the BSE signal is strongly related to the atomic number (Z) of the
specimen, BSE images can provide information about the distribution of different elements in
the sample. Characteristic X-rays are emitted when the electron beam removes an inner shell
electron from the sample, causing a higher energy electron to fill the shell and release energy.
These characteristic X-rays are used to identify the composition and measure the abundance
of elements in the sample.
Chemical analysis in the scanning electron microscope is performed by measuring the energy
or wavelength and intensity distribution of x-ray signal generated by a focused electron beam
on the specimen. With the attachment of the energy dispersive spectrometer (EDS) or
wavelength dispersive spectrometer (WDS) the precise elemental composition of material can
be obtained with high spatial resolution. When we work with bulk specimen in the SEM very
precise accurate chemical analysis (relative error- 1-2%) can be obtained from larger areas of
the solid (0.5-3 µm dia) using an EDS or WDS.
Figure: 3.13 SEM MICROPHOTOGRAPHS OF FLY ASH UNDER DIFFERENT
MAGNIFICATION
Magnification at 1000x
42
Magnification at 5000x
3.4 SLUMP TEST (ASTM Method C143 CAN3-A23.2-M77)
Determining the consistency of concrete by filling a conical mold with a sample of concrete,
then inverting it over a flat plate and removing the mold; the amount by which the concrete
drops below the mold height is measured and this represents the slump.This test method
describes the procedure for determining the slump of fresh concrete mixtures.
Test Procedure
 Dampen the mold and place it on a flat, moist, non-absorbent rigid surface.
 Hold firmly in place by standing on the two foot pieces.
 Fill the cone 1/3 full and uniformly rod the layer 25 times to its full depth.
 Fill the cone with a second layer until 2/3 full by volume and rod 25 times uniformly,
ensuring that the rod just penetrates into the first layer.
 Overfill the cone with the third layer and rod uniformly, 25 times, with the rod just
penetrating into the second layer.
 Strike off the excess concrete level with the top of the cone by a screening and rolling
motion of the tamping rod.
43
 Remove any spilled fly ash from around the bottom of the cone.
 Immediately remove the mold from the fly ash by raising it carefully in a vertical
direction without lateral or torsional motion.
 Measure the difference between the height of the mold and the height of the specimen
at its highest point to the nearest 6.3 mm. This distance will be the slump of the fly
ash.
44
CHAPTER-4
RESULT AND DISCUSSION
CONCLUSION
45
4.1 RESULTS & DISCUSSION
4.1.1 SEM (Scanning Electron Microscope)
The SEM data indicated intermixing of Fe and Al-Si mineral phases and the predominance
of Ca non-silicate minerals. The fly ash samples consisted mainly of amorphous aluminosilicate spheres with a lesser number of iron-rich spheres. The majority of the iron-rich
spheres consisted of two phases: an iron oxide mixed with amorphous alumino-silicate. The
calcium-rich material was distinct in both elemental composition and texture from the
amorphous alumino-silicate spheres. It was clearly a non-silicate mineral possibly calcite,
lime, gypsum or anhydrite. In spite of the inherent variability of fly ash samples, this analysis
indicated that the primary mineral/morphological structures are fairly common. Quartz and
alumino-silicates are found as crystals and as amorphous particles.
4.1.2 SPECIFIC GRAVITY (ASTM D 854)
The specific gravity of the fly ash collected from Jindal Steel Plant was found to be 2.275.
4.1.3 TRUE DENSITY
The true density of fly ash collected from Jindal Steel plant was found to be 2.29.
4.1.4 MOISTURE CONTENT
The moisture content of fly ash collected from Jindal Steel Plant was found to be 0.175%.
The moisture content of the sample were found out to be around 0.175% indicating that all
the moisture have been evacuated and they are suitable for the construction works etc.
46
4.1.5 SPECIFIC SURFACE AREA
The specific BET surface area of the fly ash collected from the Zindal Steel Plant was found
to be 0.44 square meter per gram.
4.1.6 PARTICLE SIZE ANALYSIS
The particle size of fly ash collected from Jindal steel plant was found to be
Minimum-3.44 µm Mean-8.080 µm Max-18.585 µm
The size, density, type of reinforcing particles and its distribution have a pronounced effect
on the properties of particulate composite. The size range of the particles is very wide i.e. 0.1
micron to 100 micron. The size ranges of the fly ash particles indicate that the composite
prepared can be considered as dispersion strengthened as well as particle reinforced
composite. Thus the strengthening of composite can be due to dispersion strengthening as
well as due to particle reinforcement.
4.1.7 SETTLING CHARACTERISTICS OF FLY ASH
Fly ash: 40 gm and water: 60ml
Upper reading: 79 ml, lower reading: 78 ml
The total time taken for settling of fly ash in the mixtures was found to be 2 hours 47 minutes
at the reading 45ml of the mixtures in the flask.
Fly ash: 50 gm and water: 50 ml
Upper reading: 74 ml, lower reading: 72 ml
The total time taken for settling of fly ash in the mixtures was found to be 3 hours and 15
minutes at the reading 55ml of mixtures in the flask.
Fly ash: 35 gm and water: 65 ml
Upper reading: 82 ml, lower reading: 80ml
The total time taken for settling of fly ash in the mixtures was found to be 2 hours 40 minutes
at the reading 39ml of the mixtures in the flask.
47
Fly ash: 25 gm and water: 75ml
Upper reading: 87 ml, lower reading: 84 ml
The total time taken for settling of the fly ash in the mixture was found to be 1 hours and 50
minutes at the reading 31ml of mixtures in the flask. From the above figure, the composition
of fly ash: 25 gm and water: 75ml was found to be the better parameter among the other
parameters for the separation of solid-liquid in slurries during the filling activity.
4.1.8 SLUMP TEST
The slump-height of the fly ash collected from Jindal Steel Plant was found to be 80mm.
4.2 CONCLUSION
 From the compositions of fly ash sample collected, it can be concluded that the fly ash
sample belongs to ASTM class F.
 Visual observations of the SEM images show a distinct spherical nature for the grains
for the fly ash samples.
 The specific gravity attribute to the mineralogical composition i.e presence of silica
content and CaO.
 The moisture content of the samples were found out to be around 0.175% indicating
that all the moisture have been evacuated and they are suitable for the construction
works etc.
48
 Due to the fine grained nature of the solid constituents, the fly ash slurries exhibit
marked sluggishness for settling and also did not provided clear supernatant solutions.
 The composition of fly ash: 25 gm and water: 75ml is the good parameter for the
separation of solid-liquid in slurries during the filling activity.
 Pozzolanic properties of fly ash can be identified by presence/absence of calcium
oxide. So class F fly ash is the weak in pozzolanic as very less amount of calcium
oxide present.
 The chemical, physical and mineralogical properties of fly ash had appreciable effects
on performance of fly ash in filling low lying and mine void areas.
 Bituminous (pozzolanic) fly ash is more frequently used to construct embankments
and structural backfills than sub bituminous or lignite (self-cementing) fly ash. This is
due in part to the self-cementing characteristics of the latter type, which hardens
almost immediately after the addition of water.
49
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