Value Added Ceramic
Products from Fly Ash: A
Review
Swapan Kumar Das
Chief Scientist
Refractories Division
CSIR-Central Glass & Ceramics Research Institute
Kolkata – 700032, India
ICUFA Conference, Kolkata,January 10-11, 2013
Phase Analysis
Microstructure
SOME CONVENTIONAL USES
FLY ASH BRICKS
CELLULAR LIGHT WEIGHT CONCRETE
FLY ASH CEMENT
ROADS, FILLS & EMBANKMENTS
FILLING OF OPEN LAND AND MINES
AGRICULTURE APPLICATIONS
ASH BASED COMPONENTS FOR
CONSTRUCTION INDUSTRY
PAVEMENT BLOCK
SUBSTITUTE OF CLINKER & NATURAL
AGGREGATES
WOOD SUBSTITUTE - DOORS & PANELS
GRANITE SUBSTITUTE
PAINTS & ENAMELS
NON CONVENTIONAL USES
WEAR RESISTANT TILES
REFRACTORY AGGREGATE
MODERATE HEAT DUTY
REFRACTORY BRICK (IS 6)
TRADITIONAL PORCELAIN TILES
METAL MATRIX COMPOSITES (MMC)
GLASS CERAMICS
Fly Ash Based Wear Resistant Liner
Background
 Industrial material handling equipments used for
transport of highly erosive and abrasive media
particles undergoes
 Heavy erosion & abrasion by MECHANICAL
PROCESS
 Corrosion by CHEMICAL PROCESS
 BHEL and few private organizations in India
manufacturing high alumina liners to provide a cost
effective solution to such wear problem.
AUTHOR OF THIS PAPER SHARING HIS R&D
EXPERIENCES OF BHEL, NML & CGCRI
“WEAR”
IN MATERIAL HANDLING SYSTEM
A Phenomena of removal of material from
surfaces in relative motion by:
EROSION
1. MECHANICAL PROCSS
ABRASION
2. CHEMICAL PROCESS
CORROSION
A SOLUTION TO THE WEAR PROBLEM
USING CERAMIC LINER
WHY CERAMICS?
Remarkable resistance to both sliding &
impact abrasion
Exceptionally tough & harder than tool
steel
Extraordinary durability
Low friction coefficient
Outlasts metal from ten to fifteen times
Lower cost : Performance ratio
RECOGNISED WEAR RESISTANT CERAMIC
MATERIALS
 Boron carbide
 Silicon nitride
 SIALON
 Alumina (>85%)
 Fused Cast
 Basalt
 AZS
Sequential order of erosion rate:
Boron Carbide <Silicon
Nitride<Sialon<Alumina<AZS<Basalt
OPTIMUM SELECTION
BC
Si3N4
Sialon
NOT A COST EFFECTIVE SOLUTION
AZS
Basalt
Inhomogeneity in structure & presence
of tapped cavities created from gas
entrainment due to a high melt viscosity
MOST COST EFFECTIVE MATERIAL
ALUMINA FAMILY
(85 – 95% Al2O3)
FOR SLIDING ABRASION
Crystal size : 2 – 16 micron
FOR EROSION (IMPINGEMENT WEAR)
Crystal size : 2-8 micron
Part Replacement of costly
alumina by Fly Ash
Properties of Wear resistant liners (study at CSIRCGCRI)
Properties
Fly Ash Based
Fly Ash content
10%
20%
40%
Al2O3 85%
Bulk Density, gm/cc
3.38
2.85
2.75
3.47
2.90
App. Porosity (%)
0.8
0.5
0.3
0.3
0.9
Moh’s hardness
9.0
9.0
~9
9.0
~9
>10000
>10000
>10000
2300
14.10
19.04
16.72
44.2
Erosion rate (vol. 0.0135
loss, cc/kg erodent)
0.0151
0.0162
0.0155
0.0985
Phase content
Mullite,
Corundum
Mullite,
Corundum,
Corundum Mullite,
Comp.
(kg/cm2)
strength >10000
Abradability index
6.17
Mullite,
Corundum
Alumina
Based
Basalt
View of some applications
View of some applications
Possible Waste Incorporated Batches
S. K. Das, CSIR-CGCRI
A
B
Calcined
alumina,
basalt
powder, iron
ore tailing,
clay and fly
ash.
Low grade
bauxite, fly
ash, clay,
iron ore
tailing and
basalt
powder.
Results of Different Batches
Properties
A
B
Vitrification Temperature
(OC)
1275
1275
% Linear Shrinkage
15-17
18-20
Bulk Density (g/cc)
2.85-2.90
2.80-2.85
% Water Absorption
<0.50
<0.50
Abradibility index
10-12
12-14
Cold Crushing Strength
(Kg/cm2)
>4000
>4000
Mohs’ Scale Hardness
Closer to 8
Closer to 8
S. K. Das, CSIR-CGCRI
A Comparison of the Abrasive
Hardness of Different Materials
Material
Abrasive Hardness*
Raw basalt
Cast basalt, vitreous
Cast basalt, Crystallized
Sintered basalt
Hard porcelain
Sintered corundum
90% Sintered alumina
Waste based ceramics (as
presented in the current study)
700-900
750-800
1600-2700
2500-2900
1300-1400
4500-5000
4000-4500
3000-3500
* Measured by a wet grinding method using a load of 4.5 kg, at a velocity of
50meters/min. Abrasive materials used was 60 mesh SiC grains.
Areas of Waste Incorporated Liner
Application
S. K. Das, CSIR-CGCRI
REFRACTORY AGGREGATE
 Refractories - withstand high load at higher
temperatures.
 Most common refractories are alumina and silica.
 Alumina, silica and their compounds are the prime
candidate as refractory, except high basic environment,
 Fly ash, containing mainly alumina and silica, can
also replace many of the refractory products if iron
oxide, lime and alkalis are negligible in amount.
 Fly ash contains these oxides (harmful for any
refractory item) to an extent of 15 to 25%. Hence fly
ash can not be directly used as refractory material.
 It can be used in combination with alumina and can
replace part of costly alumina without deteriorating the
properties.
Aggregate Property (study at CSIR-CGCRI)
Properties
Fly Ash Kyanite
Fly Ash Use
28%
--
Bulk Density
(gm/cc)
2.81
3.13
App. Porosity
(%)
0.35
0.43
Refractoriness >1804 >1804
(oC)
Aggregate as gel bonded self flow high alumina
castable (study at CSIR-CGCRI)
Properties
Fly
based
ash Kyanite
based
Aggregate amount
5% Fly ash 5% kyanite 70% WTA
+65% WTA +65%WTA
Self flow value (%)
Green Density (gm/cc)
Fired Density (gm/cc)
Green CCS (kg/cm2)
78
2.98
2.96
440
83
2.96
2.94
430
80
3.00
2.98
450
Fired CCS (kg/cm2)
950
980
1050
1400oC MOR (kg/cm2) 62
Phase content
Corundum,
Mullite
WTA based
65
48
Corundum, Corundum,
Mullite
Mullite
MODERATE HEAT DUTY REFRACTORY
BRICK (IS 6)
 Important not as hot face refractory but mainly as a back
up refractory lining.
 Withstand temperature up to 1300oC without any
deterioration.
 Prevents heat loss of the furnace and to bear the load.
 Based on fire clay, which is a refractory grade clay
material containing only the alumino silicates.
 Fly ash, similar in composition, can replace fireclay.
 Non consistency in composition and presence of
higher extent of iron and alkaline earth oxide
materials do not allow fly ash to be used singly. Up to
40% of fly ash can be successfully used in combination
with fire clay materials for these bricks.
Properties of Fly Ash Based IS 6 Brick (study at
CSIR-CGCRI)
Property
Alumina content (%)
Bulk density (gm/cc)
Apparent porosity (%)
Specific gravity
CCS (MPa)
Refractories (ASTM No.)
R. U. L.,Ta (oC)
PLCR (%), 1350oC for 5 h
IS
Specification
30 min
40% Fly Ash
Based brick
33.4
1.99
25
22.3
2.59
30
>32
1340
Nil
20 (min)
30
1300
1.0, max.
TRADITIONAL PORCELAIN TILES
A traditional porcelain batch consists of clay,
quartz and feldspar.
Gradual depletion of good quality natural raw
materials has increased the price of the product.
Alternative sources are being exploited.
Fly ash, alumino-silicate material, can replace the
kaolinitic clays partly.
Resemblance with clay in chemistry and
inherently containing microcrystalline
components like quartz and mullite, a replacement
of 25 – 30% of kaolinitic clay by fly ash hardly
affects the properties.
BACKGROUND….

Gradual depletion of natural minerals calls for
alternative source for raw materials

Converting industrial waste into wealth

mitigating environmental pollution is a global need

Industry require scientific understanding of the
waste incorporated K2O-Al2O3-SiO2 SYSTEM
RESEARCH CHALLENGE FOR INDUSTRY
Fly Ash in Traditional Porcelain Tiles
 Substitution of quartz in common porcelain tile (quartz
acts as a filler) by fly ash up to 15% showed an increase in
the density and shrinkage and corresponding decrease in
porosity.
 Again this can be achieved at a lower temperature than
the normal porcelain firing temperature.
 This is due to the formation of low viscous glassy phase
in fly ash containing compositions, resulting better
sintering / densification through liquid phase sintering
process.
 Addition of fly ash also increases the amount of mullite in
the composition and reducing the free quartz content, thus
increasing the strength of the sintered products.
 Better interlocking and uniform distribution of smaller
sized mullite needle crystals in a glassy matrix supports the
evidence of this improvement.
Compositions studied
Normal porcelain
0 1.0
0.0
P
qu
a
ng
ci
re
y
Fl
FSP-3
PS10
12
0.8
12
0.2
FSP-4
FSP-5
FSP-6
1.0
15 wt %
Fly ash
r
PF10
9
0.4
pa
%
FSP-2
ds
9
0.6
l
fe
as
g
h
in
ac
pl
PS5
0.6
6
ag
la
FSP-1
PF5
6
0.4
Sl
re
p
3
0.8
%
rtz
3
0.2
0.0
PF15
PS15
15 wt %
Slag
SEM of FA incorporated tile
 Well developed mullite
needles embedded in
the matrix significantly
enhanced the strength
 Firing
reduced
temperature
 Low cost of production
 Utilized waste material
Microstructure of fly ash based tiles
Synergistic addition of FA and slag in
porcelain
 Addition of fly ash and
blast furnace slag in
the proportion of 1:1
and 1:2 beneficial
Flexural strength (MPa)
FSP-2
70
FSP-3
FSP-1
SP
60
50
FP
40
30
NP
20
0
R
2
1
O/
R
2
/
O
3
4
5
6
3.2
2.9 3.0 3.1
2.6 2.7 2.8
2.5
2.4
2.3
SiO 2/Al 2O 3
Kausik Dana & Swapan Kr.Das, Ceramics International , (in press, 2004)
View of Waste Based Tiles
METAL MATRIX COMPOSITES (MMC)
 MMCs are engineered materials formed by the
combination of two or more materials, at least
one of which is a metal.
 MMCs have the advantages of higher strength,
density and stiffness density ratio compared
to monolithic metals.
 They also perform better than the polymer
matrix composites at elevated temperatures.
 Use of MMCs is limited due to the cost factor.
Fly Ash in MMC
 Addition of fly ash in cast aluminium (composite) have the
potential of cost competitiveness, lightness and
application potential.
 These composites are suitable for automotive
components, machine parts and related industries.
 Dispersion of coal fly ash in common aluminium metal
improves the mechanical characteristics as hardness.
 3 vol% of fly ash in aluminium metal improves the
abrasive wear resistance of aluminium alloy, specific
wear rate of the composite decreased with increasing load
and sliding velocity.
 Specific abrasive wear rate decreased with increasing size
of the abrading particles.
 Friction coefficient of the composite decreased with
increasing time, load and size of the abrading particles.
 Fly ash particles in the composite blunt the abrading SiC
particles, thus reducing the extent of ploughing.
Advantages of Fly Ash based MMCS
Use of fly ash metal composites can reduce the
consumption of aluminium metal.
For automobile industries, it reduces the weight
of the vehicle.
Correspondingly improves mileage of the vehicle.
In totality fly ash reduces the energy requirement
for metal production, disposal hazards of fly ash, oil
consumption, etc.
 These composites are also important for foundries,
manufacturing, transportation, construction,
electrical and consumer goods industries.
GLASS CERAMICS
 Glass ceramics are polycrystalline solids
produced by controlled crystallization of glass.
 For glass ceramics it is important to nucleate the
crystal first from a glassy melt and then allow to
grow to specific size as per the requirement.
 Glass ceramics normally contain around 50 – 90%
crystalline materials by volume and the rest being
a residual un-crystallized glassy phase.
 Crystal type in the glassy matrix can be controlled
by selection of the parent glass composition.
 Final properties can also be tailored for specific
application.
Fly Ash in Glass Ceramics
 The chemical composition of fly ash is typical of the
most common glassy ternary system (CaO-Al2O3SiO2) and useful for glass ceramics.
 Significant amounts of transition metal oxides
present in fly ash, act as nucleating agents, for
nucleation and crystallization.
 From the disposal point of view of the fly ash,
conversion to glass is beneficial. As inorganic
glasses can incorporate large amount of heavy
metal ions inside the random network structure
and the chemical stability of glass is very high
against leaching in water. Again, vitrification
results in a very large reduction in volume.
Glass Ceramics using only Fly Ash
Generally melting of fly ash based glasses is
done around 1400oC and nucleation around
650 – 750oC and growth between 850 – 980oC.
Temperatures are composition dependent
and can also be altered as per the requirement
criteria.
Generally crystals of alkaline earth oxides –
alumina – silica system are nucleated in the fly
ash based glass ceramics, namely gehlenite
(Ca2Al2SiO7), anorthite (CaAl2Si2O8), diopsidealumina [Ca (Mg, Al) (Si,Al)2O6], etc.
Phase Analysis of Glass Ceramics using only Fly Ash
Phase Analysis of Glass Ceramics using only Fly
Ash with varying heat treatment temperature
Properties Glass Ceramics using only Fly Ash (Other
Author’s work)
Properties
Density (gm/cc)
850oC 900 oC
2.03 2.07
950 oC 1000 oC 1050 oC
2.17
2.26
2.21
Porosity (%)
25.67 22.66
19.29
11.76
11.92
Water absorption (%)
12.63 10.82
7.41
5.18
5.87
Compressive strength 41.44 56.29
(MPa)
4
point
bending 19.96 22.57
strength (MPa)
53.96
38.75
31.31
17.00
12.09
11.99
Electrical resistivity 11.72
(-cm))
Th. Exp. co- effficient 8.63
(a) x10-6/oC
4.59
3.23
2.95
1.05
8.61
9.19
9.18
10.21
Microstructure of glass ceramics using only Fly Ash, heat-treated at
(a) 850 °C, (b) 900 °C, (c) 950 °C, (d)1000oC and (e) 1050oC for 2 h
(Other Author’s work)
Fly Ash as a Component in Glass Ceramics
Utilization of fly ash up to certain extent as a
component in glass / glass ceramics
composition in combination with other oxides for
the development of a specific glass ceramics is
also important.
Addition of MgO, Al2O3 and SiO2 to fly ash can
develop cordierite based glass ceramic, utilizing
around 70% of fly ash.
Cordierite-based glass-ceramics are important due
to their good mechanical properties, low dielectric
constant and low thermal expansion coefficient.
They are used as kiln furniture in white ware
industry as well as in micro-electronic packaging
industry.
Properties of Fly Ash Based Cordierite
Glass Ceramics
Property
Fly
ash Fly ash based Industrial
based glass cordierite
cordierite
glassceramics
Vickers Micro hardness 4020
6250
(MPa)
Density (gm/cc)
2.34
Bending
strength 65
(MPa)
Thermal Expansion co- 79
effficient (a) x10-7/oC
2.49
2.50
90
110
35
25
Phase Analysis of Fly Ash Based
Cordierite Glass Ceramics
Summary & Conclusion
Utilization of fly ash has been successfully being
practiced by various sectors and the extent of utilization is
increasing with time.
However ever increasing generation of ash due to our
higher dependence on power & electricity, using coal
based thermal route, forcing us to develop new ways of
utilizing the same.
Use of fly ash up to 40% replacing alumina in wear
resistant ceramic products shows no deterioration in
properties.
Use of fly ash up to 28% replacing alumina for the
development of refractory aggregate, used in high
alumina castable, shows no deterioration in the property,
however an increase in the hot strength was obtained due
to the presence of mullite in the composition from fly ash.
Summary & Conclusion
 For automobile industries, it reduces the weight of the


vehicle and the correspondingly improves mileage
Utilization of fly ash up to 40% replacing fire clay in
moderate heat duty refractory brick shows better
properties than the specification in all the items.
Use of fly ash up to 15% replacing quartz in traditional
porcelain composition results better density, and strength
at lower temperatures. This is due to liquid phase
sintering of the product and presence of higher amount of
mullite.
Use of fly ash in metal (aluminium) matrix composite
improves the abrasive wear resistance, decreases
specific wear rate. This application can reduce the
consumption of aluminium metal in automobile industries
and can reduces the weight of the.
Summary & Conclusion
Use of fly ash for glass ceramic material helps in many
ways. Chemical composition of fly ash is typical to glassy
ternary system (CaO-Al2O3-SiO2).
Presence of transition metal oxides act as nucleating
agents, for nucleation and crystallization. Conversion to
glass is beneficial for disposal. As it can incorporates
large amount of heavy metal ions and has very high
chemical stability. Again, vitrification results in a very
large reduction in volume.
Fly ash can be used solely or in combination with other
oxides for manufacturing of glass ceramics. The
properties are no way inferior than the conventional
products.
Fly ash with other waste can be utilized to make hard
ceramics as a liner material.