It consists of: 1. ash from minerals and 2.
coke from coal
Mostly porous coke or censpheres from coal
Combustion ash residues from coal minerals such as
clay, other silicates, carbonates, quartz pyrite.
consists of inorganic matter present in the coal that has been fused
during coal combustion. This material is solidified while suspended in
the exhaust gases and is collected from the exhaust gases by
electrostatic precipitators. Since the particles solidify while
suspended in the exhaust gases, fly ash particles are generally
spherical in shape (Ferguson et. al., 1999). Fly ash particles those are
collected in electrostatic precipitators are usually silt size (0.074 -
Fly Ash Classification
Fly ash is a
pozzolanic material and has been classified into two classes, F and C,
based on the chemical composition of the fly ash.
According to ASTM C 618, the chemical requirements to classify any fly
ash are shown in Table 3.1.
Table 3.1. Chemical
Requirements for Fly Ash Classification
Silicon dioxide (SiO2)
plus aluminum oxide (Al2O3) plus iron oxide
(Fe2O3), min, %
Sulfur trioxide (SO3),
Moisture Content, max, %
Loss on ignition, max, %
* The use of class F fly ash containing up to
12% loss of ignition may be approved by the user if acceptable
performance results are available
Class F fly ash
is produced from burning anthracite and bituminous coals. This fly ash
has siliceous or siliceous and aluminous material, which itself
possesses little or no cementitious value but will, in finely divided
form and in the presence of moisture, chemically react with calcium
hydroxide at ordinary temperature to form cementitious compounds (Chu
et. al., 1993). Class C fly ash is produced normally from lignite and
sub-bituminous coals and usually contains significant amount of Calcium
Hydroxide (CaO) or lime (Cockrell et. al., 1970). This class of fly ash,
in addition to having pozzolanic properties, also has some cementitious
properties (ASTM C 618-99).
Color is one of
the important physical properties of fly ash in terms of estimating the
lime content qualitatively. It is suggested that lighter color indicate
the presence of high calcium oxide and darker colors suggest high
organic content (Cockrell et. al., 1970).
Fly Ash Chemistry
Chemical constituents of
fly ash mainly depend on the chemical composition of the coal. However,
fly ash that are produced from the same source and which have very
similar chemical composition, can have significantly different ash
mineralogies depending on the coal combustion technology used. Because
of this, the ash hydration properties as well as the leaching
characteristic can vary significantly between generating facilities.
The amount of
crystalline material versus glassy phase material depends largely on the
combustion and glassification process used at a particular power plant.
When the maximum temperature of the combustion process is above
approximately 12000 C and the cooling time is short, the ash
produced is mostly glassy phase material (McCarthy et. al., 1987). Where
boiler design or operation allows a more gradual cooling of the ash
particles, crystalline phase calcium compounds are formed.
The relative proportion
of the spherical glassy phase and crystalline materials, the size
distribution of the ash, the chemical nature of glass phase, the type of
crystalline material, and the nature and the percentage of unburned
carbon are the factors that can affect the hydration and leaching
properties of fly ash (Roy et. al., 1985). The
primary factors that influence the mineralogy of a coal fly ash are
Chemical composition of the coal
Coal combustion process including coal pulvarization, combustion,
flue gas clean up, and fly ash collection operations
Additives used, including oil additives for flame stabilization and
corrosion control additives.
The minerals present in
the coal dictates the elemental composition of the fly ash. But the
mineralogy and crystallinity of the ash is dictated by the boiler design
Hydration of Fly Ash
cementitious material by the reaction of free lime (CaO) with the
pozzolans (AlO3, SiO2, Fe2O3)
in the presence of water is known as hydration. The hydrated calcium
silicate gel or calcium aluminate gel (cementitious material) can bind
inert material together. For class C fly ash, the calcium oxide (lime)
of the fly ash can react with the siliceous and aluminous materials (pozzolans)
of the fly ash itself. Since the lime content of class F fly ash is
relatively low, addition of lime is necessary for hydration reaction
with the pozzolans of the fly ash. For lime stabilization of soils,
pozzolanic reactions depend on the siliceous and aluminous materials
provided by the soil. The pozzolanic reactions are as follows:
Ca(OH)2 => Ca++ +
Ca++ + 2[OH]- + SiO2 => CSH
2[OH]- + Al2O3 => CAH
tricalcium aluminate in the ash provides one of the primary cementitious
products in many ashes. The rapid rate at which hydration of the
tricalcium aluminate occurs results in the rapid set of these materials,
and is the reason why delays in compaction result in lower strengths of
the stabilized materials.
chemistry of fly ash is very complex in nature. So the stabilization
application must be based on the physical properties of the ash treated
stabilized soil and cannot be predicted based on the chemical
composition of the fly ash.
Leaching from Fly Ash
The total metals content
for a specific ash source depends on the composition of the coal. The
potential for leaching of these metals not only depends on the total
metals content but also influenced by the crystallinity of the fly ash,
as this would dictate whether the metals are incorporated within the
glasseous phase or within crystalline compounds, which will hydrate (ACAA).
The metals in the glasseous phase are expected to leach at much lower
rate than that from the crystalline phase.
Since the degree of
crystallinity is a function of boiler design and remains relatively
constant for a given source, leachable materials remain relatively
constant for a given ash source. A number of state regulatory agencies
have issued source approval for specific generating facilities after the
consistency of these materials had been demonstrated.
For stabilized soil, the
leachability of metals not only depends on the property of the fly ash
but also the soil that are used for stabilized soil. Some part of these
metals leached from the fly ash will be adsorbed on the clay minerals of
XRD Analysis of Coal Combustion By-Products by the Rietveld Method. Test
Mixtures,” R.S. Winburn, S.L. Lerach, B.R. Jarabek, M. Wisdom, D.G.
Grier, and G.J. McCarthy, Adv. X-Ray Anal., 2000, 42, 387.
Quantitative X-Ray Diffraction of NIST Fly Ash Standard Reference
Materials,” R.S. Winburn, D.G. Grier, G.J. McCarthy, and R.B. Peterson,
Powd. Diff., 2000, 15, 163.
Combustion By-Product Diagenesis,” D.G. Grier, G.J. McCarthy, R.S.
Winburn, and R.D. Butler, Proceedings of the 1997 International Ash
Utilization Symposium, 1997.