Raw maetial ingridients

1. 10
CHAPTER-2
2.1 CALCIUM OXIDE
The calcium Oxide is the most important component of the all other components used in the
cement production. The calcium oxide is mostly derived from the decomposition of the
calcium carbonate CaCO3 and drive of the carbon di-oxide CO2. The calcium carbonate
occurs in the nature in various form of purity as limestone, chalk, marble etc.
The chemical reaction of the calcium carbonate depend upon the temperature that it is
prepared. It attains the dissociation pressure equal to the surrounding pressure at 900 o
C-930
o
C.
The chemical decomposition of the calcium carbonate carried out in the suspension preheater,
in the rotary kiln or using any other form of heating.
CaCO3 + heat  CaO + CO2 + h (2.1)
where h = Cp * T is the standard reaction enthalpy.
2.2 ALUMINA OXIDE.
Alumina Oxide or Alumina Al2O3, in a combined state, is an important constituent of the
cement production. The Alumina sources are in the most cases, the same as the source of the
other materials such as the limestone, clay and shellac. It also occurs free in a hydrated form,
mixed with a proportion of Ferric Oxide, in bauxite and small amount of silica.
In some case, the limestone does not have a sufficient amount of the Alumina constituent,
therefore it is necessary to add an amount of a material which have a concentration of the
Alumina, such as the Kaolinite and Bauxite.
2.3 MAGNESIUM OXIDE
Magnesium usually is derived from the Magnesium Carbonate which is present in the original
state of limestone in the form of Dolomite CaCO3 MgCO3. Clay and shellac generally
contain Magnesium Oxide either as carbonate or it is tied up in various silicate minerals. The
ratio of Ca and Mg is depends on the chemical condition during the dolomitization period. In
some case, the Magnesium content in the limestone is related to Magnesium silicate.
The use of the Magnesium Oxide in the Portland cement clinker is world wide limited. For
good cement the MgO content is limited up to 2 % by weight. The maximum allowable
content of the MgO permitted by the various countries is vary between 2.5 % and 6 %.
The Magnesium oxide, within the permissible content act as a flux at the sintering
temperature and facilitate the burning. The MgO contribute, during the burning process, to
the formation of the liquid and may be looked as a minor component in the raw mix.
An increase of the MgO, within the permissible limit, tend to increase the liquid. The raw
mix rich with MgO generate the clinker balls in the burning zone and affect the kiln
operation.

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If the glassy phase which is formed in the liquid is cooled quickly ( quenched), after
discharging from the kiln, the formation of the periclase (free MgO) crystals from the liquid
will disappear. If the cooling procedure is carried out slowly, the MgO tend to crystallize out
and form in the clinker the periclase. The concentration of the periclase crystals in the clinker
bring to unsoundness in the cement.
The Magnesium Oxide react with the water MgO + H2O  Mg (OH2) , but this reaction
proceed slowly, where as the other reactions of the hardness are already achieved. After a
period of time at the ambient temperature, and after several years, the concrete made from the
cement with significant amount of the periclase crystals will expand (Magnesium expansion)
because the Mg (OH2) occupies a large volume than MgO. And cause a destructive and
cracking in the concrete of any construction, buildings, bridges etc. Therefore it is necessary
to select the raw materials to keep the MgO content in the clinker with in the permissible
limit, and the autoclave expansion test should be proceeded to give an advance warning of
the unsoundness potential level of the cement. Chapter 14 presents a separate study
concerning the Cement manufacture with high MgO content.
2.4 SULFUR
The raw materials used for the Portland cement manufacturing may contain sulfur. The sulfur
may occur in the raw mix either as sulfate or sulfide. In general the sulfur content in the raw
materials is analyzed as sulfate, and its concentration is expressed as SO3 in the laboratory
reports. In practice it is necessary to distinguish between the sulfide S2 and the Sulfate SO3.
The Sulfide are present in the sedimentary rocks deposits and occur mineralogically as Pyrite.
The oxidation of the Iron Sulfides is accelerated at low temperature in the range of 400 o
C
and 450 o
C and the SO2 is released. It means, this reaction is carried in the upper stages of
the preheater.
The concentration of the Sulfate in the raw material used in the cement industry is in general
in the range between 0.1 % and 0.6 %, but lower than 1 %.
The fuel used in the kiln / Flash calciner is the major source for the Sulfur presence. The
sulfur content of the fuel varies in wide limits from zero in natural gas to about 3.5 % in
heavy fuel. Most of the various type of the fuel contain the sulfur which react during the
combustion to S2 and to the sulfates respectively. For that it is of great importance to know
the amount of the sulfide emitted from the fuel when during the preparation of the raw mix.
In the preheater, the excess of the SO2 is able to react with the Ca CO3. A certain part of
these materials escaping with the kiln dust and the remaining part continue the road to the
kiln as CaCO3.
The result of the fuel combustion in the kiln, the sulfur evaporate in the burning zone as SO2
which in the kiln atmosphere combine with the alkali and with the Oxygen to generate the
alkali sulphate vapor, which condenses on the raw mix particles in the colder zone as well as
in the preheater.
In the sintering zone, the sulfur di-oxide is formed again as a result of the reaction of the
calcium carbonate. If a high values of the sulfur di-oxide content occurs in the kiln gases,

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then the calcium sulfate may appear in the clinker as un-decomposed CaSO4 and pass through
the kiln sintering zone.
In the kiln burning zone, the CaSO4 decompose again and the Sulfur of the fuel will evaporate
to SO2 and circulate with the kiln gas. A certain part will combine again with the clinker and
leave the kiln to the clinker cooler.
2.4.1 Effect of the Sulfur.
The presence of high amount of the Sulfur composition in the clinker raw materials create a
problem in the kiln system and mainly in the preheater. The excess of the SO2 molar over
the alkalis in the kiln gas phase will cause the growing of the SO2 emission. But as long as
the molecular ratio of the Sulfur to the alkali is maintained in the range of 0.8 and 1.2 , the
Sulfur can be handled without too much trouble. But with the higher sulfur content, it cause
an excessive liquid formation and it occurs the formation of the calcium sulfate and alkali
double salts these give rise to build-up in the back end of the kiln and increase the emission of
the SO2. These materials are sticky compounds that remain liquid over a wider temperature
range and as consequence it help the formation of the kiln coating ring and help to make
chocking of the preheater and it is difficult to remove the hard sticky coating.
The reasonable content of the alkali sulfate in the clinker is an advantage for the early
strength of the cement.
In re-inforced concrete, the sulfur in combination with other factor, accelerate the erosion of
the re-inforcement.
2.5 CHLORIDE
The Chloride in the raw materials is present as NaCl. Salty ground water can also be a source
of the increase of the Chloride in the limestone and in the clay. The chloride is liberated
during the heating in the preheater and in the kiln during the fuel combustion and react with
the alkalis to form the alkali chlorides which is evaporated at the sintering temperature in the
kiln and it condenses in the preheater mainly at the kiln feed end area, then it return back to
the kiln and re-circulate between cold zone of the kiln and the preheater. With that the
coating is increasing which necessitate later to shut down the kiln . usually and to prevent
this coating formation phenomena, part of the kiln gases are by passed to not allow the
chloride to return back to the kiln.
2.5.1 Effect of the Chloride.
The chloride in the range of 1.2 % to 1.6 % is helping to boost the initial strength of the early
cement. It has a very bad corrosion effect not only on the plant structure, i.e preheater
cyclones and ducts, but also on the concrete structure where the chloride attacks the re-
inforcement steel in the concrete. But the presence of 0.1 % is allowed in the raw materials.
2.6 MANGANESE OXIDE Mn2O3

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The Manganese Oxide , Mn2O3 constituent is present in the raw materials of the Portland
cement, in very small amount. The Mn2O3 gives a brown color to the produced cement. The
raw materials with less than 3 % of Mn2O3 produce a good cement, whereas when the
manganese oxide is exceeding this limit, the cement strength will be reduced.
2.7 POTASH
The potash mineral is present as KCl. The potash constituent is rare in the raw material used
in the cement industry.
2.8 TITANIA (TiO2)
The Titanium (TiO2) constituent appears very small amount in the clay and shellac raw
materials, which is not exceeding 0.3 %. It has been found that in Portland cement, the
substitution of the TiO2 for SiO2 in small amounts slightly increase the strength of the
cement.
2.9 PHOSPHOROUS (P2O5)
Phosphorous appears in very low amounts in the raw materials used for the Portland cement
industry. In the most of the cement the P2O5 presence within the limit of 1.6 %, but for a
good cement and with out much effect on the clinker quality, the presence of the P2O5 is
within the range of 0.05 % to 0.25 %. In case when the content of the Phosphorus exceed the
tolerated limit, this results in longer setting time and lower strength, mainly the early strength
of the cement, because the Phosphorous P2O5 decompose the C3S in favor of a C2S. Also the
excess of the P2O5, will increase the level of the free lime in the clinker.