Page 53 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Composition 37

• In the burning zone (1000–1450°C), partial fusion of the
mix occurs, with the formation of C S, C S, and clinker.
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• In the cooling zone (1450–1300°C), crystallization of
melt occurs with the formation of calcium aluminate and
calcium aluminoferrite.
After firing the raw materials for the required period, the resultant clinker
is cooled and ground with about 4–5% gypsum to a specified degree of
fineness. Grinding aids, generally polar compounds are added to facilitate
grinding.

2.0 COMPOSITION

The major phases of portland cement are:

• Tricalcium silicate (3CaO•SiO )
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• Dicalcium silicate (2CaO•SiO )
2
• Tricalcium aluminate (3CaO•Al O )
2 3
• Ferrite phase of average composition (4CaO•Al O •Fe O )
2 3 2 3
In a commercial clinker these phases do not exist in a pure form. The
3CaO•SiO phase is a solid solution containing Mg and Al and is called
2
alite. In the clinker, it consists of monoclinic or trigonal form, whereas
synthesized 3CaO•SiO is triclinic. Alite is the most important constituent
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of normal portland cement, constituting 50–60% and promoting strength
development. The 2CaO•SiO phase occurs in the ß (belite) forms and
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contains, in addition to Al and Mg, some K O. Four forms, α, α´, ß, and γ,
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of C S are known although in clinker only the ß form with a monoclinic unit
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cell exists. It reacts slowly with water and contributes little to strength
development in the first 28 days. At one year, pure alite and belite yield the
same strengths. The aluminate phase C A constitutes 4–12% in most
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portland cements and is substantially modified by ionic substitution. In
some clinkers small amounts of calcium aluminate of formula NC A may
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also form. The ferrite phase, designated C AF, is a solid solution of variable
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composition from C F to C A F. Potential components of this compound
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are C F, C AF , C AF, and C A F. The MgO content in cement is usually
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4
2
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limited to 4–5% because in the form of crystalline periclase it may cause
slow expansion. Free lime behaves similarly. Excessive SO can also lead
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to expansion. Alkalis such as K O and Na O in excess of 0.6% equivalent
2
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