clinker chemistry

1. 26
4. CHAPTER-4
4.1 Clinker Potential
The Clinker quality is characterized by its chemical composition, by its physical properties
and by its grain crystallization which appears with the reaction of the materials decomposition
under the influence of the different degree of the heating and the procedure of the cooling.
The most important factors which characterize the clinker are :
- Tricalcium Silicate C3S
- Dicalcium Silicate C2S
- Tricalcium Aluminate C3A
- Tricalcium Aluminoferrite C4AF
The tricalcium Silicate and the dicalcium Silicate are the solid phase present in the Portland
cement clinker, where as the tricalcium aluminate and tricalcium alumino ferrite occur in the
liquid phase.
4.2 Tricalcium Silicate - Alite - C3S.
The C3S is the important factor of the Portland clinker which is only formed when the
temperature of the clinker is above of its lower stability limit of 1250 o
C. At this temperature,
the liquid phase is starting to be formed, consequently, the formation of the C3S is a liquid -
Solid reaction. The reaction through the liquid medium is faster than by solid state process.
The rate of the reaction is increasing with the increase of the burning temperature.
At the temperature above 1250 o
C, the formation of the alite increases rapidly with the
increase of the temperature and the alite will be formed very rapidly when the temperature in
the burning zone will reach 1450 o
C to 1500 o
C. But at the same time and with the same
clinker composition, the increase of the burning temperature generate a free lime formation
even at the high temperature. The formation of the alite is the rate of the reaction between the
formation of the belite and the free lime, and which determines the amount of the free lime
remaining in the clinker when it leave the burning zone.
Due to the characteristic of the crystal structure, the C3S is responsible for the early strength
development of the cement. The size of the alite grains gives the indication of the burning
rate.
Below the temperature of 1250 o
C, the C3S will decompose into two solids : C3S and CaO.
Since the cement compounds are not analyzed directly during the daily routine work, their
potential composition can be calculated from the Oxide compounds analysis using the
formula which was developed by R.H. Bogue.
For good Portland Clinker, the C3S value is in the range of 45 % - 65 %.
4.2.1 Effect of the C3S.
  (4.1)
SO
2.852
+
O
Fe
1.42864
+
O
6.71719Al
SiO
7.5986
-
CaO
4.07001
=
S
C 3
3
2
3
2
2
3 

2. 27
The effect of the Tricalcium Silicate are :
- The increase of the C3S require more heat to burn the clinker rapidly, and to cool it
rapidly, with that the fuel consumption will increase. But with the same clinker
composition, the increase of the burning temperature generate free CaO formation even at
the high temperature. With the cooling equilibrium, the fine grain size of the free lime
will re-combine with the liquid and form either C3S or C3A. With the increase of C3S
the free lime will decrease and lower the potential alkali content of the clinker.
- The clinker having C3S content up
to 65 % is accompanied by an
increase of the strength. But the
strength will decrease in the case
when the C3S content surpass the
value 65 %.
- The high value of the C3S results
an high early strength of the
cement and high heat generation.
- For the high early strength, the
ratio C3S / C2S will increase,
owing to the capacity of the C3S
to hydrate and to set rapidly.
- Upon addition SiO2 to the base raw mix, the ratio C3S / liquid is considerably reduced.
- The fluidity of the materials is approximately twice as high for the raw mix with the lower
ratio of the C3S / Liquid.
4.3 Dicalcium Silicate (Belite) C2S
The dicalcicum Silicate is a function of the raw mix composition and the burning
temperature.
The dicalcium Silicate start to be formed at low temperature say between 700 o
C - 800 o
C
depending on the fineness of the raw mix. The Belite can be formed even with lower
temperature with the fine raw mix-powder. The Belite is mainly formed by solid-state
reaction. The Belite formation is the result of the combination between the calcite and the
Silica components of the raw mix at this low temperature according to the Bogue formula.
C2S = 2.867 SiO2 - 0.7544 C3S (4.2)
The presence of the free lime, and with the increase of the temperature at which the alite
formation ( 1250 o
C), the Belite further react to yield the Alite.
Figure 4.1 Compressive Strength development
of C3S and C2S.
0
100
200
300
400
500
600
700
800
0 100 200 300 400 500 600 700 800
days
Kp/cm
2
C2S
C3S

3. 28
The reaction in the sintering zone to form the C3S from the C2S and free CaO will proceed
faster when the grain size is fine. With smaller particles, the reaction time is smaller and this
reaction may start even at lower temperature.
4.3.1 Effect of the Dicalcium Silicate C2S.
- The C2S reacts with the water at a lower rate than C3S, therefore it provides a durability
for the concrete because it is very slow set and it continues to hydrate over a long time
and the strength of the concrete may reach that of the C3S after long time say after 3 years
or more.
In the ordinary Portland Cement the C2S value is in the range of 10 % to 30 %.
4.4 Tricalcium Aluminate C3A.
The C3A is appreciated in the rotary kiln during the burning process, because it acts as a flux.
Melting and helping for the heat transfer and for the formation of the other cement
compounds.
The C3A is presented by Bogue in the form.
C3A = 2.6504 AL2O3 - 1.6922 Fe2O (4.3)
4.4.1 The effect of the C3A
- In the kiln and during burning process the C3A act as a flux.
- The C3A react rapidly with water and does not show strong hydraulic properties.
- Due to its rapid hydration, the C3A help for rapid strength development.
- The C3A is responsible for the workability of the clinker.
- It is fully hydrated, probably after 28 days, and after that it does not gain more strength.
- The content of the C3A in the ordinary Portland Clinker is most preferred at the value
around 10 %. The resistance of the cement against the sulfate attack will reduce when
the C3A increase more than 10 %.
- The cement with low C3A content have a higher resistance to sea water and certain
other destructive agencies with a low heat hydration.
- The C3A, in connection with the Silicates, increase the initial strength of the cement.
- With the increase of the C3A, the shrinkage after setting is most claimed.
4.5 Tetracalcium Aluminaferrite (C4AF).
The formation of the C4AF start at low temperature say at 1200 o
C, and it is melted at a
relative temperature of 1350 o
C. But with heating of very fine power, the C4AF may be

4. 29
formed at a lower temperature - 1100 o
C. When the presence of the Iron Oxide is more than
the Alumina, the both constituents will combine with the Lime to form the Calcium
Aluminaferrite. The C4AF is presented by Bogue as :
C4AF = 3.0432 Fe2 O3 (4.4)
- The content of the C4AF in the ordinary Portland clinker is preferred to be in the range of
5 % to 15 %.
- The C4AF is responsible for the color cement. With higher value of the C4AF ( in the
indicated range), the cement color become darker, but the color will not affect the strength
of the cement. In this case the color of the cement depends on the local market , if it
accept the gray color or dark gray color of the cement.