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Composition of Portland cement

The basic components of cement are:

SiO2 17-25 %
Al2O3 4-8%
Fe2O3 0.5-0.6 %
CaO 61-63 %
MgO 0.1-4.0 %
SO3 1.3-3.0 %
Na2 + K2O 0.4-1.3 %
Cl 0.01-0.1%
IR 0.6-1.75 %
There are four major compounds in cement and these are known as C2S, C3S, C3A & C4AF, and their composition varies from cement to cement and plant to plant.

In addition to the above, there are other minor compounds such as MgO, TiO2, Mn2O3, K20 and N2O. They are in small quantity. Of these K2O and Na2O are found to react with some aggregates and the reaction is known as Alkali Silica Reaction (ASR) and causes disintegration in concrete at a later date.

The silicates C3S and C2S are the most important compounds and are mainly responsible for the strength of the cement paste. They constitute the bulk of the composition. C3A and C4AF do not contribute much to the strength., but in the manufacturing process they facilitate combination of lime and silica, and act as a flux. In a typical Portland cement, the composition of mineralogical compounds could be

Table 1 : The extent of chemical compounds in cement

S no Compound Composition as %
1 C3S 48-52 %
2 C2S 22-26 %
3 C3A 6-10 %
4 C4AF 13-16 %
5 Freelime 1-2 %
Role of compounds on properties of cement
Characteristic C3S C2S C3A C4AF
Setting Quick Slow Rapid -
Hydration Rapid Slow Rapid -
Heat Liberation (Cal/gm) 7 days Higher Lower Higher Higher
Early Strength High up to 14 days Low up to 14 days Not much beyond 1 day Insignificant
Later Strength Moderate at later stage High at later stage after 14 days - -

Heat of Hydration

Most of the reactions occurring during the hydration of cement are exothermic in nature (heat is generated). This heat is called heat of hydration. It is desirable to know the heat producing capacity of cement in order to choose the most suitable cement for a given purpose.

For Ordinary Portland Cement, half of the total heat is liberated between 1-3 days, about ¾th in 7 days and nearly 90% in 28 days. The rate at which the heat is produced is important for practical purposes. It may lead to cracks if not properly dissipated. The sum total heat produced, if spread over a longer period can be dissipated  to a greater degree with fewer problems. The hydration of C3S produces higher heat as compared to the hydration of C2S. Fineness of cement also affects the rate of heat development. The heat of hydration generated is generally as follows at 28 days.

Comparative table of heat of hydration produced at the end of 90 days

S no Compound Heat of hydration (calories per gram)
1 C3S 100-110
2 C2S 50-60
3 C3A 300-315
4 C4AF 95-105
It may be seen that the heat produced by C3S is twice that of C2S and that by C3A is still higher. It follows that, reducing the proportions of C3S and C3A, the heat of hydration and its rate can be reduced.
What is hydration of cement?
When water is added to cement, the paste is formed due to chemical reaction, which hardens into rock like mass over a period of time. Compounds like C3S and C2S react in the presence of moisture and fully hydrated reaction can be expressed as

2C3S +6H -> C3S2H3 +3Ca(OH)2
2C2S +4H -> C3S2H3 +Ca(OH)2

C3S2H3 (Calcium Silicate Hydrate) becomes a hard mass over a period of time and normally called as C-S-H gel. While C3S contributes to most of the strength development during the first two weeks, C2S influences gain of strength after two weeks. However, in the long term (over a year) the contribution to strength is same for equal individual weight.
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