Composition of cement

Introduction
Portland cement gets its strength from chemical reactions between the cement and water. The process is known as hydration. This is a complex process that is best understood by first understanding the chemical composition of cement.

Manufacture of cement
Portland cement is manufactured by crushing, milling and proportioning the following materials:

The materials, without the gypsum, are proportioned to produce a mixture with the desired chemical composition and then ground and blended by one of two processes - dry process or wet process. The materials are then fed through a kiln at 2,600º F to produce grayish-black pellets known as clinker. The alumina and iron act as fluxing agents which lower the melting point of silica from 3,000 to 2600º F. After this stage, the clinker is cooled, pulverized and gypsum added to regulate setting time. It is then ground extremely fine to produce cement.

Chemical shorthand
Because of the complex chemical nature of cement, a shorthand form is used to denote the chemical compounds. The shorthand for the basic compounds is:
 

Compound
Formula
Shorthand form
Calcium oxide (lime)
Ca0
C
Silicon dioxide (silica)
SiO2
S
Aluminum oxide (alumina)
Al2O3
A
Iron oxide
Fe2O3
F
Water 
H2O
H
Sulfate
SO3
S
 Chemical composition of clinker
The cement clinker formed has the following typical composition:
 
Compound
Formula
Shorthand form
% by weight1
Tricalcium aluminate Ca3Al2O6 C3A 10
Tetracalcium aluminoferrite Ca4Al2Fe2O10 C4AF 8
Belite or dicalcium silicate Ca2SiO5 C2S 20
Alite or tricalcium silicate Ca3SiO4 C3S 55
Sodium oxide Na2O N

)Up to 2

Potassium oxide K2O K
Gypsum CaSO4.2H2O CSH2 5
Representative weights only. Actual weight varies with type of cement.
Source: Mindess & Young

Properties of cement compounds
These compounds contribute to the properties of cement in different ways
 

  • Tricalcium aluminate, C3A:-

  • It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant.
     
  • Tricalcium silicate, C3S:-

  • This compound hydrates and hardens rapidly. It is largely responsible for portland cementís initial set and early strength gain.
     
  • Dicalcium silicate, C2S:

  • C2S hydrates and hardens slowly. It is largely responsible for strength gain after one week.
     
  • Ferrite, C4AF:

  • This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000o F to 2,600o F). It hydrates rapidly, but does not contribute much to strength of the cement paste.

    By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.

    References:
    Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 671.

    Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.

    Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc.,