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.
Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous
Silica, SiO2: from sand, old bottles, clay or argillaceous rock
Alumina, Al2O3: from bauxite, recycled aluminum,
Iron, Fe2O3: from from clay, iron ore, scrap iron
and fly ash
Gypsum, CaSO4.2H20: found together with limestone
Because of the complex chemical nature of cement, a shorthand form
is used to denote the chemical compounds. The shorthand for the basic compounds
Chemical composition of clinker
|Calcium oxide (lime)
|Silicon dioxide (silica)
|Aluminum oxide (alumina)
The cement clinker formed has the following typical composition:
Representative weights only. Actual weight varies with type
% by weight1
|Belite or dicalcium silicate
|Alite or tricalcium silicate
)Up to 2
Source: Mindess & Young
Properties of cement compounds
Tricalcium aluminate, C3A:-
These compounds contribute to the properties of cement in different
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.
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
By mixing these compounds appropriately, manufacturers can produce different
types of cement to suit several construction environments.
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.,