Composition of cement

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:

Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock
Silica, SiO₂: from sand, old bottles, clay or argillaceous rock
Alumina, Al₂O₃: from bauxite, recycled aluminum, clay
Iron, Fe₂O₃: from from clay, iron ore, scrap iron and fly ash
Gypsum, CaSO₄.2H₂0: found together with limestone

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	) )Up to 2
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 cement’s 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 3,000^o F to 2,600^o 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.,