## Composition of cement Introduction Portland cement gets its strength fr om 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, mill ing and proportioning the following materials:

◦ Lime or calcium oxide, CaO: from limest one, chalk, shells, shale or calcareous rock

◦ Silica, SiO 2 : from sand, old bottles, cl ay or argillaceous rock

◦ Alumina, Al 2 O 3 : from bauxite, recy cled aluminum, clay

◦ Iron, Fe 2 O 3 : from from clay, iron ore, scrap iron and fly ash

◦ Gypsum, CaSO 4 .2H 2 0: 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 pe llets 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, pul verized 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) SiO 2 S Aluminum oxide (alumina) Al 2 O 3 A Iron oxide Fe 2 O 3 F Water H 2 O H Sulfate SO 3 S Chemical composition of clinker The cement clinker formed has the following typical composition: Compound Formula Shorthand form % by weight 1 Page 1 of 3 Composition of cement

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...Tricalcium aluminat e C a 3 A l 2 O 6 C 3 A 10 Tetracalcium aluminoferrite Ca4 Al 2 Fe 2 O 10 C 4 AF 8 Belite or dicalcium silicate Ca2 SiO 5 C 2 S 20 Alite or tricalcium silicate Ca 3 SiO 4 C 3 S 55 Sodium oxide Na2 O N ) )Up to 2 Potassium oxide K2 O K Gypsum CaSO 4 .2H 2 O CSH 2 5 Representative weights only. Actual we ight varies with type of cement. Source: Mindess & Young Properties of cement compounds These compounds contribute to the prope rties of cement in different ways • Tricalcium aluminate, C 3 A:- It liberates a lot of heat during the early stages of hydration, but has litt le strength contribution. Gypsum slows down the hydration rate of C 3 A. Cement low in C 3 A is sulfate resistant. • Tricalcium silicate, C 3 S:- This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain. • Dicalcium silicate, C 2 S: C 2 S hydrates and hardens slowly. It is largely responsible for strength gain after one week. • Ferrite, C 4 AF: This is a fluxing agent which reduces the melting te mperature of the raw materials in the kiln (from 3,000 o F to 2,600 o F). It hydrates rapidly, but does not c ontribute much to strength of the cement paste. By mixing these compounds appropriately, manufacturers can produce di fferent 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. Page 2 of 3 Composition of cement

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...Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc., Page 3 of 3 Composition of cement

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