Table of Contents
- 1 Definition.
- 2 Raw Materials of Portland Cement.
- 3 Composition of Portland Cement.
- 4 Functions of Ingredients.
Portland cement is a product obtained by the calcination at a very high temperature, an intimate mixture of correctly proportioned calcareous and argillaceous materials. The calcined product which is called clinker. Clinker is then finally pulverized by grinding into a very fine powder and is finally mixed with calcium sulphate or gypsum to obtain cement.
Raw Materials of Portland Cement.
The commonly raw material included in cement is as follow.
(a) Calcareous materials and (b) argillaceous materials.
The Calcareous materials include compounds of calcium and magnesium, such as Limestone.
And Argillaceous ones include mainly silica, alumina and oxide of iron such as clay and shale. Cements are manufactured in several varieties, but the most common one is used is normal setting or ordinary cement, usually called Portland cement. These are manufactured with two or more raw materials. They have to be correctly proportioned and thoroughly mixed. Lime, silica and alumina are the important ingredients.
Composition of Portland Cement.
To understand the technique of proportioning of raw materials, it is necessary to know the following.
(i.) Composition of Portland cement
(ii.) The Constituents that go to form it.
(iii.) The function played by the Constituents.
The Chemistry of cement is very complicated. An engineer should understand the functions played by each constituent in imparting the properties of setting and hardening of cement.
( a. ) Mineral Constituents.
The important constituents of Portland cement could be represented as shown in following table.
( b. ) Acidic Alkaline Constituents:
The acidic constituents are SiO2, Al2O3, Fe2O3 and the alkaline ones are CaO and MgO. Omitting the rest, The Percentage of acidic Constituents is round about 32 and that of the alkaline ones is 63.5.
Acidic and Alkaline Constituents.
Thus, from the above composition for a normal setting Portland Cement, the values are :
(2.8 X 0.37 + 1.1 X 0.07 + 0.7 X 0.02) / (1.0 X 1.1 + 1.4 X0.04)
= (1.3 + 0.08 + 0.02) / (1.1 + 0.05) = (1.13/1.15) = 1 (approximately)
These ratio are termed as “Cement Modulii”.
( c. ) Necessity of proportioning
It will therefore, be seen that in order to have constituents of definite composition as mentioned above, it is necessary that the calcareous and the argillaceous materials should be carefully selected and proportioned. It is possible theoretically to make a mixture of raw materials and to calcine or to heat the same to a very high temperature to obtain Clinker. But it is necessary that a thorough and an intimate mixing of the raw materials is essential to ensure a thorough and a complete chemical reaction between the ingredients while clinkering.
Functions of Ingredients.
( i. ) Lime (CaO).
Lime forms nearly two-third of cement and therefore, its proportion has an important effect on its qualities. A sufficient quantity of lime must be present in the raw materials to form the required silicates and aluminates of calcium. A deficiency in lime reduces the strength and causes it to set quickly; an excess will make cement unsound and causes it to expand and disintegrate. Usually lime is kept below the required value.
( ii. ) Silica (SiO2).
The quantity of silica should be enough to form dicalcium silicate and tricalcium silicate. Silicate imparts strength to cement and is present usually to the extant of about one-fifth of the cement.
( iii. ) Alumina (Al2O3).
Alumina has the property of imparting quick setting quality. It lowers the clinkering temperature. Since a specific high temperature is necessary to form clinker of required composition, an excess of alumina weakens the cement. Compunds of alumina react readily with water. Alumina forms about one-fifteenth.
( iv. ) Magnesia ( MgO ).
This is usually kept at a very percent. The permissible percentage is 1.5 to 3. Within these values it has no bad effects, but when present in large percentages, magnesia is harmful.
( v. ) Calcium Sulphate ( CaSO4 ).
This is in the form of gypsum and has an effect of slowing down the setting action. Gypsum is added after the clinker is formed. and during the process of grinding clinker.
( vi. ) Oxide of Iron ( Fe2O3 ).
Oxide of iron require a very high temperature to enter in to chemical reaction with calcium and aluminium to form tricalcium alumino-ferrite, which imparts hardness and strength. The color of cement is due to iron compounds.
( vii. ) Sulphur.
As calculated on the basis of sulphuric and hydride. SO2 is limited to 2 percent. An excess quantity of sulphur causes the cement to become unsound.
( viii. ) The Alkalies.
Alkalies present in the raw materials are carried away ordinarily in the flue gases during clinkering, so that cements contains very small quantities of the same, often less than 1 percent. An excess of alkaline matter causes efflorescence.