Functions of Ingredients of Cemmet

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The function of various ingredients of an ordinary Portland cement, as mentioned above, are as follows :--

         1)  Lime (CaO): 

    This is the important ingredient of cement and I s proportion is to be carefully maintained. The lime in excess makes the cement unsound and causes the cement to expand and disintegrate. On the other hand, if lime is in deficiency, the strength of cement decreases and it causes cement to set quickly.

Functions of Lime:-

·         Presence of lime in a sufficient quantity is required to form silicates          and aluminates of calcium.

·         Deficiency in lime reduces the strength of property to the cement.

·         Deficiency in lime causes cement to set quickly.

·         Excess lime makes cement unsound.

·         Excessive presence of lime cause cement to expand and disintegrate.

2)  Silica (Si02): 

     This is also an important ingredient’ of cement and it gives or imparts strength to the cement due to the formation of dicalcium and tricalcium silicates. If silica is present in excess quantity, the strength of cement increases but at the same time, its setting time is p. longed.

Functions of Silica:-

·         Sufficient quantity of silica should be present in cement to dicalcium and tricalcium silicate.

·         Silica imparts strength to cement.

·         Silica usually present to the extent of about 30 percent cement.

3)  Alumina (AI203): 

     This ingredient imparts quick setting property to the cement. It acts as a flux and it lowers the clinkering temperature. However the high temperature is essential for the formation of a suitable type of cement and hence the alumina should not be present in excess amount as it weakens the cement.

Functions of alumina:-

·         Alumina imparts quick setting property to the cement.

·          Clinkering temperature is lowered by the presence of the requisite quantity of alumina.

·         Excess alumina weakens the cement.

4)  Calcium sulphate (CaS04): 

     This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement.

Functions of Calcium sulphate:-

·         This is present in cement in the form of gypsum(CaSO4.2H2O)

·         It slows down or retards the setting action of cement.

5)  Iron oxide (Fe203): 

      This ingredient imparts color, hardness and strength to the cement.

Functions of Iron oxide:-

·         Iron oxide imparts color to cement.

·         It acts as a flux.

·         At a very high temperature, it imparts into the chemical reaction with calcium and aluminum to form tricalcium alumino-ferrite.

·         Tricalcium alumino-ferrite imparts hardness and strength to cement.

6)  Magnesia (MgO): 

       This ingredient, if present in small amount, imparts hardness and color to the cement. A high content of magnesia makes the cement unsound.

Functions of Magnesia:-

·         Magnesia should not be present more than 2% in cement.

·         Excess magnesia will reduce the strength of the cement.

7                7) Sulphur (S): 

                   A very small amount of sulphur is useful in making sound cement. 

                   If it is in excess, it causes cement to become unsound. 

               8) Alkalis: 

                   The most of the alkalis present in raw materials are carried away by the                flue gases during heating.

             Function of  Alkalis:- 

·         Should not be present more than 1%.

·         Excess Alkaline matter causes efflorescence.