Monday, August 26, 2013

Functions of various Constituents of Cement

Functions of various cement ingredients:

  •      Lime (CaO) :This is the important ingredient of cement and its proportion is to be carefully maintained.  The lime in excess makes the cement unsound and cause the expansion and disintegration of the cement. On the other hand if the lime is deficient it will decrease the strength of the cement and will cause it to set quickly.
  •  Silica (SiO2) :   This is an important ingredient of the cement as it imparts strength to the cement due to formation of di-          calcium silicate and tri-calcium silicate. If silica is present in excess quantity its setting time is prolonged          however strength is increased.
  •  Alumina (Al2O3) :      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 in alumina should not be present in excess amount as it weakens cement.
  • Calcium Sulphate (CaSO4) :This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement.
  • Iron Oxide (Fe2O3) :This imparts color, hardness and strength to the cement.
  • 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.
  • 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.
  • Alkalies :The most of the alkalies present in raw materials are carried away by the flue gases during heating and the cement contains only a small amount of alkalies. If they are in excess in cement, they cause a number of troubles such as alkali-aggregate reaction, efflorescence and staining when used in concrete, brickwork or masonry mortar.

Harmful Constituents of Cement:
The presence of the following two oxides adversely affects the quality of cement:
        1) Alkali Oxides K2O and Na2O    2)    Magnesium oxide MgO.

    If the amount of alkali oxides exceeds 1 per cent, it leads to the failure of concrete made from that cement.     Similarly, if the content of magnesium oxide exceeds 5 per cent, it causes cracks after mortar or concrete        hardens. It is due to the fact that magnesium oxide, burned at a temperature of about 1500 degree            Celsius, slakes very slowly, when mixed with water.



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