The Ordinary Portland cement contains different ingredients with varied proportions. Each ingredients imparts different property to the cement. To produce good quality of cement, we must know the proportions, functions and limitations of different ingredients of cement.
Proportion of Ingredients of Cement
Different ingredients of cement and their proportions are as follows:
- Lime (CaO)
- Silica (SiO2)
- Iron (Fe2O3)
- Magnesia (MgO)
- Calcium sulfate (CaSO4)
- Sulfur (SO3)
Functions and Limitations of Cement Ingredients
Lime or calcium oxide is the most important ingredient of cement. The cement contains 60 to 67% of lime in it. It is obtained from limestone, chalk, shale etc. Adequate quantity of lime in cement is helpful to form the silicates and aluminates of calcium.
If lime is added in excess quantity the cement becomes unsound as well as expansion and disintegration of cement will occur.
If lime content is lower than the minimum requirement strength of cement will reduce and also setting time of cement will decrease.
Silica or silicon dioxide is the second largest quantity of cement ingredients which is about 17 to 25%. Silica can be obtained from sand, argillaceous rock etc. Sufficient quantity of silica helps for the formation of di-calcium and tri-calcium silicates which imparts strength to the cement.
Excess silica in cement will increase the strength of cement but at the same time setting time of cement also increased.
Alumina in cement is present in the form of aluminum oxide. The range of alumina in cement should be 3 to 8%. It is obtained from bauxite, alumina contain clay etc. Alumina imparts quick setting property to the cement.
In general, high temperature is required to produce required quality of cement. But alumina when added with cement ingredients it behaves as a flux and reduces the clinkering temperature which finally weakens the cement. So, to maintain the high temperature alumina should not be used in excess quantity.
4.Iron oxide (Fe2O3)
Iron oxide quantity in cement is ranges from 0.5 to 6%.It can be obtained from fly ash, iron ore, scrap iron etc. The main function of iron oxide is to impart color to the cement.
At high temperatures, Iron oxide forms tricalcium aluminoferrite by reacting with aluminum and calcium. The resultant product imparts the strength and hardness properties to the cement.
Cement contains Magnesia or Magnesium oxide in the range of 0.1 to 3%. Magnesia in cement in small quantities imparts hardness and color to the cement.
If it is more than 3%, the cement becomes unsound and also strength of the cement reduces.
6.Calcium sulfate (CaSO4)
Calcium sulfate is present in the form of gypsum in the cement. It is found together with limestone. It ranges between 1 to 3%.
The function of calcium sulfate in cement is to increase the initial setting time of cement.
Sulfur or sulfur trioxide in the cement is about 1 to 3%. Its function is to make the cement sound. If it is in excess quantity the cement becomes unsound.
Alkalis like soda and potash are present in the cement which normally ranges from 0.1 to 1%. During manufacturing process of cement most of the alkalis are carried away by the flue gases at the time of heating. Hence cement contains very small quantities of alkalis in it.
If alkalis content is more than 1% then it will cause several problems like alkali aggregate reaction, efflorescence, staining etc.