C ements are materials that exhibit characteristic properties of setting and hardening when mixed to a paste with water. This makes them join rigid masses into coherent structures. It is powdery bonding material having adhesive and cohesive properties.
Chemically it is a finely ground mixture of calcium silicates and aluminates which set to a hard mass when treated with water. These are called as Hydraulic Cements (Portland Cement) and those setting in air are Non Hydraulic cements (Ordinary Lime).
It was first made by Joseph Aspdin in 1824 in England.
Natural Cement : Obtained by calcinating and pulverizing natural cement rock of argillaceous and clay with limestone. It does not have sufficient strength and is cheap and quick setting & have hydraulic properties.
Pozzolana Cement : Volcanic ash containing silicates of calcium, iron and aluminum when mixed with lime and heated produces this cement.
Slag Cement : Mixture of blast furnace slag (Ca and Al Silicates) and hydrated lime. Sometimes accelerators like clay, salt, caustic soda are added to hasten hardening process.
Portland Cement : It consists of compounds of lime, silica, alumina and iron. When mixed with water it forms a paste which binds the rock, sand and gravel to form concrete.
PORTLAND CEMENT COMPOSITION Lime (CaO) 60-66% Excess reduces strength and make cement disintegrate and in less amount reduces strength and makes it quick setting. Silica (SiO 2 ) 17-25% Provides strength to cement Alumina (Al 2 O 3 ) 3-8% Helps in quick setting Calcium Sulphate (Gypsum) Enhances initial setting of cement Iron Oxide 2-6% Gives color, Strength and hardness. Sulphur trioxide (SO 3 ) 1-3% Provides soundness Alkali Oxides (Na 2 O and K 2 O) 0.5-1.5% in excess makes cement efflorescent Magnesium Oxide (MgO) 1-5%
This is lowest portion of Klin with a temperature of about 1400-1600 ° C. Lime and clay nodules melts with chemical fusion and gives calcium aluminates and silicates. These silicates and aluminates then fuse together to form small hard stones called Clinkers which than fall down from lower end of the Klin.
CaCO 3 CaO + CO 2 Calcium Oxide
2CaO + SiO 2 Ca 2 SiO 4 Dicalcium Silicate
3CaO + SiO 2 Ca 3 SiO 5 Tricalcium Silicate
3CaO + Al 2 O 3 Ca 3 Al 2 O 6 Tricalcium Aluminate
4CaO + Al 2 O 3 +Fe 2 O 3 Ca 4 Al 2 Fe 2 O 10 Tetracalcium Aluminoferrite
Clinkers are finally grinded in ball mill and tube mill to a fine powder. Additives added are as follows.
Gypsum CaSO 4 .2H 2 O or Plaster of Paris CaSO 4 .½H 2 O acts as retarder to prevent quick setting. After initial setting gypsum retards the dissolution of tricalcium aluminate by forming tricalcium sulphoaluminate (3CaO.Al 2 O 3 .xCaSO 4 .7H 2 O).
Sodium salts and polymers of condensed napthlene or sulphonic acid are added to prevent the formation of lumps and cakes in the cement.
Vertical shaft klin technology has been successfully employed by mini cement plants. The VSK have pan noduliser or pelletiser. The uniform pellets so formed can be fed into the vertical shaft Klin and the cement can be produced. Proper proportions of raw material must be taken to get good quality of clinkers.
Black Metal Process : Raw materials are ground along with coal and then converted into pellets with about 15% water.
Fuel Slurry Process : Raw materials are dry ground and coal is wet ground separately and then mixed to get pellets which move down VSK.
Cement forms paste like mass with water resulting in hydration of compounds of cement. This mass then becomes stiff and hard. This change of fluid to solid is known as setting of cement.
The process of solidification comprises of three stages:
Initial Setting – It is mainly due to the hydration of tricalcium aluminate and gel formation of tricalcium aluminoferrite. Dicalcium silicate also contributes to initial setting.
Final Setting – mainly due to the formation of tobermonite gel and crystallization of calcium hydroxide and hydration of tricalcium aluminate. The concrete can neither be moulded into any shape nor it can be remixed.
During hardening the solid cement material begins to gain strength. It depends upon the chemical combination of cement and water. Hardening starts with great speed but finally its speed reduces. The hardening of concrete stops and it is dried out.
The hydration reactions are exothermic and the volume of cement increases with hydration.
Hydrated cement dissociates on heating by destroying the bond which held the mass together.