21-02-2012, 04:42 PM
17-12-2012, 12:57 PM
CEMENT
What is cement?
Cements in a general sense are adhesive and cohesive materials which are capable of bonding together particles of solid matter into a compact durable mass.
Cements used in construction industry may be classified as hydraulic and non hydraulic.
Today cement finds extensive use in all types of construction works; in structures where high strength is required e.g. bridge piers, light houses, lofty towers, and large structures such as bridges, silos, chimneys
Artificial cement
Artificial cement is obtained by burning at a very high temperature a mixture of calcareous and argillaceous materials.
Mixture should be intimate and in correct proportions.
The calcined product is known as clinker.
A small quantity of gypsum is added to the clinker and it is then pulverized into very fine powder which is known as cement.
FUNCTIONS OF CEMENT INGREDIENTS
Lime: if in excess it makes the cement unsound and causes cement to expand and disintegrate. If deficient, then strength of cement is decreased.
Silica: imparts strength due to formation of dicalcium silicates and tricalcium silicates. But setting time is prolonged.
Alumina: imparts quick setting property to the cement.
Calcium sulphate: increase the initial setting time.
Iron oxide: imparts colour, hardness and strength.
Magnesia: imparts hardness and colour.
Harmful constituents of cement
Alkali oxides K2O and Na2O: if it exceeds 1% leads to failure of concrete.
Magnesium oxide MgO: if exceeds 5 % cause a cracks may appear.
Setting action of cement
Tricalcium aluminate (3CaO.Al2O3): formed within 24 hours after addition of water. It is about 5-11% of cement. It rapidly reacts with water and is responsible for setting.
Tetra calcium alumino ferrite(4CaO.Al2O3.Fe2O3): Also formed within 24 hours after addition of water. Contains 8–14% of cement.
Tricalcium silicate(3CaO.SiO2): formed within a week. It is about 25-50% of cement. It renders the clinker easier to grind, increases resistance to freezing and thawing, hydrates rapidly generating high heat and develops an early hardness and strength.
Dicalcium silicate (2CaO. SiO2): formed very slowly. It imparts progressive strength. Contains about 25-40% of cement. It imparts resistance to chemical attack
Wet process
Limestone is crushed and stored in silos. Clay is thoroughly mixed with water in a container known as wash mill, then stored in basins.
They are allowed to fall from silos and basins in correct proportions. This channel leads the materials to grinding mills where they are brought in contact to form slurry.
Grinding is carried out in ball mill or tube mill. Slurry is led to correcting basin and is constantly stirred. Then It is stored.
Process of burning
Slurry is injected at the upper end of the kiln and hot gases or flames are forced through the lower end of kiln.
Portion of kiln near the upper end is called as dry zone and here the water in the slurry is evaporated.
Slurry gradually descends, there is rise in temperature and the carbon dioxide from slurry is evaporated.
Small lumps are known as nodules.
Tube mill
It is in the form of a longitudinal horizontal steel cylinder of dia about 150cm.
Its length is about 7m to 10 m.
Cylinder is filled partly with steel balls of size varying from 20mm to 25mm.
Fine grinding is achieved due to balls of smaller size. Action is same as that of ball mill.
A worm is provided to feed the material to the mill.
What is cement?
Cements in a general sense are adhesive and cohesive materials which are capable of bonding together particles of solid matter into a compact durable mass.
Cements used in construction industry may be classified as hydraulic and non hydraulic.
Today cement finds extensive use in all types of construction works; in structures where high strength is required e.g. bridge piers, light houses, lofty towers, and large structures such as bridges, silos, chimneys
Artificial cement
Artificial cement is obtained by burning at a very high temperature a mixture of calcareous and argillaceous materials.
Mixture should be intimate and in correct proportions.
The calcined product is known as clinker.
A small quantity of gypsum is added to the clinker and it is then pulverized into very fine powder which is known as cement.
FUNCTIONS OF CEMENT INGREDIENTS
Lime: if in excess it makes the cement unsound and causes cement to expand and disintegrate. If deficient, then strength of cement is decreased.
Silica: imparts strength due to formation of dicalcium silicates and tricalcium silicates. But setting time is prolonged.
Alumina: imparts quick setting property to the cement.
Calcium sulphate: increase the initial setting time.
Iron oxide: imparts colour, hardness and strength.
Magnesia: imparts hardness and colour.
Harmful constituents of cement
Alkali oxides K2O and Na2O: if it exceeds 1% leads to failure of concrete.
Magnesium oxide MgO: if exceeds 5 % cause a cracks may appear.
Setting action of cement
Tricalcium aluminate (3CaO.Al2O3): formed within 24 hours after addition of water. It is about 5-11% of cement. It rapidly reacts with water and is responsible for setting.
Tetra calcium alumino ferrite(4CaO.Al2O3.Fe2O3): Also formed within 24 hours after addition of water. Contains 8–14% of cement.
Tricalcium silicate(3CaO.SiO2): formed within a week. It is about 25-50% of cement. It renders the clinker easier to grind, increases resistance to freezing and thawing, hydrates rapidly generating high heat and develops an early hardness and strength.
Dicalcium silicate (2CaO. SiO2): formed very slowly. It imparts progressive strength. Contains about 25-40% of cement. It imparts resistance to chemical attack
Wet process
Limestone is crushed and stored in silos. Clay is thoroughly mixed with water in a container known as wash mill, then stored in basins.
They are allowed to fall from silos and basins in correct proportions. This channel leads the materials to grinding mills where they are brought in contact to form slurry.
Grinding is carried out in ball mill or tube mill. Slurry is led to correcting basin and is constantly stirred. Then It is stored.
Process of burning
Slurry is injected at the upper end of the kiln and hot gases or flames are forced through the lower end of kiln.
Portion of kiln near the upper end is called as dry zone and here the water in the slurry is evaporated.
Slurry gradually descends, there is rise in temperature and the carbon dioxide from slurry is evaporated.
Small lumps are known as nodules.
Tube mill
It is in the form of a longitudinal horizontal steel cylinder of dia about 150cm.
Its length is about 7m to 10 m.
Cylinder is filled partly with steel balls of size varying from 20mm to 25mm.
Fine grinding is achieved due to balls of smaller size. Action is same as that of ball mill.
A worm is provided to feed the material to the mill.