Cement

CEMENT Dr. Prashant Mehta Assistant Professor, National Law University, Jodhpur

DEFINITION
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.

CLASSIFICATION OF CEMENT
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%

RAW MATERIALS
Calcareous Materials : Supplies Lime to cement
Lime Stone (65-80% CaCO 3 ), Marl, Chalk, Shale, Calcite, Alkali waste. It should contain less than 3.3% of MgO and 3-4% of SiO 2 , Fe 2 O 3, and AlO 2 combined.
Argillaceous Materials : Supplies Silica, Alumina and Iron Oxide. Clay, Marl, Shale, Blast Furnace Slag, sand etc. Here Silica provides strength, Alumina imparts quick setting, iron provides color, strength and hardness.
Gypsum: increases setting time.
Powdered Coal and Fuel Oil: For generating required temperatures.

MANUFACTURE OF PORTLAND CEMENT
Crushing
Mixing (Wet Process)
Mixing (Dry Process)
Grinding (Ball Mill and Tube Mill)
Storage of Ground Materials
Burning
Drying Zone
Calcination Zone
Clinkering Zone
Grinding
Retarder
Dispersing Agent
Water Proofing
Packaging

CRUSHING
This is the first step in the manufacture of Portland Cement.
Jaw crushers of various sizes are employed for the crushing purpose.
Raw materials are crushed by crushers till the size of the raw material reduces to ¾ of an inch.
It is than send for either Wet process or Dry process. Wet process is universally employed.

MIXING WET PROCESS
Calcareous materials are crushed, powdered and stored in bins.
Argillaceous materials is mixed with water and washed. This removes any adhering organic impurities.
Powdered Calcareous and Washed Argillaceous materials are mixed in proper proportions to get a slurry.
Chemical composition is analyzed and corrected if necessary by addition of the deficient materials.
This slurry is then fed into the rotary klin.

MIXING DRY PROCESS
Hard raw materials like cement rock or blast furnace slag are first crushed to 50mm pieces in ball mill, then dried and stored.
Crushing is done by gyratory crushers and drying is done by rotary driers.
Separate powdered ingredients are mixed in required proportions to get the raw mix which is then fed to rotary klins.

GRINDING
Grinding can be done in two stages
Ball Mill
Consists of cast iron drum containing iron and steel balls of different sizes. The principle used in ball mill s impact and shear produced by large no. of tumbling and rolling balls.
Tube Mill
Ball mill grinding is followed by tube mill grinding. Tube mill is conical at the discharge end with separate inlet and outlet.
Slower is the feeding speed finer is the product coming out of the tube mill.

STORAGE OF GROUND MATERIALS
The ground materials containing 30 – 40% of water is stored in separate tanks equipped with agitators.
This step is followed by process of burning.

BURNING
Slurry is burnt in rotary klin where actual chemical changes takes place.
Klin is long steel cylinder 30-40 meter in length, 2-4 meter in diameter, lined by refractory bricks. It is inclined at gradient of 0.5-0.75 inch and can be rotated at the desired speed.
The material is introduced in the klin from the upper end as the klin rotates material passes slowly towards the lower end.
Klin is heated by burning pulverized coal or oil and temperature is maintained at about 1400-1500°C. At clinkering temperature actual chemical reactions takes place.

BURNING ZONE
A. DRYING ZONE
The upper part of the klin is known as drying zone.
The temperature is about 200-500 ° C.
Most of the water gets evaporated from the slurry by means of hot gases.
B. CALCINATING ZONE
This the middle zone of the klin with temperature around 1000 ° C.
Organic matter burns away and CaCO 3 decomposes to quick lime and CO 2 escapes out. The material forms small lumps called as nodules.

BURNING ZONE
C. CLINKERING ZONE
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

CONSTITUTION OF CLINKERS
Clinkers are cooled under controlled condition to produce a definite degree of crystallization.
Clinker Constitution
Ca 2 SiO 4 Dicalcium Silicate
Ca 3 SiO 5 Tricalcium Silicate
Ca 3 Al 2 O 6 Tricalcium Aluminate
Ca 4 Al 2 Fe 2 O 10 Tetracalcium Aluminoferrite
MgO Magnesia
Cao When burning is incomplete

GRINDING
Clinkers are finally grinded in ball mill and tube mill to a fine powder. Additives added are as follows.
Retarder
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).
Dispersing Agent
Sodium salts and polymers of condensed napthlene or sulphonic acid are added to prevent the formation of lumps and cakes in the cement.
Water proofing agents are also added.

PACKAGING
The ground powder is packed by automatic machines in a 50kg bag.
This is then dispatched to the markets where it is sold for constructions of cities.

CHEMICAL COMPOSITION OF PORTLAND CEMENT
Ca 2 SiO 4 Dicalcium Silicate 25% 28 days 420KJ/Kg
Ca 3 SiO 5 Tricalcium Silicate 45% 7 days 880KJ/Kg
Ca 3 Al 2 O 6 Tricalcium Aluminate 1% 1 day 879KJ/Kg
Ca 4 Al 2 Fe 2 O 10 Tetracalcium Aluminoferrite 9% 1 day 418KJ/Kg
MgO Magnesia 4%
Cao Incomplete burning 2%
CaSO 4 Calcium Sulphate 5%

SPECIFICATIONS OF PORTLAND CEMENT
Lime saturation fraction = CaO / 2.8 SiO 2 + 1.2 Al 2 O 3 + 0.65 Fe 2 O 3 = 0.66. To 1.02
The ratio of Al 2 O 3 / Fe 2 O 3 < 0.66
SiO 2 / Al 2 O 3 = 2.5 to 4.0
Weight of insoluble residue < 1.5%
Weight of Magnesia < 5%
Total sulphur content calculated as SO 3 < 2.75%
Total loss of ignition should be less than 4%

OTHER SPECIFIC REQUIREMENTS
Fineness generates lot of heat quickly hence cement mortar or concrete is likely to develop cracks.
Setting time initially - 30 minutes and finally not mare than 600 minutes.
Tensile Strength after 3 days  300lbs.sq. inch and after 7 days  375 lbs/sq. inch.
Compressive Strength after 3 days  300lbs.sq. inch and after 7 days  375 lbs/sq. inch.
Heat of Hydration within 7 days  65 cal/gm and in 28 days  75 cal/gm.
Soundness is the ability of a cement to maintain stable volume after setting. A sound cement resists cracking and disintegrating.

VERTICAL SHAFT KLIN TECHNOLOGY
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.

ADVANTAGES OF VERTICAL SHAFT KLIN
Vertical Shaft Klin has following advantages:
No dust discharge in the atmosphere due to pellets.
Clinker clogging is avoided because no low melting constituents are formed.
Wearing of grinder machinery and cost of grinding is removed
Plant is quite compact.

CHEMISRTY OF SETTING
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.

CHEMISRTY OF HARDENING
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.

REACTIONS INVOLVED
2(3CaO.SiO 2 ) + 6H 2 O  3CaO.2SiO 2 .3H 2 O + 3Ca(OH) 2
2(2CaO.SiO 2 ) + 4H 2 O  3CaO.2SiO 2 .3H 2 O + Ca(OH) 2
3CaO.Al 2 O 3 + 30H 2 O + 3(CaSO 4 .2H 2 O)  3CaO.Al 2 O 3 .3CaSO 4 .36H 2 O
4CaO.Al 2 O 3 .Fe 2 O 3 + 10H 2 O + 2Ca(OH) 2  6CaO.Al 2 O 3 .Fe 2 O 3 .12H 2 O
Decay of cement : Cements are susceptible to attack by salty water and acidic solutions

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Cement

by Prashant Mehta

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Cement

by Prashant Mehta

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