CEMENT

Dr. Prashant MehtaAssistant Professor,

National Law University, Jodhpur

DEFINITION

Cements 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 (SiO2) 17-25%

Provides strength to cement

Alumina (Al2O3)

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 (SO3)

1-3% Provides soundness

Alkali Oxides (Na2O and K2O)

0.5-1.5%

in excess makes cement efflorescent

Magnesium Oxide (MgO)

1-5%

RAW MATERIALS

• Calcareous Materials : Supplies Lime to cementLime Stone (65-80% CaCO3), Marl, Chalk, Shale, Calcite, Alkali waste. It should contain less than 3.3% of MgO and 3-4% of SiO2, Fe2O3, and AlO2 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 CaCO3 decomposes to quick lime and CO2

escapes out. The material forms small lumps called as nodules.

BURNING ZONE

C. CLINKERING ZONEThis 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.

– CaCO3 CaO + CO2 Calcium Oxide

– 2CaO + SiO2 Ca2SiO4 Dicalcium Silicate

– 3CaO + SiO2 Ca3SiO5 Tricalcium Silicate

– 3CaO + Al2O3 Ca3Al2O6 Tricalcium Aluminate

– 4CaO + Al2O3+Fe2O3 Ca4Al2Fe2O10 Tetracalcium Aluminoferrite

CONSTITUTION OF CLINKERS

Clinkers are cooled under controlled condition to produce a definite degree of crystallization.

Clinker Constitution

– Ca2SiO4 Dicalcium Silicate

– Ca3SiO5 Tricalcium Silicate

– Ca3Al2O6 Tricalcium Aluminate

– Ca4Al2Fe2O10 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.

RetarderGypsum CaSO4.2H2O or Plaster of Paris CaSO4.½H2O acts as retarder to prevent quick setting. After initial setting gypsum retards the dissolution of tricalcium aluminate by forming tricalcium sulphoaluminate (3CaO.Al2O3.xCaSO4.7H2O).

Dispersing AgentSodium 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

– Ca2SiO4 Dicalcium Silicate 25% 28 days 420KJ/Kg

– Ca3SiO5 Tricalcium Silicate 45% 7 days 880KJ/Kg

– Ca3Al2O6 Tricalcium Aluminate 1% 1 day 879KJ/Kg

– Ca4Al2Fe2O10 Tetracalcium Aluminoferrite 9% 1 day 418KJ/Kg

– MgO Magnesia 4%

– Cao Incomplete burning 2%

– CaSO4 Calcium Sulphate 5%

SPECIFICATIONS OF PORTLAND CEMENT

Lime saturation fraction = CaO / 2.8 SiO2 + 1.2 Al2O3 + 0.65 Fe2O3

= 0.66. To 1.02

The ratio of Al2O3 / Fe2O3 < 0.66

SiO2 / Al2O3 = 2.5 to 4.0

Weight of insoluble residue < 1.5%

Weight of Magnesia < 5%

Total sulphur content calculated as SO3 < 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.SiO2) + 6H2O 3CaO.2SiO2.3H2O + 3Ca(OH)2

2(2CaO.SiO2) + 4H2O 3CaO.2SiO2.3H2O + Ca(OH)2

3CaO.Al2O3 + 30H2O + 3(CaSO4.2H2O) 3CaO.Al2O3.3CaSO4.36H2O

4CaO.Al2O3.Fe2O3 + 10H2O + 2Ca(OH)2

6CaO.Al2O3.Fe2O3.12H2O

Decay of cement : Cements are susceptible to attack by salty water and acidic solutions