Cement | Definition, Composition, Manufacture, Types.

Cement:-

·    Cement is a most important binding material, which is used to bind the aggregates to form the desired civil engineering structure.

·    It mixed with fine aggregate (Sand) to form mortar or with fine aggregate (sand) and coarse aggregate (gravel) to form concrete.

·      If it is mixed with water it forms a paste, Because of their hydrating properties, which gets hard to form a hard and durable mass like a natural stone occurring at Portland in England that’s why it is called Portland cement.

·       It was invented by Joseph Aspdin of UK in 1824.

·       It is classified as – Hydraulic cement and Non-hydraulic cement.

·    Hydraulic cement- it set and hardens due to chemical action of cement with water known as hydration and the resultant stable product is water-resistant and also safe from chemical attack. eg- Portland cement.

·   Non-hydraulic cement- it is derived by calcination of limestone and gypsum. Its hydrated product isn’t water-resistant. eg Plaster of Paris.

·      Cement which is formed by grinding of natural cement stones called natural cement. eg Roman cement, Puzzolana cement and Medina cement.

·       Cement which is formed artificially by using Argillaceous and Calcareous materials. 

Calcareous materials	Limestone, Chalk, Marine shells
Argillaceous materials	Shale and clay, Cement rock, Blast furnace slag, Marl

Raw material composition:-

·        The relative proportions of the compositions are responsible for various properties of cement.

·        The main constituents of cement are Lime, Silica, Alumina, and Iron oxide.

·        The various chemical composition in cement are -  

Chemical	Composition	Average	Function
Lime (CaO)	60-65%	63%	Control strength and soundness Deficiency- Reduce strength and setting time Excess- Cause unsoundness
Silica (SiO2)	15-25%	21%	Gives strength Excess- Cause slow setting
Alumina (Al2O3)	3-8%	6%	Cause quick setting Excess- Lowers the strength
Iron Oxide (Fe2O3)	0.5-6%	3%	Gives Colour Helps in fusion of other ingredients
Magnesia (MgO)	0.5-4%	2%	Impart colour and hardness Excess- Cause cracks and unsoundness
Sulphur trioxide (SO3)	1-2%	1.5%	Makes cement sound
Soda and/or Potash	0.5-1%	1%	These are residues Excess- Cause efflorescence and crack

·       Alkalis present in the cement accelerate the setting of cement paste.

·    The variation in composition in cement largely depends upon the ratio of CaO to SiO₂ in raw materials.

·    By regulating the ratio of SiO₂ to (Al₂O₃+Fe₂O₃), the rate of setting of cement paste can be controlled.

·     To control the development of undesirable heat of hydration, Silica content is increased to about 21% and the Alumina and Iron oxide is limited to 6% each.

·     To make the cement sulphate resistant, Silica content rise to 24% and the Alumina and Iron oxide is limited to 4% each.

Chemical composition of cement:-

·       Cement has four major chemical compounds called Bague compound.

·      Besides Bague compound, two minor compounds are also formed K₂O and Na₂O, which takes part in Alkali-Aggregate reaction.

·      Ordinary Portland Cement (OPC) achieves its 70% of final strength in 28 days and about 90% of final strength in 1 year.r.

 

S. No.	Bague compound	Name	Symbol	Formula
1	Tricalcium silicate	Alite	C3S	3CaO.SiO2
2	Dicalcium silicate	Belite	C2S	2CaO.SiO2
3	Tricalcium aluminate	Celite	C3A	3CaO.Al2O3
4	Tetracalcium alumina ferrite	Felite	C4AF	4CaO.Al2.Fe2O3

1. Tricalcium silicate:-

·     It occurs in cement about 25-50% (Normally 40%)

·     It is best cementing material.

·     It is well-burnt cement.

·     It hydrates rapidly, generating high heat of hydration.

·      It is responsible for early strength and hardness (7 days of strength and hardness).

·     It makes clinker easy to grind, increase resistance to freezing and thawing.

·     Increasing of C₃S content beyond limits increases the solubility of cement in water and also increases the heat of hydration.  

·    Heat of hydration is 500 J/gm.

2. Dicalcium silicate:-

·     It occurs in cement about 20-40% (Normally 32%)

·     It is responsible for ultimate strength and hardness.

·     It makes cement resistance to chemical attack.

·     It has low rate of hydration, takes long time to add to the strength (After one year or more).

·   Increasing C₂S content makes clinker harder to grind, reduces early strength, and decrease the heat of hydration.

·   It has very low strength at an early age and after one year it has strength almost equal to the strength of C₃S.

·     Heat of hydration 260 J/gm

3. Tricalcium aluminate:-

·     It occurs in cement about 5-12% (Normally 11%)

·     It responsible for initial set.

·     It has high heat of hydration

·     It has greater tendency to volume change which causing cracking.

·     It rapidly reacts with water and is responsible for Flash setting.

·    Flash setting of C₃A is controlled by the addition of 2-3% of gypsum at the time of grinding of clinker.

·    Increasing of C₃A content reducing the setting time, reducing the ultimate strength, and weakens resistance to sulphate attack.

·      Heat of hydration is 865 J/gm

4. Tetracalcium alumina ferrite:-

·      It occurs in cement about 6-10% (Normally 9%)

·      It is poorest cementing material.

·       Increasing C₄AF content reduces strength.

·      Heat of hydration is 420 J/gm.

                                   Contribution of bogue compound to the strength of cement

Manufacturing of cement:- 

• There are 4 stages of manufacturing of cement-

1.      Crushing and grinding the raw materials.

2.      Blending the materials in the correct proportions.

3.      Burning the prepared mix in a kiln.

4.      Grinding the burned product (clinker) with gypsum.

• Raw Material for manufacturing of cement are-

Calcareous materials	Limestone, Chalk, Marine shells
Argillaceous materials	Shale and clay, Cement rock, Blast furnace slag, Marl

 

• Clay and Limestone are mixed in the ratio of 1: 3 in grinded form before feeding to kiln.
• During the manufacturing process, Lime, silica, alumina, iron-oxide etc which are obtained from raw material, combine to form bogue compound.
• There are two manufacturing process-

1.      Dry process

2.      Wet process