Chemical composition of cement clinker | Compound of cement clinker

Chemical composition of cement clinker

                    

Clinker

When the calcined products reach the bottom of the Rotary kiln, they gradually cool down, and the temperature rises to about 1000 °C and this is the case for Tri Calcium Aluminate (C3A) and Tetra Calcium Aluminoferrite (C4AF). ) is formed, and the calcined product is obtained in the form of hard and dark greenish blue noodles, which is called clinker.

 The diameter of clinker is 3 - 25 mm, and four types of compounds are found in it, which are called Bough compounds.

	Compound	Formula	Symble	Name
1.	Tricalcium Silicate	3CaO.SiO2	C3S	Alite
2.	Dicalcium Silicate	2CuO.SiO2	C2S	Belite
3.	Tricalcium Aluminate	3CaO.Al2O3	C3A	Celite
4.	Tetra calcium Alumino-ferrite	4CaO.Al2O3Fe2O3	C4AF	Felite

 Solid State Reaction Zone ( 900-1300^॰C)

 In this zone all the ingredients remain in the solid state, but in this zone, reactive silica reacts with CaO to form di calcium silicate (C3S), as well as intermediate compounds from calcium aluminate and calcium ferrite, which act as slurry. 

This means that these intermediate compounds melt at low temperatures (13 degrees centigrade) and increase the rate of action. If intermediate compounds are not present, the rate of formation of clinker compounds is greatly reduced.

Cooling of clinker 

  The clinker obtained from the rotary kiln whose temperature is 1000 degree centigrade is cooled by the cooler at medium rate, in moderate rate cooling the temperature is brought to 1200- 1500 degree centigrade in 15 minutes and in 10 minutes It is brought to room temperature from 500 degree centigrade.

Granding of clinker

 The clinker obtained from Rotary Kiln is ground in ball mill or tube mill, and 3 to 5% gypsum is added at the time of grinding itself.

                    

Ball Mill and Tube Mill 

The ball mill is in the shape of a steel cylinder, whose diameter is 2 -2.5 m and length 1.8 - 2 m, this cylinder is partially filled with steel balls, whose diameter is 50 - 120 mm. It is less finely milled, whereas the size of steel ball used in tube mill is 20 mm to 25 mm, it is milled more finely.

               Weight of 1 Bag of Cement = 50 Kg

                Volume = 0.35 m3 or 35 liters

The height of the cement bag is 30 inches, the width is 20 inches and the thickness is 4 inches.

Compounds of Clinker

 1. C₃S 

Cement compound is the best cementing material, its quantity in cement is about 25 to 50% and normal is 40%. It reacts faster with water i.e. the rate of hydration is high, due to the heat released as a result of the process of hydration, the cement hardens and gives strength quickly.

Initial strength (mainly 7 to 28 days) is due to a compound, its hydration releases heat of about 500 joules/g, i.e., due to the high heat released, if the cement has a higher concentration, then there is a chance of cracking. Chances increase.

·         Less resistance to chemicals.

Better resistance to freezing and thawing.

About 24% water is required for its hydration.

2. C₂S 

Its amount is 25 to 40%, and the normal is 32%. Its hydration is gradual and contributes to strength after a long time. After 1 year, the contribution of C3S and C2S to strength is equal.

• Its hydration releases 260 joules/g of heat
•  It has good resistance to chemicals.
•  Registration is less good towards freezing and thawing.
•  21% water is required for its chemical reaction.
•  The ultimate strength of cement is due to this compound C2S.

 Note - The combined amount of C3S and C2S in cement is about 70 to 80%.

3. C₃A 

Its quantity is 5 to 11%, and the normal is 10.5 percent. It reacts very quickly with water and is responsive to flash set, releasing 865 joules/gram of heat in its hydration. Hence it is an undesirable compound of cement. Initial setting of cement is due to hydration of this compound.

• If its quantity is high, more heat is released, due to which the possibility of cracking increases, that is, the ultimate strength decreases.
• The hydration of C3A requires 40% water, but due to the relatively small amount of C3a, the amount of water is less.

4. C₄AF

Its quantity is normally 9%, it is also responsible for flash set of cement, but relatively less heat is released in its hydration, so it is not considered, in its hydration, heat of 420 joules/gram is released, It is a stable compound with C3A.

Heat of hydration-

C3A > C3S > C4AF > C2S

 

Hydration of Cement 

The chemical reaction of cement compound with water is called Hydration of Cement. This process is exothermic, that is, heat is released as a result of chemical reaction, which is called heat of hydration.

Hydration lasts for a very long time i.e. cement retains strength for a long time, but the rate of hydration is very high in the initial stage but as time passes, the rate of hydration decreases.

 Hydration of C₃S and C₂S 

When c3s and c2s react with water, a gel of C-S-H is formed, which is called calcium silicate hydrate, and Ca(OH)2 is formed.,

2 C₃S + 6H₂O ------------ C-S-H + 3Ca(OH)₂

2 C₂S + 4 H₂O ------------- C-H-S + Ca(OH)₂  

• The calcium silicate hydrate gel formed by hydration of C3S and C2S has a binding property that binds different ingredients of concrete together and provides strength.
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The hydration of C3S produces a relatively large amount of C-S-H gel, but its quality and density are of slightly inferior quality to the C-S-H gel formed in the hydration of c2s, also the specific surface area of ​​C-S-H water formed in the hydration of c2s is more.

•   Hydration of C3S releases a relatively high amount of Ca(OH)2, which is nondegradable is |
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• addition, if concrete is in contact with soil or in contact with water, it absorbs sulphate present in it and this sulphate reacts with Ca(OH)2 to form calcium sulphate and this sulphate reacts with c3a.
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•  Sulfur forms aluminate, whose volume is about 227 times more than the original aluminate volume, that is, due to the increase in the volume of hard concrete, cracks occur in the concrete, this is called the effect of sulfate or the effect of chemicals.
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• Calcium hydroxide Ca(OH)2 has only one advantage that it maintains the PH value of concrete, which reduces the chances of rusting in steel bars.
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•  When c3a reacts with water, calcium forms aluminate system (CaO.Al2O3. H2O), but in the presence of calcium sulfate or gypsum, calcium sulfur forms aluminate which stops the direct hydration of c3a for a short time but When calcium sulphate is depleted, monosulphate is formed, which is the final product of c3a hydration. Similarly, the hydration of c4af forms a calcium chloride system (CaO.Fe2O3.H2O), which is more stable but has no significant contribution to strength. does not exist so it cannot be considered

Rate of Hydration

As soon as water is added to the cement c3a reacts with water very rapidly i.e. it is responsible for flash set of cement i.e. the cement hardens without strength, but if CaSO4 is present in the cement then it immediately dissolves in water. dissolves and reacts with c3a to form calcium sulfur aluminate which is deposited on c3a as a colloidal membrane

 and c3a reduces the rate of direct hydration, but as the volume of the membrane increases, it breaks down and caso4 interacts with c3a to form a re-membrane. This cycle process continues until caso4 is present in the solution when all caso4 is exhausted then calcium sulphate min converts to monosulphate and after that direct hydration of c3a and water starts and cement sets rapidly.

Note- When hydration has added full caso4, tricalcium aluminate should always be left in the access, which is why the amount of c3a is always greater than 5%.

C4AF > C3A > C3S > C2S