ABOUT CEMENT

                                    ABOUT CEMENT

  

Hey friends!Have you ever wondered off, how a cement which is just like a powder  when  mixed with water is able to bind the materials together .Let's have a look about the magical power of  cement and its binding factors.

           
             ............ This great idea first struck in the mind of a british mason named JOSEPH ASPDIN who finally brought ordinary portland cement into existence on 21st october ,1824............

             Cement is made from limestone, clay ,Alumina ( Al₂O₃),Iron Oxide(Fe₂O₃).These raw materials are extracted from the quarry crushed to a very fine powder and then blended in the correct proportions.This blended raw material is called the 'raw feed' or 'kiln feed' and is heated in a rotary kiln where it reaches a celcius temperature of about 1400  to 1500 .The material formed in the kiln is described as 'clinker' .After cooling, the clinker may be stored temporarily in a clinker store, or it may pass directly to the cement mill.

      The cement mill grinds the clinker to a fine powder. A small amount of gypsum - a form of calcium sulphate - is normally ground up with the clinker. The gypsum controls the setting properties of the cement when water is added. After grounding, our ordinary portland cement is ready to for use and transportation.

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This is simple overview how cement made. After this we come to our main topic how cement bind the material together?  

For to know more we go to chemical composition  of clinker.In clinker main compounds are 

• Alite: C₃S, or tricalcium silicate.
• Belite: C₂S, or dicalcium silicate.
• Aluminate phase: C₃A, or tricalcium aluminate.
• Ferrite phase: C₄AF, or tetracalcium aluminoferrite.  are also called the bogue's compound.

Tricalcium aluminate, C₃A:-

It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C₃A. Cement low in C₃A is sulfate resistant.
 

Tricalcium silicate, C₃S:-

This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain.
 

Dicalcium silicate, C₂S:

C₂S hydrates and hardens slowly. It is largely responsible for strength gain after one week.
 

Ferrite, C₄AF:

This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 1400 to 1500^o C). It hydrates rapidly, but does not contribute much to strength of the cement paste.

By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.

 When we mix the water into the cement then this process is called the hydration of cement.

let's have a brief look in the part of hydration. 

• The tricalcium aluminate reacts with the gypsum in the presence of water to produce ettringite and heat:
Tricalcium aluminate + gypsum + water ® ettringite + heat
C₃A + 3CSH₂ + 26H ® C₆AS₃H₃₂, D H = 207 cal/g
Ettringite consists of long crystals that are only stable in a solution with gypsum. The compound does not contribute to the strength of the cement glue.
 
• The tricalcium silicate (alite) is hydrated to produce calcium silicate hydrates, lime and heat:
Tricalcium silicate + water ® calcium silicate hydrate + lime + heat
2C₃S + 6H ® C₃S₂H₃ + 3CH, D H = 120 cal/g
The CSH has a short-networked fiber structure which contributes greatly to the initial strength of the cement glue.
 
• Once all the gypsum is used up as per reaction (i), the ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals:
Tricalcium aluminate + ettringite + water ® monosulfate aluminate hydrate
2C₃A + 3 C₆AS₃H₃₂ + 22H ® 3C₄ASH₁₈,
The monosulfate crystals are only stable in a sulfate deficient solution. In the presence of sulfates, the crystals resort back into ettringite, whose crystals are two-and-a-half times the size of the monosulfate. It is this increase in size that causes cracking when cement is subjected to sulfate attack.
 
• The belite (dicalcium silicate) also hydrates to form calcium silicate hydrates and heat:
Dicalcium silicates + water ® calcium silicate hydrate + lime
C₂S + 4H ® C₃S₂H₃ + CH, D H = 62 cal/g
Like in reaction (ii), the calcium silicate hydrates contribute to the strength of the cement paste. This reaction generates less heat and proceeds at a slower rate, meaning that the contribution of C₂S to the strength of the cement paste will be slow initially. This compound is however responsible for the long-term strength of portland cement concrete.
 
• The ferrite undergoes two progressive reactions with the gypsum:
• in the first of the reactions, the ettringite reacts with the gypsum and water to form ettringite, lime and alumina hydroxides, i.e.
• Ferrite + gypsum + water ® ettringite + ferric aluminum hydroxide + lime
• C₄AF + 3CSH₂ + 3H ® C₆(A,F)S₃H₃₂ + (A,F)H₃ + CH

• the ferrite further reacts with the ettringite formed above to produce garnets, i.e.

• Ferrite + ettringite + lime + water ® garnets
• C₄AF + C₆(A,F)S₃H₃₂ + 2CH +23H ® 3C₄(A,F)SH₁₈ + (A,F)H₃

The garnets only take up space and do not in any way contribute to the strength of the cement paste.

By these process of hydration the CSH gel is formed which bind the material together  and after the setting of cement cement get harden and then form a solid rock in which other material like sand and aggregate are held together.