CEMENT CLINKER AND GYPSUM

COMPOSITION OF CEMENT CLINKER AND THEIR PROPERTIES

Cement clinker

ü Raw materials required: Calcareous and argillaceous materials

ü Burnt in: Rotary Kiln

Composition of cement clinker

SN	Compound	Abbreviations	Common proportion
1	Tricalcium Silicate (3CaO SiO2)	C3S	40%
2	Dicalcium Silicate (2CaO SiO2)	C2S	32%
3	Tricalcium aluminate (3CaO Al2O3)	C3A	10.5%
4	Tetracalcium aluminium ferrite (4CaO Al2O3 Fe2O3)	C4AF	9%

Besides these composition

ü Gypsum (CaSO₄): 3 to 4 % is added while grinding

ü Gypsum is added to retard/slow the initial setting action or increase the initial setting time

PROPERTIES OF COMPOUNDS OF CLINKER

1. Tricalcuim silicate (C₃S)

ü Generates more heat rapidly

ü Hydrates more rapidly

ü Possess less resistant to chemical attack

ü Develops early strength

2. Dicalcium silicate (C₂S)

ü Imparts ultimate strength to the cement

ü Offers more resistant to chemical attack

ü Hardens more slowly

ü Hydrates slowly

3. Tricalcium alumiate (C₃A)

ü Weak against Sulphur attack

ü Reacts fast, generating large amount of heat

ü Does not contribute to develop strength

ü Causes initial setting of cement

ü First to react with water

4. Tetra calcium aluminium ferrite (C₄AF)

ü Poor cementing value

ü Reacts slowly, generating small amount of heat

ü Comparatively inactive

HOW gypsum retards setting action of cement?

What is gypsum?

•      Chemically, gypsum rock is calcium sulphate with two molecules of water.

•      Gypsum = CaSO₄.2H₂O

What is initial setting time?

•      When cement is mixed with water, stiff and sticky paste is formed and remains plastic for a short period

•      As the time passes, plastic cement becomes stiff due initial hydration of cement

•      The phenomenon by virtue of which plastic cement changes into solid mass is setting of cement.

•      Generally Initial setting is the time elapsed between the moment water is added to the cement to the time at which paste starts losing its plasticity.

How Gypsum Retards Setting action of Cement?

•      C₃A is the compound responsible for initial setting of cement and is the first compound to react with water

•      Gypsum reacts with C₃A and turns it into Calcium Sulpho Alumintate

•      Since Gypsum reacts with C₃A and C₃A reduces

•      Thus, C₃A is not available for reaction with water and hence Setting action is reduced

•      Reduction in setting action means elongation of setting of cement i.e. Setting time increases

Advantage of retarding setting action

•      As setting action retards, setting time increases and gives sufficient time for

•      Mixing

•      Transporting

•      Placing