Portland cement

It is defined as an extremely finely ground product.
It is obtained by heating a mixture of argillaceous (clay containing ) and calcareous (lime containing ) raw materials to about 1500 c. It is then mixed with gypsum to increase the quick setting and hardening property.
MANUFACTURE OF PORTLAND CEMENT
Raw materials :
(i)  Calcareous materials , CaO   Ex: Limestone, chalk.
(ii) Argillaceous materials, Al2O3 and SiO2    Ex: clay, slate etc
(iii) Powdered coal (or) fuel oil.
(iv) Gypsum (CaSo4.2H2O)
 Manufacture of Portland cement involves the following steps:

• (i) Mixing of raw materials
• (ii) Burning
• (iii) Grinding
• (iv) Storage and Packing
• (i) Mixing of raw materials:

(a)   Dry Process        (b) Wet Process

• (a) Dry Process:  In dry process, the raw materials  like limestone and clay(3:1) are dried, and mixed in definite proportions 
• (b) Wet process : In wet process, the raw materials in definite proportions are finely ground with water and the slurry  ( past like) is fed at the top of the rotary kiln.

(II) Burning
The burning process is usually done in rotary kiln which is a long horizontal steel cylinder coated with refractory bricks and capable of rotating  at 1 rpm 9 Revolution per minute) . The rotary kiln is set at a slight inclination of about 5-60  in order to allow the raw materials fed at one end to travel slowly to the firing and discharge exit end.
The slurry of raw materials is allowed to enter from the top end of the rotary kiln. Simultaneously the burning fuel ( like powdered coal or oil) and air are introduced from the lower end of kiln . The slurry gradually comes down in the kiln into the different zones ( Drying Zone at
400o  :Calcination zone at 700 -1000 o C and clinkering zone at 1250-1500 o C of increasing temperatures.

• (a) Drying Zone: The upper part of the rotary kiln is known as drying zone ,where the temperature is about 400 o C . Due to the presence of hot gases in this zone, water is evaporated from the slurry.
• (b) Calcinations zone:  The middle part of the rotary kiln is known as calcining zone where the temperature ranges from 700 -1000 o C. In this zone lime stone is decomposed into CaO and CO2

CaCO3     700 -1000 o C  CaO +CO2
Lime Stone   Quick lime
(c) Clinkering Zone : The lowest part of the zone is called as clinkering zone, where the temperature is maintained about 1250-1500 o C. In this zone lime reacts with clay              ( Containing  Al2O3, Fe2O3 and SiO2) and forms aluminates and silicates
2CaO+    SiO2 -------- 2CaO.SiO2
Di calcium Silicate
3CaO+    SiO2 -------- 3CaO.SiO2
Tri calcium Silicate
The mixture is then finely powdered and fed into the top of the rotary kiln.
3CaO+    Al2O3-------- 3CaO.Al2O3
Tri calcium Aluminate
4CaO   + Al2O3   + Fe2O3--------- 4CaO.Al2O3.Fe2O3
Tetra calcium alumino Ferrate

• (ii) Cooling : the hot clinker is cooled with atmospheric air and the hot air thus produced is used for drying the coal before grinding.
• (iii) Grinding : The cooled clinker is then finely pulverized with 2-6% gypsum acts as a retarding agent for quick setting of cement.
• (iv) Storage and Packing:  The cement coming out from the grinding mills is stored in a concrete storage silos.

Then the cement is packed in jute bags by automatic machines. Each bag contains 50kgs of cement.
PROPERTIES
Setting and Hardening of cement:
When the cement is mixed with water, hydration and hydrolysis of cement begin, resulting in the formation of gel and crystalline products.
Setting:    It is defined as the stiffening of the original plastic mass, due to initial gel formation.
Hardening:   It is defined as the development of strength, due to crystallization.
Chemical reactions involved in setting and hardening of cement:
When water is mixed with cement , hydration of tricalcium  aluminate occurs rapidly and the paste becomes quite hard within a short time. This process is known as initial setting of cement.
3CaO.Al2O3  +6H2O-------3CaO.Al2O3.6H2O
Role of gypsum in cement:
(i)   In initial setting process gypsum is added during grinding of cement clinkers to retardt
The rapid hydration of C3A
Gypsum reacts with C3A to form insoluble calcium sulphoaluminate complex.
C3A    + 3CaSO4.2H2O--------C3A.3CaSO4.2H2O

• (ii) After the hydration of C3A,C3S begins to hydrate to give tobermonite gel and crystalline Ca(OH)2.  The hydration of C3S takes place within 7days.

         2(3CaO.SiO2)   + 6H2O--------- 3CaO.2SiO2.3H2O   + 3Ca(OH)2  + 500kj/kg 

• (iii) Dicalcium silicate reacts with water slowly and gets finished 7-28days.

2(2CaO.SiO2)    + 4 H2O --------     3CaO.2SiO2.3H2O   +Ca(OH) 2 +    250kj/kg

• (iv) Hydration of tetra calcium aluminoferritetakesplace  initially, the hardening takes place finally through crystallization along with C2 S.

4CaO.Al2O3.Fe2O3   + 7H2O®3CaO.Al2O3.6H2O  + CaO.Fe2O3.H2O +  420KJ
Crystalline gel
Thus the final setting and hardening of cement is due to the formation of tobermonite gel plus crystallization of Ca(OH)2 and hydrated tricalcium aluminate.
SPECIAL CEMENT
Water Proof Cement :
It is obtained by adding water proofing agents like calcium stearate and gypsum with tannic acid to ordinary Portland cement during grinding.
Functions of water- Proof cement:
Functions of water- proof cement

• (i) To make concrete impervious to water under pressure.
• (ii)  To resist the adsorption of water.

 White cement or White Portland cement
It is obtained by heating the raw materials free from iron oxides.  It is white in color due to the absence of ferric oxide.
It issued for making tiles, mosaic works with some coloring agents like yellow ochre, Venetian red etc.
It is used for repairing and joining marble pillars and blocks.