Explain the process of manufacturing ordinary Portland cement by vertical shaft rotary kiln method and write chemistry of hardening and setting of it with chemical equations. Describe the process of setting and hardening of cement concrete, giving the reactions involved in such process. What is the difference, if any, between “setting” and “hardening”?

Manufacture of cement by vertical shaft kiln technology:- Large big cement plants with large capital investment use rotary kiln technology but certain cement plant use vertical shaft kiln technology of long horizontal kilns used in dry and wet process. In VSK technology, pellets of raw material are made by pan nebuliser or palletise. Uniform pellets are then fed into VSK and good quality cement cements are obtained. Lime (calcareous), clay (argillaceous) and fuel (low volatile) are used as main raw material. In VSK, the fuel (coal) is mixed with proportioned batch of raw materials either by (1) Black meal process or (2) Fuel slurry process and it is converted to pellets with the help of palletise.

Black meal process: Raw materials are ground with coal in the calculated proportional to black meal. This powdered material is then converted into pellets by using (15%) water.

Fuel slurry process: Raw material is ground by separately while coal is ground wet separately with equal amount of water. Dry ground material and coal are mixed properly and converted into pellets.

Process: The ready pellets are fed into the vertical shaft kiln through a hopper. The kiln is lined with refractory and is filled up with clinker leaving about 2 feet. The pellets pass through the kiln from the top to downwards, and get heated and sintered by ascending hot gases. The whole coal charge composed of unburnt modules and clinker rest on a specially designed gate, rotating slowly at the bottom of the kiln. Various zones of reaction starting from the top of the kiln are:

1.      Preheating zone

2.      Calcining zone

3.      Sintering zone

4.      Cooling zone

The cold air is blown through the discharge unit to cool the sintered clinkers of cement. The air gets heated in cooling the clinkers which is then utilized in combustion of coal present in the pellets. The clinker is finally, extracted from kiln bottom through discharge gate and finally ground with 4.5% gypsum before packing.

Advantages of vertical shaft kiln technology:

1.      Due to use of pellets and low velocity of exit gases, no dust is discharged into the atmosphere. So air pollution is less.

2.      Control over the shape of pellets reduces the wear of grinding machinery.

3.      The plant is quite compact since the kiln is usually 10 meters tall.

4.      It has lower maintenance cost and requires less capital investment.

Reactions:

  CaCO3 = CaO + CO₂

2CaO + SiO₂ = 2CaO.SiO₂

3CaO + SiO₂ = 3CaOSiO₂

3CaO + Al₂O₃ = 3CaO.Al₂O₃

4CaO + Fe₂O₃ + Al₂O₃ = 4CaO.Fe₂O₃.Al₂O₃

Setting and hardening of Portland cement:-

When water is mixed with cement, it forms a paste like mass, a hydration of compounds in cement take place. This fluid like paste eventually becomes stiff and then hard. This action of change of mixture from a fluid (paste) to solid is known as setting of cement. During the process the gel is formed which adheres firmly to stones sand etc. Binding them together to form a solid mass which on setting and hardening has a great mechanical strength and forms valuable building material. The process of solidification comprises 3 stages.

1.      Initial set: When cement is mixed with water a plastic mixture is formed which can moulded as desired as the action of setting take place gradually in 60 minutes or so. But with the time mixture begins to lose its plasticity marked by slight jellifying of the cement paste. Further it should not be disturbed until it has hardened. In initial set stage paste attains stiffness.

2.      Final set: It takes place after few hour of mixing. Then concrete mixture can neither be moulded into any shape nor can it be remixed. As the final setting of cement take place the mass attains such a stiffness that the standard needle cannot penetrate in the cement mass at all.

Hardening: CaCO3 = CaO + CO₂

2CaO + SiO₂ = 2CaO.SiO₂

3CaO + SiO₂ = 3CaOSiO₂

3CaO + Al₂O₃ = 3CaO.Al₂O₃

4CaO + Fe₂O₃ + Al₂O₃ = 4CaO.Fe₂O₃.Al₂O₃

3.      Hardening: After final setting of the solid cement material begins to gain strength and it is this increase in strength which is known as hardening. The hardening or development of strength depends on chemical. Combination of cement and water and thus continue with a great speed in the beginning but afterward speed is reduced. The hardening of concrete stops as it is dried out. But if it is kept moist, the hardening process can continue for years. However, the rate of hardening decreases as the time passes. The hydration reactions are exothermic and the volume of cement increases with hydration. In the larger structures of concrete, the heat evolved is somehow removed and allowances for expansion are provided. Hydrated cements dissociate on heating by destroying the bond which held the mass together.