TOP 10 PSU IN INDIA

CEMENT-

  Cement is a commonly used binding material in the construction. The cement is obtained by burning a mixture of calcarious (calcium) and argillaceous (clay) material at a very high temperature and then grinding the clinker so produced to a fine powder. It was first produced by a mason Joseph Aspdin in England in 1924. He patented it as portland cement.

Type of Cement-

In addition to ordinary Portland cement there are many varieties of cement.

(i)   White Cement-

The cement when made free from colouring oxides of iron, manganese and chlorium result into white cement. In the manufacture of this cement, the oil fuel is used instead of coal for burning. White cement is used for the floor finishes, plastering, ornamental works etc. In swimming pools white cement is used to replace glazed tiles. It is used for fixing marbles and glazed tiles.

(ii)   Coloured Cement-

The cement of desired colours are produced by intimately mixing pigments with ordinary cement. The chlorium oxide gives green colour. Cobalt produce blue colour. Iron oxide with different proportion produce brown, red or yellow colour. Addition of manganese dioxide gives black or brown colour cement. These cements are used for giving finishing touches to floors, walls, window sill, roofs etc.

(iii)  Quick Setting Cement-

Quick setting cement is produced by reducing the percentage of gypsum and adding a small amount of aluminium sulphate during the manufacture of cement. Finer grinding also add to quick setting property. This cement starts setting within 5 minutes after adding water and become hard mass within 30 minutes. This cement is used to lay concrete under static or slowly running water.

(iv)  Rapid Hardening Cement-

This cement can be produced by increasing lime content and burning at high temperature while manufacturing cement. Grinding to very fine is also necessary. Though the initial and final setting time of this cement is the same as that of Portland cement, It gains strength in early day. This property helps in earlier removal of form works and speed in construction activity.

(v)   Low Heat Cement-

In mass concrete works like construction of dams, heat produced due to hydration of cement will not get dispersed easily. This may give rise to cracks. Hence in such constructions it is preferable to use low heat cement. This cement contains low percentage (5%) of tricalcium aluminate (C₃A) and higher percentage (46%) of dicalcium silicate (C₂A).

 (vi) Pozzulana Cement-

Pozzulana is a volcanic power found in Italy. It can be processed from shales and certain types of clay also. In this cement pozzulana material is 10 to 30 percent. It can resist action of sulphate. It release less heat during setting. It imparts higher degree of water tightness. Its tensile strength is high but compressive strength is low. It is used for mass concrete works. It is also used in sewage line works.

(vii)  Expanding Cement-

This cement expands as it sets. This property is achieved by adding expanding medium like sulpho aluminate and a stabilizing agent to ordinary cement. This is used for filling the cracks in concrete structure.

(viii) High Alumina Cement-

It is manufactured by calcining a mixture of lime and bauxite. It is more resistant to sulphate and acid attack. It develops almost full strength within 24 hours of adding water. It is used for under water works.

(ix) Blast Furnace Cement-

In the manufacture of pig iron, slag comes out as a waste product. By grinding clinkers of cement with about 60 to 65 percent slag, this cement is produced. The properties of this cement are more or less same as ordinary cement, but it is cheap, since it utilise waste product. This cement is durable but it gains the strength slowly and hence needs longer period of curing.

(x)  Acid Resistant Cement-

This cement is produced by adding acid resistant aggregated such as quartz, quartzite, sodium silicate or soluble glass. This cement has good resistance to action of acid and water. It is commonly used in the construction of chemical factories.

 (xi)  Sulphate Resistant Cement-

By keeping the percentage of tricalcium aluminate C₃A below five percent in ordinary cement this cement is produced. It is used in the construction of structures which are likely to be damaged by alkaline conditions. Example of such structures are canals, culverts etc.

(xii) Fly Ash Blended Cement-

Fly ash is a byproduct in thermal stations. This particles of fly ash are very minute and they fly in the air, creating air pollution problems. Thermal power stations have to spend lot of money to arrest fly ash and dispose safely. It is found that one of the best way to dispose fly ash is to mix it with cement in controlled condition and derive some of the beneficiary effects on cement. Now-a-day cement factories produce fly ash in their own thermal stations or borrow it from other thermal stations and further process it to make it suitable to blend with cement. 20 to 30% fly ash is used for blending.