WHAT IS CEMENT?

Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th.
Cement is a binder, a substance, which sets and hardens independently and can bind other materials together.  Cement is a mixture of compounds, consisting mainly of silicates and aluminates of calcium, formed out of calcium oxide, silica, aluminium oxide and iron oxide.  Cement is manufactured by burning a mixture of limestone and clay at high temperatures in a kiln, and then finely grinding the resulting clinker along with gypsum. The end product thus obtained is called Ordinary Portland Cement (OPC). 

In India, OPC is manufactured in three grades, viz. 33 grade, 43 grade and 53 grade, the numbers indicating the Compressive Strength obtained after 1 day, 3 days, 7 days, 15 days and 28 days when tested as per the stipulated procedure and as per Bureau of Indian Standards.
Apart from OPC, there are several other types of cements being manufactured and most of them meant for special purposes, e.g. sulphate resistant cement, coloured cement, oil well cement, etc. However, there are some general purpose cements, the commonest one being Portland Pozzolana Cement (PPC).  Every grade of cement has its own strength and role to play according to its fineness.

CEMENT MANUFACTURING PROCESS :

Portland cement, the basic ingredient of concrete, is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other ingredients to which gypsum is added in the final grinding process to regulate the setting time of the concrete. Lime and silica make up about 85% of the mass.  Common among the materials used in its manufacture are limestone, shells, and chalk or marl combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore.

Two Manufacturing Processes
Two different processes, "dry" and "wet,"  are used in the manufacture of portland cement.

When rock is the Principal Raw Material, the first step after quarrying in both processes is the primary crushing. Mountains of rock are fed through crushers capable of handling pieces as large as an oil drum. The first crushing reduces the rock to a maximum size of about 6 inches. The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller.

In the wet process, the raw materials, properly proportioned, are then ground with water, thoroughly mixed and fed into the kiln in the form of a "slurry" (containing enough water to make it fluid). In the dry process, raw materials are ground, mixed, and fed to the kiln in a dry state. In other respects, the two processes are essentially alike.

The raw material is heated to about 2,700 degrees F in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter ­ large enough to accommodate an automobile and longer in many instances than the height of a 40-story building. Kilns are mounted with the axis inclined slightly from the horizontal. The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil or gas under forced draft.

As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance with new physical and chemical characteristics. The new substance, called clinker, is formed in pieces about the size of marbles.

Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency.
The cement shall be stored in such a manner as to permit easy access for proper inspection and identification of each shipment, and in a suitable weather-tight building that will protect the cement from dampness and minimize warehouse set.