Cement today, is by far the most useful of all the building materials. It is being used in all countries throughout the world as a binding material in the construction of buildings and civil engineering. It is a grey coloured powder. When mixed with water and allowed to settle for some time, it becomes as hard as stone. The principal minerals in cement are lime (which comes mainly from limestone), silica and alumina (which are provided by clay). The Romans discovered cement around 250 B.C. They made it by mixing slaked lime and sand with volcanic ash. As it was obtained from a place named Pozzuoli, it was called ‘pozzolana’. Up to the middle of the 18th century, it was in common use. In 1757, an improved version was developed by John Smeaton, a British engineer. It was obtained by heating a mixture of slaked lime and clay. It remained in use for many years. The most common kind of cement was made by an Englishman, Joseph Aspdin, in 1824. When set, its colour would become similar to that of portland stones. Hence, it was called ‘portland cement’. Portland cement is made by mixing one part of clay and three parts of limestone in a very hot oven. Both the raw materials are powdered with the help of a crushing machine and mixed together by a blending mill. This mixture is then sent to the hot oven called a ‘kiln’. It is heated to about 1500°C-1600°C to form a hard mass called ‘clinker’. When the clinker cools down, it is grounded into a fine powder with a mineral called gypsum in a grinder. About 3% of ‘gypsum’ is added to it to regulate the time that the cement takes to harden. The final product is known as the ‘portland cement’. In India, the Indian Cement Company was established at Porbunder, in 1914. Today, cement is being manufactured on a large scale to meet an increasing demand. As a result of its increasing demand, scientists have also been able to develop a special kind of cement from rice husk.
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