The credit for introducing cement goes to an English mason, Joseph Aspdin, who prepared it in 1824 by heating a mixture of finely powdered clay and limestone. The product resembled Portland rock, a popular building stone of England. The name 'Portland Cement' was thus given to it. In India, its manufacture began in 1914 at Porbandar. We are now producing several million tons of this important building material. In good quality cement, the following ratios of constituents are maintained.
In the manufacture of cement, the basic raw materials, namely, clay (that provides Al2O3 and SiO2) and limestone (that provides lime) are mixed in appropriate amounts, pulverized, homogenized and fed into a rotary kiln (Fig) and heated to 1700 K.
The roasted product, thus obtained, is called cement clinker. It is mixed with gypsum ( 2-3% of its weight to regulates the setting ) and ground to a fine powder.
Fig. Manufacture of cement
When water is mixed with the cement, a gelatinous mass is obtained which soon sets to a hard mass. The reactions taking place are quite complicated but essentially the strength of the cement is due to the formation of three-dimensional cross links between - Si-O-Si-O- and -Si-O-Al-chains.