The natural Cement is obtained by burning and crushing the stones containing clay, carbonate of lime and some amount of carbonate of magnesia. The clay content in such stones is about 20 to 40 percent. The natural Cement is brown in colour and its best variety is known as the Roman Cement. The natural Cement resembles very closely eminent hydraulic lime. It sets very quickly after addition of water. It is not so strong as artificial Cement and hence it has limited use in practice.
It was in the eighteenth century that the most important advances in the development of Cement were made which finally led to the invention of Portland Cement. In 1756, John Smeaton showed that hydraulic lime, which can resist the action of water, can be obtained not only from hard limestone but from a limestone which contained substantial proportion of clay.