Earliest manmade cement (called Hydraulic Cement)

There was a general decline in the quality & use of cement in Middle ages. Advancement in the knowledge of cements could really take place only in 18^th century. In mid-eighteenth century, an English engineer named John Smeaton needed a strong material to rebuild the Eddystone lighthouse (shown in the picture on your right ) off the coast of Devon. He needed a hydraulic mortar that would set and develop some strength in the twelve hour period between successive high tides.

Smeaton concluded after investigations that lime-stone which contained considerable proportion of clayey matter yielded better lime possessing superior hydraulic properties. Thus was born, in 1756, the earliest manmade cement, called hydraulic cement. This was made by calcining nodules of argillaceous lime-stones.

However, it can not be called as an entirely new discovery. In fact, it was well known to builders in the earliest historic times that certain limes would, when set, resist the action of water, i.e. were hydraulic. It was also known that this property could be conferred on ordinary lime by admixture of siliceous materials such as pozzolana.