how is cement made?

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modern cement is made by heating a crushed mixture of limestone and clay or slag-a by product of metal refining-to a very high temperature...this mixture is heated in a huge furnace until large,glossy winders called clinkers are formed....the clnkers are then ground into a powdered...when water is added to the powder,a complicated chemical reaction takes place....the result is a durable artificail stone that will not dissolve into water...in fact,this type of cement will actuaaly harden under water....for this reason it is called hydraulic cement,from the greek work hydro,meaning water....before hydraulic cement was discovered,a kind of cement was made from lime and water....lime is white,powdery substance that is made by heating limestone...plain lime cement hardened very slowly in the air...it would not harden at all underwater....it was not a strong as hydraulic cement...the romans discovered how to make a type of hydraulic cement in the second or third century b.c. by mixing volcanic ash with there lime...this dicovery is still ranked as one of romes greatest achievements....soon after the romans had learned how to make their cement,they used it to make concrete....many of their impressive structures were built of concrete...some of this are still standing today....it was not always possible to get volcanic ash or cement rock....a kind of limestone to make hydraulic cement....so people began to try to make it artificially....by the early 19th century several french and english builders had succeeded in this task....the best of these cement was made by an english brick layer named joseph aspdin....i 1824 aspdin inventede a cement that he called portland cement....he gave it this name because it looked like a kind of stone found on the aisle of portland...since the enf of the 19th century all building cement has been made artificially... cement manufacturers no longer to depend on supplies of natural cement rock or volcanic ash....by preparing their own mixtures,the manufacturers can make cements that are specially suited to almost any use....