Cement

Hydraulic cements are much used in building permanent fortifications. The cement used by the Romans in their great sea-walls, aqueducts, etc., which are still standing as monuments of their civil engineering, was pozzuolana , a volcanic earth from near Baiæ , Italy. It is still an article of export from Italy. The most noted modern cement is Portland, made artificially in England by burning a mixture of the chalk and clay from the valley of the Medway.