"A cement is any compound which, under certain conditions, is plastic, and under others develops tenacity, and can be used for holding together various materials." By far the most important, structurally and commercially, are the hydraulic cements, compounds of lime, silica, and alumina that have the property, when mixed with water to a paste, of cohering or setting, and finally becoming stone hard, even under water. There are four general classes of structural cement: ( 1) hydraulic lime; (2) hydraulic or natural rock cement; (3) Portland cement; (4) Puzzolan or slag cement. 15 p.