OLA88IIWWATION OF LIMN AND OEniENT. Considered as ma terials for use in the builders' art, the products of calcination of limestone are classified as common lime, hydraulic lime, and hydraulic cement. If the limestone is nearly pure carbonate of lime, the prod uct is common lime, which will slake upon the addition of water, and mortar made of it will harden by absorbing carbonic acid from the air, but will not harden under water. If the limestone contains from 10 to 20 per cent of clay (silica and alumina) the product is hydraulic lime, which will slake upon the addition of water, and mortar made of it will harden either in air or under water by the chemical action between the hydraulic lime and the water used in making the mortar. If the limestone contains from 20 to 23 per cent of clay, the product is hydraulic cement, which will not slake upon the addition of water but must be reduced to a paste by grinding, and which will set either in air or under water by the chemical action between the cement and the water used in making the mortar.
Common lime is sometimes called air-lime, because a paste or mortar made from it requires exposure to the air to enable it to "set," or harden. The hydraulic limes and cements are called water limes and water-cements, from their property of hardening under water. Notice that common lime differs from hydraulic lime and hydraulic cement in that common lime will not set or harden under water, while both hydraulic lime and hydraulic cement will set under water. Common lime and hydraulic lime differ from hydraulic cement in that the two former as they come from the kiln will absorb water and crumble to a powder, and if more water is added will slake or reduce to a paste; while cement after being burned is practically unaffected by water until it is first reduced to a fine powder by grinding.
Lime and cement are important materials of engineering construction since they are the only substances that are used to bind together bricks and blocks of stone to form masonry structures.