CEMENTS 181 iut the Babylonians, not having such materials, covered their bricks with plaster, on which they made their designs. It was upon the " plaster of the wall " of Belshazzar's palace that the mystic hand traced the fatal letters. Under Nebuchadnezzar Babylon became the first city in the world, and mortars and ce- mts of various kinds, of a bituminous and rthy character, were used in enormous quan-
- ies in the construction of edifices and public
orks. The Greeks gave the subject much .telligent attention, as is evidenced by the .emical composition and state of preservation mortars and cements which have been found their ancient temples; and it is a matter history that in the earlier development of the architectural and engineering arts by the Eomans, the Greeks were often consulted by them. The Romans, however, attained to the greatest distinction for the magnitude and du- rability of their works. They prepared an ex- cellent cement for hydraulic purposes, which they used in making concrete with broken stones for the construction of various piers and harbors on the Mediterranean. They early be- came acquainted with the properties of poz- zuolana, which mixed with burned lime gave them a hydraulic cement that can scarcely be said to have been since excelled. The mole or breakwater of Pozzuoli is one of the monu- ments of the durability of their hydraulic struc- tures. It was composed of 24 arches, sustained upon piers, built of brick faced with stone, and held together with cement made of pozzuolana and lime. Thirteen of the piers are still above the water, although they were built more than 1,800 years ago. The arched construction was for the purpose of preventing a collection of sand behind the mole. Vitruvius, in his work De Ar- chitecture^ says : " There is found in the neigh- borhood of Baise, and the municipal lands lying at the foot of Vesuvius, a kind of powder which produces admirable effects ; when mixed with lime and small stones it has not only the advantage of giving great solidity to common buildings, but possesses the further property of forming masses of masonry which harden under water." Cements may be divided into those which are chemical and those which are mechanical, or into the stony and the resinous and glutinous. The stony cements may be again subdivided into those which harden on exposure to the air, such as common building mortar ; those which harden when immersed in water, as the hydraulic cements; and those which harden principally by combining with water, as gypsum and gypseous cements. Com- mon building mortar is made of lime and sand. The preparation of lime by calcination from limestone, chalk, marble, and other forms of carbonate of lime, is treated of in the article LIME. Many kinds of limestones contain car- bonate of magnesia as well as carbonate of lime, and are called dolomitic limestones. When the proportion of carbonate of magnesia is 46 per cent., the stone is called dolomite, and has been pronounced unfit for making building mortar. It does not become so hot in slaking nor set so soon as pure lime; but those who use it assert that in time it becomes harder. Some of the best lime in this country is made from stone obtained along the Hud- son river, much of which is almost pure dolo- mite. Before mixing with sand, the lime is slaked, a process that requires to be carefully performed in order to secure good results. Three volumes of quicklime moistened with one of water slakes with much violence and the evolution of heat, which often reaches 300 F. If enough water is used to cover the lime, it will be made to boil. Slaked lime is a hydrate of the protoxide of calcium, and is two and a half or three times the volume of the quicklime used. All the water used in slaking should be added at once. Less than is required to convert all the lime to a hydrate will cause a granular powder to be formed, which is of inferior value. After the slaking is completed the mass should be allowed to lie one or two days before it is made into mortar. In the south of Europe it is often put in boxes and kept for months, a practice which is thought to increase its power of conferring hydraulic energy to pozzuolanas. When used, the slaked lime should be made into a creamy paste by the addition of about an equal quantity of water. The quantity of sand to be added de- pends upon the use which is to be made of the mortar, and also upon the quality of the sand. In order to prevent shrinking, there should not be much more lime paste than is sufficient to fill the void spaces between the particles of sand. Coarse sharp sand will therefore take more lime than that which is fine, and will in time generally become harder. For bricklay- ing and the rough or foundation coat of wall plastering, the proportions vary from two to three parts of sand to one of lime paste. In the rough coat in wall plastering about one sixth part by measure of cows' hair is mixed, to aid in binding. Mortar may be mixed by hand, in which case a common hoe is the im- plement generally used. A mill, however, mixes the materials more thoroughly, and when a large quantity is required will repay the out- lay. A common pug mill, such as is used for grinding clay for brick, answers the purpose very well, or one having a pair of heavy rollers mov- ing round an axis in a circular pan. A form used in Europe is a conical mill, which empties into a trough or tube in which a spiral band wound round a shaft revolves, while it mixes the mortar and carries it to the end, when it is delivered into tubs. The setting and hardening of mortar are two distinct processes. Setting is caused partly from the adhesion that takes place between the particles of sand and lime, and which is no doubt increased by incipient chemical action, partly from the formation of more or less crystalline hydrate, and probably to some extent by evaporation. The hardening of common building mortar consists in the
