Page:The New International Encyclopædia 1st ed. v. 04.djvu/478

* CEMENT. 408 CEMENT. ties, is eniployod for uniling metals, stone, glass, wood, or other materials. Solder, gums. l)Utty, imieilage. glue, plaster of Paris, limes, and hy- draulic cements are all comprehended in this defmitioTi. The most importiint da-ss of cements, structurally and commercially, is that com- prising lime, hydraulic lime, and hydraulic cement. The cements of this class will be de- scribed first. Lime. Lime, common lime, quicklime, or caus- tic lime, as it is variously named, is produced Hy burning limestone in kilns until the carbonic acid has been driven off. The clinker resulting from this burning ])Ossesses the property of dis- integration or slaking upon being treated with a sufficient quantity of water. The slaking of lime is due to its rapid hydration when in con- tact with water, and the process is accom]ianied by a material increase in volume and a con- siderable evolution of heat. If the quantity of water be just sufficient to cause the hydration of the lime, it is reduced to a dry powder, while if the water l)e in excess it becimies a paste. The slaked lime thus formed, when mixed to a paste with water luid allowed to stand in the air, has the pro|X'rty of hardening and firmly adhering to any surface with which it may be in contact. This hardemng of common limes will take place only ill air. When lime is very pure and its activity is very great, it is known as fat lime; if the lime contains, either mixed with it or in combination, considerable amoinil.s of inert im- pirities which lessen the activity of the lime, lause a partial loss of the property of slaking, and diminish its power of hardening, it is known as meagre lime.- The conunon meth- od of slaking lime consists in covering it with from two to three times its volume of water, and allowing it to stand until all the lumps are reduced and the mixture is in the con- dition of a thick paste. For use in c'onstruc- tion, this paste is mixed with from two to three times its volume of sand, when it is called lime mortar. The process of hardening of lime mor- tar consists in the gradual formation of car- bonate of lime through the absorption of car- bonic acid from the air, ac<<impanied by the crystallization of the mass of hydrated lime as it gradually dries out. The hardening process is a slow one at best, and the lime mortar used in the interior of thick masses of masonry, where the air canniit get at it, will take 3'ears to become hard. Lime mortar should be used only in niasonn' exposed to the air. See Bitiluixg. HYnR.ii.ic Lime. Hydraulic lime is obtained by burning limestone containing enough silica and alumina to impart to it the ability to hard- en under water. In calcination the silica and alumina combine with a portion of the lime to form silicates and aluminates of lime, leaving the remainder of the lime as free lime in an luicombined state. 'hen treated with water the free lime is shaked. The manufacture of hydraulic lime is practically confined to I'lurope. and consists, after the quarrving of the rock, of burning, slaking, and bolting the material. The burning is accomplished in kilns, and is a proc- ess requiring considerable skill and careful at- tention. To slake the clinker it is spread in layers from four inches to eight inches deep and sprinkled with water. The object is to slake the free lime without hvdrating the silicates and aluminates. .After the lime has been reduced to powder by slaking, it is passed through sieves and packed for shipment. Hydraulic lime is used in the same manner as common lime, being mixed with wat*"r and sand to a paste. When in the air hydraulic lime acts like common lime, slow- ly absorbing carbonic acid, di-ying. and harden- ing. In water the action of hydraulic lime is altogether dill'erent from that of conunon lime, since, owing to the presenc<' of the silicates and aluminates of lime, the hydraulic lime hardens under water, while the common lime does not. Hyor.mlic Cement. Hydraulic cements are classified as natiiral cements, Portland cements, and puzzolanic cements, yatiiial cement is the product obtained by calcining at a low tempera- ture a natural limestone without pulverization or admixture of other materials, and finely grinding the clinker. In Kurope these cements are called Roman cements, and they were first manufactureing 2.000.000 l)arrels and 8,800.000 barrels annually. The principal centres of natural-ce- ment manufacture in the United Stat<'s are: Ulster County. N. Y. : Cuml)erland. Md. : Louis- ville, Ky. : Milwaukee, Wis.; and Utica, 111. The rock employed is an argillaceous limestone. The process of manufacture consists in mining and quarrying this limestone; breaking it into lumps about the size of one's hand ; calcining these lumps with coal in kilns, and finally crushing and grinding the clinker. Natural cements are characterized l)y a very rapid set and slowness in gaining strength subsecpiently ; they have less strength than Portland cement. I'ortUinil cement is the product obtained by calcining to incipient vitrifaction an intimate artificial admixture of two or more raw mate- rials, and finally grinding the clinker. Chem- ically Portland cement is a combination consist- ing principally of silicates and aluminates of lime, and the raw materials must necessarily contain silica, alumina, and lime. Within these limitations, a great variety of raw materials are capable of being utilized for cement making; in England chalk and cl;iy are used iirinci))ally ; in Germany and France marl and clay and lime- stone and slate are employed. In the United States marl and day, limestone and day. and argillaceous limestone of different compositions are ised. The first process in the manufacture of Portland cement is the mixing of the r;iw ma- terials. This mixture iiuist be uniform and homogeneous, and the respective ingredients jiropeily proportioned, which requires that they 1h' rc<hiced to a fine powder. The method of re- duction practiced dein-nds up(m the character of the raw nuiterials; when soluble in water they are usually reduced by one of the wet processes. The wet process ])roper, formerly extensively used in England, consists in applying an excess of water to the clay and chalk, mixing them in a sort of pug mill to a thin paste, which is run into settling basins, where the water is decanted off as the solid matter settles until the mixture is dry enough to lie cut into blocks or bricks.