Page:The New International Encyclopædia 1st ed. v. 04.djvu/479

* CEMENT. 409 CEMENT. In tlio senii-wot process only enoujili wntor is iulileil to ii'ihK'e the mixture to a plastic condi- tion in the pii;; mill. When hanl mat<'rials incapable of dissolution hy water are emploved. they are ground dry in grinding mill?, the powder being then either slightly moistened and made into bricks, or, where the rotary kiln is used, stored in powdered form. In the wet and scnii wet processes the bricks or blocks are dried and then burned in kilns, of which there arc several varieties. (See Kll..s.) In the dry process, drying preliminary to burning is usually unnecessary. In the I'nit- cd States, where the rotary kiln is used, the dry powder or the wet paste is run directly into the kiln without previous brick-making or dry- ing. The burning or calcination is continued until incipient vitrifaction of the raw mixture occurs, the resulting clinker being dark-green or black in color. The clinker is erushed anil then ground to an impalpable powder, when, after a ]ieriod of curing, it is ready for use. (See C'Risiiixn AXD Grindino JIaciiinerv. ) Ex- treme fineness of grinding is a prime essential of good Portland cement, many brands of which are ground so fine that from 00 to !)G per cent, of the ])owder will pass through a sieve ^having 10,000 meshes per square inch, Portland cement sets slower than natural cement, and attains a greater final strength. I'iiZ7oIanic cement, or puzzolana, is a term applied to a combination of silica and alumina, which, when mixed with common lime and made into mortar, has the ])roperty of hardening under water. Natural puzzolanic cements have been used in Italy from very early times, and are made by grinding certain volcanic tufas and mixing the powder with slaked lime. These cements are still made and useTi in Italy and some other parts of Europe. The most common- ly employed artificial puzzolanic cement is made from blast-furnace slag and lime. The hot slag, as it comes from the furnace, is run into cold water, and becomes granulated. It is ground to fine powder, and then this powder is ground with lime. The chemical composition of three principal classes of cement is about as follows: and sand to an extreinely fine powder and a very intimate mixture. Siirl.i. a <'emi'nt nnieli used in India, is nuide of one ]iart slaked lime and one- half to one ])arl of finely ground brick-dust. When hydraulic cement powder is mixed with water to a plastic i-ondition and allowed to stiind, it gradually combines into a solid mass, and this process of eombinatitm is known as the sittiiifi of the cement. Cements of different char- acters didcr very widely in their rate and man- ner of setting: some occupy but a few minutes in the operation, while others require several hours: some begin setting immediately, and take considerable time to complete the set, while others stand for a considerable time with no ap- parent action, and then set very quickly, .fter the completion of the setting of the cement, the mixture continues to increase in cohesive strength for a considerable period of time, and this subse- quent development of strength is called the hnrdenhuj of the cement. The properties of set- ting and hardening of cement by which a plastic ])aste is transformed into a hard stone are those upon which the value of cement as a structviral material depends. Setting and hardening pro- ceed under water as elfectively as in air. Previous to its use in structural work, cement is usually tested. The usual tests made are for specific gravity, fineness of grinding, rajjidity" of setting, tensile strength, and soundness. Other tests sometimes made are for compressive and transverse strength, adhesive strength, re- sistance to abrasion, permeability to water, and chemical composition. The test for fineness of grinding consists in determining the proportion of the cement powder which will pass standard sieves. The sieves commonly employed have 2500, 10.000. and 40.000 meshe"s per square inch, but the 40,000-niesh sieve is not frequently used in making ordinary working tests. The standard test for rate of setting consists in making cakes of neat cement about two or three inches in diameter and one-half inch thick to a stiff paste, observing the time wlien they will bear a needle one-twelfth inch in diameter, sustaining a weight of one-fourth of a povuid, aiid noting this as the beginning of .setting: then continuing the observations with a needle one-twentv-fourth ELE,ME.T silica per cent. Ahiniina, •' •'. Iron oxide, *' " . Lime, *' ". Majrnesia, " ". {Sulphuric acid. " '•, Portland 20.3 to 26.1 6.2 to 10.6 2.1 to 6.3 6S. to 67. 0.33 to 2.3 0.26 to 1.78 Natural 21.1 to 35.4 6.3 to U.5 1.7 to 6.1 32.3 to 63.5 l.U to 20.9 Slag 18. to S8. 9. to 19. 0.33 to 2.8 36. to 55. 0.6 to 7. 0.18 to 2.7 In addition to the three principal classes of hydraulic cements just described, there are sev- eral other classes of minor importance. Mixed cements include a considerable number of ce- ments which are formed of admixtures of dif- ferent grades of other cement ; of the overburned or underburned portions of the clinker, or of foreign material added to the cement, dnip- lilern ccmeitts are made by grinding to powder the grappiers left from the slaking and Ixdting of hydraulic lime, Mixed cements and grap- piers cements are European products solely. Kniiil cement is the name given to the material formed by grinding together Portland cement inch in diameter, carrying a weight of one pound initil the material is sufficiently firm to bear tliis, when it may be called fully set. Tests for tensile strength are made by break- ing test pieces, briquettes, whose smallest sec- tion is one inch square, in special tensile testing machines. The cement j)aste or mortar is made into l)ri(|uettes in small molds, and is allowed to remain in the molds until set; the briquettes are then removed and kept in a moist atmoe- phere for 24 hours, and then innnersed in water, where they remain until tested. Tests are usu- ally made when the briquettes are 7 and 28 days old, and they consist in pulling the briquette