RSHING AND I :RINDING YIACIIINERY.1 Ex treme fineness of grinding is a prime essential of good Portland cement. many brands of which are ground so fine that from 90 to 96 per eent. of the powder will pass through a sieve ,,having 111.000 meshes per square inch. Portland cement sets slower than natural cement, and attains a greater final strength.
I'mn-olunie 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 property of hardening under water. Natural puzzolanie 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 used in Italy and some other parts of Europe. The most common ly employed artificial puzzolanic cement is made inn]] blast-funmee 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: rind sand to an extremely line powder and a very intimate mixture. Narki, it cement much used in India, is made of one port slaked lime and one half to one put of finely ground brick-dust.
When hydraulic cement powder is mixed with water to a plastic condition and allowed to stand. it gradually combines into a solid mass, and this process of combination is known as the st-t t fait of the cement. Cements of different char acters differ very widely in their rate and Inan ityr of setting: some occtlpy but a few minutes in the operation. while others require several hours; sonic 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. _After the completion of the setting of the cement. the mixture continues to Myren se in cohesive strength for a considerable period of time, and this subse quent development of strength is called the burdening of the cement. The properties of set
ting and hardening of cement by which a plastic paste is transformed into a hard stone are those upon which the value of cement as a structural material depends. Setting and hardening pro ceed under water as effectively as in air.
Previous to its use in structural work, cement is usually tested. The usual tests Made are for speeitie gravity. fineness of grinding.. rapidity 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 eommonly employed have 2500, 10.000. and 10,000 meshes per square inch. but the 10.000-mesh 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 when they will bear a needle one-twelfth inch in diameter. sustaining a weight of one-fourth of a pound. and noting this as the beginning of setting; then continuing the observations with a needle one-twenty-fourth In addition to the three principal classes of hyilraulie cements just described, there are sev eral other classes of minor importance. Mixed mitts 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. 4rnp pit ntn arc made by grinding to powder the grapplers left front the slaking and of hydraulic lime. :Mixed cements and grap piers are European products solely. el mint is the Mune given to the material formed by grinding together Portland cement inc.]] in diameter. earrying a weight of one pound until the material is sufficiently firm to bear this, when it may be called fully set.