Cements

In ,strength, natural cement falls consider ably below Portland. Its hydraulic activity, on the other hand, is considerably greater; that is, it sets much more quickly than Portland cement —which may or may not be an advantage, de pending on the time available and whether a quick or slow setting is desirable. The setting of natural cement can be retarded, and thus regulated to some extent, by the addition of a small proportion of lime in mixing the mortar or concrete.

For use under sea water, a good quality of Rosendale cement seems better adapted than the ordinary Portland. The former is of con stant volume in sea water, probably due to the absence of free lime and to the presence of a larger amount of magnesia than can safely be used in Portland cement; while the latter has a tendency to expand or swell and disintegrate, probably because of the chemical action of the sulphates in the water upon the constituents of Portland cement, and the consequent formation of soluble salts.

Natural cement costs less than Portland, and for this reason is still used to a large extent in construction where no very great degree of tensile strength is required. It is well adapted, for example, for use in the interior of heavy masonry. The lower cost of natural cement as compared with Portland is explained by the fact that its raw material is wholly a natural product used just as it comes from the quarry, and in its manufacture a much lower temperature is re quired for burning than is needed for the making of Portland cement.

Natural cements will not carry so large a proportion of sand with good results as will Portland, and they require more water than the latter for proper hydration; they are more plas tic, however, and work more smoothly under the trowel or other tools.

The Western natural cements, such as "Louisville" and "Akron," weigh 265 lbs. to the barrel; but "Rosendale" from New York or Pennsylvania weighs 300 lbs. to the barrel. Natural cements are thus lighter than Port lands, which weigh about 380 lbs. per barrel; and they are usually sold in cloth bags, three bags to the barrel, instead of four bags, as is the case with Portland cements.

For many years, as already noted, natural cement enjoyed a practical monopoly of the con structive field in concrete work. In recent years, however, the natural cement industry has shown a remarkable decline. This is readily explained by improvements in American machinery and by skilled technical supervision of the processes of manufacture, which have resulted in the pro duction of absolutely reliable Portlands; by the greater adaptability of the latter to construc tions requiring the utmost possible strength; and by the gradual lowering that has been effected in the cost of Portland cement. Port

land cement is superseding natural almost all along the line. Statistics show that the manu facture and sale of natural cement, within the last five or six years, has fallen off from sixty to eighty per cent. In 1906, for example, of the total production of all kinds of cement, amount ing to over 50,000,000 barrels, valued at over $54,000,000, only 3,935,275 barrels, valued at $2,362,140, consisted of natural cement; and 481,224 barrels, valued at $412,921, were Poz zuolana. The Portland cement industry, on the other hand, has expanded by leaps and bounds; and, as before intimated, notwithstanding its already unprecedented development, the future holds out the promise of still greater attain ments whose possibilities can as yet be but dimly foretold.

Slag Cement or Pozzuolana The use of slag cement dates back to ancient times. The cement used by the Romans was made for the most part from a simple mixture of lime and volcanic lava. The lava first used for this purpose appears to have been obtained at Pozzuoli, a village lying near the base of the volcano Vesuvius. For this reason the name as it is very commonly spelled, Puzzolan cement—was first given to this form of cementing material, and has since been ex tended so as to include all forms of slag cement which are made without calcining.

In Italy and some other parts of Europe, Pozzuolana is still made from lava; but in America, furnace slag is used instead. This material is simply mixed mechanically and thor oughly with slaked lime. No burning is re quired, but the mixture is ground exceedingly fine.

In chemical composition, slag cement con sists of a combination of silica and alumina, mixed with hydrated lime. Like Portland cement, cement, and hydraulic lime, it has the property of hardening under water; but in hardness and strength it falls far below the standard of true Portland cement. It is of a pinkish hue in color, and of lower specific grav ity than Portland cement. It is not used to any great extent in building operations. It cannot be depended on in freezing weather, and is not well adapted for dry work which is to be left exposed above ground to the action of the atmosphere. Its principal use is for under ground work and in moist locations. Sea water appears to affect it less than it does Portland cement; and for that reason slag cement is sometimes used instead of Portland in cases where large masses of masonry or concrete are to be deposited under sea water, and where great strength is not the prime consideration.