Page:Popular Science Monthly Volume 37.djvu/265

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HE cements now in the market are of two kinds: natural, made directly from stone; and artificial, commonly called Portland cement. The manufacture of the former consists simply in burning and grinding the cement stone, a magnesian limestone containing about fifteen per cent of silica and a little silicate of alumina. The burning drives off the small amount of combined water and all the carbon dioxide from the stone, leaving the lime and magnesia as oxides, while the grinding to a powder puts it into the best possible condition for mixing with sand and gravel, and moistening to form a mortar. Artificial cement consists of about sixty-two per cent of lime mixed with silica and silicate of alumina in nearly the same proportions as those found in the cement stone, and it is free from magnesia. This seems to be the whole difference in its constitution. In use, the artificial cement sets rapidly and attains maximum hardness in a comparatively short time; the natural cement hardens rather slowly and reaches its maximum hardness only after a long period of exposure to the air.

The increasing use of cement in modern construction, either alone or more commonly as mixed with sand and gravel, demands that the qualities of the different kinds, and the means of testing, both roughly and accurately, should be generally understood. The foundations of all important structures, in situations where they can not rest directly upon solid rock, owe their strength to cement. They are usually made of concrete, cement mixed with sand or gravel, and they are often strengthened by iron beams so as to bind the whole into one continuous mass. Tunnels under rivers, sewers, cable trenches, and all the numerous subways of our large cities, are either concrete or masonry laid in cement mortar. Their strength, again, is the strength of the cement used. And even the piers of most of the large bridges are now made in part or wholly of concrete. Oftentimes, even the walls of stone and brick buildings are rendered more secure by being laid up with mortar of which cement forms a large ingredient. Used for so many purposes, the necessity of uniform quality, and proper knowledge of the quality of the cement used, become plain.

Before examining the methods of testing now employed and comparing the results, the process of hardening needs to be comprehended. Some things are not yet quite clear in it, but it is certainly in the main a chemical process. Mixed with water, the lime and magnesia of the cement unite to form a hydrate, and it