38. History

The American product called natural cement was first manufactured at Fayetteville, Onondaga County, N. Y., in 1818, and used in the construction of the Erie Canal. Other early dates of manufacture are given as 1823, near Rosendale, N. Y., and 1824 at Williamsville, Erie County, N. Y., the products being used in the construction of the Erie and the Delaware & Hudson Canals. Factories were soon started in other states, and at present nearly every State in the Union has one or more natural cement factories, the total annual production being now about nine million barrels.

39. Materials Required

The composition of rock from which natural cement may be made varies within wide limits. Natural cement is essentially a combination of acid constituents, silica, alumina, and iron oxide, with the basic constituents, lime and magnesia, but the proportions in which the several materials shall occur in the natural cement rock is not confined to the narrow limits we have seen were necessary in Portland cement manufacture. When argillaceous limestone is used the product is sometimes referred to as aluminous natural cement, its essential ingredient being a bisilicate of alumina and lime, while the product made from argillo-magnesian limestone is called magnesian cement, and is probably composed of a triple silicate of lime, magnesia, and alumina. The Maryland cements are typical of the former or aluminous variety, containing only one to five per cent, of magnesia, while the Rosendale and Milwaukee are magnesian cements containing 15 to 25 per cent, magnesia. (See Table 3, page 11).

Rocks suitable for the manufacture of natural cement are widely distributed throughout the United States. If this were not so the use of natural cement would be very much curtailed, for if it must be transported long distances it cannot successfully compete with Portland cement, since the Cost of transportation makes a much greater proportionate increase in the price of the product than is the case with Portland.

In the following table are given a number of analyses of raw cement rock quoted by Mr. Eckel in his report on Cement Materials and Industry of the United States.1

1 Bulletin No. 243, U. S. Geological Survey.

Analyses Of Natural Cement Rock

The materials found at any locality may vary considerably as to chemical composition, especially among the several strata. In some cases the different strata are utilized to make two or more brands, which differ somewhat in their characteristics as to time of setting, etc. It is common also to mix two or more layers together in the manufacture, with the idea that the ingredients lacking in one stratum will be supplied by the others.

40. Description Of Process

As the ingredients have been incorporated by nature in such proportions as to yield a natural cement, much of that portion of the process of Portland cement manufacture preliminary to the burning is unnecessary. The rock occurs in strata and is quarried in open cut where near the surface, or is mined in case the stripping is heavy. In open cut a face of 20 feet or more is sometimes worked. As has already been stated, the strata vary in chemical composition, and while two or more brands are sometimes made at the same mill, the rock from several strata are frequently mixed in the production of one brand, the idea being that if one layer has an excess of acid constituents it may be corrected by another containing too much lime or magnesia. As the rock is not finely pulverized before it enters the kiln, each lump burns by itself and makes a certain cement; the piece of cement next it must make as distinct a product as though burned in a separate kiln. What is obtained, then, by this method is a mixture of several cements, and it would seem that the mere mechanical mixing of an over-limed and an over-clayed cement can not produce a well-balanced product, but that whatever advantage may be gained by this process is due to the pozzo-lanic reaction which plays a part in the hydration of natural cements. Moreover, the fact that wide variations in composition must call for similar variations in the temperature of burning precludes the probability that the best results are obtained in this way. This practice undoubtedly accounts, in a great degree, for the large variations that occur in the cement from a single mill, variations which are often, however, more noticeable in short-time tests than in longer ones.

41. Burning

As the rock is quarried it is broken into pieces varying in size up to six inches. To secure uniformity in burning, the pieces should be approximately of the same size, and to this end the rock should be passed through the ordinary rock crusher. It is then conveyed to the kilns, usually by tramway.

The style of kiln almost universally used in burning natural cement is the cylindrical dome kiln, briefly described under Portland cement manufacture (§ 23). These kilns are from 20 to 50 feet high and 8 to 16 feet in diameter, built of masonry or steel lined with brick, and are operated as continuous kilns. The coal used for fuel is about pea size, either bituminous or anthracite being used, according to the locality. The rock and fuel are spread on the top of the kiln in alternate layers, the proportion of fuel being usually regulated by the man in charge, though a machine is sometimes employed to govern automatically the amount of coal used.

The temperature reached in the burning varies quite widely according to the character of the rock. Materials high in alumina, iron oxide, and silica, that is, with a low lime-acid ratio (§ 9), are most readily broken up. The carbon dioxide is first driven off, then takes place the combination of the lime and magnesia with alumina and iron oxide, and finally with the silica. Such rocks may be burned at a temperature of about 1000° to 1200° C, while those having a high lime-acid ratio may require 1200° to 1400°.