Hydraulic cements are made by calcining limestones containing from 30 to 60 per cent, of clay.

They do not slake or break up like lime, and their paste sets very quickly, either in air or water.

They may be divided into two classes:

1. Natural cements.

2. Artificial cements.

110. Natural Cements are made from a natural rock, of which the principal ingredients are carbonate of lime, carbonate of magnesia and clay. The stone, after being quarried, is broken into pieces of a suitable size and mixed with anthracite coal and burned in kilns specially constructed for the purpose. Great care is required in selecting and preparing the stone for the kiln and in burning it to a consistent degree of calcination.

After calcining the material is drawn out of the kilns and carefully inspected. That which is properly burned is sent to the mill to be finely ground between ordinary millstones, and the underburned or over-calcined thrown away.

Natural cements weigh about two-thirds as much as Portland cement, are very quick setting and have less ultimate strength. They attain their full strength, however, sooner than the Portland cements, and are sufficiently strong for all ordinary building operations.

They have been used in many of the largest building and engineering works in this country with perfectly satisfactory results, and their use is extending every year.

They are sold at a less price than Portland cements, and in localities where the cost of transportation is not excessive are almost exclusively used for cement mortar.

III. Distribution of Natural Cements. - "In no other country in the world is there to be found cement rock formations which are at all to be compared with those so well distributed throughout the United States. . . . Here we have immense beds of cement rock absolutely free from any extraneous substances, perfectly pure and clean, with layer upon layer, extending for thousands of feet without appreciable variation in the proportion of the ingredients."*

Natural cements are manufactured in very many localities throughout this country, the cement being commonly known by the name of the place from which the stone is obtained, although, as there are often several manufactories in the same locality, there may be several brands of cement made from the same rock. The difference in the quality of such brands is generally due to the care exercised in their manufacture.

The localities in which natural cements are made on an extensive scale are as follows:

Rosendale, N. Y. - Natural cement was first made in this country in the town of Rosendale, Ulster County, N. Y., during the year 1823, for use in building the Delaware and Hudson Canal. Since then inexhaustible deposits have been found of the fine-grained natural stone out of which Rosendale cement is made, and there are several companies which manufacture cement from this rock, each having a special brand for their product. Owing to the length of time for which they have been used, and the special advantages enjoyed for transportation and nearness to the great building centres of the country, Rosendale cement is more widely known than any other of the natural cements. It is generally of a very good quality and well suited for building operations.

Very good natural rock cements are also made at Buffalo, Akron and Howe's Cave, N. Y.

Louisville, Ky. - Louisville cement, made from natural rock quarried at this place, is probably the leading natural cement beyond the Alleghenies, the product being exceeded only by the production from the Rosendale district. There are several brands of this cement in the market, and they find their way as far west as the Rocky Mountains.

At Utica, Ill., a natural cement has been manufactured since 1838. This cement has always stood well in public favor, and is largely used throughout the West.

At La Salle, Ill., a natural cement is manufactured from the same rock formation as that running through Utica, Ill.

The Milwaukee Cement Co. manufactures a natural cement from rock obtained near Milwaukee, Wis., which is extensively used.

Mankato, Minn. - A cement rock of the very best quality exists at this place, and the manufactured product has obtained a strong foothold in the markets of the Northwest.

Cement, Ga. - The cement manufactured from stone quarried at this place "probably has no superior in this country. Used as an exterior plaster on a house in Charleston in 1852, the stucco still remains unimpaired, while the sandstone lintels over the windows have long since been worn away."

Fort Scott, Kan. - A natural cement has been manufactured at this place since 1867. The product resembles that of Cement, Ga.

* Uriah Cummings in the Brickbuilder.

Natural cements are also manufactured at Siegfried's Bridge, Lehigh Valley, Pa.; Balcony Falls, Va., and Cumberland, Md., and to a limited extent at several localities in the West.

[An extended description of the natural cements manufactured in this country is given in a series of articles by Uriah Cummings in the Brickbuilder for 1895.]