Page:Popular Science Monthly Volume 60.djvu/155

as the 'Natural Cement Industry' but locally in the neighborhood of New York City as the Rosendale Cement Industry.

Continuing these experiments it was found that the rocks found in a few localities produced cements of a quality superior to those in general use. This was particularly true of a stone found in the Island of Portland. At length artificial mixtures of limestone and clay were made and burned under such conditions that the resulting cement very closely resembled the natural cement made from the Portland rock. These results led to the adoption of the name of Portland for all cements of this class whether made in England, where the name originated, or elsewhere. The first attempts to prepare a cement by artificial mixtures, in imitation of the natural Portland rock, were made in France about 1802.

Portland cements, as at present manufactured, were first made in England by a process that was patented in 1824, although there had been several patents for 'Portland Cements' previously issued. In the patent specifications of 1824 occurs the following description of the process used:

I take a specific quantity of limestone and calcine it. I then take a specific quantity of clay and mix it with water to a state approaching impalpability. After this proceeding I put the above mixture into a slip-pan for evaporation till the water is entirely evaporated. Then I break the said mixture into suitable lumps and calcine them in a furnace similar to a lime-kiln until the carbonic acid is entirely expelled. The mixture, so calcined, is to be ground to a line powder, and it is then in a fit state for cementing. The powder is to be mixed with a sufficient quantity of water to bring it into the consistency of mortar, and this applied to the purposes wanted.

It will thus be seen that at the middle of the last century there were in use: Common lime mortar, consisting of slacked lime and sand; made all over the world and used for common masonry and plastering.

Also Roman cement, made by mixing common lime and some dry aluminous material, like pulverized tufa, brick or slag. This was stronger than common mortar and slightly hydraulic.

Also, natural cement, called Rosendale cement in the United States, made by burning and grinding a natural limestone containing clay. These natural cements are of very varying quality and are hydraulic, i. e., will set or harden under water.

Also, Portland cements, called also Artificial cements, made by grinding limestone or marl, both of which are nearly pure carbonate of lime, and mixing it in proper proportions with ground clay, which is a silicate of alumina containing a variable proportion of the oxides of iron. This mixture is calcined at a temperature that will produce semifusion and the resulting clinker is ground to a fine powder. The powder is 'aged' in order to partially slack the lime. The powder is mixed with