The most useful of artificial cements is that known as Portland cement.
The first Portland cement was made by Joseph Aspdin, of Leeds, England, who obtained a patent on it, dated October 21, 1824. For making his cement he used powdered limestone and a certain quantity of clay, which he mixed together with water to a paste, then evaporated in pans. After evaporation the mixture was broken up into lumps, calcined at a high temperature and ground.
"Portland cement requires a homogeneous mixture containing in proper proportions carbonate of lime, alumina (clay), silica and iron. . . . This mixture must be subjected to a heat sufficiently high to produce a vitrified, dense and heavy clinker, and afterward ground to a fine powder."
The proper proportions of the above ingredients are rarely found in a natural stone, so that it is necessary to obtain the lime and alumina from separate sources and mix them in the proper proportions artificially.
At the present time the bulk of the English cement, and much of the German cement, is manufactured from chalk instead of the hard limestones. This chalk is mixed with clay in the proper proportions, before burning, in a large wash mill, and the slurry is then run off and dried, either by artificial means or sun evaporation. After drying the mixture is burned at a fixed temperature into a scoriaceous mass, resembling pumice stone, to which the name of "clinker" is applied. This "clinker" being dried, ground to powder and passed through sieves, furnishes the finished product.
The quality of the cement depends upon the quality of the raw materials, the proper proportion of the mixture, the degree to which it is burnt, the fineness to which it is ground, and constant and scientific supervision of all the details of manufacture.
The first American Portland cement was manufactured by Mr. David O. Saylor in the year 1874 at Coplay, Pa. Since that time several factories have been established in the United States, and in the year 1894 there were nineteen factories, which made about 700,000 barrels; this amount, however, being but 18 per cent, of what was imported. Most of the American Portland cement is manufactured in the neighborhood of Coplay, Pa., the largest factory being that of the Atlas Cement Co., where 1,800 barrels a day are now manufactured. "All the factories located in this region make cement under the dry process from an argillaceous limestone. There are several factories in New York State, along the Erie Canal, and in Ohio, where marl and clay or limestone and clay are used. Practically nine-tenths of the Portland cement manufactured in this country is made in the States of Pennsylvania, New York and Ohio. Other States where small quantities are manufactured are Texas, Colorado, Dakota, Oregon,. California and the Territory of Utah. There is plenty of raw material suitable for making the highest grade of Portland cement, in almost every State in the Union." *
Several of the American Portland cements have been shown by thousands of carefully conducted tests to be equal in quality to any of the imported cements, and they have been used with perfectly satisfactory results in many of the largest engineering works in this country, as well as many of our largest buildings. The Mississippi jetties were built with American Portland cement, and they have successfully withstood the most severe test to which cement concrete can be subjected.
"The induration, or ' setting,' of Portland cement consists in the formation of a real mineral of a crystalline rock species, analogous to natural zeolites."
Owing to the greater expense in manufacturing Portland cement, its market price is nearly three times that of the Rosendale cements, but where great strength is required, as in brick or stone piers, or for concrete footings, Portland cement should always be preferred to any of the natural cements.