Portland Cement

Manufacturing Processes.

Portland cement is obtained by first grinding and mixing to gether, in carefully determined proportions, the principal raw ingredients, then heating the mix ture to a temperature at which melting begins (2,000° to 4,000° F.), and finally grinding the resulting cement clinker to an impalpable pow der, a small percentage (not over three per cent) of gypsum or other material being subsequently added in some instances, to retard the setting or otherwise regulate the activity of the product.

The degree of fineness to which the clinker is ground has an important bearing on the strength of the cement. The finer the cement, the higher the test it will stand, both in tension and in compression. The extreme degree of fineness considered necessary for the best results may be seen from the fact that in the case of one of the leading American brands of Portland cement, the manufacturers guarantee that 95 per cent of their product will pass through a sieve having 10,000 meshes to the square inch, and 80 per cent through a sieve having 40,000 meshes to the square inch.

In addition to the characteristic properties developed under test, as explained later, a feature that distinguishes Portland from all other cements is the accurate and wholly arti ficial proportioning and mixing of its raw ingre dients. All the Portland cement mills have chemists in charge of laboratories, who deter mine the proportions of the materials that are being used from day to day, and thereby main tain a standard of mixture as nearly perfect as possible.

Another distinguishing feature of Portland cement is the intense heat necessary in calcining the pulverized raw materials, as compared with the temperatures which are found sufficient in the manufacture of the so-called "natural" cements.

In the mixing of the materials for Portland cement, two processes are used—the wet and the dry—depending on the amount of water pres ent. Where marl is the calcareous ingredient, the "wet" mix is almost invariably employed. In either case, the materials are first crushed and mixed, and then finely ground, whereupon they are ready to be passed to the calcining kilns.

The Rotary Kiln.

In the early days of the Portland cement industry, a simple, vertical kiln like those used for lime burning was used. This type of kiln, however, while fairly efficient as to fuel cost, required too much labor, and the output was small. The result was the develop ment of the rotary or revolving kiln, which is the type of kiln universally used at the present day.

The rotary kiln was evolved only after much experiment. As at first perfected, it was ap

plicable only to "dry" mixtures, and fuel oil and gas were the only fuels that could be used. Rotary kilns are now, however, used for cal cining both "wet" and "dry" mixtures; and in many cases powdered coal, blown in by forced air-blast, is employed as fuel. In the making of "white" Portlands, an oil or gas flame is still used, in order to avoid staining the product.

The rotary kiln consists of a steel cylindrical tube, five to eight feet in diameter. It is lined, except at its upper end, with a highly resistant firebrick, to withstand both the high temper atures generated and the destructive action of the molten clinker. Its length, for "dry" mate rials, is 60 to 150 feet; and for "wet," 80 feet or longer. In its mounting, the tube is set with one end higher than the other, so as to give it a gentle incline—about one-half inch to the foot. It is mounted on geared bearings which revolve it slowly on its axis. The fire is applied at the lower end. The mixture of the raw materials is fed in at the upper end; is heated to the melting point (2,000° to 4,000° F.) as it passes slowly down the tube; and is discharged as cement clinker at the lower end, being then ready to pass to the final grinding.

It was the development of the rotary kiln that rendered possible the very great increase in the annual production of Portland cement that has been recorded since the early nineties; and to it, also, in large measure, may be attrib uted that marked improvement in the quality of the product which has compelled the world to acknowledge that the leading brands of American-made Portland cement, in point of quality, are superior to all others manufactured at the present day.

While the fundamental processes in the man ufacture of Portland cement are essentially the same for the various brands, there may be minor differences among the different plants in the details of machinery, methods of handling, etc., depending on the location and arrangement of the plant, the particular kind of raw material used, etc. "Atlas" cement, for example, is made from cement rock and limestone; "Berkshire Snow White," from white marble and clay; "Buckeye," from Maryland clay; "Chicago A A," from rock and clay; "Lehigh," from marl and limestone; "Universal," from limestone and granulated blast-furnace slag from the Bessemer furnaces of the Illinois Steel Com pany; "Vulcanite," from cement rock (Trenton limestone or argillaceous limestone) and car bonate of lime; etc.