Why is it called Portland Cement, and how is it processed?
The invention of Portland cement is generally credited to Joseph Aspdin, an English mason. In 1824, he obtained a patent for his product, which he named Portland cement because it produced a concrete that resembled the color of the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel. The name has endured and is used throughout the world, with many manufacturers adding their own trade and brand names.
Although Aspdin was the first to prescribe a formula for Portland cement and the first to have his product patented, calcareous cements had been used for many centuries. Natural cements were manufactured in Rosendale, New York, in the middle 1800’s. The first recorded shipment of Portland cement to the United States was in 1868 and the first Portland cement made in the
United States was produced at a plant in Coplay, Pa., in 1871.
Selected raw materials are crushed, milled, and proportioned in such a way that the resulting mixture has the desired chemical composition. The raw materials are generally a mixture of calcareous (calcium oxide) material, such as limestone, chalk or shells, and an argillaceous (silica and alumina) material such as clay, shale, or blast-furnace slag. Either a dry or a wet process is used. In the dry process, grinding and blending are done with dry materials. In the wet process, the grinding and blending operations are done with the materials in a slurry form. In other respects, the dry and wet processes are very much alike. After blending, the
ground raw material is fed into the upper end of a kill. The raw mix passes through the kiln at a rate controlled by the slope and rotational speed of the kiln. Burning fuel is forced into the lower end of the kiln where temperatures of 2600 to 3000 degrees F change the raw material chemically into cement clinker approx ½” in diameter. The clinker is cooled and then pulverized.
During this operation a small amount of gypsum is added to regulate the setting time of the cement. The clinker is ground so fine that nearly all of it passes through a No. 200 mesh (75 micron) sieve with 40,000 openings per square inch. This extremely fine gray powder is Portland cement.