Cement quite similar to the type in use today was originally developed in ancient Rome. This cement was so durable that many a Roman bridge, building, road, and box culvert constructed two millennia ago are still in existence. To make their cement they mixed volcanic ash with slaked lime (lime mixed with water). This type of cement hardened under water. When the Roman empire fell in the fifth century A.D. the knowledge of cement making died also. It wasn't until the mid-eighteenth century that a British engineer named John Smeaton rediscovered how to manufacture cement. The first great demand for cement in the U.S. was the building of the Erie Canal in the 1820's. An American engineer named Canvass White discovered rock in upstate New York which had the same hardening properties that volcanic ash served in Roman cement. Portland cement was invented in 1824 by an English bricklayer named Joseph Aspdin, who named it. This cement was manufactured by grinding, mixing, burning, and regrinding limestone and clay, which produced a superior cement than earlier types. The first Portland cement plant in America was founded in 1871.
Portland cement is manufactured of approximately 60% lime, 25% silica, and 5% alumina, with small amounts of gypsum and iron oxide added. The function of the gypsum is to regulate the hardening time. Lime can made from burned limestone, marl clay, or even oyster shells. Alumina and silica come from silica sand, shale, slate, clay, or slag from blast furnaces. The iron oxide comes from pyrite or iron ore. Many cement plants are conveniently located close to limestone quarries. In the plant the raw materials are stored in bins made of precast T panel and are processed by grinding and crushing, burning, and regrinding. The limestone as it comes from the quarry is crushed by machines which can take large rocks and smash them into small pieces; then the smaller pieces are broken by hammer mills into gravel. The gravel is mixed with the other raw materials and ground in mills into fine dust; or by a wet process using water to form a slurry. The product is then placed in a large cylindrical kiln constructed of steel lined with firebrick, and burned. Cement kilns are fired with powdered coal, gas, or oil and rotate at about one r.p.m. The materials are loaded into the top of the kiln and slowly slide down to the bottom in a period of about four hours at temperatures in excess of 3000 degrees Fahrenheit (1500 degrees Celsius), which transforms the materials into small chunks of clinker. The clinker is ground again together with gypsum to produce the Portland cement powder which is stored in silos constructed of agricultural T panel sections. It can be shipped either in bulk, or else packed in paper bags for shipping.
Originally plants producing Portland cement for manufacture into a precast T panel or an agricultural T panel used their own formulas. However in 1917 the National Bureau of Standards published a universal formula for producing Portland cement in the U.S. Air entrained concrete for manufacturing items such as a prefabricated box culvert, block, or brick was perfected early in the 1940's.
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