If construction has a signature material, it's concrete -- the familiar stone-like substance that forms some part of nearly everything we build. And the key to concrete is portland cement- the fine gray powder that binds sand and gravel into concrete's rock-like mass.
The mining, production and uses of cement, clay and shale are quite similar. Cement is made from gypsum, shale or clay, and limestone. The term "cement" is most often used to refer to Portland cement. Portland cement, when mixed with sand, gravel, and water, makes concrete, which is an essential element of the construction industry. Portland cement accounts for more than 95% of all cement produced. To make Portland cement, clay, shale and limestone is ground to a powder and baked in a kiln. The baked mixture forms clods (clinkers), which are then ground up and mixed with gypsum. Most of the raw materials are mined in open pits. Michigan traditionally ranks in the five states in terms of cement production. One of the largest cement plant in the state is in Alpena. Making Portland cement requires lots of heavy raw materials and a tremendous amount of energy.
Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Portland cement owes both its name and origin to Joseph Aspdin, a British stone mason. Aspdin's quest for a manufactured counterpart to natural or Roman cement - a crude formulation of lime and volcanic ash used as early as 27 BC - led to his discovery and patent of portland cement in 1824. Aspdin heated a mixture of finely powdered limestone and clay in a small furnace to produce hydraulic cement - one that would harden with the addition of water. He named his invention "portland cement" not only to distinguish it from Roman cement, but also as a marketing tool: Concrete made from his new cement resembled a highly prized building stone quarried on the Isle of Portland off the British coast.
Today's portland cement still relies on Aspdin's raw materials for its basic components of calcium, silica, alumina, and iron. The most common combination is limestone, clay, and sand. In today's cement production, minerals from a quarry near the plant are ground to fine powder, then blended to the exact proportions needed for the final cement product. Raw materials are processed in a rotating, cylindrical furnace called a kiln. At the hottest part of the kiln, a 1870 C flame heats the raw materials to about 1480 C. Under this intense heat, a series of chemical reactions converts the partially molten raw material to pellets called clinker. With the addition of a small amount of gypsum, the clinker is ground to an extremely fine gray powder. It is now considered portland cement - ready for shipment to concrete producers and then to construction sites.
One of the largest Portland Cement plants in the US is located in Alpena, and is shown below. The advantages of placing such a plant in Alpena include: nearness to large supplies of high-quality limestone, gypsum and shale, and access to Lake Huron, thereby facilitating easy shipping of the cement by freighter.
Source: Photograph by Randy Schaetzl, Professor of Geography - Michigan
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