civil engineering-->>PORTLAND CEMENT

PORTLAND CEMENT-->>

Portland cement is the chief ingredient in cement paste - the binding agent in portland cement concrete (PCC). It is a hydraulic cement that, when combined with water, hardens into a solid mass. Interspersed in an aggregate matrix it forms PCC. As a material, portland cement has been used for well over 175 years and, from an empirical perspective, its behavior is well-understood. Chemically, however, portland cement is a complex substance whose mechanisms and interactions have yet to be fully defined. ASTM C 125 and the Portland Cement Association (PCA) provide the following precise definitions:

hydraulic cement	An inorganic material or a mixture of inorganic materials that sets and develops strength by chemical reaction with water by formation of hydrates and is capable of doing so under watER
portland cement	A hydraulic cement composed primarily of hydraulic calcium silicates.

Manufacturing

Although there are several variations of commercially manufactured portland cement, they each share many of the same basic raw materials and chemical components. The chief chemical components of portland cement are calcium, silica, alumina and iron. Calcium is derived from limestone, marl or chalk, while silica, alumina and iron come from the sands, clays and iron ore sources. Other raw materials may include shale, shells and industrial byproducts such as mill scale (Ash Grove Cement Company, 2000).

The basic manufacturing process heats these materials in a kiln to about 1400 to 1600�C (2600 - 3000�F) - the temperature range in which the two materials interact chemically to form calcium silicates (Mindess and Young, 1981). This heated substance, called "clinker" is usually in the form of small gray-black pellets about 12.5 mm (0.5 inches) in diameter. Clinker is then cooled and pulverized into a fine powder that almost completely passes through a 0.075 mm (No. 200) sieve and fortified with a small amount of gypsum. The result is portland cement. The Portland Cement Association (PCA) has an excellent interactive illustration of this process on their website.

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