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.