Basically, cement is produced in two steps: in the first step cement clinker is produced from raw materials. In the second step cement is produced from the cement clinker. The first step can be dry, wet, semi-dry or semi-wet process, according to the state of the raw materials. The raw materials are crushed, and homogenized into a paste which is fed into the kiln. This is a kind of enormous rotating pipe, 60 to 90 m long and up to 6 m of diameter. This 'oven’ is heated by a 200 ℃ flame to a temperature which can reach 1400)1600℃. The kiln is slightly inclined to allow materials to slowly reach the other end, where it is quickly cooled to 100)200℃. The final product of this phase is called 'clinker'. The second phase takes place in a cement grinding mill, which may be located in a different place from the clinker plant. Gypsum (calcium sulphates) and possibly additional raw materials are added to the clinker. All elements are ground (crushed) to lead to a fine and homogenous powder. The finished cement is then stored in silos before being delivered to users.
Chromium in the cement can originate from:
1) raw materials or fuel,
2) magnesia)chrome kiln refractory brick, if used,
3) wear metal from raw mill grinding process, if chromium alloys are used, and
4) additions such as gypsum, ground granulated blast furnace slag, mineral components, and cement kiln dust.
The cement process, specifically kiln conditions, can influence how much Cr (VI) will form. The source of chromium input in the kiln feed is primarily as Cr (III). The conditions in the kiln include high amount of CaO, free lime, and alkalis due to the internal circulation of volatiles. Such conditions are favorable for oxidation of chromium to Cr (VI), the amount of which will depend on the oxygen pressure in the kiln atmosphere. In the kiln, oxidizing atmosphere will play the largest role, with more oxygen in the burning zone leading to increased Cr (VI) formation. Alkali concentration is also of importance, since Cr (VI) in clinker is primarily in the form of chromates. In the finish mill, thermodynamically favorable conditions for oxidation to Cr (VI) exists, including high air sweep, moisture from gypsum dehydration, cooling water injection, and grinding aids, along with the high pH of the cement. Portland cement contains Cr (VI), so operations that create concrete dust may lead to worker exposures. In one operation, the use of water to suppress dust during cleanup was observed to result in visibly lower dust concentrations. Dermal exposure to Cr (VI) through skin contact with Portland cement or other Cr (VI) containing products may lead to sensitization or allergic contact dermatitis. This condition, while not life)threatening, is debilitating and marked by significant discomfort and long)lasting adverse effects; it can have adverse occupational and social consequences and should be considered a material impairment to the health of affected workers. Occupational Safety and Health Administration (OSHA's) Advisory Committee for Construction Safety and Health (ACCSH) advised OSHA to include construction cement work under the proposed standard because of the known hazards associated with wet cement and the large number of workers exposed to wet cement in construction work settings. In particular ACCSH advised OSHA that only certain provisions might be necessary for workers exposed to wet cement (e.g., protective work clothing, hygiene areas and practices, medical surveillance for signs and symptoms of adverse health effects only, communication of hazards and recordkeeping for medical surveillance and training). Exposures can occur from working with a variety of construction materials that contain Portland cement. These include: concrete, mortar, stucco, and terrazzo. Examples of construction trades with potential exposure to wet cement include: bricklayers, cement masons, concrete finishers, construction craft laborers, plasterers, terrazzo workers, and tile setters. At a minimum, containers of water should be available to exposed workers so that they may wash skin that has come in contact with Portland cement (either dry or wet).