Portland cement
Portland cement is arguably the most important type of cement in general usage, as it is a basic ingredient of concrete and mortar. It consists of a mixture of oxides of calcium, silicon and aluminum. Portland cement and similar materials are made by heating limestone (as source of calcium) with clay or sand (as source of silicon) and grinding the product. The resulting powder, when mixed with water, will become a hydrated solid over time.Portland cement was first manufactured in Britain in the early part of the 19th century, and its name is derived from its similarity to Portland Stone, a type of building stone that was quarried near Portland, England. The patent for Portland cement was issued to Joseph Aspdin in 1824, a British bricklayer.
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2 Use 3 Safety |
There are three fundamental stages in the production of Portland cement:
Production
The chemistry of cement is very complex, so cement chemist notation was invented to simplify the formula of common molecules found in cement.
The raw materials for Portland cement production are a slurry of calcium oxide (44%), silicon oxide (14.5%), aluminum oxide (3.5%), ferric oxide (3%), and magnesium oxide (1.6%). The raw materials are usually quarried from local rock, which in some places is already practically the desired composition and in other places requires the addition of clay and limestone, as well as iron ore, bauxite or recycled materials.
The raw mixture is heated in a kiln, a gigantic slowly rotating and sloped cylinder, with temperatures increasing over the length of the cyclinder up to ~1480ÃÂðC. The temperature is regulated so that the product contains sintered but not fused lumps; too low a temperature causes insufficient sintering, but too high a temperature results in a molten mass or glass. In the lower temperature part of the kiln, calcium carbonate (limestone) turns into calcium oxide (lime) and carbon dioxide. In the high temperature part, calcium oxides and silicates react to form dicalcium and tricalcium silicates (C2S C3S). Small amounts of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF)are also formed. The resulting material is clinker, and can be stored for a number of years before use, prolonged exposure to water decreases the reactivity of cement produced from weathered clinker.
The energy required to produce clinker is ~1700 J/g, however because of heat loss during production actual values can be much higher. The high energy requirements and the release of significant amounts of carbon dioxide makes cement production a concern for global warming.
In order to achieve the desired setting qualities in the finished product, about 2% gypsum is added to the clinker and the mixture is pulverized very finely. This powder is now ready for use, and will react with the addition of water.
The finished cement has approximately the following composition: calcium oxide 64%, aluminum oxide 5.5%, silicon oxide 21%, ferric oxide 4.5%, magnesium oxide 2.4%, sulfate 1.6%, with a loss of ignition about 1% (mostly water).
The most common use for portland cement is the production of concrete. Concrete is a composite material consisting of aggregate, cement, and water. Concrete is an important construction material because it can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element.
When water is mixed with Portland cement, the product sets in a few hours and hardens over a period of weeks. The initial setting is caused by a reaction between the water, gypsum, and tricalcium aluminate (C3A), forming the crystaline hydration products calcium-alumino-hydrate (CAH), ettringite (Aft), and monosulfate (Afm). The later hardening and the development of cohesive strength is due to the reaction of water and tricalcium silicate (C3S), forming an amorphous hydrated product called calcium-silicate-hydrate(CSH gel). In each case the hydration products surround and cement together the individual grains. The hydration of dicalcium silicate (C2S) proceeds more slowly than that of the above compounds slowly increasing later-age strength. The ultimate cementing agent is probably gelatinous silica (SiO2), and it is thought by some that the value of the aluminate lies in its action as a flux in the burning of the clinker. All three reactions mentioned above release heat.
When cement is mixed with water a highly alkaline solution (pH ~13) is produced by the dissolution of calcium, sodium and potasium hydroxides. Gloves should be worn to protect the skin. Hands should be washed after contact.
In Scandinavia, chrome hexavalent, which is a major skin irritant, shouldn't exceed 2 ppm (mg/kg). The total chrome level may be higher.Use
Safety