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 Aspin in 1824, an British bricklayer.

There are three fundamental stages in the production of Portland cement:

1. Preparation of the raw mixture
2. Production of the clinker
3. Preparation of the 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 1480°C. 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 (3CaO·SiO₂). Small amounts of tricalcium aluminate (3CaO·Al₂O₃) and tetracalcium aluminoferrite are also formed. The resulting material is clinker, and can be stored indefinitely; it is not affected by exposure to water.

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.

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 and tricalcium aluminate (3CaO·Al₂O₃), present in the cement, accompanied by the separation of gelatinous hydrated product. The later hardening and the development of cohesive strength is due to the reaction of water and tricalcium silicate (3CaO·SiO₂). In each case the gelatinous hydration product surrounds and cements together the individual grains. The hydration of dicalcium silicate (2CaO·SiO₂) proceeds more slowly than that of the above compounds. 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 set off heat.

The finished cement has 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).

When used in concrete, sand and gravel are added as aggregate. In mortar, only sand is added. After hardening, the aggregate particles are surrounded and held together by the cement.