Portland cement
It was developed from other types of hydraulic lime in England in the early 19th century by Joseph Aspdin, and is usually made from limestone.
Its most common use is in the production of concrete, a composite material consisting of aggregate (gravel and sand), cement, and water.
[5] In 1811 Edgar Dobbs of Southwark patented a cement of the kind invented 7 years later by the French engineer Louis Vicat.
In the 1840s William Aspdin, apparently accidentally, produced calcium silicates which are a middle step in the development of portland cement.
[8] In 1859, John Grant of the Metropolitan Board of Works, set out requirements for cement to be used in the London sewer project.
[3] The Hoffmann "endless" kiln which was said to give "perfect control over combustion" was tested in 1860 and shown to produce a superior grade of cement.
Clinkers make up more than 90% of the cement, along with a limited amount of calcium sulphate (CaSO4, which controls the set time), and up to 5% minor constituents (fillers) as allowed by various standards.
Clinkers are nodules (diameters, 0.2–1.0 inch [5.1–25.4 millimetres]) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature.
The key chemical reaction distinguishing portland cement from other hydraulic limes occurs at these high temperatures (>1,300 °C (2,370 °F)) as belite (Ca2SiO4) combines with calcium oxide (CaO) to form alite (Ca3SiO5).
The aluminium, iron, and magnesium oxides are present as a flux allowing the calcium silicates to form at a lower temperature,[15] and contribute little to the strength.
For special cements, such as low heat (LH) and sulphate resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3 CaO·Al2O3) formed.
The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface area.
Cement plants normally have sufficient silo space for one to 20 weeks of production, depending upon local demand cycles.
The cement is delivered to end users either in bags, or as bulk powder blown from a pressure vehicle into the customer's silo.
Concrete can be used in the construction of structural elements like panels, beams, and street furniture, or may be cast-in situ for superstructures like roads and dams.
Portland cement is also used in mortars (with sand and water only), for plasters and screeds, and in grouts (cement/water mixes squeezed into gaps to consolidate foundations, road-beds, etc.).
The typical compound compositions of this type are: 55% (C3S), 19% (C2S), 10% (C3A), 7% (C4AF), 2.8% MgO, 2.9% (SO3), 1.0% ignition loss, and 1.0% free CaO (utilizing cement chemist notation).
This type is used in concrete to be exposed to alkali soil and ground water sulphates which react with (C3A) causing disruptive expansion.
*Constituents that are permitted in portland-composite cements are artificial pozzolans (blast furnace slag (in fact a latent hydraulic binder), silica fume, and fly ashes), or natural pozzolans (siliceous or siliceous aluminous materials such as volcanic ash glasses, calcined clays and shale).
As Fe2O3 contributes to decrease the melting point of the clinker (normally 1450 °C), the white cement requires a higher sintering temperature (around 1600 °C).
Other metallic oxides such as Cr2O3 (green), MnO (pink), TiO2 (white), etc., in trace content, can also give colour tinges, so for a given project it is best to use cement from a single batch.
As a result, wet cement is strongly caustic and can easily cause severe skin burns if not promptly washed off with water.
Similarly, dry cement powder in contact with mucous membranes can cause severe eye or respiratory irritation.
[24] In Scandinavia, France, and the United Kingdom, the level of chromium(VI), which is considered to be toxic and a major skin irritant, may not exceed 2 parts per million (ppm).
[12] The powder can cause irritation or, with severe exposure, lung cancer, and can contain a number of hazardous components, including crystalline silica and hexavalent chromium.
Environmental concerns are the high energy consumption required to mine, manufacture, and transport the cement, and the related air pollution, including the release of the greenhouse gas carbon dioxide, dioxin,[citation needed] NOx, SO2, and particulates.
[26] The International Energy Agency has estimated that cement production will increase by between 12 and 23% by 2050 to meet the needs of the world's growing population.
[27] There are several ongoing researches targeting a suitable replacement of portland cement by supplementary cementitious materials.
[29]An independent research effort of AEA Technology to identify critical issues for the cement industry today concluded the most important environment, health and safety performance issues facing the cement industry are atmospheric releases (including greenhouse gas emissions, dioxin, NOx, SO2, and particulates), accidents, and worker exposure to dust.
[clarification needed] The thrust of innovation for the future is to reduce sources 1 and 2 by modification of the chemistry of cement, by the use of wastes, and by adopting more efficient processes.
