In general, any substance used to adhere to each of two materials which cannot themselves adhere, and therefore to effect a join. More usually, it refers to Portland cement, an artificial mineral substance used in building and engineering construction. This is made by heating clay and limestone in retorts to form a clinker which is then finely ground. The addition of water produces a soft manageable substance which sets hard, through the formation of hydrated silicates. It is commonly mixed with other mineral substances (sand, stone) to form various grades of concrete, or used as a mortar for joining brickwork.
In the most general sense of the word, cement is a binder, a substance which sets and hardens independently, and can bind other materials together. Most important cements are hydraulic cements, materials which set and harden after combining with water, as a result of chemical reactions with the mixing water and, after hardening, retain strength and stability even under water. (Note that "hydraulic cements" as commonly sold in U.S. hardware stores represent a different use of the term "hydraulic." "Hydraulic cement" is a fast setting cement formulated in such a way that it expands slightly on setting in contrast to common cement which does the opposite. As such,"hydraulic cement" is used for watertight patches of small cracks or holes in concrete.)
The most important use of cement is the production of mortar and concrete - the bonding of natural or artificial aggregates to form a strong building material which is durable in the face of normal environmental effects. By far the most common and most important hydraulic cement in modern construction is Portland cement, which is made primarily from limestone, certain clay minerals, and gypsum, in a high temperature process that drives off carbon dioxide and chemically combines the primary ingredients into new compounds.
The name "cement" goes back to the Romans who used the term "opus caementitium" to describe masonry which resembled concrete and was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick additives which where added to the burnt lime to obtain a hydraulic binder were later referred to as cementum, cimentum, cäment and cement.
Setting and hardening
Setting and hardening of hydraulic cements is caused by the formation of water containing compounds, forming as a result of reactions between cement components and water. The Greeks later improved on this recipe and finally the Romans produced a very durable hydraulic cement from pozzolanic ash and lime. They sometimes used broken clay pots and jars to make the cement stronger for important buildings.
The art of creating cement was lost for hundreds of years during the middle ages. Portland cement was patented in England by Joseph Aspdin in 1824. When William Aspdin, the son of Joseph Aspdin, started to produce Portland cement in 1843 in a newly established works at Rotherhithe near London it became apparent, especially during the construction of the Houses of Parliament in London, that this was far superior to "Roman cement", because a considerable proportion of the mix had been sintered during the sintering process.
Types of manufactured cements
Portland cement
Portland cement is the most common type of cement in general usage, as it is a basic ingredient of concrete, mortar and most non-speciality grout. The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregate (gravel and sand), cement, and water.
Portland cement clinker is a combination of calcium, aluminum, silicon and iron oxides in specific combinations, made from a raw material mix of limestone, chalk, and clay, or their natural blend, lime marl. These are certain calcium silicates and calcium aluminates which confer on the cement its characteristic features of setting in the presence of water.
Natural gypsum and/or anhydrite cover most of the demand for sulphate agents, which serve to adjust the working properties of the cements.
Apart from natural raw materials, also alternative raw materials can be utilised, such as lime sludge, used foundry sand and fly ash. The preconditions to be met by the material composition of an alternative raw material primarily depend on the raw material situation prevailing at a cement works, i.e.
Portland cement blends
These are often available as inter-ground mixtures from cement manufacturers, but similar formulations are often also mixed from the ground components at the concrete mixer.
Portland Blastfurnace Cement contains up to 70% ground granulated blast furnace slag, with the rest Portland clinker and a little gypsum. Used as an economic alternative to Portland sulfate-resisting and low-heat cements. Where good quality cheap flyash is available, this can be an economic alternative to ordinary Portland cement.
Portland Pozzolan Cement includes fly ash cement, since fly ash is a pozzolan, but also includes cements made from other natural or artificial pozzolans. Italy, Chile, Mexico, the Philippines) these cements are often the most common form in use. Addition of silica fume can yield exceptionally high strengths, and cements containing 5-20% silica fume are occasionally produced. However, silica fume is more usually added to Portland cement at the concrete mixer.
Masonry Cements are used for preparing bricklaying mortars and stuccos, and must not be used in concrete. Subtle variations of Masonry cement in the US are Plastic Cements and Stucco Cements.
Expansive Cements contain, in addition to Portland clinker, expansive clinkers (usually sulfoaluminate clinkers), and are designed to offset the effects of drying shrinkage that is normally encountered with hydraulic cements.
Non-Portland Hydraulic Cements
Pozzolan-lime cements. Mixtures of ground pozzolan and lime are the cements used by the Romans, and are to be found in Roman structures still standing (e.g. The hydration products that produce strength are essentially the same as those produced by Portland cement.
Slag-lime cements. Ground granulated blast furnace slag is not hydraulic on its own, but is “activated” by addition of alkalis, most economically using lime. They are similar to pozzolan lime cements in their properties. water-quenched, glassy slag) is effective as a cement component.
Supersulfated cements. These contain about 80% ground granulated blast furnace slag, 15% gypsum or anhydrite and a little Portland clinker or lime as an activator. They produce strength by formation of ettringite, with strength growth similar to a slow Portland cement.
Calcium aluminate cements are hydraulic cements made primarily from limestone and bauxite. The active ingredients are monocalcium aluminate CaAl2O4 (CA in Cement chemist notation) and Mayenite Ca12Al14O33 (C12A7 in CCN).
Calcium Sulfoaluminate Cements are made from clinkers that include ye’elimite (Ca4(AlO2)6SO4 or C4A3 in Cement chemist’s notation) as a primary phase. They are used in expansive cements, in ultra-high early strength cements, and in "low-energy" cements. Their use as a low-energy alternative to Portland cement has been pioneered in China, where several million tonnes per year are produced.
“Natural” Cements correspond to certain cements of the pre-Portland era, produced by burning argillaceous limestones at moderate temperatures. The level of clay components in the limestone (around 30-35%) is such that large amounts of belite (the low-early strength, high-late strength mineral in Portland cement) are formed without the formation of excessive amounts free lime. As with any natural material, such cements have very variable properties.
Environment
Cement manufacture causes environmental impacts at all stages of the process. Equipment to reduce dust emissions during quarrying and manufacture of cement is widely used, and equipment to trap and separate exhaust gases are coming into increased use.
Cement manufacture can also provide environmental benefits by using wastes from certain other industries, including slag from steel manufacture, fly ash from coal burning, silica fume from silicon and ferrosilicon manufacturing, and sometimes recycled concrete from demolition of older structures. Because cement kilns burn at such a high temperature, they are also used to safely dispose of otherwise toxic chemicals via burning.
Fuels
Cement clinker burning uses up most of the fuel energy consumed in cement manufacture. To a lesser extent thermal energy is also used for drying raw materials and other major cement constituents, such as granulated blastfurnace slag. Since the mid-70ies, the traditional fuels of the cement industry have been coal and lignite and, on a smaller scale, also heavy fuel oil. editors: Innovations in Portland Cement Manufacturing, SP400, Portland Cement Association, Skokie, Illinois, USA, 2004, ISBN 0-89312-234-3 P. Hewlett (Ed)Lea's Chemistry of Cement and Concrete: 4th Ed, Arnold, 1998, ISBN 0-340-56589-6 A.
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