Cement

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For other uses, see Cement (disambiguation).

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. The significance of the clay content for the hydraulic properties of the hydraulic lime produced from a natural mixture of limestone and clay was discovered by the Englishman John Smeaton (1724-1792) when he was preparing to build the Eddystone lighthouse near Plymouth and was looking for a binder for water-resistant mortar. The significance of the sintering process had apparently been first recognized in 1844 by Isaac Charles Johnson (1811-1911).

1 Setting and hardening
2 History
3 Types of manufactured cements
4 Environment
- 4.1 Fuels
5 Literature
6 See also
7 External links

[edit] 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 reaction and the reaction products are referred to as hydration and hydrates or hydrate phases, respectively. As a result of the immediately starting reactions, a stiffening can be observed which is very small in the beginning, but which increases with time. After reaching a certain level, this point in time is referred to as the start of setting. The consecutive further consolidation is called setting, after which the phase of hardening begins.

[edit] History

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The Egyptians discovered and used lime and gypsum mortar as a binding agent for building such structures as the Pyramids. This was far superior to the clay that had been used by the Assyrians and Babylonians. 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.^{[citation needed]}

The art of creating cement was lost for hundreds of years during the middle ages. The proper formula was not truly discovered again until the early 19th century. Portland cement was patented in England by Joseph Aspdin in 1824. However, this material at first corresponded in composition and properties to a Roman lime, as it had not yet been burnt to the sintering point. The artificial rock produced from it resembled Portland stone, an oölitic limestone which is quarried on the Portland peninsula in the county of Dorset and the Channel coast, thus giving it its name. 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.

[edit] Types of manufactured cements

[edit] Portland cement

Main article: 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. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element.

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. The iron oxide is either contained in the clay minerals in the form of ferrous hydroxide, or it is added in the form of iron ore.

The raw material mix is heated up to a temperature of approximately 1,450 °C in a rotary kiln until it starts sintering. This results in the starting materials forming new compounds known as clinker phases. 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. Gypsum from flue gas desulphurisation, however, is very well suited to substitute the natural resources.

Apart from natural raw materials, also alternative raw materials can be utilised, such as lime sludge, used foundry sand and fly ash. They contain silicon dioxide, aluminium oxide, iron oxide and/or calcium oxide as main constituents as well and are combined with the raw materials in quantities which will ensure compliance with the clinker composition specified. 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. the composition of the limestone and marl deposits, respectively.

[edit] 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. All compositions produce high ultimate strength, but as slag content is increased, early strength is reduced, while sulfate resistance increases and heat evolution diminishes. Used as an economic alternative to Portland sulfate-resisting and low-heat cements.

Portland Flyash Cement contains up to 30% fly ash. The flyash is pozzolanic, so that ultimate strength is maintained. Because flyash addition allows a lower concrete water content, early strength can also be maintained. 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. In countries where volcanic ashes are available (e.g. Italy, Chile, Mexico, the Philippines) these cements are often the most common form in use.

Portland Silica Fume cement. 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. They are usually complex proprietary formulations containing Portland clinker and a number of other ingredients that may include limestone, hydrated lime, air entrainers, retarders, waterproofers and colouring agents. They are formulated to yield workable mortars that allow rapid and consistent masonry work. Subtle variations of Masonry cement in the US are Plastic Cements and Stucco Cements. These are designed to produce controlled bond with masonry blocks.

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. This allows large floor slabs (up to 60 m square) to be prepared without contraction joints.

[edit] 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 Pantheon in Rome). They develop strength slowly, but their ultimate strength can be very high. 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. Only granulated slag (i.e. 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. They exhibit good resistance to aggressive agents, including sulfate.

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). Strength forms by hydration to calcium aluminate hydrates. They are well-adapted for use in refractory (high-temperature resistant) concretes, e.g. for furnace linings.

Calcium Sulfoaluminate Cements are made from clinkers that include ye’elimite (Ca₄(AlO₂)₆SO₄ or C₄A₃<math>\bar \mathrm{S}</math> 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. Hydration produces ettringite, and specialized physical properties (such as expansion or rapid reaction) are obtained by adjustment of the availability of calcium and sulfate ions. Their use as a low-energy alternative to Portland cement has been pioneered in China, where several million tonnes per year are produced. Energy requirements are lower because of the lower kiln temperatures required for reaction, and the lower amount of limestone (which must be endothermically decarbonated) in the mix. In addition, the lower limestone content and lower fuel consumption leads to a CO₂ emission around half that associated with Portland clinker. However, SO₂ emissions are usually significantly higher.

“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.

Geopolymer cements are made from mixtures of water-soluble alkali silicates and aluminosilicate mineral powders such as metakaolin.

[edit] Environment

Cement manufacture causes environmental impacts at all stages of the process. These include emissions of airborne pollution in the form of dust, gases, noise and vibration when operating machinery and during blasting in quarries, consumption of large quantities of fuel during manufacture, release of CO₂ from the raw materials during manufacture, and damage to countryside from quarrying. 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. Environmental protection also includes the re-integration of quarries into the countryside after they have been closed down by returning them to nature or re-cultivating them.

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.

Used car tires, generally considered a waste product, are also used as fuel, in addition to wood chips, shredded plastics, and even solid waste(used diapers, etc). Recycling of wastes generally allows reduction of use of raw materials and fuel for processing. Because cement kilns burn at such a high temperature, they are also used to safely dispose of otherwise toxic chemicals via burning.

[edit] 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-1970s, the traditional fuels of the cement industry have been coal and lignite and, on a smaller scale, also heavy fuel oil. A significant portion of coal has been replaced by petcoke since the 1990s. Petcoke is a coal-like fraction of mineral oil generated in crude oil processing. In addition to that, light and heavy fuel oil and gas are used for kiln start-up and drying processes. Many major improvements have been made to the power efficiency of the process over the years.

[edit] Literature

Friedrich W. Locher: Cement : Principles of production and use, Duesseldorf, Germany: Verlag Bau + Technik GmbH, 2006, ISBN 3-7640-0420-7
Javed I. Bhatty, F. MacGregor Miller, Steven H. Kosmatka; editors: Innovations in Portland Cement Manufacturing, SP400, Portland Cement Association, Skokie, Illinois, USA, 2004, ISBN 0-89312-234-3
P. C. Hewlett (Ed)Lea's Chemistry of Cement and Concrete: 4th Ed, Arnold, 1998, ISBN 0-340-56589-6
A. M. Neville Properties of Concrete: 4th Ed, Wiley, 1996, ISBN 0-582-23070-5