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A cement is a binder, a substance that sets and hardens and can bind other materials together. The word "cement" can be traced back to the Roman term ''opus caementicium'', used to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were later referred to as ''cementum'', ''cimentum'', ''cäment'', and ''cement''. Cements used in construction can be characterized as being either hydraulic or non-hydraulic, depending upon the ability of the cement to set in the presence of water (see hydraulic and non-hydraulic lime plaster). Non-hydraulic cement will not set in wet conditions or underwater; rather, it sets as it dries and reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting. Hydraulic cements (e.g., Portland cement) set and become adhesive due to a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack. The chemical process for hydraulic cement found by ancient Romans used volcanic ash (activated aluminium silicates) with lime (calcium oxide). The most important uses of cement are as a component in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material. ==Chemistry== Non-hydraulic cement, such as slaked lime (calcium hydroxide mixed with water), hardens by carbonation in the presence of carbon dioxide which is naturally present in the air. First calcium oxide is produced by lime calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure: :CaCO3 → CaO + CO2 The calcium oxide is then ''spent'' (slaked) mixing it with water to make slaked lime: :CaO + H2O → Ca(OH)2 Once the excess water is completely evaporated (this process is technically called ''setting''), the carbonation starts: :Ca(OH)2 + CO2 → CaCO3 + H2O This reaction takes a significant amount of time because the partial pressure of carbon dioxide in the air is low. The carbonation reaction requires the dry cement to be exposed to air, for this reason the slaked lime is a non-hydraulic cement and cannot be used under water. This whole process is called the ''lime cycle''. Conversely, the chemistry ruling the action of the hydraulic cement is hydration. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main components being: :Belite (2CaO·SiO2); :Alite (3CaO·SiO2); :Tricalcium aluminate (3CaO·Al2O3) (historically, and still occasionally, called 'celite'); :Brownmillerite (4CaO·Al2O3·Fe2O3). The silicates are responsible of the mechanical properties of the cement, the tricalcium aluminate and the brownmillerite are essential to allow the formation of the liquid phase during the kiln sintering (firing). The chemistry of the above listed reactions is not completely clear and is still the object of research.〔(Cement's basic molecular structure finally decoded (MIT, 2009) )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cement」の詳細全文を読む スポンサード リンク
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