Cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick additives that were added to the burnt lime to obtain a hydraulic binder were later referred to as cementum, cimentum, cäment, and cement. Cement is made by grinding together limestone and shales (a mixture of aluminosilicates) and heating the mixture to about 1500oC. The chemical reaction releases carbondioxide and partially melts the components to form solid lumps called clinker. The clinker is ground to powder, and a small quantity of calcium sulfate is mixed in. This mixture is known as Portland Cement. Chemically, its main components are 26% dicalcium silicates ( Ca2SiO4); 51% tricalcium silicates (Ca3SiO5); and 11% tricalcium aluminate (Ca3Al2O6). When water is added, a number of complex hydration reactions take place. A typical idealized reaction can be represented as 2 Ca2SiO4(s) + 4 H2O(l)
Ca3Si2O7. 3H2O(s) + Ca(OH)2(s).
The hydrated silicate, called tobermorite gel, forms strong crystals that adhere by means of strong silicon-Oxygen bonds to the sand and aggregate (small rocks) that are mixed with the cement. Because the other product in this reaction is Calcium hydroxide, the mixture should be treated as a corrosive material while it is hardening. Cements used in construction can be characterized as being either hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) harden because of hydration, a chemical reaction between the anhydrous cement powder and water. Thus, they can harden underwater or when constantly exposed to wet weather. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water. Non-hydraulic cements do not harden underwater; for example, slaked limes harden by reaction with atmospheric carbon dioxide. The most important uses of cement are as an ingredient in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material. Non-hydraulic cement such as slaked limes (calcium hydroxide mixed with water), harden due to the reaction of carbonation in presence of the carbon dioxide naturally present in the air. 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 mixing it to water to make slaked lime: CaO + H2O → Ca(OH)2
Once the water in excess from the slaked lime 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 small. The reaction of carbonation requires the air be in contact with the dry cement, hence, for this reason the slaked lime is non-hydraulic cement and cannot be used under water. Conversely, the chemistry ruling the action of the hydraulic cement is the hydration. Hydraulic cements (such as the Portland cement) are made of a mixture of silicates and oxides, the four main components being: Belite (2CaO·SiO2);
The reactions during the setting of the cement are:
(3CaO·Al2O3)2 + (x+8) H2O → 4 CaO·Al2O3·xH2O + 2 CaO·Al2O3·8H2O (3CaO·Al2O3) + 12 H2O + Ca(OH)2 → 4 CaO·Al2O3·13 H2O
(4CaO·Al2O3·Fe2O3) + 7 H2O → 3 CaO·Al2O3·6H2O + CaO·Fe2O3·H2O And during the hardening (the chemistry of the reaction of hydration is still not completely clear): (3CaO·SiO2)2 + (x+3) H2O → 3 CaO2·SiO2·xH2O + 3 Ca (OH)2 (2CaO·SiO2)2 + (x+1) H2O → 3 CaO2·SiO2·xH2O + Ca (OH)2 The silicates are responsible of the mechanical properties of the cement, the...
References: 1. Hewlett, Peter (2003). Lea 's Chemistry of Cement and Concrete. Butterworth-Heinemann.
2. Justnes, H.; Ronin, V. Performance of Energetically Modified Cement (EMC) and Energetically Modified Fly Ash (EMFA) as Pozzolan. SINTEF. Retrieved 5 November 2013.
3. Van Oss, Hendrik G.; Padovani, Amy C. (2002). "Cement Manufacture and the Environment, Part I: Chemistry and Technology". Journal of Industrial Ecology.
4. Van Oss, Hendrik G.; Padovani, Amy C. (2003). "Cement Manufacture and the Environment, Part II: Environmental Challenges and Opportunities". Journal of Industrial Ecology.
5. Geoff Ratner – Canham, and Tina Overton. ‘’Descriptive Inorganic Chemistry’’. 3rd edition (2002).
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