A deposit of cement was formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes. Perhaps the earliest known occurrence of cement is from twelve million years ago. The carbonation reaction requires that the dry cement be exposed to air, so the slaked lime is a non-hydraulic cement and cannot be used under water. This reaction is slow, because the partial pressure of carbon dioxide in the air is low (~ 0.4 millibar). This single chemical reaction is a major emitter of global carbon dioxide emissions. įirst, the limestone (calcium carbonate) is burned to remove its carbon, producing lime (calcium oxide) in what is known as a calcination reaction. The chemistry of these reactions is not completely clear and is still the object of research. The silicates are responsible for the cement's mechanical properties - the tricalcium aluminate and brownmillerite are essential for the formation of the liquid phase during the sintering ( firing) process of clinker at high temperature in the kiln.
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