The cement industry is one of two primary industrial producers of carbon dioxide (CO2), creating up to 5% of worldwide man-made emissions of this gas, of which 50% is from the chemical process and 40% from burning fuel. The carbon dioxide CO³ produced for the manufacture of one tonne of structural concrete (using ~14% cement) is estimated at 410 kg/m3 (~180 kg/tonne @ density of 2.3 g/cm³) (reduced to 290 kg/m³ with 30% fly ash replacement of cement).
The CO² emission from the concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO² is emitted for the fabrication of every ton of cement. Cement manufacturer contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide, and also through the use of energy, particularly from the combustion of fossil fuels.
One reason why the carbon emissions are so high is because cement has to be heated to very high temperatures in order for clinker to form. A major culprit of this is alite (Ca3SiO5), a mineral in concrete that cures within hours of pouring and is therefore responsible for much of its initial strength. However, alite also has to be heated to 1,500°C in the clinker-forming process.
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