Environmental impact of concrete....
Carbon dioxide emissions and climate change:
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[li]The cement industry is one of the two largest producers of carbon dioxide (CO2), creating up to 8% of worldwide man-made emissions of this gas, of which 50% is from the chemical process and 40% from burning fuel.[1][4] The CO2 produced for the manufacture 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).[5] The CO2 emission from the concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO2 are emitted for the fabrication of every ton of cement, accounting for 88% of the emissions associated with the average concrete mix.[6][7] Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide,[8] and also through the use of energy, particularly from the combustion of fossil fuels.[/li]
[li]One area of the concrete life cycle worth noting is the fact that concrete has a very low embodied energy per unit mass. This is primarily the result of the fact that the materials used in concrete construction, such as aggregates, pozzolans, and water, are relatively plentiful and can often be drawn from local sources.[9] This means that transportation only accounts for 7% of the embodied energy of concrete, while the cement production accounts for 70%. With a total embodied energy of 1.69 GJ/tonne concrete has a lower embodied energy per unit mass than most common building material besides wood. However, concrete structures have high mass, so this comparison is not always directly relevant to decision making. It is worth noting that this value is based on mix proportions for concrete of no more than 20% fly ash. It is estimated that one percent replacement of cement with fly ash represents a .7% reduction in energy consumption. With some proposed mixes containing as much as 80% fly ash, this would represent a considerable energy saving.[7][/li]
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