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The environmental impact of concrete, its manufacture and applications, is complex. Some effects are harmful; others welcome. Many depend on circumstances. A major component of concrete is cement, which has its own environmental and social impacts and contributes largely to those of concrete. In spite of the harm that badly planned use of concrete can do, well-planned concrete construction can have many sustainable benefits. The cement industry is one of the primary producers of carbon dioxide, a major greenhouse gas. Concrete is used to create hard surfaces which contribute to surface runoff that may cause soil erosion, water pollution and flooding. Conversely, concrete is one of the most powerful tools for proper flood control, by means of damming, diversion, and deflection of flood waters, mud flows, and the like. Concrete can reduce the urban heat island effect, due to its high albedo. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution. The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicity and radioactivity. Wet concrete is highly alkaline and should always be handled with proper protective equipment. Concrete recycling is increasing in response to improved environmental awareness, legislation, and economic considerations. ==Carbon dioxide emissions and climate change== The concrete industry is one of two largest 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 Cement Sustainability Initiative: Progress report ), ''World Business Council for Sustainable Development'', published 1 June 2002〕 The carbon dioxide 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 are emitted for the fabrication of every ton of cement, accounting for 88% of the emissions associated with the average concrete mix.〔 〕〔Nisbet, M., Marceau, M., VanGeem, M. (2002). Environmental Life Cycle Inventory of Portland Cement Concrete. http://www.nrmca.org/taskforce/item_2_talkingpoints/sustainability/sustainability/sn2137a.pdf〕 Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide,〔(EIA – Emissions of Greenhouse Gases in the U.S. 2006-Carbon Dioxide Emissions )〕 and also through the use of energy, particularly from the combustion of fossil fuels. One area of the concrete life cycle worth noting is the fact that concrete has an very low embodied energy relative to the quantity that is used. 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.〔Building Green. (1993). Cement and Concrete: Environmental Considerations. Retrieved 2 November 2015.http://www.wbcsdcement.org/pdf/tf2/cementconc.pdf〕 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 is lower than any other building material besides wood. 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 fly ash with cement represents a .7% reduction in energy consumption. With some mixes proposed mixes containing as much as 80% fly ash, this would represent a considerable energy savings.〔 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「Environmental impact of concrete」の詳細全文を読む スポンサード リンク
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