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Cement is the essential ingredient in concrete, which is the world's second most consumed substance after water. Portland cement is a local product made in Britain and even invented here. No school, house, road, hospital or bridge could be built without it.
Though “cement” and “concrete” are often used interchangeably, concrete is actually the final product made from cement. The primary component of cement is limestone. To produce cement, limestone and other clay-like materials are heated in a kiln at 1400°C and then ground to form a lumpy, solid substance called clinker; clinker is then combined with gypsum to form cement.
Cement plant manufacturing is highly energy – and – emissions intensive because of the extreme heat required to produce it. Producing a ton of cement requires 4.7 million BTU of energy, equivalent to about 400 pounds of coal, and generates nearly a ton of CO2. Given its high emissions and critical importance to society, cement is an obvious place to look to reduce greenhouse gas emissions.
The production of cement releases greenhouse gas emissions both directly and indirectly: the heating of limestone releases CO2 directly, while the burning of fossil fuels to heat the kiln indirectly results in CO2emissions.
The direct emissions of cement occur through a chemical process called calcination. Calcination occurs when limestone, which is made of calcium carbonate, is heated, breaking down into calcium oxide and CO2. This process accounts for ~50% of all emissions from cement production.
Indirect emissions are produced by burning fossil fuels to heat the kiln. Kilns are usually heated by coal, natural gas, or oil, and the combustion of these fuels produces additional CO2 emissions, just as they would in producing electricity. This represents around 40% of cement emissions. Finally, the electricity used to power additional plant machinery, and the final transportation of cement, represents another source of indirect emissions and account for 5-10% of the industry’s emissions. ball mill:http://www.crusher-machine.com/21.html
Alternatively, efficiency measures can reduce the demand for fuel by addressing the production process itself (such as switching from inefficient wet kilns to dry ones) or through technical and mechanical improvements (such as preventative maintenance to repair kiln leaks). While some estimate that energy efficiency improvements could achieve emission reductions of up to 40%, some industry analyses suggest that producers may have already exhausted this potential. Without additional financial incentives (such as subsidies or a tax on carbon), further breakthroughs could be difficult.
Reducing emissions from the calcination process means looking to a material other than limestone. Blended cement replaces some of the limestone-based clinker with other materials, primarily coal fly ash and blast furnace slag. Blended cement could reduce CO2 emissions by as much as 20%, but widespread use of blended cement is limited by other environmental regulations (these substitutes can contain toxic heavy metals); the limited availability of substitute material; and some building code restrictions (blended cement can take longer to set).
Finally, CO2 emissions can be captured after they are produced through carbon capture and storage (CCS).In addition to traditional CCS methods, which are already employed in some power plants, concrete producers can actually use their own product as a sink for CO2. Through the process of accelerated carbonation, CO2 penetrates concrete and reacts with calcium hydroxide in the presence of water to form calcium carbonate; the result is stable, long-term CO2 storage. As a mitigation technology, accelerated carbonation can be achieved by exposing freshly mixed concrete to flue gases with high CO2concentrations.
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