A single industry accounts for around 5 percent of global carbon dioxide (CO2) emissions. It produces a material so ubiquitous it is nearly invisible: cement. It is the primary ingredient in concrete, which in turn forms the foundations and structures of the buildings we live and work in, and the roads and bridges we drive on. Concrete is the second most consumed substance on Earth after water. On average, each year, three tons of concrete are consumed by every person on the planet.
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Concrete is used globally to build buildings, bridges, roads, runways, sidewalks, and dams. Cement is indispensable for construction activity, so it is tightly linked to the global economy. Its production is growing by 2.5 percent annually, and is expected to rise from 2.55 billion tons in to 3.7-4.4 billion tons by .
Manufacture of Cement
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 °C and then ground to form a lumpy, solid substance called clinker; clinker is then combined with gypsum to form cement.
Cement 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.
Cement 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 CO2 emissions.
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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 percent 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 percent 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 percent of the industry's emissions.
Opportunities to Mitigate
Indirect emissions from burning fossil fuels to heat the kiln can be reduced by switching to alternative fuels, including natural gas, biomass and waste-derived fuels such as tires, sewage sludge and municipal solid wastes. These less carbon-intensive fuels could reduce overall cement emissions by 18-24 percent from levels by .
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 percent, 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. It could reduce CO2 emissions by as much as 20 percent, but its widespread use 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 CO2 concentrations.
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