The Cement Industry's Emissions

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A single industry accounts for about 5 percent of global carbon dioxide (CO2) emissions. It produces a material so ubiquitous it's almost invisible: cement. It is the main 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, every year, each inhabitant of the planet consumes three tons of concrete.

 Concrete is used throughout the world to construct buildings, bridges, roads, runways, sidewalks, and dams. Cement is essential for construction activity, which is why it is closely linked to the world economy. Its production is growing at 2.5 percent per year and is expected to rise from 2.55 billion tonnes in 2006 to 3.7-4.4 billion tonnes in 2050.

Cement Manufacturing

Although "cement" and "concrete" are often used interchangeably, concrete is actually the final product made from cement. The main component of cement is limestone. To produce cement, limestone and other clay-like materials are heated in a kiln to 1,400°C and then ground to form a solid, lumpy substance called slag; The clinker is then combined with gypsum to form cement.

Cement manufacturing is very energy and emissions intensive due to the extreme heat required to produce it. Producing a ton of cement requires 4.7 million BTUs of energy, equivalent to about 400 pounds of coal, and generates almost 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

Cement production releases greenhouse gas emissions both directly and indirectly: heating limestone releases CO2 directly, while burning fossil fuels to heat the kiln indirectly generates CO2 emissions.

Direct emissions from cement occur through a chemical process called calcination. Calcination occurs when limestone, which is made of calcium carbonate, is heated and breaks 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 furnace. Furnaces are typically heated with coal, natural gas, or oil, and burning these fuels produces additional CO2 emissions, just as they would when producing electricity. This represents about 40 percent of cement emissions. Finally, the electricity used to power the plant's additional machinery and the final transport of the cement represents another source of indirect emissions and accounts for 5 to 10 percent of the industry's emissions.

Opportunities to mitigate

Indirect emissions from burning fossil fuels to heat the furnace can be reduced by switching to alternative fuels, including natural gas, biomass, and waste-derived fuels such as tires, sewage sludge, and municipal solid waste. These less carbon-intensive fuels could reduce total cement emissions by 18-24 percent from 2006 levels by 2050.

Alternatively, efficiency measures can reduce fuel demand by addressing the production process itself (such as switching from inefficient wet kilns to dry kilns) or through technical and mechanical improvements (such as preventative maintenance to repair leaking kilns). While some estimate that improvements in energy efficiency could achieve emission reductions of up to 40 percent, some industry analysis suggests that producers may have already exhausted this potential. Without additional financial incentives (such as subsidies or a carbon tax), progress could be difficult.

Reducing emissions from the calcination process means looking for 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 up to 20 percent, but its widespread use is limited by other environmental regulations (these substitutes may contain toxic heavy metals); the limited availability of substitute material; and some building code restrictions (mixed cement may 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,