Emissions Cement Industry

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hydraulic CEMENT is the binding and strength-contributing agent in concrete and most mortars. The cement is hydraulic because it sets and develops strength through the hydration (chemical combination with water) of its component minerals or compounds. Among hydraulic cements, the most common are Portland cement and similar cements having Portland cement as a base.

Concrete is a very common construction material and is a proportioned mix of fine and course aggregates (such as sand and gravel or crushed stone), hydraulic cement (only about 10-12 percent by volume, or about 11-14 percent by weight), and sufficient water to fully hydrate the cement.

In 2006, hydraulic cement was manufactured in about 150 countries and world output was about 2.5 billion metric tons (Gt) per year, enough for about 20 Gt per year of concrete (including mortar) or about 3 tons of concrete per year per person on the planet. The most important environmental issue with respect to global warming stems from cement rather than concrete manufacture: nearly one ton of carbon dioxide (CO2) is emitted for every ton of Portland cement made. Although generally well below the total CO2 emissions from fossil fuel-fired power plants and motor vehicles, the cement industry is overall the world's largest single industrial source of CO2 emissions.

Portland cement is made by finely intergrinding Portland cement clinker (about 95 percent by weight) with about 5 percent calcium sulfate (mainly as the mineral gypsum). The CO2 emissions come from the clinker's manufacture. The chemical composition of clinker is fairly uniform worldwide, and averages about 65 percent calcium oxide (CaO), 22 percent silicon dioxide, 6 percent aluminum oxide, 3 percent iron oxide, and 4 percent other oxides. These oxides are derived by the high-temperature breakdown in a kiln of a variety of (mostly geologic) raw materials. The clinker is then formed by chemically recombin-ing these oxides into certain hydraulically reactive compounds (mainly calcium silicates).

The dominant CaO component of clinker is chiefly derived from limestone, an abundant rock made up primarily of the mineral calcite (calcium carbonate, or CaCO3). In the kiln, the calcite is broken down by the calcination reaction: CaCO3 + heat CaO + CO2. Calcite is 56 percent CaO; thus, it takes 1.16 tons of calcite per ton of clinker of 65 percent CaO content, assuming no other source of CaO.

This amount of calcite yields 0.51 tons of CO2; this emissions factor is commonly boosted to 0.52 tons to account for some loss of calcined raw materials in exhaust dust from the kiln, and the possible presence of small amounts of other carbonate minerals in the raw materials.

Although calcination occurs at temperatures 1,292-1,832 degrees F (700-1,000 degrees C) below those needed for the subsequent formation of the hydraulic calcium silicates in clinker 2,562-2,642 degrees F (about 1,350-1,450 degrees C), it is calcination that consumes much the largest share of the total heat energy requirements.

Depending on the kiln technology and factors such as heat loss through the kiln shell, about 3-7 GJ of heat energy is required per ton of clinker manufactured. This enormous amount of heat is supplied by burning large quantities of fuels (typically coal and/or petroleum coke), which also releases CO2, typically about

0.43-0.48 tons of CO2 per ton of clinker, depending on the fuel's overall carbon content.

Thus, counting calcination and combustion, total CO2 emissions are about 0.95-1.0 tons per ton of clinker, or about 0.90-0.95 tons per ton of Portland cement. Based on typical issues affecting the accuracy of clinker and cement production data, estimates of CO2 output using the foregoing emissions factors accurate to about plus or minus 5 percent. Other fairly high-volume emissions from cement plants (such as kiln dust, sulfur oxides, and non-N2O nitrogen oxides) are not significant to the issue of global warming.

There are four main strategies to reduce CO2 emissions from cement manufacturing. The first involves reducing overall fuel (heat) consumption by upgrading the kiln technology (for example, conversion or replacement of wet kilns with preheater-precalciner dry kilns). The second involves switching among fossil fuels and/or incorporating a proportion of waste fuels; the latter reduce primary fossil fuel consumption, commonly have lower carbon contents, and may offer carbon emissions credits. The third strategy is to source some of the requisite CaO from non-carbonate materials such as iron and steel slags, or coal combustion ashes; these not only do not directly involve calcination emissions, but also require far less heat in the kiln to process.

The fourth strategy is to encourage the use of blended cements rather than straight Portland cement. Blended cements are integral mixes of Portland cement and pozzolans. Pozzolans are siliceous materials that have little initial cementitious character, but which become hydraulically cementitious when reacted with calcium hydroxide (such as that released through the hydration of Portland cement). Although the first three strategies can reduce plant-level CO2 emissions, the main emphasis (of four strategies) is to reduce the emissions per ton of cement produced by keeping the kilns working at full capacity (their most efficient condition), to make more cement using less raw material and fuel.

sEE ALso: Carbon Dioxide; Carbon Emissions; Industrialization.

BIBLioGRApHY. Intergovernmental Panel on Climate

Change (IPCC), 2006IPCC Guidelines for National Green house Gas Inventories (IPCC CD-ROM; v.3, ch. 2.2, 2007); H.G. van Oss and A.C. Padovani, "Cement Manufacture and the Environment; Part I—Chemistry and Technology," Journal of Industrial Ecology (v.6/1, 2002); H.G. van Oss and A.C. Padovani, "Cement Manufacture and the Environment; Part II—Environmental Challenges and Opportunities," Journal of Industrial Ecology (v.7/1, 2003).

Hendrik G. van Oss U.S. Geological Survey

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