Cement and concrete and climate change

Definition

Concrete, at its most basic, is composed of aggregates and a binding material (cement). Concrete aggregates are coarse, greater than 4.75 millimeters; fine aggregates are less than 4.75 millimeters in size. Aggregates—which are free of silt, organics, sugars, and oils—include sand, gravel, crushed stone, and iron blast-furnace slag. By volume, they make up about 75 percent of a concrete-cement mixture. The aggregates’ size plays an important role in achieving maximum particle packing. Optimum packing reduces the amount of cement needed; with less cement, the durability and mechanical properties of the concrete are improved.

Compressive strength, the measured maximum resistance to axial loading, is one of the outstanding properties of cement. Tensile strength, a measure of resistance to stretching, is much lower for concrete, so it is often reinforced with steel bars to provide additional tensile strength. The durability of concrete is high because it can be designed and manufactured for resistance to freeze-thaw cycles, seawater exposure, chemicals, and corrosion.

Cement, in the broadest sense, binds concrete elements together in the presence of water. Cement is instrumental in determining the quality of concrete. In properly manufactured concrete, every particle of aggregate must be surrounded by cement, and all voids must be filled with cement.

Early cements, known as soft lime cements, were prepared by burning slabs of limestone in a vertical kiln. After burning, the crumbly slabs were used immediately, slaked to produce a powder form that, when combined with sand and water, created soft lime mortars used for brickwork/masonry, or packed into barrels for later use.

There are three classes of hydraulic cement: Pozzolana (Pozzola or Trass), natural cement, and Portland cement. Pozzolana, a volcanic deposit, is finely ground, then mixed with lime, sand, and water, creating strong cement that hardens underwater (hydraulic) and is impervious to salt water. Natural cement is produced by low-temperature burning of clay- or magnesium-rich limestone; upon completion of the burn, the limestone is crushed into smaller fragments, then pulverized, producing very strong cement.

Portland cement, patented by Joseph Aspdin in England in 1824, combines limestone and clay, then grinds them with water into a fine slurry. The dried slurry is burned in a kiln and the calcined material is again ground to a fine powder. By the 1850s, the strength and setting qualities of Portland cement were improved by burning the mixture at very high temperatures—close to the fusion point within the kiln. This improvement and the ability to chemically analyze successful cement products allowed the Portland cement industry to grow. Portland cement began production in the United States between 1875 and 1890, with mills in Texas, Oregon, Michigan, New York, Maine, and the Lehigh District of Pennsylvania.

Significance for Climate Change

The basic makeup of Portland cement is lime (CaO) from limestone and cement rock, silica (SiO₂) from clay and fly ash, alumina (A1₂O₃) from aluminum ore refuse, and iron oxide (Fe₂O₃) from iron ore. The proportions of these crushed elements are closely defined by industry standards. A mix or slurry of limestone and shale or clay is prepared for burning and final cooling in an inclined, rotating kiln. The dry cement mix slowly heats to 1,260° Celsius, and the carbonates (limestone and cement rock) burn and lose carbon dioxide (CO₂). Lime, alumina, and iron oxide fuse between 1,427 and 1,482° Celsius to complete the cement. Approximately 60 percent of all CO₂ emissions are from the lime-burning process; the remaining 40 percent of CO₂ emissions originate from used for combustion.

Originally, oil was used to heat kilns; the use of pulverized coal began in the late 1890s and has continued. Electricity, used in plant operation, is often generated by coal, and diesel or gasoline is used for quarrying raw materials. All are fossil fuels—all release CO₂ into the atmosphere. US CO₂ emission data from the Energy Information Agency (EIA) for cement manufacture show that atmospheric CO₂ has risen from 33 million metric tons in 1990 to 46.1 metric tons in 2005, an increase of 13.1 percent in fifteen years. It was estimated that 5 percent of all global atmospheric CO₂ was derived from cement manufacturing.

According to climate scientists, elevated levels of CO₂ increase the Earth’s temperature. CO₂ and other greenhouse gases absorb and prevent the longer wavelength heat radiation from leaving Earth’s surface. This heat builds up and warms Earth’s surface, atmosphere, and climate. This warmer climate may substantially melt glaciers and polar ice sheets, causing sea-level rise, increased evaporation, drought, flooding, and heat waves. In 2023, the cement and concrete industry was responsible for roughly 8 percent of the world's carbon emissions. This posed a challenge for world governments, as the cement manufacturing industry has been notoriously difficult to decarbonize. Despite this, numerous companies were investing in the development of low-carbon concrete and cement manufacturing methods.

Bibliography

“Concrete as a CO₂ Sink?” Environmental Building News 4, no. 5 (September/October, 1995): 5.

Energy Information Administration. Emissions of Greenhouse Gases in the U.S. Washington, D.C.: U.S. Department of Energy, 2006.

Friedman, Jamie. "Cement and Concrete Companies Leading the Net-Zero Transition." CAP 20, 11 July 2024, www.americanprogress.org/article/cement-and-concrete-companies-leading-the-net-zero-transition/. Accessed 13 Dec. 2024.

Kosmatka, S. H., B. Kerkhoff, and W. C. Panares. Design and Control of Concrete Mixtures. Skokie, Ill.: Portland Cement Association, 2002.

Long, Douglas. Global Warming. New York: Facts On File, 2004.