We make more concrete than any other material in the world. It is used in our roads, dams, bridges, and buildings because of its versatility, strength, and durability. Yet producing the portland cement that binds concrete together is energy intensive and emits enormous amounts of carbon dioxide (CO2) as well as numerous other pollutants.
Fly ash can replace a percentage of portland cement in concrete, thus reducing the impacts of concrete while using a material that would otherwise be landfilled. The mercury and other heavy metals contained in fly ash have made the design and construction industry nervous about its use, however. This article explores the environmental footprint of portland cement production and future emissions regulations and looks at the issues surrounding its most common replacement, fly ash.
Portland Cement Manufacturing
Concrete is typically made up of 41% crushed rock, 26% sand, 16% water, 11% portland cement, and 6% entrained air. When combined, the cement and water form a slurry that flows between the aggregate and cures through a “hydration” process into a solid, rock-like mass.
Portland cement, the key ingredient, was patented in 1824 by British bricklayer Joseph Aspdin and now accounts for about 95% of the cement market. It is made from calcium carbonate (primarily from locally quarried limestone or chalk), silicon, aluminum, and iron (from clay, sand, and a variety of other materials). About 1.6 tons of these raw materials are required to make 1 ton of cement. In 2009, over 78 million tons (71 million metric tons or mmt) of portland cement were produced in the U.S., and 3 billion tons (2.8 billion metric tons) were produced in the rest of the world. That’s about 900 pounds (400 kg) of cement for every person on the planet.
The raw materials for portland cement are typically mined locally, crushed, sorted, analyzed for chemical composition, and carefully combined before entering a rotary cement kiln. These kilns are the world’s largest pieces of moving industrial equipment, with some reaching 25 feet (7.6 m) in diameter and 1,000 feet (305 m) in length; they rotate one to three times per minute and slope gently toward a heat source. There are two main types of kilns in use today. Older, inefficient “wet” kilns combine raw materials into a slurry before processing, while shorter dry kilns combine raw ingredients in powder form and use about 25% less energy than wet kilns. Of the 153 cement kilns currently operating in the U.S., 118 are dry and 35 are wet, according to Bruce McCarthy, president and CEO of the Portland Cement Association (PCA).
These kilns heat the raw materials to about 1,650ºF (900ºC), transforming the limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide through a reaction called calcination. Then, as the materials reach about 2,700ºF (1,480ºC), the calcium oxide reacts with the other raw materials. Carefully controlled temperatures ensure that the compounds combine, melt, and cool properly to form marble-sized pellets called clinker. After cooling, this clinker is ground into a fine powder and gypsum is added (typically 5% by weight) to aid the setting time, storage, and workability of the cement. The portland cement is then bagged at the plant or sent to precast or ready-mix facilities, where it is mixed with aggregates and water to form concrete.
The Inventory of Carbon and Energy at the University of Bath in the U.K. estimates that it takes 4 million Btu to produce one ton (0.9 metric tons) of portland cement. According to the U.S. Department of Energy, portland cement production accounts for about 0.33% of the annual energy consumed in the U.S, roughly equivalent to the amount of energy in 13 million tons (12 mmt) of coal.
As high as these numbers are, it’s important to keep them in perspective. According to the University of Bath, cement has lower embodied energy per pound than steel, aluminum, fiberglass insulation, and many other building materials, and concrete’s embodied energy is significantly less—about the same as cellulose insulation. While these “cradle-to-gate” figures are not directly comparable because the materials have different properties and functions, the numbers offer some perspective.
Manufacturing portland cement accounts for approximately 5% of the anthropogenic CO2 emissions worldwide and about 2% of total CO2 emissions in the U.S. As construction materials go, cement production is one of the largest CO2 emitters. Some of the CO2 released during cement production comes from burning coal and other carbon-intensive fuels as a heat source in kilns that can run non-stop for over a year at a time. But portland cement is unique in that about half of its carbon emissions comes from calcining limestone, which emits CO2 during the chemical reaction. This calcination process alone generated approximately 45 million tons (41 mmt) of CO2 emissions in 2008. According to the EPA’s report, "Quantifying Greenhouse Gas Emissions from Key Industrial Sectors in the United States," energy-intensive iron and steel production generates more CO2 than cement (114 vs 83 mmt, respectively), but less than 50% of that steel is used for construction compared with 95% of cement, making construction-based emissions approximately 57 mmt for iron and steel and 79 mmt for cement.