Concrete is probably not the first thing that comes to mind when thinking of green building materials, but given that concrete is the most common building material in the world, making the concrete manufacturing process greener is extremely important. Manufacturing the portland cement that binds concrete together is a serious CO2 emitter and energy consumption hog, and replacing a portion of portland cement required for concrete mix with fly ash both reduces the environmental impact of concrete and finds use for material that would otherwise end up in a landfill.
The mix that makes up concrete is typically 41% crushed rock, 26% sand, 16% water, 11% portland cement, and 6% entrained air. The portland cement and water form a paste that combines with rock and sand to harden into concrete. Portland cement is not the only type of cement available for making concrete, but it does make up about 95% of the cement market. Portland cement is made from calcium carbonate, silicon, aluminum, and iron, which are mined, crushed, sorted, and then heated to extremely high temperatures in a cement kiln. The heat sources for these cement kilns emit carbon dioxide, but about half of the carbon emissions from manufacturing portland cement come from the chemical reaction that turns limestone into lime (calcination).
In addition to emitting CO2, cement kilns also emit dangerous toxins, including high levels of mercury. Cement kilns have been largely unregulated in the United States, with the Environmental Protection Agency declining to set mercury emission standards for cement kilns until 2008.
There are no commercially viable alternatives to completely replace portland cement, but there are some greener materials that can replace a portion of the portland cement required to make concrete. These materials come from natural sources, such as rice husks, and as byproducts from industrial processes, which keep industrial waste out of landfills but may also introduce other problems into the concrete supply chain.
Fly ash is a by-product of burning coal in a coal-fired power plant, and it is the most widely used portland cement replacement. It can replace up to 40% of portland cement in a standard concrete mix, but can be used to replace up to 70% when used for building larger structures. Like most portland cement substitutes, fly ash improves the performance of concrete by making it less permeable, and it also reduces the amount of water required to make concrete.
Fly ash is, however, far from a perfectly green building material. Fly ash contains trace amounts of heavy metals, such as mercury and arsenic. In 2014, the EPA found that, although concentrations of potentially harmful substances is higher in concrete made with fly ash, these higher levels of heavy metals are below “relevant regulatory and health-based benchmarks.” There is some evidence that suggests that contaminants are less likely to leach out of concrete that is made with fly ash than out of concrete made entirely with portland cement.