Fly ash in concrete— no worries
(Structural Engineer April 2012) For years structural engineers have incorporated fly ash into their concrete mix designs to produce high quality concrete, and its use as a cement replacement is increasing as engineers seek to reduce CO2 emissions. Questions have arisen recently over the appropriateness of incorporating fly ash into concrete due to concerns about the trace amounts of mercury found in the byproduct of coal combustion. Just as structural engineers were feeling that they were part of the solution to reduce CO2 emissions a monkey wrench was thrown into the works. This article hopes to answer some of the concerns regarding using fly ash in concrete.
Questions about the mercury content in fly ash arose following the billion gallon spill of fly ash sludge from a holding pond into the Clinch River, a tributary of the Tennessee River, outside of Knoxville, Tenn., in 2008. Up until this spill no one paid much attention to fly ash. Following this disaster, fly ash got on everyone's radar, and the Environmental Protection Agency considered adding coal combustion residuals (CCRs), including fly ash, to its list of hazardous materials. Such designation would have complicated the use of fly ash in concrete since engineers, architects, contractors and owners would have been reticent to include it in their concrete mix designs. The EPA still exempts CCRs from hazardous waste regulations and fly ash continues to be used in concrete.
Fly ash is collected from flue gases leaving coal combustion furnace stacks. Trace amounts of mercury already in the coal are collected along with the fly ash, rather than being released into the air in the furnace smoke. Mercury concentration in fly ash is on the order of 0.01 to three parts per million (www.epa.gov/wastes). Thus, fly ash includes trace amounts of mercury that, depending on transport, storage, and end use, might expose workers to mercury and might introduce mercury into the environment. Because of this, some argue that following the precautionary principle, we should not assume that using fly ash in concrete is safe, and that its use should be prohibited or at least closely regulated. While this argument may have its merits, it assumes that fly ash transport and handling is not controlled or is poorly controlled and that mercury leaches out of cured concrete.
We contacted industry representatives to learn more about fly ash transportation and they describe fly ash transportation and handling methods that indicate that very little fly ash is released. From initial collection to concrete batch plant, fly ash transportation is a controlled process from one sealed container (be it silo, rail car, or truck) to another, and very little is lost in the process of transfer from one locale to another. Due to its extremely fine size and spherical shape, fly ash flows like water. So, transfers and transport cannot allow leakage, particularly in states such as California, where leakage would violate very strict particulate pollution regulations. Fly ash releases are essentially no more than what occurs with cement particulates and mercury release is accordingly small. At 3 parts per million of mercury within the fly ash, a small amount of fly ash release would result in a very small mercury release. The EPA limits fine particulates in the air to 15μg/m3, and does not typically have limits for airborne mercury, as mercury exposure is predominantly waterborne. (As of Dec. 16, 2011, the EPA does regulate airborne mercury from power plants, but expresses these limits in pounds per GWh, which is not relevant to mercury releases from fly ash.)
Occupational Safety and Health Administration-mandated safety procedures should guard workers against any greater risk when handling fly ash concrete as compared to all-cement concrete, since cement dust alone poses a health hazard, requiring workers to follow a detailed safety protocol. OSHA requires that workers be equipped with waterproof boots, gloves, and respirators when working with Portland cement, and recommends duct-taping these to long sleeves and pant legs to minimize skin exposure to cement. These procedures would help to protect workers from mercury contained in fly ash.
We researched the available studies on mercury leaching from fly ash concrete and they demonstrate virtually no aqueous or aerial release of mercury from fly ash concrete under any tested circumstances. McCann et al., 2007, Golightly et al., 2009, and Regennitter, 2007, compared mercury emissions from concrete with fly ash to identical concrete mixes without fly ash and found that the fly ash concrete samples typically emitted or leached less mercury than the non-fly ash concrete samples. The studies suggested that this was due to the decreased porosity of the fly ash concrete.
Golightly et al. tested for mercury leaching from cured concrete on integral block samples as well as crushed concrete, including all dust. The fly ash concrete mix leached less mercury than the cement-only concrete mix, at 4.1 ng/kg (parts per trillion) maximum versus 9.9 ng/kg (ppt), measured after seven days' cure. (Both levels are well below the EPA limit of 2 parts per billion of mercury in drinking water.) The fly ash concrete likely leached less mercury than all-cement concrete because fly ash typically produces less permeable concrete. Thus, available data indicates that using fly ash in concrete will not increase the amount of mercury leached into the environment and that it may even have the opposite effect. It appears that concerns about mercury contamination from fly ash concrete are unwarranted and that the AEC industry can continue to safely include fly ash in their concrete mix. However, discussion alone will likely not put the question to rest. It would behoove the concrete industry to answer concerns directly by adopting nationwide industry standards for storage and transportation of fly ash and by funding additional studies addressing potential environmental impacts from mercury contained in fly ash as it is used in concrete construction.
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