Groups Set Mercury Limits for Flyash in Concrete
In 2006, the most recent year for which data is available, the U.S. produced 72 million tons of coal flyash, a waste material left over after coal is burned to generate electricity. While most of that flyash was put in landfills, about 15 million tons went into concrete, often as a substitute for the more carbon-intensive portland cement. Flyash is made largely of silica and lime, but it also contains trace amounts of heavy metals from the coal, including mercury.
The reuse—or “beneficial use,” in industry parlance—of some materials containing toxic substances like mercury, lead, and arsenic is encouraged by industry partnerships and by U.S. Environmental Protection Agency (EPA) policies. Now, two prominent rating systems, the U.S. Green Building Council’s LEED for Healthcare and California’s Collaborative for High-Performance Schools (CHPS), are putting limits on the mercury content for flyash that can be used to earn points in those systems.
CHPS has included flyash requirements in the draft 2009 version of its standards, which is now under review. To qualify for the recycled-content credit under CHPS, California schools must avoid flyash if it contains mercury at a level higher than 11 parts per billion (ppb); projects outside of California must avoid flyash with mercury levels above 5.5 ppb.
According to Heinen, CHPS chose the 5.5-ppb level based on research conducted by a Hawaii utility and published in 2007. That research tried to establish an appropriate level for mercury in flyash by measuring leaching of mercury from flyash concrete. It failed to find any, and established the 5.5-ppb level by raising the previous cap. The 11-ppb level that applies in California is based on a test method used there, which yields different results from the EPA test method used elsewhere.
To further avoid contaminants, CHPS also prohibits flyash from plants burning municipal solid waste, hazardous waste, medical waste, or tires.
Although CHPS was motivated partly by concerns about mercury emissions from the concrete and direct exposure to occupants, “actual dermal or inhalation exposure is really small,” said Kristen Heinen, assistant director of CHPS. The larger concern is the end of life for the concrete, she explained. “Twenty years from now the new problem is going to be how to dispose of mercury in concrete,” she told EBN.
In the initial draft of LEED for Healthcare, coal flyash used as a substitute for portland cement must have “verified” mercury content no greater than 2 ppb in order to quality as recycled content, and must not come from plants burning municipal or hazardous waste. Gail Vittori, chair of the LEED for Healthcare core committee, said the limit of 2 ppb is based on EPA’s drinking water standard, which uses the same level. LEED for Healthcare is now being revised after a public comment period, and the mercury level is among the elements under review.
David Goss, executive director of the American Coal Ash Association, an industry group, discounted any perceived dangers. “When concrete contains flyash there are low levels of offgassing from mercury—fractions of nanograms—so that occupants would not be exposed to any measurable health risks of any sort,” he told EBN. Goss also argues against any danger from concrete disposal, saying that concrete binds up any toxic substances.
Goss worried that the green standards “potentially discourage the use of concrete,” although he speculated that projects would continue to use flyash in concrete but not seek credit for it. That’s not because the flyash isn’t up to the standard, he argued. “We think that most of the flyash can beat that 5.5-ppb level,” he said, “but it would impose a testing program that is not currently in place.”
The issue is likely to heat up in the coming years, as changing regulations could increase the mercury concentrations in flyash. Vittori said that newer smokestack technologies that limit mercury emissions from coal-burning plants raise the concerns for flyash. “If it’s not going out the stack, where’s it going?” she said. “It’s ending up in flyash.” For the time being, however, Goss says that’s not a concern. “It [the mercury level] is probably the same that has been present in flyash for years and probably will remain that way for several more years.”
For more information:
Collaborative for High-Performance Schools
LEED for Healthcare
U.S. Green Building Council