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The use of fly ash in building materials diverts millions of tons of ash from the landfills each year, increases the constructability and durability of concrete and brick, partially replaces energy-intensive Portland cement, and is recognized by the U.S. Green Building Council’s LEED rating system as a post-industrial recycled material.


On the other hand, environmentalists question whether the industry has enough testing data to conclusively determine that this byproduct of power-producing coal combustion is environmentally safe.


Furthermore, as the U.S. Environmental Protection Agency deliberates over whether to reclassify fly ash disposal under a hazardous waste statute, the future of fly ash has turned into quite a hot-button issue.


At the same time, one big point in favor of fly ash’s use in concrete and brick is the idea that a chemical reaction is believed to effectively trap any minute levels of fly ash pollutants within the building material.


In fact, when concrete is produced, “much of the fly ash reacts with the Portland cement products of combustion to become, get this—calcium silicate hydrate— the same mineral that gives concrete its strength,” explains Michael Chusid, RA, FCSI, CCS, principal of the Tarzana, Calif.-based architectural technology consulting firm, Chusid Associates. “Any trace amount of heavy metal gets entrapped in the hydrated cement crystals and will have a very difficult time becoming liberated.”


Filling in more details on this unique chemical reaction, David Shepherd, AIA, LEED AP, sustainable development director, Portland Cement Association, Skokie, Ill., explains, “unlike some encapsulation techniques which coat a contaminant with material to ‘glue’ it into place, fly ash chemically reacts with cement during the hydration process and becomes an integral part of the new crystalline structure.”


Consequently, many independent building professionals, and even some environmental groups, are on board with the EPA’s current position that the encapsulated use of fly ash is a very good alternative to sending the ash to the landfills where it faces a greater risk of environmental catastrophe such as the collapse of a Tennessee Valley Authority’s fly ash containment structure in 2008, which sent 5.4 million cubit yards of toxic sludge across 300 acres in Kingston, Tenn.


For example, the active Washington, D.C.-based Environmental Integrity Project—a non-partisan, non-profit group that advocates on behalf of communities for stronger environmental protections—agrees that encapsulating fly ash in cementitious products, such as concrete and brick, is beneficial.


“However,” explains, EIP Attorney Lisa Widawsky, “this is a question of science, not law, and EIP believes EPA should apply the most accurate, up-to-date leach testing procedures to ensure that the products made with coal combustion waste products are safe.”


While many agree that more testing needs to be done, a number of studies have provided some encouraging data. For instance, a 2009 study, “Fly Ash Properties and Mercury Sorbent Affect Mercury Release from Curing Concrete,” performed by chemists at Ohio State University, discovered that concrete made with a 55% fly ash replacement emitted basically the same trace amount of mercury vapor during curing as concrete made without fly ash, according to Alan Kren, SE, LEED AP, a senior associate with the San Francisco, Calif.-based structural and geotechnical engineering firm Rutherford & Chekene.


 


Boosting durability and constructability, clay and concrete pavers made from fly ash reduce, and in some cases, eliminate the need for CO2-producing Portland cement.

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