By Sue McCraven
Most people on the street have no idea what fly ash is or how it it is used. But for concrete manufacturers, fly ash and the current regulatory controversy spinning around it are a very big deal. After seven decades of beneficial use in concrete structures, the U.S. Environmental Protection Agency (EPA) is currently reassessing the classification, storage, disposal and reuse of fly ash to ensure that it is properly managed to minimize potential harm to human health and the environment.
Structural engineers, infrastructure builders, fossil-fuel power generators, environmental activists and the U.S. Federal Highway Administration (FHWA) are also invested in the ongoing debate over the EPA’s recently proposed ruling to reclassify fly ash – after years endorsing its positive contribution to the concrete industry – as a hazardous waste. A balanced discussion of the pros and cons of recycling fly ash (aka coal ash or CCP) in concrete from a global perspective is in order. Let’s start with the source of the massive amounts of fly ash: the world’s electrical power industry (i).
What is the main source of electrical power?
The No. 1 source of electrical generation has always been, is today, and likely will continue to be coal (ii). And the burning (combusting) of pulverized coal generates electric power and tons and tons of powdery fly ash residue. Fly ash is captured with electrostatic precipitators, scrubbers or filter fabric baghouses in power plants, then sluiced to settling basins (wet) prior to disposal or stored in silos (dry) for sale or disposal. In our endless need for power, it makes sense that fly ash has become the earth’s largest industrial waste byproduct, but fortunately some of it is diverted for a good purpose. Indeed, certain classes of fly ash are an important component of reinforced concrete, the best composite structural material in the world (iii).
Major role of fly ash in concrete structures
Fly ash is a very fine-particulate material that looks and feels like talcum powder and can be a tan to gray color, depending on its source. It is classified as a pozzolan and with its high silica content is used by concrete producers as a component in the range of 10 to 25% of the cementitious portion of concrete mixtures (iv). Fly ash forms calcium silica hydrate (cementitious material) in addition to that produced by hydration of portland cement.
Worldwide, concrete is used twice as much as all the other building products combined. Concrete is everywhere: in our bridges, roads, buildings, work places and neighborhoods. And underground, concrete foundations support our infrastructure.
Here’s where the importance of fly ash in concrete becomes clear: More than 75% of all concrete is made with fly ash. In some states, fly ash is specified for all concrete structures. And the concrete industry is only one of the important entities using this abundant waste material.
Other beneficial uses of fly ash
Fly ash production and its repurposing in various markets are quantified in data from FHWA (v) and EPA (vi) (see the sidebar “Fly Ash Reuse”). From these data, we see that the lion’s share of recycled fly ash goes into making concrete, and the reason is because fly ash delivers measurable economic and structural benefits. Its worldwide availability, outstanding structural contributions (strength and durability) and relatively economical cost create a constant demand for fly ash in the construction industry.
Several NPCA publications explain fly ash in more technical detail, including its important environmental benefits, its material properties, chemical reactions and guidelines for production (vii).
The following is a summary list of fly ash’s contribution to concrete:
- Increases ultimate concrete strength
- Increases concrete durability
- Is more economical than portland cement
- Reduces the heat of hydration (first used in mass concrete construction in the building of Hungry Horse Dam, Montana,1948)
- Reduces risk of alkali-silica reaction (ASR)
- Increases resistance to sulfate attack
- Reduces concrete bleeding (water loss at the surface after placement)
- Reduces concrete shrinkage during curing
- Reduces the amount of water required in mixtures
- Reduces permeability (increases concrete’s resistance to water penetration)
- Improves workability (microscopic, spherical-shaped particles create a more flowable, easier-to-finish concrete)
- Lightens the color of concrete
- Fulfills LEED points (LEED MR 4.1, Reclaimed Materials/Recycled Content) and is routinely specified on many green projects
- Meets the guidelines of many building codes, design guidelines and standards that encourage fly ash recycling in concrete.
- Meets ASTM standards and test methods (ASTM C618-08, C1240 and C 311-07)(viii)
- Is environmentally beneficial (ix).
Opposition to fly ash
Environmentalists and the EPA are opposed to the wet handling of fly ash (storage of waste CCP slurry in holding ponds) at power plants because of potential violations of The Clean Water Act. A recent report (x) by a coalition of environmental organizations includes this statement from the EPA:
“Coal-fired power plants are the largest source of toxic water pollution in the United States based on toxicity, dumping billions of pounds of pollution into America’s rivers, lakes and streams each year (xi). The waste from coal plants, also known as coal combustion waste, includes coal ash and sludge from pollution controls called ‘scrubbers’ that are notorious for contaminating ground and surface waters with toxic heavy metals and other pollutants. (xii)”
But it wasn’t always this way. Opposition to fly ash is relatively new for the EPA. Prior to 2008, the EPA actually promoted the beneficial use of CCP (including fly ash) in its C2P2 Program, a cooperative partnership with ACAA and USWAG (xiii):
“Fly ash is a CCP possessing unique characteristics that allow it to be used ton-for-ton as a substitute for portland cement in making concrete. Through the reuse of fly ash, the GHG emissions associated with the production of portland cement are avoided.”
As we’ll see, it was an unfortunate event in 2008 that gave the EPA good reason to reassess its position on fly ash.
The best laid plans…
EPA’s sudden change of heart is the result of a massive slurry spill at The Tennessee Valley Authority’s (TVA) Kingston plant in 2008. Failure of Kingston’s coal ash retention basins resulted in serious local flooding and the pollution of 300 acres including the Clinch and Emory rivers. Consequently, the EPA reassessed its fly ash management position and proposed a new ruling that, for the first time, would regulate waste storage and disposal from coal-fired power generators.
EPA is currently considering two management options for CCPs. Both options fall under the Resource Conservation and Recovery Act (RCRA). One option would list CCPs (stored in settling basins or slated for landfill disposal) as special wastes subject to regulation under subtitle D of RCRA. In the second proposal – a ruling that the concrete, power and coal ash industries dread – the EPA would regulate CCPs under subtitle C of RCRA and classify fly ash as a hazardous waste. A long-awaited final ruling is expected in 2014. EPA also proposed a ruling to establish standards for wastewater management at power plants.
Concrete arguments from ACAA
Opposition to listing fly ash as a hazardous waste is based on the economic, environmental and market concerns of producers and associations representing concrete, electric power and CCPs. Thomas H. Adams, executive director of the ACAA, has extensive experience in the concrete industry and is a strong proponent for the continued recycling of fly ash. Adams says, “On Dec. 22, 2008, the EPA decided to re-examine its 1993 and 2000 determinations that coal ash did not warrant management as a hazardous waste for disposal purposes. The mere suggestion that fly ash and other coal combustion products would be viewed as hazardous waste has had a chilling effect on those who have come to recognize the value of fly ash in enhancing concrete durability. A hazardous waste label would create significant issues. This stigma continues to linger with the regulatory uncertainty created by the EPA.”
Some federal legislators aren’t waiting for the 2014 ruling. House Bill H.R. 2218 would establish minimum federal requirements, administered by states, for the management of fly ash to protect human health and the environment. The bill passed the House in July and was sent to the Senate, where its status is uncertain.
What is the future for fly ash?
It should be noted that environmentalists’ concerns about potentially toxic water pollution (particularly mercury) have no relevance to the use of fly ash in concrete production. The mercury level in most fly ash is the same as that found in most virgin soils. Further, if CCPs are high in mercury (mercury has an affinity for carbon), its carbon content would make it unsuitable for concrete. EPA’s main concern with fly ash has been certain unlined landfills and specific massive earth fills. Failures of fly ash settling ponds, like the 2008 TVA spill, are extremely rare events, and in any case, only dry fly ash is used in concrete – not wet material in settling ponds. And lastly, the EPA has not indicated any desire to restrict fly ash use in concrete.
After seven decades of reducing the environmental impact of portland cement production and providing a means of recycling industrial waste, fly ash and concrete stand strong together.
Sue McCraven, NPCA technical consultant and Precast Inc. technical editor, is a civil and environmental engineer.
(i) Coal Combustion Products (CCP) include fly ash, bottom ash and flue gas desulfurization solids. Fly ash is also called CCR (Coal Combustion Residuals).
(ii) Coal currently provides 40% of the world’s electricity needs. It is the second source of primary energy in the world after oil, and the first source of electricity generation. Since the beginning of the 21st century, coal has been the fastest-growing global energy source. The last decade’s growth in coal use has been driven by the economic growth of developing economies, mainly China. Source: www.iea.org/topics/coal
(iii) “Coal-fired power plants produce huge amounts of fly ash and generate the world’s largest quantity of industrial solid wastes, creating severe waste disposal problems.” Recycling of Waste Fly Ash: A Rheological Investigation, by M. Sharma, C. Guria, A. Sarkar and A.K. Pathak. International Journal of Science, Environment & Technology, Vol. 1, No. 4. 2012. p. 285.
(iv) A pozzolan is a siliceous and aluminous material made up of very finely divided particles; when pozzolans are mixed with lime and water, they react to form a strong cementitious product, essentially slow-hardening cement.
(v) Source: FHWA Fly Ash Facts for Engineers, April 2011.
(vii) “Using Fly Ash in Concrete” by Arnie Rosenberg, May/June 2010 Precast Inc.; “Ultrafine Fly Ash” by K. Foody, May/June 2010, Precast Inc.; “Fly Ash: A Hazardous Material?” by C. Goguen, P.E., LEED AP, Precast Inc., July/Aug 2010; “If You Use Fly Ash in Your Mix, You Need to Read This!” NPCA Staff, Oct/Nov 2010, Precast Inc.
(viii) ASTM C618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete”; ASTM C1240, “Standard Specification for Silica Fume Used in Cementitious Mixtures”; and ASTM C311, “Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete”
(ix) By replacing cement, fly ash reduces energy and greenhouse gas emissions from cement clinker production and significantly reduces industrial waste destined for landfills.
(x) “Closing the Floodgates: How the Coal Industry is Poisoning our Water and How We Can Stop It,” authored by a coalition of environmental organizations, July 2013.
(xi) EPA, Environmental Assessment for the Proposed Effluent Limitation Guidelines and Standards for the Steam Electric Power Generating Point Source Category 3-13 (April 2013) .
(xii) EA 3-34, 3-38.
(xiii) Utility Water Act Group