Fly Ash benefits and concerns
Fly Ash is a waste by-product from the combustion of coal. It is a very fine powder composed predominantly of silica, and is generally a light tan color. When combined with water, it produces a cementations mix that can be used as a substitute for a portion of the quantity of Portland cement in a blended concrete mix, generally 0 – 30%. However, the fly ash must meet strict standards, including standards on the fineness of the grain sizes, and that the composition is >20% lime. These standards are typically represented by Class C fly ash. Figure 5 provides a summary of the benefits and concerns associated with the use of Fly Ash.
1.1 Performance Impacts
The use of fly ash can impact performance in ways that are different from the use of traditional Portland Cement. The use of high volume fly ash creates a stronger, denser, and more durable concrete. Fly ash concrete is also less permeable to water, particularly seawater and sulfates, making it ideal for use in those conditions. Concrete with fly ash is at less risk of micro cracking due to less heat created during hydrations, and also does a better job at protecting reinforced steel. Due to the fine, round particles in fly ash, it is less likely to clump together during hydration, leading to greater workability, and because of its small particle size, it fills voids that otherwise would be filled with water in a traditional mix, leading to a lower water/cement ration. Fly ash also produces a concrete that is white in color which reflects heat and reduces the urban “heat island” effect.
However, there are also concerns associated with the use of fly ash. Fly ash concrete takes longer to set into the glue that holds the concrete together than traditional Portland cement. The longer set times reduces the early strength of concrete with fly ash compared to Portland cement. Longer set times have the potential to delay construction schedules and are regarded as one of the main obstacles to using more fly ash. It’s difficult to estimate the exact difference in set times since there are many other factors that affect the set time such as air temperature, water temperature, sub-grade temperature (if applicable), admixtures, and cement type.
Fly ash has the potential to reduce the costs of concrete mixes. A report for Sound Transit Authority in 2014 interviewed a number of concrete suppliers in the Pacific Northwest and found that the cost of using SCM will range from a 8% cost savings to a 3.5% cost premium. In most cases, using SCM reduces initial costs but like any commodity, the cost of these materials changes over time. Concrete producers along the west coast procure their fly ash from a number of different sources, making transportation costs among the largest price variables. (Sound Transit, Dec 2014).
Fly ash is a byproduct of coal combustion. Major coal plants and sources of fly ash in Washington and Oregon will be shutting down by 2025, possibly constraining the local supply of fly ash. Other sources of fly ash mainly come from Canada and the Eastern United States. Although there is not expected to be a major shortage of fly ash, there may be an increase in price due to the longer transport distances required to obtain the fly ash. Fly ash is, however, subject to some degree of seasonal availability pressures based on hydropower production in certain regions. Specifically, if power plants are able to rely on more hydroelectricity, then they’ll burn less coal to satisfy baseline power demands. Additionally, there is increasing pressure on coal fired power plants to switch to natural gas fired plants to cover their base load as the price of natural gas decreases.
Slag is procured either from Asian steel mills or Eastern US steel mills. Slag is not subject to the same potential supply challenges as fly ash and is regarded to have a stable supply chain.
1.4 Slag benefits and concerns
Granulated Blast Furnace Slag, also known as “slag,” is a byproduct of iron an steel manufacturing that can typically replace 20-80% of Portland cement in concrete, depending on the application. Slag is a glassy, grainy material that, once ground into a fine powder, will react with water to produce a cementations mix.
1.5 Performance Impacts
The use of slag can impact operations in ways different from the use of traditional Portland cement. Concrete containing slag has longer set times and therefore greater workability than concrete containing only Portland cement, which helps particularly during hot weather.xiii It also has a lower heat of hydration compared to traditional Portland cement, thus reducing the risk of micro cracking.xiii Slag produces a lighter colored concrete, which some might find more aesthetically pleasing, but can also reflect more light and reduced trapped heat.xiv Slag concrete has a higher strength compared with straight Portland cement.xvi This is because the mixture is denser and contains more strength-enhancing calcium silicate hydrates. Concrete with slag is preferrable when specific safety and strength standards need to be met.
However, there are some operational impacts of using slag that raise concerns. Slag concrete has a longer set time, and thus takes longer to develop initial strength.xiii Concrete with a high amount of slag is more likely to experience salt scaling, or flaking during freeze-thaw cycles than traditional mixes.xiiiSalt scaling remains one of the largest concerns when slag accounts >25% of the mixture. When working around slag, additional care should be used to prevent unnecessary exposure to the skin, eyes, and lungs. Excessive, unprotected exposure to high concentrations of slag can cause coughing, sneezing, and respiratory inflammation, or silicosis if the slag has a high concentration of silica particles less than 5 microns in diameter. However, slag is not classified as a hazardous materialxvii, and the use of proper dust respirators minimize risk.xviii
Slag has the potential to reduce the costs of concrete mixes. A report for Sound Transit Authority in 2014 interviewed a number of concrete suppliers in the Pacific Northwest and found that the cost of using SCM will range from a 8% cost savings to a 3.5% cost premium. In most cases, using SCM reduces initial costs but like any commodity, the cost of these materials changes over time. Slag prices have historically trended along with cement prices (Sound Transit, Dec 2014).
Toxicity Concerns for SCM
There are concerns about the toxicity of fly ash and its use in concrete. Fly ash contains several chemicals of concern including numerous metals. While some of these compounds are also found in traditional Portland cement, the real question is whether they pose an unacceptable risk to human or ecological health. The Environmental Protection Agency (EPA) released a report titled “Coal Combustion Residual Beneficial Use Evaluation: Fly Ash Concrete and FGD Gypsum Wallboard" (PDF) in February of 2014 that stated the use of fly ash in concrete does not pose a risk that is greater than the existing regulatory and health based benchmarks for human and ecological receptors. EPA concluded that they support the beneficial use of fly ash in concrete.