Fly ash from coal burning is an almost essential component of concrete mixtures. But with coal on the decline for power production, the concrete industry is looking for alternatives.

Burning coal is one of the primary means of generating electricity in the United States. The coal-burning process produces residual, incombustible materials. Fly ash—fine, glassy, rounded particles rich in silicon, aluminum, calcium, and iron oxides—is one of these residual materials, captured from the flue gas by precipitators and bag filters. Because of its chemical and physical characteristics, fly ash can substitute for a portion of portland cement in concrete mixtures as a supplementary cementitious material (SCM). Fly ash also improves many concrete properties such as workability. The spherical shape of fly ash particles compared to the shape of other SCMs can be seen in microscopic images.

The concrete industry has used fly ash as an SCM for decades, thereby diverting it from landfills and impoundments, providing a benefit to both the power and concrete industries. According to the American Coal Ash Association's (ACAA) 2017 Production and Use Survey, 111.3 million tons of coal combustion products were produced that year, with 71.8 million tons beneficially used. In 2017, concrete manufacturers used 14.1 million tons of fly ash in concrete.

Fly ash is the primary SCM used in concrete in the United States. Because fly ash is a byproduct material and cement is a manufactured material, the cost of fly ash is generally lower than the cost of cement. Therefore, substituting fly ash for cement reduces the cost of concrete.

Recent environmental regulations that require emissions-control systems and the abundance of natural gas as an alternative fuel to coal have led to a decline in coal-fired power plants—a trend that is likely to continue. The U.S. Energy Information Administration (EIA) forecasts that 42 percent of existing coal-fired generation capacity will retire by 2050. As fly ash becomes less and less available, State departments of transportation and their contractors will need to seek alternatives. Many are already considering other options, such as natural pozzolans.

The Benefits of SCMs

Fly ash and other SCMs enhance concrete properties. The spherical particles of fly ash reduce friction during mixing when concrete is in its early, fluid state, enabling the reduction of mixing water or chemical additives used to improve flow. SCMs chemically react over time in a pozzolanic reaction with calcium and hydroxyl ions in the concrete pore solution to form calcium silicate hydrates. The calcium silicate hydrates increase concrete's long-term strength and reduce porosity and permeability. The slow reaction is especially beneficial in thick concrete pavement and mass concrete applications to prevent cracking and develop long-term strength.

SCMs can reduce the risk of thermal cracking and subsequently provide good long-term mechanical properties. SCMs also help protect concrete from long-term chemical degradation. For example, SCMs reduce porosity through the pozzolanic reaction, slowing the intrusion of chlorides that can cause corrosion of steel reinforcement and sulfates that can cause expansive reactions and cracking. Lastly, SCMs can reduce expansion and cracking from alkali-silica reaction in aggregates containing reactive siliceous materials.

An Uncertain Future

In 2011, the Environmental Protection Agency (EPA) issued a rule regulating the amount of mercury and other air toxins emitted by power plants in response to the 1990 Amendments to the Clean Air Act. To meet these regulations, coal-fired power plants have had to install emission control systems to reduce emissions primarily from sulfur oxides, nitrogen oxides, and mercury. These systems often contaminate the fly ash produced by treating the flue gas with various substances, such as limestone powder, to react with the sulfur producing gypsum and activated carbon to absorb the mercury. Often the products of emission control systems become mixed with the fly ash, reducing its quality and performance in concrete.

In addition to the contamination of fly ash, installation and maintenance of emission control systems can be costly for smaller or older plants, forcing many to be retired. The EIA reports approximately 475 coal-fired power generator closures since the EPA finalized the 2011 regulation. Coal-fired power plants that can make the necessary modifications incur higher costs to produce electricity, which increases the cost of electricity generated by the plants. This has created a more competitive market for other energy sources, such as natural gas, solar, and wind energy.

According to the ACAA's Production and Use Surveys, the amount of fly ash used in concrete products has increased 5 percent between 2011 and 2017; however, the amount of fly ash produced has dropped 36 percent. The American Road and Transportation Builders Association (ARTBA) estimates that concrete production will increase more than 50 percent by 2033. Dwindling fly ash production necessitates the search for alternative sources of this material.

"The Texas Department of Transportation [TxDOT], [like] many other DOTs, has relied heavily on fly ash to improve long-term durability of concrete," says Andy Naranjo, rigid pavements and concrete materials branch manager of TxDOT. "Seasonal power plant outages, changes in coal sources, and power plant closures have significantly impacted the supply of fly ash making it challenging for the fly ash industry to meet the fly ash demand. TxDOT has worked closely with fly ash marketers as they bring in new sources of fly ash from other States and countries, and some unconventional options to ensure the immediate demand is met."

Fly Ash Beneficiation and Harvesting

According to the ACAA, only 64 percent of the fly ash produced in the United States in 2017 was beneficially reused. The large unused quantities of fly ash produced per year are often landfilled or ponded onsite at power plants. Therefore, opportunities exist for excavating or dredging and recovering these materials, a process referred to as harvesting. In addition, coal combustion produces other residuals, such as bottom ash and economizer ash, which may be untapped resources for SCMs.

The primary obstacle to using underutilized coal combustion residuals in concrete is material quality. ASTM International (ASTM) C618 and the American Association of State Highway and Transportation Officials (AASHTO) M295 specify the chemical and physical properties that fly ash must meet for use in concrete mixtures. However, beneficiating or remediating fly ashes that do not meet these specifications can make them acceptable for use. For example, coarse material can be post-processed by classifying or grinding to increase fineness and fly ash with high unburned carbon content can be thermally, electrostatically, or chemically treated to remove carbon or reduce its absorptivity.

Beneficiation is not limited to as-produced fly ash. According to the EPA, more than 310 active landfills onsite at power plants have an average size of more than 120 acres (48.5 hectares) and an average depth of more than 40 feet (12 meters). In addition, more than 735 active surface impoundments have an average area and depth of 50 acres (20 hectares) and 20 feet (6 meters). Presumably, these landfills and impoundments hold vast reserves of materials that operators or fly ash distributors could harvest and beneficiate for use in construction.

As reported in the July–August 2019 issue of the American Concrete Institute Materials Journal, research indicates that applying thermal, mechanical, and/or chemical treatment to fly ashes harvested from landfills can result in fly ashes with very similar performance to the as-produced material. Similarly, bottom ash and economizer ashes benefit from treatments to improve their performance in concrete. After the performance of these materials is proven, the limiting obstacle is modification of the relevant standards and specifications to enable their use. To this end, several research projects recently begun through the Federal Highway Administration's Exploratory Advanced Research Program, the National Cooperative Highway Research Program, and by industry associations and SCM producers and suppliers to better define performance requirements of harvested and beneficiated fly ash and other coal combustion products.

Natural Pozzolans

Another solution to extend the resources for SCMs is to increase production of natural pozzolans. Natural pozzolans are quarried minerals with similar compositions to fly ash, making them also pozzolanically reactive. Minerals in this category include unaltered volcanic minerals such as pumice, perlite, and volcanic ash; altered volcanic minerals such as zeolites; and calcined sedimentary minerals such as clays and shales.

Natural pozzolans have a strong history of use in the United States in the early 20th century for the construction of many landmark bridges and dams. Their use decreased as fly ash came into favor during the late 20th century, but they are experiencing a renaissance as fly ash production decreases and demand for high-quality SCMs increases. In the United States, natural pozzolan producers formed the Natural Pozzolan Association (NPA) in 2017 to represent their growing industry. The NPA reports adding 500,000 tons of new production capacity in North America in 2018 and estimates producing 500,000 tons more in 2019.

Use and research on both raw and calcined natural pozzolans demonstrate excellent performance as SCMs in concrete in terms of fresh and hardened state properties and long-term durability.

Blended Ashes

Another opportunity for extending SCM resources comes from blending materials from different sources. Blending facilitates the use of underutilized materials and conserves the use of high-quality materials, enabling the production of a larger quantity of good quality material for use in concrete. For example, blending an SCM that does not meet the ASTM C618/AASHTO M295 specification for fineness with a finer material can produce an acceptable alternative. Similarly, blending an SCM with a high carbon content with one having a lower carbon content can yield an acceptable level of carbon content.

Blending of SCMs is permitted under ASTM C1697. However, the specification currently only allows the blending of materials that meet specifications for fly ashes, natural pozzolans, silica fume, and slag cement. Off-specification materials are not allowed despite research that shows off-specification fly ashes blended with natural pozzolans or other fly ashes perform quite well in concrete mixtures, as long as the blended material meets the chemical and physical requirements for a fly ash. Furthermore, blending materials such as milled bottom ash with fly ashes and other SCMs presents the opportunity to include more underutilized coal combustion residuals.

"Changes in electricity generation will continue to impact concrete mixture designs into the future," says Michael Praul, P.E., senior concrete engineer with the FHWA Mobile Concrete Technology Center. "However, there are many promising approaches to solving this problem—from beneficiating underutilized or landfilled materials to searching for new sources of SCM materials and optimizing blends for targeted performance. Now is the time to develop viable means to assure the long-term availability of SCMs so we can continue to produce high-quality concrete for the Nation's infrastructure in the future."

Assessment of New Rapid Alkali-Silica Reaction (ASR) Tests. Ongoing research in the TFHRC Concrete Laboratory and the TFHRC Aggregate and Petrographic Laboratory (APL) tests the reliability of two new test methods—the concrete cylinder test and miniature concrete prism test—in assessing ASR mitigation measures. This research is in collaboration with the University of Texas and Oregon State University. Researchers plan to present the results of this project, comparing the performance of reactive aggregates in the lab with field exposure blocks containing SCMs (Class F and Class C fly ash, slag cement, or silica fume), at the Transportation Research Board's 2020 Annual Meeting.

Assessment and Refinement of Concrete Durability Testing Procedures. TFHRC is one of the research organizations looking at durability testing procedures for concrete with and without SCMs in support of the new AASHTO PP84-19 for Performance Engineered Mixtures (PEM) for concrete. The PEM project is assessing a suite of new test procedures for practicality in the lab and relation to performance. One test is electrical resistivity of a concrete cylinder as an indicator of the quality of the pore system. Concrete resistivity depends both on the pore structure and on the pore solution in the concrete. Aspects of this research, including data on concrete with fly ash and slag cement SCMs, are explained in Formation Factor Demystified and Its Relationship to Durability (FHWA-HRT-19-030) at www.fhwa.dot.gov/publications/research/infrastructure/pavements/19030/index.cfm.

Saif Al-Shmaisani is a Ph.D. student in civil engineering at the University of Texas at Austin. Al-Shmaisani has B.S. and M.S. degrees in civil engineering from the University of Texas at Austin.

Maria Juenger, Ph.D., is a professor of civil, architectural, and environmental engineering at the University of Texas at Austin. Juenger received a B.S. in chemistry from Duke University and a Ph.D. in materials science and engineering from Northwestern University.