October 18, 2023
The significant lifespan, durability, strength, and cost advantages of concrete make it the world’s most widely used material for constructing our buildings, roads, bridges, and other vital infrastructure. Unfortunately, paired with all the attributes of this essential building material comes a high carbon footprint. In fact, making concrete’s key ingredient—ordinary Portland cement (OPC)—is responsible for an estimated 8 percent of the world’s anthropogenic CO₂ emissions.
Optimizing the use of lower-emission cement blends is critical to meeting the construction industry’s sustainability ambitions. One proven way to achieve these goals is to use supplemental cementitious materials (SCMs), such as fly ash, as a partial replacement of OPC. Doing so yields significant environmental benefits, as each ton of fly ash that is substituted for OPC in concrete mix designs saves approximately one ton of CO2 emissions. It also improves concrete’s performance properties while significantly reducing the amount of coal-ash waste from power plants that is disposed of in landfills.
Coal is the fuel source for approximately one-quarter of electricity generation in America and produces large volumes of coal combustion products (CCPs)—primarily fly ash and bottom ash. As the name suggests, fly ash, with its extremely fine and lighter particle sizes, flies up with the exhaust gases during combustion and is captured by emission control devices. Bottom ash, with its larger and heavier particle sizes, falls through open grates to the bottom of the furnace.
Used as an SCM in concrete for decades, fly ash is classified based on its chemical and physical composition. Class F—produced by burning anthracite or bituminous coal—is considered pozzolanic and contains a high percentage of silicon, aluminum, and iron oxides, as well as up to 18 percent calcium oxide. Class C—produced by burning lignite or sub-bituminous coal—exhibits both pozzolanic and some cementitious properties and contains less silicon, aluminum, and iron oxides, as well as calcium oxide in excess of 18 percent.
Bottom ash has very similar chemical and pozzolanic properties to fly ash. Some elements, such as silicon and iron, are generally slightly higher than in fly ash, while others, such as sodium, potassium, and calcium, are generally slightly lower. Its coarse particles require grinding to uniform shape, size, and specified fineness levels to achieve desired pozzolanic activity and workability in concrete.
Coal Ash Pond
Sourcing reliable supplies of fly ash has become increasingly challenging for concrete producers as electric utilities steadily decommission their coal-fired power plants in the U.S. due to stringent environmental regulations and rising operating expenses. At the same time, the amount of fly ash used in concrete has increased significantly—approximately 29% from 2009 to 2019—and annual beneficial recycling volumes have now grown to nearly 12 million tons annually.
As the availability of fly ash continues to decline and industry demands for green building materials increasingly intensify, ash marketers are continually exploring opportunities to boost dwindling supplies of this important SCM. A current focus is on harvesting the estimated two billion tons of CCPs in landfills and surface impoundments. With nearly 4.5 million tons of CCPs reclaimed in 2021—a 12 percent increase over 2020—the trend of using harvested ash for beneficial reuse in concrete is rapidly gaining momentum throughout the building and construction sector.
ASTM E3183–18, “Standard Guide for Harvesting Coal Combustion Products Stored in Active and Inactive Storage Areas for Beneficial Use”, provides a framework for harvesting CCPs from a power plant’s active and inactive storage areas. This resource includes a wealth of information covering the planning and scoping of projects, detailed characterization of wet or dry material, harvesting implementation and more.
The CCPs in landfills and retention ponds can be fly ash, bottom ash, or a comingled combination of both. While ash harvested from these sites is expected to retain pozzolanic properties, it often does not have the proper chemical and physical characteristics to achieve the expected properties of concrete and meet target specifications. For example, one common problem is a high loss-on-ignition (LOI), which is typically required to be below 1 percent for use in concrete (ASTM specifications require less than 6 percent). Moisture content also needs to be reduced to specified limits of 3 percent, larger-sized particles require milling to improve fineness and uniformity, and sulfur trioxide concentrations may exceed specified limits of 5 percent due to the presence of FGD materials in the reclaimed ash.
Because of the wide-ranging variations in chemical and physical characteristics, a beneficiation step is commonly necessary to condition harvested ash into a high-quality, concrete-grade product that meets regulatory standards and commercial expectations. Well-established beneficiation technologies and processing techniques are available to remove excess moisture and carbon, passivate the effects of carbon that remains, and mitigate the effects of emissions control technologies on the material.
A comprehensive evaluation of the quality of CCPs—derived from boring samples taken at various locations and depths of a storage area—is important in the scoping of harvesting opportunities and determining the feasibility of excavation. This also establishes the beneficiation techniques that are needed to meet specification standards and market requirements for use in concrete. The findings from this detailed characterization of the wet or dry material disposed under varying conditions and over long periods of time are used to create a roadmap for harvesting activities.
ASTM C618, “Standard Specification for Coal Ash and Raw and Calcined Natural Pozzolan for Use in Concrete”, is the is the most widely used coal-ash specification in the U.S. It establishes the chemical and physical property requirements that the coal ash material must meet and the laboratory testing that is required for moisture and oxide content, loss of ignition (LOI), fineness, strength activity index, and other material characteristics. Under AASHTO M295, the American Association of State Highway and Transportation Officials makes available virtually the same coal ash specification.
Comprehensive Evaluation of Coal Ash
In the ongoing pursuit to move the market to lower-emission cement blends, most major U.S. cement manufacturers have transitioned away from producing Type I/II OPC, instead supplying Type IL PLC (portland limestone cement) exclusively. With Type IL PLC, ASTM and AASHTO specification standards allow up to 15 percent of OPC clinker to be replaced with finely ground limestone, which reduces the CO2 footprint of concrete by 10 percent. Similar percentages of SCMs, including “as-produced” fly ash and beneficiated CCPs harvested from disposal sites, can also be used to further reduce the carbon intensity of concrete.
Product-development testing and application evaluations by the cement and concrete industry have shown that Type IL PLC has similar performance to Type I/II OPC. Slight behavioral differences have been noted by users, however, that may require adjustments to the mix formulation and further testing to achieve desired performance properties. Based on the reported field observations, additional data and investigative research on Type IL PLC—with and without the inclusion of CCPs in the concrete mix design—are needed to assess potential behavioral nuances and their impact on long-term performance.
As the construction sector progresses in its journey to decarbonize by midcentury, the market will increasingly shift to lower-emission cement blends that rely on PLC and SCMs, such as CCPs. The availability of materials, such as fly ash, is continuously changing and new low-carbon cementitious alternatives, such as harvested CCPs, are continuously evolving. As the industry continues to explore innovative concrete mix formulations, extensive testing and in-depth performance assessments will be critical to providing stakeholders with the confidence to fully embrace these changes.
