Fly Ash and Innovation in Concrete

Search the words “fly ash” and you’ll see a lot of results. It's an essential part of modern construction. It’s used in concrete, hot mix asphalt, grout fill, wallboard manufacturing and soil stabilization. Fly ash is an integral part of building around the world, and it’s a great utilization of waste materials, helping to expand the circular economy. 

Coal—the most abundant fossil fuel on the planet—is used in coal-fired power plants around the world. Fly ash is a by-product of burning pulverized coal in these plants. During combustion, mineral impurities in the coal (clay, feldspar, quartz, and shale) fuse in suspension. The fused material cools and solidifies into spherical glassy particles called fly ash that are carried away from the burning zone in the boiler by the flue gases, and then collected by either mechanical or electrostatic separators. 

Using fly ash in construction keeps it out of our landfills, encouraging sustainability across the value chain. Years of research show that fly ash can decrease concrete’s carbon footprint—and its cost—while increasing its strength and workability. Fly ash is typically less expensive than cement, making it a very important ingredient in concrete mix designs for most concrete producers.

It’s flexible, too—depending on the application, the type of fly ash, specification limits, geographic location and climate, fly ash can be used at levels ranging from 15% to 25% (most common) to 40% to 60% (when rapid setting time is not required), reducing emissions by roughly the same amount. This also helps to keep concrete products at an affordable price.

Sounds great, right? But there’s more to the story.

Declining Supply of Fly Ash

We’ve been using fly ash in construction materials since the mid-1900s, but at the same time, we’ve learned that coal-fired power plants have a significant negative impact on our planet’s atmosphere. For us to reduce these emissions, we’ve got to reduce our reliance on fossil fuels. Governments around the world are taking steps to reduce the impact of coal-fired plant emissions, which in turn is affecting the availability of fly ash in many regions. Several market forces are contributing to the decline in the supply of fly ash and the need for concrete producers to seek viable alternatives.

In the United States, coal production peaked in 2008 at 1,172 million short tons and has since declined significantly, falling to 577.9 million short tons in 2023, a decrease of approximately 50.7% over fifteen years. This comes as coal-fired power plants are declining due to the increasing popularity of cheaper renewable energy and cleaner forms of fuel like natural gas.

According to the most recent survey by the American Coal Ash Association (ACAA), "use of coal fly ash in concrete [in 2022] declined from 11.9 million tons to 10.9 million tons. Concrete producers and consumers indicated a desire to use more fly ash, but several regional markets continued to be affected by shifting supply dynamics associated with closures of coal-fueled power plants." Future forecasts also suggest that, in 2026, coal consumption will fall by 7%, and coal production will fall by 3%.

So what does it all mean? It offers concrete producers an excellent opportunity to continue to design performance mixes that optimize cement content, while continuing to innovate with other SCMs and solutions in their quality concretes.

Fly Ash Alternatives

In a CarbonCure-hosted webinar, Adam Auer, President and CEO of the Cement Association of Canada, and Matt Dalkie, Technical Services Engineer at Lafarge Canada Inc., discussed several new cement and concrete technologies that are helping producers meet the demands of the growing low-carbon concrete trend. These technologies can also act as fly ash alternatives.

1. Blended Cements

Portland Limestone Cements (PLCs) use uncalcified limestone in the cement grinding phase of the manufacturing process and can reduce the carbon footprint of concrete by 5-10% and may be more cost-efficient.

2. Other SCMs

Other SCMs also reduce the amount of cement required in a concrete mix, thereby reducing the carbon emissions by up to 30%.

Slag

Slag is a by-product of steel production. Slag reacts with both the water (latent hydraulic reaction) and the hydrated cement paste (pozzolanic reaction) in concrete and can replace 40-50% of the cement in a mix and up to 90% for some specialty applications. The carbon reduction from slag can be up to 30% depending on the replacement level specified. 

Silica fume (SF)

Silica fume (SF) is a by-product of silicon alloy production. The addition of SF decreases the permeability and diffusion of the concrete and is typically used at replacement levels between 3% and 10%. Replacement levels above 10% can lead to further durability improvements, but the workability and finishability of the concrete can be problematic.

Metakaolin

Metakaolin is a dehydroxylated form of the clay mineral kaolinite and can be used as cement replacement in concrete. Typical replacement levels for metakaolin range from 5% to 10%.

3. Carbon mineralization

Innovation in carbon capture, utilization and storage (CCUS) technologies enable captured carbon dioxide (CO₂) emissions to be injected into concrete, reducing the need for cement. This solution can be stacked alongside other alternative SCMs in concrete to reduce dependency on fly ash.

CarbonCure is one such solution. CarbonCure’s retrofit technologies can be installed in any ready mix or precast concrete plant today. It injects CO₂ into concrete in order to maintain its strength and performance. These improvements enable concrete producers to realize cost savings through cement efficiencies.

Carbon Credits for Carbon Utilization

Unlike fly ash, concrete producers utilizing carbon utilization technologies can tap into additional revenue through CarbonCure's generation and sale of high-value carbon credits. These credits, each representing a metric ton of avoided CO₂ emissions, are then purchased by corporations in the voluntary carbon market seeking to offset their emissions. CarbonCure manages the entire carbon credit process and shares the revenue with its producer partners, providing a direct financial incentive beyond the material science benefits of carbon mineralization.

This carbon credit revenue opportunity is not available with the use of fly ash or other SCMs due to their widespread and longstanding use across the industry. In contrast, carbon credit buyers are eager to incentivize the adoption and increased production of lower carbon concrete, catalyzing newer solutions such as CarbonCure to advance concrete sustainability.

If you’re interested in learning more about CarbonCure, contact us.