One way to lower the emissions of concrete is to reduce the amount of cement. Cement is also one of the most expensive ingredients in concrete so any material that can replace it efficiently is of great interest to concrete producers.
Fly ash, sometimes called flue ash, has been a popular supplementary cementitious material (SCM) since the mid-1900s. Years of research show that fly ash can decrease concrete’s carbon footprint—and its cost—while increasing its strength and workability.
For most concrete producers, fly ash is an important ingredient in concrete mix designs. 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—and helping to keep concrete products at an affordable price.
However, several market forces are contributing to the decline in the supply of fly ash and the need for concrete producers to seek viable alternatives.
Declining Supply of Fly Ash
Fly ash is a by-product of burning pulverized coal in power-generating 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.
According to a survey by the American Coal Ash Association (ACAA), approximately 37 million tons of fly ash were produced in the U.S. in 2018. Of this, approximately 20 million tons were used in concrete and other applications, the remainder was disposed of in landfills. There are several reasons for the disposal:
- Construction cycles vary so in times of decreased demand, power plants have to dispose of fly ash they can’t sell or store.
- The Clean Air Act requires the injection of activated carbon into the combustion steam, which makes the fly ash unusable in concrete.
- The chemical composition of fly ash varies with the type of coal from which it was derived—not all the fresh material from a coal plant fulfills concrete industry requirements.
Overall, the ACAA estimates that the gap between demand and supply of concrete-grade fly ash is about 25% in the U.S. As well as the disposal of unusable fly ash, the decline of coal as a power source is also contributing to the decreased supply. Coal-fired power plants are declining due to the increasing popularity of renewable energy and cleaner forms of fuel like natural gas.
According to LafargeHolcim, however, availability is actually a regional issue and the gap between demand and supply is much greater in some areas. Even producers operating in areas where fly ash is still abundant should be looking to the future for viable alternatives.
Fly Ash Alternatives
In a recent webinar, Adam Auer, Vice President of Environment and Sustainability at 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 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)
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 is a dehydroxylated form of the clay mineral kaolinite and can be used as cement replacement in concrete and 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 back into concrete to strengthen it, reducing the need for cement. This technology can be stacked alongside other alternative SCMs in concrete to reduce dependency on fly ash.
CarbonCure is one such solution. CarbonCure’s retrofit technology can be installed in any ready mix concrete plant today. It injects CO2 into wet concrete in order to improve its strength and performance. These improvements enable concrete producers to realize cost savings through mix optimization.
If you’re interested in learning more about CarbonCure, contact us.