Keep in mind clinker replacements such as fly ash often contain heavy metals that are hazardous to human health.- Tori Wickard, Perkins&Will
Can Concrete Become Carbon Neutral?
The Global Cement and Concrete Association (GCCA) recently released a “carbon ambition statement” announcing that its 40 members aspire to deliver carbon-neutral concrete by 2050. A laudable goal, but given concrete’s chemistry, is it even possible?
One problem is the cement. The majority of cement’s CO2 emissions (around 60%) come from the materials themselves, according to GCCA. Limestone emits CO2 when it is heated to produce Portland cement clinker. And there’s currently no alternative to limestone—just various ways of using less clinker by adding more additives like slag, fly ash, calcined clays, etc.
The remaining 40% of CO2 emissions mostly come from the fuel needed to heat the limestone. GCCA notes that conventional fossil fuels burned for this purpose could be replaced by alternative fuels—in fact, the rate of replacement is already 5.6% globally. But lest you think all these cement manufacturing facilities are running on solar and wind power, the replacements are typically highly combustible byproducts of industrial processes, the most common being petcoke, sewage sludge, and bone-meal, according to IntechOpen. Renewable electricity sources would more likely be used to replace indirect emissions.
In addition to relying on alternative fuels, GCCA says that it will achieve carbon-neutral concrete by enhancing recarbonation and deploying carbon-capture technologies. Carbon-capture technologies do currently exist (see CarbonCure-Capturing Carbon in Concrete Blocks) and there’s hope for bringing them to scale. As for recarbonation, concrete carbonizes as it dries, absorbing CO2 from the air, but once the surface hardens, the concrete becomes less permeable, limiting more re-absorption. By its own measure, GCCA says this mechanism currently sequesters 25% of the CO2 emissions released during clinker processing. More could be sequestered if the concrete is crushed, stockpiled, and “left exposed to the air before reuse,“ according to GCCA. But waiting for a concrete structure to be demolished and sit exposed to the air is a long time to wait for recarbonization given the time value of carbon.
GCCA has set a high goal, but at least its ambition statement acknowledges that innovation and ingenuity will be required. Furthermore, the statement’s policy framework and overview on technologies clearly articulates which levers the concrete industry has available to it today, offering a clear path to make progress.
Published November 9, 2020 Permalink Citation
Pearson, C. (2020, October 22). Can Concrete Become Carbon Neutral?. Retrieved from https://www.buildinggreen.com/newsbrief/can-concrete-become-carbon-neutral
I agree with Tori's comment. We should be very wary of putting coal fly ash in building materials given how toxic it is.
fly ash and other things
And remember that fly ash comes from the burning of coal, a non-renewable and climate-wrecking source of energy.
There are a lot of other substitutes for Portland cement in research and development. Does the GCCA see any promise in these?
I don't see any accounting in this about transportation emissions - the 60/40 appears to be about the materials.
Fly Ash is Beneficial in Concrete and Safe
The use of fly ash and other supplemental cementing materials like ground granulated blast furnace slag (GGBFS), have been used in concrete for decades and will continue so long as there are economical sources of each product. Not only do these by-products of other industries reduce the amount of cement, and therefore embodied CO2 within a paticular mixture, they greatly improve the physical properties of the concrete, including strength, durability, permeability, resistance to certain chemcial attacks, etc. Concrete containing fly ash or slag will last longer thereby reducing the need to replace a paticular structure or element. Keep in mind, when using these products, portland cement must also be used as the fly ash and slag rely on the cement chemistry to function properly.
I encourage you to check out the following webites about fly ash and concrete or speak to your local ready mix producer to learn more about these products and how they are used as well as safety. In my engineering opinion, concrete should have as much fly ash and or slag as possible provided the structural intent, constructibilty and costs are met.
What about end of life?
@John What about at end of life when the concrete is broken up and used for fill? I am not aware of any tests that address how much fly ash leaches out under those conditions.
Fly Ash End of Life
@Brent It's an interesting question to which I do not know the direct answer. However, I can speculate that since the fly ash undergoes a chemical reaction during hydration, it's not really fly ash anymore once hardened. The way I look at it, we currently have byproducts of coal-fired power plants. We can either directly landfill that byproduct, fly ash, or we can put it to work in concrete where it can lower concrete's carbon footprint by replacing a portion of carbon-dense cement while at the same time producing higher quality, more durable, longer-lasting, better-performing concrete before it's eventually landfilled decades from today in the form of demolition rubble.
Fly Ash and Leaching
@brent Good question. The American Concrete Institute (ACI) 232.2R "Report on the Use of Fly Ash in Concrete" addresses this topic (see link below). To my knowlegde there have been no issues with leaching of materials when a concrete contains fly ash. In fact, we are all driving on, landing planes, etc. on pavements that contain fly ash every day and leaching has never been a concern. As I stated before, concrete containing fly ash is less permeable and in fact becomes less and less permeable with age, therefore it is less likely to have leaching. My contention is that concrete with fly ash, whether in service or being used as fill at the end of it's life, poses no leaching concerns. Here are a couple of excerpts from the ACI document in regards to leaching:
Section 4.2.11 - Calcium hydroxide produced by hydrating cement is water-soluble and may leach out of hardened concrete, leaving voids for the ingress of water. Through the pozzolanic reaction, fly ash chemically combines with Ca(OH)2 and water to produce C-S-H, thus reducing the risk of leaching Ca(OH)2. Additionally, the long-term reaction of fly ash refines the pore structure of concrete to reduce the ingress of water containing chloride ions. As a result of the refined pore structure, permeability is reduced (Manmohan and Mehta 1981; Electric Power Research Institute 1984).
Section 4.2.15 - 4.2.15 Efflorescence—Efflorescence is caused by leaching of water-soluble Ca(OH)2 and other salts to external concrete surfaces. The leached Ca(OH)2 reacts with CO2 in air to form CaCO3, the source of the white discoloration on concrete. The use of fly ash in concrete can be effective in reducing efflorescence by reducing permeability as well as by consuming Ca(OH)2 in the pozzolanic reaction.
Section 9.7 dicusses how fly ashes, when used in a cement mixture, are used to stabilze wastes and wastewaters includeing heavy metals.
From Section 9.7 - Fly ash immobilizes many toxic heavy metals as relatively insoluble hydroxides or carbonates. This immobilization is accomplished by maintaining a pH in the range between 8 and 12. Other additives are sometimes used to treat the wastes and decrease leachability of various organic compounds. When solidifying hazardous wastes with fly ash, treatability studies should be conducted on the combined wastes and solidifying agents so that appropriate results are obtained (Roy et al. 1991; Roy and Eaton 1992).
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