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Yves here. Readers had a surprisingly vigorous discussion in comments to a recent post about the viability of recycling concrete. So it seemed a longer discussion of its environmental costs and possible remedies was in order.
By Wil Srubar, Assistant Professor of Architectural Engineering and Materials Science, University of Colorado Boulder. Originally published at The Conversation
Humans produce more concrete than any other material on the planet. It is the literal foundation of modern civilization – and for good reason.
Concrete is strong, durable, affordable and available to almost every community on the planet. However, the global concrete industry has a dirty little secret – it alone is responsible for more than 8% of global carbon dioxide emissions – more than three times the emissions associated with aviation. Those emissions doubled in the past two decades as Asian cities grew, and demand is continuing to expand at an unprecedented rate.
It’s also one of the most difficult industries to decarbonize, in part because manufacturers are typically hyperlocal and operate on slim margins, leaving little to invest in technologies that could lower emissions.
However, difficult does not necessarily mean impossible.
Architects, engineers, scientists and cement and concrete manufacturers around the world are investigating and piloting several new strategies and technologies that can significantly reduce the carbon footprint of cement and concrete. Here are a few of them, including one my team at the University of Colorado is working on: figuring out ways to use all-natural microalgae to solve concrete’s biggest emissions problem – cement.
It Doesn’t Have To Be 100% Cement
The primary culprit behind concrete’s climate impact is the production of portland cement – the powder used to make concrete.
Cement is made by heating limestone rich in calcium carbonate to over 2,640 degrees Fahrenheit (1,450 Celsius). The calcium carbonate decomposes into calcium oxide, or quicklime, and carbon dioxide – a climate-warming greenhouse gas. This chemical reaction, what the Portland Cement Association calls a “chemical fact of life,” is responsible for a whopping 60% or so of cement-related emissions. The remainder comes from energy to heat the kiln.
One of the most promising short-term strategies for reducing concrete’s carbon footprint uses materials like fly ash from coal plants, slag from iron production, and calcined clay to replace some of the portland cement in concrete mixtures. These are known as supplementary cementitious materials.
Using 20% to 50% fly ash, slag or calcined clay can reduce the embodied carbon of concrete mixtures by about the same percentages.
Another method uses small amounts of ground limestone to replace some of the cement and is becoming a best practice. After rigorous testing, the California Department of Transportation recently announced it would allow portland-limestone cement mixes, known as PLC, in its projects. With 5% to 15% ground limestone replacing cement, PLC can reduce emissions by about the same amount. California’s decision quickly led other states to approve the use of PLC.
Many researchers are now advocating for the adoption of limestone calcined-clay cement, which contains about 55% portland cement, 15% ground limestone and 30% calcined clay. It could cut emissions by more than 45%.
What Electrification and Carbon Capture Can Do
Cement plants have also started testing carbon capturetechnologies and electric kilns to slash emissions. But carbon capture is expensive, and scaling the technology to meet the demand of the cement and concrete industry is no easy feat.
Kiln electrification faces the same barriers. New technologies and large capital investments are required to electrify one of the world’s most energy-intensive processes. However, the promise of zero combustion-related emissions is enticing enough for some entrepreneurs and cement companies – including those interested in using 100% solar energy for cement production – who are racing to find solutions that are both technologically and economically viable at scale.
The Inflation Reduction Act, which Congress passed in August 2022, could help put some of these technologies to wider use. It includes funding for modernizing equipment and adding carbon capture capabilities, as well as tax credit incentives for manufacturers to cut their emissions.
Going Cement-Free, Possibly with Algae
Another strategy is to produce functionally equivalent materials that contain no portland cement whatsoever.
Materials like alkali-activated slag or fly ash cement concrete are produced by combining slag, fly ash or both with a very strong base. These materials have been shown to cut carbon emissions by 90% or more, and they might meet scale and cost criteria, but they still face technical and regulatory challenges.
Some examples of low-carbon, portland cement-free concrete products that have gained market traction include wollastonite-based modular components, compressed earth blocks and prefabricated biocement products – including those produced using photosynthetic, biomineralizing microalgae.
Algae have also been used as an alternative biofuel for heating cement kilns, and algae cultivation systems have also been linked with cement production to capture carbon.
My team at the University of Colorado Boulder and I are looking into the use of algae-derived limestone for portland cement production, which could help eliminate 60% of the emissions associated with cement manufacturing. This technology is appealing because it is plug-and-play with conventional cement production.
Using Concrete to Lock Captured CO2 Away
Engineers are also experimenting with injecting captured carbon dioxide into concrete as well as using aggregates made of carbon dioxide in place of gravel or sand that is mixed into concrete.
It’s an exciting concept, but so far injection has yielded limited carbon dioxide reductions, and production of carbon-dioxide-storing aggregates has yet to scale up.
A Growing Problem
Ultimately, time will tell whether these and other technologies will live up to their promise.
What is certain is that there has been a worldwide reckoning within the cement and concrete industry that it has a problem to solve and no silver bullet solution. It may take a suite of solutions tailored to both local and global markets to address the immediate and long-term challenges of keeping up with an ever-growing population and rapidly changing climate.
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Looks like an anchor is missing an end tag near the end.
I believe it’s fixed.
Yields of microalgae cultured in ponds are estimated at about 20-40 tons per Ha or 7,5-15 tons per acre and per year in terms of dry biomass. Global demand for cement is about 2Gt/year (2.000.000 t/y).
If 1 ton of dry algae biomass yields, lets say 1 ton of cement, 500.000-1.000.000 Ha of algae ponds would be theoretically needed to cover global cement demand. That is tall order but in any case microalgae ponds have really enormous potential for carbon storage. I wish the best to projects like this.
I f&cked all the numbers didn’t I?
Cement demand: 2.000.000.000 t/y divided by 20-40 t/Ha-year translates in 50-100 million Ha of algae culture ponds. Isn’t it?
100 million hectares is basically…. Texas. Not impossible when scattered around the world, but still a lot. If you could use salt water, then that would open up a lot of desert areas for carbon sequestration.
“If you could use salt water…”
Yep. That’s the key question. If we’re limited to freshwater, 75 million Ha would cover nearly 20% of the world’s freshwater bodies. That’s too much alteration of lake and river ecosystems.
But if you can do this in the oceans, it drops down to about 0.2%, which should be small enough to keep production from dominating ecosystems. On the other hand, production in salt water would undoubtedly be more expensive.
We need to start a f&cked numbers club ;-) I had an immediate flashback to nature’s cement – the humble, but almost eternal, stromatolite. ” An (ancient) calcareous mound built up of layers of lime-secreting photosynthetic cyanobacteria and trapped sediment, found in precambrian rocks as the earliest known fossils – and still being formed in lagoons in Australia.” Gotta be a “natural cement” clue there. “These microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineral microbial mats.” Shades of Roman cement recipes, with the secret ingredient being pig’s blood. Some animal protein connection going on. And why can’t cement be used to get rid of some of our mountains of plastic? Plus, I’m pretty sure that nothing makes a more tenacious glue than a soybean – so that would be easy to scale up.
A link to that would be cool :)
Possibly this starting with Solarjay’s comment?
Ups. 2 Gt is: 2.000.000.000 t/y. so 5-10 million Ha of ponds. More that tall order it is unfeasible. Yet I still consider it an excellent way to store carbon though total global CO2 emissions are about 35.000.000.000 CO2 tons according to Our world in data. Jesus!
No feasible way of creating “vertical” ponds so surface area is not an issue?
It is possible. More expensive and less productive in terms of volume as less light is available at the bottom unless artificial illumination is used. But it is possible. You can try to select LEDs with the proper wavelength. There is also reduced gas exchange unless you pump air from the bottom. If you pump CO2 enriched air it can get better.
Is Roman Concrete similar to PLC?
Roman concrete
https://en.wikipedia.org/wiki/Roman_concrete
Seems like figuring out how the Roman made it without using NG fired cement kiln would be a good idea.
+1
This was being researched by the Lawrence Livermore lab, but we don’t hear anything about it, and it should be a big deal – MUCH less CO2, better concrete (lasts millennia as opposed to mere decades)… The cement lobby?
We bought a house and outside the patio doors there was a steep step down to a small cement pad. After a few years of putting up with that, we hired a ‘cement guy’ and his crew to put in a new patio, based on a patio we had seen in the Parade of Homes. It was expensive. They busted out the little pad, created forms and replaced the pad with another much larger and several inches thicker patio. The selling point was the finish, pressed with a texturing mold, colored to look like real stone, and sealed with acrylic.
Every summer after that Husband, usually on a hot day in July, cleaned the patio and waited for it to thoroughly dry… otherwise bubbles in the finish coat. Then he put on a respirator* and used xylene to melt last summer’s coat, spreading it out in the thinner areas and added new. It wasn’t a choice, it had to be done or in the extremes of weather here in Colorado, the top coat of plastic would crack and flake, leaving the cement underneath unprotected, and we didn’t want our dog to snuffle that stuff up his schnauzle.**
This got old. The patio gathered heat all day and released it at night at a time we wanted to open up the house to let in the cooler air, and the finish looked less swell with each passing year. New products came out like Trex and others that are similar. The patio had to go. So Husband went looking for someone to do the work at a price we could afford.
Have you done this, gone looking for a contractor, who knows a ‘cement guy’, who is willing to bust out your old patio, especially in these times of Covid? There’s not a contractor building anything at all here who isn’t short-handed. Supplies short, demand higher than ever for labor, therefore even more expensive. Lots of crews willing to put in cement, very few (a different few) who are willing to stand outside with a jackhammer, bust it back out and haul it off… ’cause very limited number of places willing to recycle the old concrete.
Our contractor found such a guy, who bid $1500 for the job, with the understanding that the work would be done this September. But then other things happened and now it looks more like spring and the contractor isn’t returning our calls. Patio looks even crappier, rubber mats have to go back down to provide traction for our elderly terrier, in his haste this winter to find an acceptable poop spot pronto without slipping and falling on his old backside.
Bother, first world problems. Not surprised to see the article is from Colorado, a lot of algae research going on here, including CSU campus.
*But first there was some wooziness and a lot of brain cells died, sharp words might have been spoken (I can’t recall exactly what was said), and then the respirator was worn faithfully after that.
**Spellcheck says that word doesn’t exist, but neither party is known for it’s creativity, so it stays.
Concrete pavers over a compacted base of pulverized recycled concrete would have been the better option, 20/20 hindsight of course.
But acrylic over concrete for an application exposed to the elements? Who sold you that? It’s form over function, the demise of every aspect of longevity, from appliances to cars, for the sake of superficial bling.
One of the big issues with this and all other proposals to reduce the impact of construction materials, is that its simply easier and safer (in terms of liability) to specify virgin materials. The use of pfa in concrete has been common for a long time, but despite it being a waste product, its use is limited in many uses (mostly lower grade bricks and fill) for liability and regulatory reasons.
The obvious solution is to compel the use of low carbon concrete and let the builders choose the most appropriate type. But as always with novel construction methods, there is the risk that in 20 or 30 years time there will be a nasty surprise to be found about the longevity of new types of concrete or other structural materials. Mind you, even virgin materials can be highly problematic if you don’t do basic things like check mica content.
Thinking the same myself, especially in this current mindset of cutting and running with short term gains over long term gains at the expense of long term sustainability. Add to that the corruption inherent in approvals and production to specifications, and we’re stuck with an infrastructure falling apart at two thirds of its expected lifetime and nowhere to go but the taxpayers to recoup the cost of the failures.
Fly ash has a little problem. Some coal fly ash contains radioactive residues: “Because of coal nature, fly ash represents a significant drawback with presence of radionuclides such as Ra, Th and K.”
“Processing and uses of fly ash addressing radioactivity (criticalreview)”
https://pubmed.ncbi.nlm.nih.gov/30390998/