Environment

Coal-ash spheres keep concrete from cracking

Coal-ash spheres keep concrete...
Drexel researchers have recycled coal ash into spheres that prevent cracks forming in concrete as it cures
Drexel researchers have recycled coal ash into spheres that prevent cracks forming in concrete as it cures
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A microscope image of one of the porous SPoRA spheres
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A microscope image of one of the porous SPoRA spheres
Drexel researchers have recycled coal ash into spheres that prevent cracks forming in concrete as it cures
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Drexel researchers have recycled coal ash into spheres that prevent cracks forming in concrete as it cures

Concrete is made of a combination of cement, an aggregate such as gravel, and water. If that mixture dries too quickly, then cracks can form within it as it cures. Now, however, scientists have determined that the use of coal-ash spheres can help keep that from happening.

Typically, in order to ensure that poured concrete doesn't dry too rapidly, contractors take measures such as covering it with membranes or spraying it with water.

Doing these things can be a hassle, though, which is why some groups have developed lightweight, porous aggregate materials that initially absorb water and then slowly release it within the concrete matrix. The idea is that by using these, the concrete will dry at a more controlled, uniform rate without having to be covered or sprayed.

Looking for a cheaper, more abundant form of such porous aggregates, researchers at Philadelphia's Drexel University turned to the "fly ash" waste that is produced when coal is burned at power plants. Led by Asst. Prof. Yaghoob Farnam, they combined the ash with bonding chemicals, formed it into tiny spheres, then baked those spheres at a temperature of 1,160 ºC (2,120 ºF) for a few minutes.

The resulting material is known as SPoRA, which stands for "spherical porous reactive aggregate." Not only is it cheap and easy to manufacture, but it can absorb almost half of its weight in water – this is considerably better than some commercially-available aggregates. That water is gradually dispersed from the spheres and into the concrete matrix as it dries and cures, ensuring that no cracks form.

"The solution we came up with involved recycling this waste product, coal ash, into a porous, lightweight aggregate with excellent performance characteristics that could be produced at a lower cost than current natural and synthetic options," says Farnam. "This material and process would not only benefit the concrete industry by improving the quality of their products, but it could also help keep coal ash out of landfills."

A paper on the research was recently published in the journal Cement and Concrete Composites.

And as a side note, scientists at Rice University previously reported that fly ash could also be used as an eco-friendly alternative to traditional Portland cement.

Source: Drexel University

11 comments
CarolynFarstrider
I'd rather hope that fly ash from burning fossil fuels such as coal, generating greenhouse gases and prompting global warming, rapidly becomes a thing of the past.
Worzel
Great! How will people buy it? Will it be mixed in with cement, or supplied in separate bags? How much will be needed per Cubic Metre of concrete, how does it effect the strength of the set concrete and most importantly, what will it cost? Without answers to these and more questions, it's only of academic interest.
Aross
Right on CarolynFarstrider. I would have thought that something like pumice would also do the trick. it is readily available because there is a whole island of the stuff floating in the pacific ocean. Better still why don't we use tufa as the Romans, the inventors of concrete, did.
Sambo
While fly ash will become increasingly scarce in Canada and the US, it'll be available from China, India, etc for many years to come.
Sibyl
I would think that crushed cement blocks would work as well.
Chuck Goecke
I would hope that, rather than using fresh new fly ash, from an operating coal fire power plant, this process could use old stockpiled or landfilled fly ash, perhaps from some waste ponds. I suspect that there is billions cubic years of the stuff laying around, a century supply or more. As for China and India, making the fly ash a useful product will perhaps make them try harder to capture it. I suspect that fly ash in those countries now mostly just ... flies.
Signguy
How come nobody ever mentions hempcrete which has tons of advantages over most concrete; number one being it filters the air from toxins, it's lighter, hemp is a weed that grows anywhere, and it's incredibly strong!
lucius
@ Signguy While hempcrete has many positive qualities, its compressive strength is only 1/20 of residential grade concrete, which means it cannot be used to build foundations or load-bearing walls, and that limits its suitability for many construction purposes.
ljaques
Have the spheres been purified? We don't want all that radioactivity and heavy metals in our concrete. Hopefully, fly ash will soon diminish in the environment. Coal is cheap, and isn't worth it.
Saigvre
Fly and restricted ashes are pooled for another 50 years before any digging need be done to access it. Meanwhile there is a ton of testing to be done! The people looking for 'bags' of it to be a practical thing can take my laugh, because there's nothing to do but file the DOE application paperwork, know a source (power plant, shipper using ash as dunnage, you drew it we got it,) and have a kiln of the type described. For engineers, it's a matter of taking a safety factor of 18 and considering hitting 10 (and buying instruments for your driver, who shouldn't be picking up lanthanide slag-ash unless you totes planned it. Talk about a hard case for automated driver tech!) Looking forward to those hempcrete citations pointing out firmer, less shaggy points of performance. >has that ash been purified? No, just separated by DOE hazard class. It's kind of the point that the stuff is stuck in a bridge, footing, arch or caisson 120 years. Otherwise it's cake fill for mines, refined for urban cement, etc.