Environment

Purified fly ash used to produce greener, stronger concrete

View 2 Images
A microscope image of the heavy-metal-free fly-ash-based cement
Tour lab/Rice University
Study lead authors Wei Meng (left) and Bing Deng, with a sample of concrete made with the purified fly ash
Gustavo Raskosky/Rice University
A microscope image of the heavy-metal-free fly-ash-based cement
Tour lab/Rice University

Production of the cement used in concrete is a major source of greenhouse gas emissions, which is why some people have tried replacing it with fly ash. A new technique makes that fly ash more eco-friendly itself, resulting in concrete which is both greener and stronger.

When producing widely used Portland-style cement, calcium and other ingredients must be heated to very high temperatures. It has been estimated that the processes by which that heat is generated are responsible for 5% to 8% of all human-made CO2 emissions.

With this problem in mind, scientists have tried substituting fly ash for some of the cement used in concrete mixtures (the other ingredients being sand and/or gravel, and water).

Fly ash is a waste product left over from the burning of coal, and while it has qualities similar to those of cement, it also contains toxic heavy metals. If concrete incorporating that ash were to be used in roads or sidewalks, it's possible that those heavy metals may gradually leach out of it and into the surrounding environment.

Seeking to address that shortcoming, scientists at Rice University in Texas developed a process that begins with fly ash being mixed with electrically conductive carbon black powder.

Drawing on a technique known as flash Joule heating, that mixture is then placed between two copper or graphite electrodes (connected to a capacitor), which deliver a short electric pulse to the material. The application of that current causes the mixture to rapidly heat to a temperature of about 3,000 ºC (5,432 ºF), causing the heavy metals to evaporate into a steam which is captured in a vacuum chamber.

"By using this method, we can eliminate the heavy metals from coal fly ash with very high efficiency," said postdoctoral research associate Bing Deng, co-lead author of the study. "For different heavy metals like arsenic, cadmium, cobalt, nickel and lead, the removal efficiency is up to 70% to 90% in just one second."

Study lead authors Wei Meng (left) and Bing Deng, with a sample of concrete made with the purified fly ash
Gustavo Raskosky/Rice University

As an added benefit, concrete made with the purified fly ash is also stronger than that made with pure cement. More specifically, when the scientists replaced 30% of the cement in a concrete mixture with the ash, the resulting concrete was found to be 51% stronger and 28% more elastic than control samples.

"This is very meaningful for structural engineering and the construction industry because stronger structures can be built with less cement," said postdoctoral research associate Wei Meng, another co-lead author.

A paper on the research was recently published in the journal Communications Engineering.

Source: Rice University

  • Facebook
  • Twitter
  • Flipboard
  • LinkedIn
5 comments
guzmanchinky
WOW, those are big numbers! Hopefully we don't burn coal someday soon but until then this seems very useful.
Fifi Holeson
It will be interesting to see how this scales to industial quantities given the demand for concrete. As we try to move away from coal this will require alternate materials to replace the fly ash in concrete. Tech Ingreedients on Youtube reciently released a video on creating graphine to strengthen epoxy using a very similar technique. Depending on the amount of arsenic, cadmium, cobalt, nickel, and lead, that now shifts the burden to the processor to get rid of that waste.
TechGazer
It's based on heating the material to 3000C until the heavy metals evaporate. Flash heating doesn't reduce the total energy required to heat the material to that temperature. It's a good method for heating tiny samples briefly (no oven to heat and keep hot), but it doesn't help for bulk processing. If you heat a thick layer of flyash, the metals will take a long time to migrate out, so you'd have to keep the sample hot for a long time. Overall, I don't think it will scale up for bulk processing. Furthermore, the carbon used will probably oxidize, releasing CO2.

This looks like: "Our process works great (on microgram samples in a lab)! Buy our patent now (before you figure out that it won't work in bulk)!"
Benjamin Goldberg
I wonder what other uses we could find for purified fly ash.
Insulation?
Karmudjun
Nice article Ben! Yes, Fly ash - when clean of leeching heavy metals - is a great substitute in the building crafts. I wonder how the capacitor driven flash heating IN A VACUUM scales up so that the heavy metals are safely sequestered. Possibly they will use a partial inert gas vacuum? However they devise scaling up, it needs to be fast tracked. We have too many recycling processes that are limited or blocked by heavy metal contamination. How many? So many that I can't imagine it scaling up in bulk production. Or maybe my mind is muddled by heavy metals. Great research, an incremental step if not a solution - that is Awesome!