Concrete is responsible for a significant share of man-made greenhouse emissions. Credit: Pixabay.

MIT undergraduate students found that recycled plastic flakes can make concrete 15 percent stronger. Discarded plastic bottles could thus one day serve a new role, inside your walls for instance, instead of polluting the environment in landfills and oceans. The plastic is first blasted with gamma rays, a process which is complately harmless.

Blasting concrete pollution

Concrete is the second most widely used construction material in the world, after water. Manufacturing and transporting concrete is responsible for 4.5 percent of all man-made carbon dioxide emissions.

While researching a student project, Carolyn Schaefer and Michael Ortega were amazed by just how many emissions the concrete industry is responsible for. If they could find a way to make concrete greener, even by a fraction, the two thought, it would be possible to lessen concrete’s strain on the environment.

“There is a huge amount of plastic that is landfilled every year,” Michael Short, an assistant professor in MIT’s Department of Nuclear Science and Engineering, told MIT News. “Our technology takes plastic out of the landfill, locks it up in concrete, and also uses less cement to make the concrete, which makes fewer carbon dioxide emissions. This has the potential to pull plastic landfill waste out of the landfill and into buildings, where it could actually help to make them stronger.”

The MIT undergrads scoured the literature and found out about previous efforts that mixed recycled plastic with Portland cement. The resulting concrete, however, was weakened. Going deeper into the rabbit hole, the two found out that exposing the plastic to gamma radiation alters the material’s crystalline structure to such a degree that the plastic turns stiffer, tougher, and stronger. So, they got the bright idea to first irradiate plastic and then mix it with cement and mineral additives (fly ash and silica fume) to manufacture a potentially stronger concrete.

The plastic in question was polyethylene terephthalate, recovered from a nearby recycling plant. The flakes were irradiated with a cobalt-60 irradiator housed in one of MIT’s basements. The irradiated flakes of plastic do not leave traces of radiation afterward so they can safely be used in cement without the fear of jeopardizing human health.

After it was poured into molds, allowed to cure, and removed from the molds, cylindrical concrete samples were subjected to a battery of compression tests. The results were then compared to those performed on concrete made from regular, non-irradiated plastic, as well as plain concrete with no plastic.

According to the MIT researchers, the presence of the gamma-ray irradiated plastic and fly ash enhanced the strength of the concrete by 15 percent, as reported in the journal Waste Management.

Using X-ray diffraction, backscattered electron microscopy, and X-ray microtomography, the researchers found that irradiated plastic, particularly at high doses, exhibited crystalline structures with more cross-linking, or molecular connections. Such crystalline structure seems to trap pores within the concrete, making it denser and stronger. Tests so far suggest that the higher the dose of gamma ray radiation, the stronger the concrete, though more work needs to be carried out to find the optimal mix of materials and radiation.

Next, Ortega and Schaefer plan on experimenting with different kinds of plastic and radiation doses. They say that replacing just 1.5 percent of concrete with plastic makes it stronger, and could have a significant impact. By one calculation, 1.5 percent plastic in concrete implies 0.0675 percent of the world’s carbon dioxide emissions would be slashed.