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Close to 40 million water bottles go unrecycled in America each year. Undergraduate students at MIT have found a way to use those wasted water bottles to create a strong concrete material that could help cut down on the cement industry's carbon emissions.

The research team found that by exposing plastic flakes from discarded water bottles to small, harmless doses of gamma radiation, then pulverizing the flakes into a fine powder, they can mix the irradiated plastic with cement paste and fly ash to produce concrete that is up to 15% stronger than conventional concrete.

In their research, the students learned that those before them had tried to introduce plastic into cement mixtures, but the plastic always weakened the resulting concrete. In an effort to make the material stronger, they found that exposing plastic to doses of gamma radiation makes the material’s crystalline structure change in a way that the plastic becomes stronger, stiffer, and tougher, because the material has more more cross-linking, or molecular connections. The concrete with irradiated plastic also blocks pores within concrete, making the substance more dense and therefore stronger.

“There’s no residual radioactivity from this type of irradiation,” says Michael Short, an assistant professor in MIT’s Department of Nuclear Science and Engineering in a news release from MIT. “If you stuck something in a reactor and irradiated it with neutrons, it would come out radioactive. But gamma rays are a different kind of radiation that, under most circumstances, leave no trace of radiation.”

The team plans to experiment with different types of plastics going forward, along with various doses of gamma radiation, to determine their effects on concrete. So far they've found that substituting about 1.5% of concrete with irradiated plastic can significantly improve its strength. While Short notes that may seem like a small fraction, it would significantly reduce carbon emissions from the cement production process when implemented on a global scale.

“Concrete produces about 4.5% of the world’s carbon dioxide emissions,” Short says. “Take out 1.5% of that, and you’re already talking about 0.0675 percent of the world’s carbon dioxide emissions. That’s a huge amount of greenhouse gases in one fell swoop.”

It would also reduce waste from water and soda bottles that end up in landfills.

“There is a huge amount of plastic that is landfilled every year,” says Short. “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.”