“There is a huge amount of plastic that is landfilled every year,” says Michael Short, an assistant professor in MIT’s Department of Nuclear Science and Engineering. “Our technology takes plastic out of the landfill, locks it up in concrete, and also uses less cement to make the concrete, which means fewer carbon dioxide emissions.”
By 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.
“There’s no residual radioactivity from this type of irradiation,” Short says. “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 exposed various batches of flakes to either a low or high dose of gamma rays, which was then added to cement paste samples, each with traditional Portland cement powder and one of two common mineral additives: fly ash (a byproduct of coal combustion) and silica fume (a byproduct of silicon production). Each sample contained about 1.5% of irradiated plastic.
Once the samples were mixed with water, the researchers poured the mixtures into cylindrical moulds, allowed them to cure, removed the moulds, and subjected the resulting concrete cylinders to compression tests. They measured the strength of each sample and compared it with similar samples made with regular, nonirradiated plastic, as well as with samples containing no plastic at all.
They found that, in general, samples with regular plastic were weaker than those without any plastic. The concrete with fly ash or silica fume was stronger than concrete made with just Portland cement. And the presence of irradiated plastic strengthened the concrete even further, increasing its strength by up to 20% compared with samples made just with Portland cement, particularly in samples with high-dose irradiated plastic.
After water, concrete is the second most widely used material on the planet. The manufacturing of concrete generates about 4.5% of the human-induced carbon dioxide emissions. It’s meant the cement industry has been keen to reduce its global carbon footprint in recent years.
Reusing plastics as concrete additives also redirects old water bottles, the bulk of which currently end up in a landfill.