MIT (Massachusetts Institute of Technology) undergraduate students have found that, by exposing plastic flakes to small, harmless doses of gamma radiation, then pulverizing the flakes into a fine powder, they can mix the irradiated recycled plastic with cement paste and fly ash to produce concrete that is up to 15 percent stronger than conventional concrete.
Concrete is, after water, the second most widely used material on the planet. The manufacturing of concrete generates about 4.5 percent of the world’s human-induced carbon dioxide emissions. Replacing even a small portion of concrete with irradiated recycled plastic could thus help reduce the cement industry’s global carbon footprint.
The students learned that others have tried to introduce plastic into cement mixtures, but the plastic weakened the resulting concrete. Investigating further, they found evidence 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.
The team exposed various batches of flakes to either a low or high dose of gamma rays. They then ground each batch of flakes into a powder and mixed the powders with a series of 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 percent irradiated recycled plastic.
Once the samples were mixed with water, the researchers poured the mixtures into cylindrical molds, allowed them to cure, removed the molds, 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 along with fly ash strengthened the concrete even further, increasing its strength by up to 15 percent compared with samples made just with Portland cement, particularly in samples with high-dose irradiated plastic.
After the compression tests, the researchers went one step further, using various imaging techniques to examine the samples for clues as to why irradiated plastic yielded stronger concrete.
The team took their samples to Argonne National Laboratory and the Center for Materials Science and Engineering (CMSE) at MIT, where they analyzed them using X-ray diffraction, backscattered electron microscopy, and X-ray microtomography. The high-resolution images revealed that samples containing irradiated recycled plastic, particularly at high doses, exhibited crystalline structures with more cross-linking, or molecular connections. In these samples, the crystalline structure also seemed to block pores within concrete, making the samples more dense and therefore stronger.
Going forward, the team is planning to experiment with different types of plastics, along with various doses of gamma radiation, to determine their effects on concrete. For now, they have found that substituting about 1.5 percent of concrete with irradiated plastic can significantly improve its strength. While that may seem like a small fraction, replacing even that amount of concrete could have a significant impact.