If applied, the discovery by undergraduates at MIT could help cut carbon dioxide emissions associated with the production of cement and reduce the amount of plastic that goes to landfill.
“There is a huge amount of plastic that is landfilled every year,” said 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 makes fewer carbon dioxide emissions.”
Plastic has been introduced into cement mixtures before, but was found to weaken the resulting concrete. Exposing plastic to doses of gamma radiation, however, changes the material’s crystalline structure to make it stronger, stiffer, and tougher.
The students obtained flakes of polyethylene terephthalate — plastic material used to make bottles — from a local recycling facility and took them to MIT’s cobalt-60 irradiator, which emits the sort of gamma rays used commercially to decontaminate food.
According to MIT, 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 by-product of coal combustion) and silica fume (a by-product of silicon production). Each sample contained about 1.5 per cent irradiated plastic.
During compression tests the MIT team 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 per cent compared with samples made just with Portland cement, particularly in samples with high-dose irradiated plastic.
After the compression tests, the researchers used 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 analysed them using X-ray diffraction, backscattered electron microscopy, and X-ray microtomography.
The high-resolution images revealed that samples containing irradiated 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 denser and stronger.
“At a nano-level, this irradiated plastic affects the crystallinity of concrete,” said Kunal Kupwade-Patil, a research scientist in MIT’s Department of Civil and Environmental Engineering. “The irradiated plastic has some reactivity, and when it mixes with Portland cement and fly ash, all three together give the magic formula, and you get stronger concrete.”
The team’s paper - Irradiated recycled plastic as a concrete additive for improved chemo-mechanical properties and lower carbon footprint – is published in Waste Management.
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