Materials

Recycled and Irradiated Plastic Makes Concrete Stronger and More Eco-Friendly

MIT undergraduates have developed a process for strengthening concrete — a notoriously carbon-intensive material — by using discarded plastic bottles.

It's a familiar refrain in engineering circles — in any industry, really: You never know where the next big idea is going to come from.

An undergraduate class project at MIT may prove to be the next big idea in cement manufacturing. A group of students developed a new technique to make industrial concrete stronger and friendlier to the environment, according to newly published research. And the secret ingredient for the eco-friendly concrete is discarded plastic bottles blasted with gamma radiation.

“The thing that's most interesting to me is that undergraduates actually did this,” Michael Short, an assistant professor in MIT’s department of nuclear science and engineering, told Seeker. “We instructors helped to keep them on the rails, technically. But this was the students idea.”

The research, published in the journal Waste Management, details the project, which as an independent study exercise aimed at finding ways to lower carbon dioxide emissions using nuclear technologies.

Concrete manufacturing generates about 4.5 percent of the world’s human-induced carbon dioxide emissions. One way to reduce that footprint could be to make plastic-reinforced concrete. But introducing even small amounts of plastic tends to weaken the material’s structures.

But rooting through the literature, the students discovered that certain types of plastic actually become stronger when exposed to gamma radiation. The students' hypothesis: Irradiate the plastic before adding it to the concrete mixture.

The student researchers gathered plastic bottles from the local recycling center and took them over to an MIT lab where they crushed the plastic using a high-energy ball mill.

“That's basically a hardened canister with metal balls that shakes back and forth real fast and violently,” Short said. “But that didn't finish the job, so the students actually did it by hand with a mortar and pestle.”

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They then walked the crushed plastic to another MIT lab, which houses a cobalt-60 irradiator that emits gamma rays — a radiation source often used commercially to decontaminate food.

“It's constantly emitting radiation, only there's a shield in front of it,” Short said. “They were able to put the plastic in there, remotely open the shield, then go back in safely to retrieve it.”

After the irradiated plastic was added into standard cement mixes, the team ran tests. Sure enough, the irradiated plastic improved the concrete, increasing its strength by up to 15 percent compared with control samples. 

Short cautioned that the new technique is in a very preliminary stage, and more research is required to confirm the study's various findings.

“The paper is basically saying, here's a method that works, here's the quantification on how well it works,” Short said. “We know that the plastic makes it denser and forms particular crystalline structures in the material that make the final concrete stronger.”

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The research team — students and instructors both — intend to stay with the project and keep publishing new findings. The group is working on a proposal for the National Science Foundation for further funding.

Short, who guided the project in his role as faculty adviser, said that he's particularly proud of the student-run aspect of the project.

“Actually, I took this same class in 2004, as an undergraduate,” he said. “It was well-taught, but I found the design aspect to be lacking. I thought we were going to do something or make something physical, and we didn't.”

As an instructor, Short wanted to let the students use hands-on methods to take the idea as far as they could.

“They decided what to do, they did it, and they wrote it up,” he said. “This is a bunch of twenty-year-olds that may have just developed a new kind of cement.” 

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