Early outcomes of an experiment by researchers at Iowa State University's National Concrete Pavement Technology Center showed that a concrete roadway with a titanium dioxide coating removed 40% of the nitrogen oxides from the air, but later results showed only 4% was removed. "[A] thin layer of calcium carbonate had formed in the test area," said James Alleman, a researcher at the NCPTC. "It seemed the experimental pavement had literally sucked carbon dioxide out of the air and combined it with calcium in the concrete to create a layer that prevented sunlight from reaching the titanium dioxide."

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