The report comes from the Eindhoven University of Technology. Here's a brief description from the report on how the science works:
"Vehicle exhaust gases contain nitrogen oxides (NOx), which cause acid rain and smog. The air purifying concrete contains titanium dioxide, a photocatalytic material that removes the nitrogen oxides from the air and converts them with the aid of sunlight into harmless nitrate. The nitrate is then rinsed away by rain. These stones also have another advantage: they break down algae and dirt, so that they always stay clean. "