Junk that collects in sidewalk cracks

In my neighborhood I have noticed that many of the large cracks in the sidewalks (the spaces between the segments of concrete that make up the sidewalk) are full of an odd grayish debris. It looks fuzzy, almost like dust bunnies or dirty fiberglass insulation. Any clue what this could be?

The sidewalks where I have seen this abut an industrial area with iron recycling, if that makes a difference.

Any poplar (cottonwood) trees around? It’s about this time of year (in the northern hemisphere, anyway) that they shed their seeds, which are attached to clumps of downy white fluff. It gets everywhere.

No, there are no trees like that nearby, and anyway I notice the debris all the time, not just this time of year.

I know exactly the stuff that you mean, but I always assumed it was installed on purpose. It never occurred to me that it was sidewalk lint.

If it is about 1/2 inch wide then it is probably expansion joint filler. There is probably a better name for it. But it is there so as the temperature changes the sidewalk can expand and squeeze the stuff instead of buckling. In the side walks where I grew up this would be installed every 20 or so feet.