Really interesting way to make fdm watertight and stronger

[jamcultur]

Yes sand would work - up to a point.
bear in mind that the sand will get embedded in the plastic. And getting rid of sand is really hard. You can't 'sand' it off as it's as hard as the sanding paper.
Silicon dioxide is tough stuff.

The really clever thing about using salt is that it is water soluble.
So all you do to remove the embedded salt is to rinse or soak it off.
The other clever thing about salt is that you can grind it to different 'grit' sizes yourself.
If you only have coarse sand - you cannot grind that smaller yourself.

If you wanted to make an abrasive block or some custom shaped 'files' then you could use sand.
Of just pack the area round the print with tungsten carbide dust. And use salt or sand for the bulk.

But for non-file type uses - you do need to use salt.

The sand used for sand casting is quite different from ordinary sand. The grain size is much much smaller, and the better types (like Petrobond) have an oily coating that prevent molten metal from sticking to them. The oily ones get a bit smelly when you pour molten metal into them as some of the oil burns off, but that shouldn't be a problem with PLA. I'd guess that the non-stick coating in Petrobond would also work with PLA, but there's only one way to find out.