Wheat screenings are the residue left from cleaning grain. Mixture of small grain, cracks, fines, weed seed, etc. Commonly called chicken scratch in some places.
Cement mixed with wheat seed will work fine too. I don’t use wheat seed, because we sell and use it. I have lots of waste from the cleaning operation which I often give away. (I could sell it locally but normally don’t)
Not sure it will work with squirrels, but it probably will. The mixture of cement and grain is pretty effective on rats, works on mice but not as effective as on rats.
Thanks, I will use it in the attic, but not out in the open.