My favorite "projectile" for a blank load is a piece of a T-shirt. I always swab the chamber of my GB mortar between shots with a damp piece of a T-shirt (about a 1/4 piece). Not soaking wet ... just damp enough to kill any embers. Just for fun at the end of the day I loaded it, gently tapped it down, and fired it. It was extremely loud! The T-shirt only goes about 30 feet, it rapidly opens and quickly stops. Plus, it is self extinguishing and not a fire hazard. You can recover it and use it again for a number of shots before it is too tattered.
Edit: I don't want to HIJACK this thread so I started another thread on Favorite projectiles for blanks.