I used ash straight from the stove to fill in a low spot in front of my mailbox.
The mail & paper guy used to sink when it was wet, then light up the tires trying to get out, making it even worse. They'd pull up to the box & be tilted downward!
The ash, all by itself, hardened enough to keep this problem from happening again. The street sweeper went by & it didn't even touch the coal ash!! Even coarse, hydraulic-powered brushes wouldn't move the stuff!! It picked up everything else though -- now I can see the spots I missed!
My next concrete job at the house is definitely getting some ash mixed in!