Epoxy- and get a gallon rather than a qt, mix 1/2 pour it in- it will
permeate & consolidated the cinder..
Bear in mind that if the cinder is settling there's a good chance that
it leaking out somewhere...

prior to the second pour toss in a handful of pea gravel, or pennies to
either bring it up to floor level, and call this the finish or to - 3/4
or 1" if you are going to finish it with tile or wood...
Go for your lungs and get the nifty West System w/ the pumps and special
mixing cups or go cheapo and just get cans of this stuff @ the local
marina...