How can I make sure cement is hardened after it got applied?

I used cement (powder with water) to fill gaps indoor, not in walls but of an old cupboard thingy attached to a wall. I wanted to seal it shut and cement was the only way to do it. Problem is, after filling the holes and surface with my hand and tools, a day later toughing it leaves a bit of powder residue on my hands. does this mean its never gonna harden? And some "rocks" cement sticking out can easily be cracked. The filling itself appears to be solid but the surface surrounding it aint.