doing up old hearth
 

I live in an old terrace. In the bedroom there has been a small
fireplace which has been removed. In front of it is the remains of
the old hearth. At one time it has been tiled, but the tiles have
been removed leaving a base of cement or concrete (what is the
difference?). A large chunk of this is missing, leaving a crater
about an inch deep. What is the best thing to use to fill this corner
to bring it flush with the rest of it before I tile it?