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?