Would it be possible to build an old polished concrete casted fireplace that has damaged edges and corners?

You definitely can shutter off the broken edges and use our Craft Mix. You will need to put a primer on the existing concrete to make sure the new concrete bonds to it

The only issue with this would be that the craft mix won’t match the colour of the rest of the hearth/surround.

To remedy this, you could apply microcement to the fireplace and surrounding so it all matches.