In reply to Flinticus:
you could grind it, it you've got an angle grinder: https://www.toolstation.com/diamond-concrete-grinding-disc-100-x-22mm/p6251...
but that could be time consuming and a bit of a pita.
if you're going to break it out, I'd cut slots in it first (angle grinder with diamond disk) - this makes it easier to chisel out bigger chunks - any sds drill on hammer mode will do it.
depends a wee bit on what the plan it to finish it (I tiled the concrete ex-hearth in my bedroom, left it as a feature for standing pot-plants on)
seriously, if you're going at it with a grinder or cutting disk - get a quality mask, the rubbery type with replaceable filters, and consider setting up a extractor / blower to get the dust outside....
Post edited at 10:44