The Medway Handyman coughed up some electrons that declared:

Doing a fencing job at the mo, and its being a right embuggerance.

Posts are rotten at ground level & seem to be embedded in a mixture of
hardcore & dry mix sand/cement. The hardcore breaks out OK but the
crumbly dry mix is a PITA.

Using a big breaker (hired) but whats happening is that the point digs
into
the crumbly stuff for about 2" then jams & has be pulled out. You have to
hold the breaker back to stop the point sticking.

Having just helped a neighbour with a fence post...

Would a spade bit work better on the breaker?
Or can you try to break out at 45 degrees from the centre - ie push the base
apart, outwards?

Using a weedy SDS, I had to dig down a bit to expose the "concrete", then
knock one side out (digging as required to allow the SDS access and to give
the concrete room to break). Then we did the neighbouring side. Remaining
two sides were solid, so left to locate the new post.

The most obstructive bit was the old rotten wood - had to use the SDS and a
long bar to break it up and lever it out in bits.

Do you have a TNT license?

Cheers

Tim