Removing plaster with some super bonded material

[MartinB85]

I’m currently removing all the plaster off one wall as there was a large damp area showing through (tried every product to cover it), it never showed mould but it was always present, and slightly wet to touch.
I’ve taken a lot off but the top section in the centre of the wall has a cement like bonding in one large patch (pictures)
We had damp issues on the other side of the house in the exact same spot, spent loads trying to fix it and in the end just removed the chimney stack above the house as it was knackered and re tiled and it’s resolved that.
My thoughts are that at some point before we moved in, the other side (also had a chimney at some point before we moved in) possibly had similar issues and the previous owner maybe tried to use some sort of cement based material to try stop it. The chimney has gone, and we also have external insulation and new render on the outside so nothings getting in.
Before I take an SDS to it to try get it off I just want some opinions on wether to take it off or leave it and just get a plasterer to add a new scratch coat around it and plaster everywhere else?

 

[ree]

OP,
Briefly, take off the plaster/render back to brickwork. Hire a powerful power chisel.
But before demo use your knife to cut into the plaster at all inside corner abutments - eg. wall meets ceiling.
Look at the roof & wall in the loft, & it will show if there was a previous chimney breast?

To make good: use a two coat mix of sand & NH lime render at 3:1. And a Limelite skim finish.