What is this layer under plaster?

[andre1990]

Stripping back tiles in the bathroom of an early 1900s terraced house. Under the tiles was the layer of plaster, but there is another layer underneath that, that looks almost like a cement type render?

We're planning to install cement boards prior to retiling but we're unsure if this layer should come off or not...assuming it will without a fight.

 

[Wayners]

Wet now backing plaster.
Bonding or Hardwall.
Could even be discontinued browning plaster

 

[andre1990]

Wet now backing plaster.
Bonding or Hardwall.
Could even be discontinued browning plaster

Would you remove it from the brick before mounting cement boards?

 

[ree]

OP,
A large context pic that shows the whole bath tub, the backwall, & the walls at either end would help.

Whatever is presently on the walls is irrelevant - its usually best to hack off all plaster & render back to the bare brickwork. That way you will have space and fresh, sound surfaces to work on.
Cement boards must be fixed plumb & flat on the same plane with 90* inside corners from back wall to end walls. Consider your shower details, and any enclosure arrangements before going any further.

 

[Alastairreid]

The base coat is a cement render followed by bonding and finish plaster.
The render coat may be solid and might pull the face from the brick if you try to remove it.
Personally I would remove any loose stuff and straighten it with a 4-1 cement render before fitting the cement boards.

 

[ree]

.