Repair open concrete wall

Hi,

today a wastewater pipe, which was damaged by a drill hole, was repaired by a plumbing company. The wall was opened by the craftsman, a piece of white plastic was glued onto the black plastic pipe and then a proper amount of adhesive was applied over it.
In my opinion, this is a reinforced concrete wall (built in 1985). After closing, a piece of wallpaper is supposed to be glued over it again.
The hole has a diameter of about 30-40 cm.

Here is a picture:



My question now:
how should the substrate of the open spot be pre-treated (primer?) and what material is needed to close it?
Any other tips?

Thank you in advance!

Use plaster mortar, which is also available in a 5kg sack at the hardware store. Moisten the edges of the hole beforehand, apply the plaster, let it dry slightly. Then smooth it wet with a float board, and finally smooth it with a trowel. You don't need to prime the rough surface, wallpaper adhesive is sufficient.

Can you get plaster mortar reasonably smooth? I was maybe thinking of putty on top. But it’s more work. This is the heating room... it doesn’t have to look perfectly smooth.

No, you don't have to! With a trowel, you can make it smooth enough. Like an older eel