Help with identifying plaster

[pais]

[GALLERY=media, 106179]CA16928D-7AAD-41E5-856B-3527F00053E9 by pais posted 7 Mar 2021 at 10:28 AM[/GALLERY][GALLERY=media, 106179]CA16928D-7AAD-41E5-856B-3527F00053E9 by pais posted 7 Mar 2021 at 10:28 AM[/GALLERY]
Nobody seems to know what this is on my old terrace. Any ideas?

[GALLERY=media, 106178]B72B6D0F-BD6D-4CDE-BCA4-E10ABF5786A4 by pais posted 7 Mar 2021 at 10:28 AM[/GALLERY]

[GALLERY=media, 106177]7CA1364B-C86D-4C9B-B4B7-832CACBA2771 by pais posted 7 Mar 2021 at 10:28 AM[/GALLERY]

[GALLERY=media, 106176]52B9214F-8F64-47B9-8B04-668BC0959DBF by pais posted 7 Mar 2021 at 10:28 AM[/GALLERY]

 

[JohnD]

Is this indoors or out?

Is the wall damp?

How old is the house?

What county are you in?

Can you scratch it with your fingernail?

Chip off a bit and put it in water for a few hours. Does it soften?

Your pics do not show scale. Stand back and tale a wider pic of the wall. Include something to show scale.

 

[pais]

Indoor on an external wall upstairs.

The wall is damp, there was an issue with the roof which has been resolved so there are signs it is drying out now.

I believe the house was built in the 1930s. North east United Kingdom.

I’ll take more pictures to show scale and will check if it softens in water!

thanks.

 

[Nige F]

looking @ the thickness ( and your location ) could it be mine slag and cement/hydraulic lime ?