One of Thomas Edison's failed businesses was the construction of poured concrete houses. You may want to read up on them. Most of the existing ones are in New Jersey, I believe.

Are the walls solid concrete? There were early versions of the equivalent of blueboard that were basically concrete board, perhaps an inch thick.

Hard to say with one photo. Scratch coat plaster often looks like concrete as it was intentionally rough and made on site then covered with finish plaster.

Regardless, gets some plaster weld to seal that area and go over it with setting type joint compound.

@rwiegand,

Thanks for sharing the information about Thomas Edison's concrete houses. I looked it up. It was certainly a bold vision. He'd be glad to know that we're able to 3-D print houses today.

It probably is plaster: lath and plaster. You can read up on it. There are lots of pros:

https://www.bobvila.com/articles/lath-and-plaster/

Similar article as above.

https://www.ehow.com/info_8730943_rock-lath.html

I can't remember what the product was called but...My sister purchased a home built around 1950. It appeared to have regular drywall until he decided to do a little doorway expansion and additional power outlets. The "drywall" consisted of what appeared to be regular drywall with a thin coating of what appeared to be very hardhardhardconcrete then very thin coat of plaster. There is a name for it...it escapes me. It was used for a short time before builders just just plain drywall. It was very difficult to cut compared to drywall. He had to get special saw blades to cut some holes.