How to seal ventilation holes in concrete block?

[Vermilion]

I have in a basement room removed a lot of partition walls but now have a lot of old ventilation holes open to the outside. Too many! I need to close some of them but how? The wall is made of betonhålsten. On the outside, there is a layer of plaster (and that's how I want it to look in the end). How it will look on the inside doesn't matter at all.

How would you make it airtight?

 

[MathiasS]

Something that breaks a thermal bridge is good. Foam sealant or cellular plastic in the middle and then concrete and/or plaster on each side.

 

[Vermilion]

Something that breaks a thermal bridge is good. Foam sealant or cellular plastic in the middle and then concrete and/or plaster on each side.

Foam sealant sounds easy to spray into and then just plaster each side.