How to fill an airvent in the wall

[B Souza]

I have a hole, 10cm in diameter in my 1st floor bedroom. It's a hole through and through the wall. This hole is located say 20cms from floor level. I have no idea why this hole was drilled - other than getting air in the room
I want to block this hole as I get a lot of cold air during winter.
I've put in a rounded thermalite block which occupies 80% of space. Added an image to this post.
My question is:
1. what glue/sealant should i use around the thermalite block, to hold that block in place
2. I'll block the hole with cement on the outer wall (External). this won't come all the way into the inner wall. So, can i use expanding foam to fill the other space not filled by the cement?
3. Finally on the inner wall, can i use polyfilla to fill the inner gaps and smooth it up, before painting over.

Any other advice?

 

[Harry Bloomfield]

You need sand and cement to 'stick' it in place properly, in both the outer and inner walls (if it a cavity wall). Set the block on the inner leaf of the wall 10mm back, so as to allow it to be plastered over. You can buy bags of dry, ready to mix sand and cement.

The hole was probably the flue vent hole for a gas fire.