metal window

Hi all, I've removed the rotten wood below this metal window, and it occurred to me - why don't I fill the gap between the concrete cill and the metal frame (gap height about 45mm high) with concrete rather than wood? - any thoughts or comments greatly appreciated, thank you.

Hello, I am in the process of refurbishing the first of many metal windows in my house. I'm using this first one as a kind of experiment to find the best way of going about it. So far I've: Removed the casement/opener. Had it dipped to remove the paint. Primed it with Rustoleum 1085 Hard Hat...