u/Rahatlin

First: i live in country where all floors are concrete. Second: all bags of concrete filling i can find are 25kg, while it's not expensive it would be a waste to toss it afterwards and i got nowhere to store it.

Photo: https://postimg.cc/2b1dY85b

Story: I had plumbers move some of my radiators around for better efficiency. Whole interaction made me not want their faces ever again. So if possible id like not to call them back to fill one small hole. That they left for probably just to piss me off. All other radiators left just round pipe sized openings that they filled with foam and covered but this one was problematic to remove. Remainder of pipes lead to nowhere they were detached from whole installation.

Can anyone give some ideas how to fill?

edit: Thank you everyone! I will check availability in my area for everything mentioned.