Bubbly macro

Hello people! Do you know what is this? At first, when I saw it, I didn't know what it is, but after few seconds of looking at it I found out. This was photographed literally 2 minutes ago. It's bubbly inside as you can see, I won't tell you what it is because I want you to comment down bellow what do you think it is. :D

I'll give you a hint, it's used for construction work, mostly for isolation. Some of them also use this for building houses out of bricks. :D