Boards for building flower boxes?

[Picapica55]

I bought a house and the previous owner built flower boxes from some boards with a load-bearing construction of beams on the outside. I like them and want to build more, but he doesn't remember what the boards were called. Light gray, reminiscent of asbestos cement, smooth surface. Could it be fiber gypsum, Fermacell? Does it withstand water that well?

 

[Nybyggarn]

It looks like fiber cement boards, available in slightly different models and appearances.

 

[Claes Sörmland]

Maybe those fiber cement boards that are sold as subfloor boards?

 

[Picapica55]

I looked at pictures online, I think you are right, thank you for the help!