Portland Cement on Model Railroad Layouts?

[Charlie B]

In the Linka building group there was some discussion about various cements for outdoor use (waterproof). One fellow mentioned he was using "Tuff Stone" gypsum cement from U S Gypsum, some others recommend the vinyl concrete patching cement. some of this sets up fairly fast, (Sackreet makes some varieties) and is inexpensive compared to some other products.
I have not tried them myself, just passing the information along.
Charlie