Challenge accepted. Everything melts under the right conditions.
https://en.wikipedia.org/wiki/Pykrete
Pykrete-reinforced igloo: https://en.wikipedia.org/wiki/Pykrete#/media/File:Ice_Dome_-...
Not that these don't melt; they're mostly fiberglass and polyurethane.
edit: It seems it doesn't melt even then.
https://m.reddit.com/r/askscience/comments/1rwxne/could_i_me...
That sounds like as close as we'll get to melting the stuff.