Concrete expert's chime in.

[Fargo]

From what I remember (I don't work on that side of the state) ERNST makes good mud. You should be fine. Some plastic sheeting and a regular blanket wouldn't hurt if you're feeling frisky...but probably not necessary.

Yeah, what he said. You are talking about a great deal of mass, largely underground, creating heat as it cures. IIRC freezing on concrete is around 29-30 degrees. Even if a bit of the surface freezes, which is IMO unlikely, you are looking at surface cosmetic issues. The innards are going to be fine. Poly and a blanket won't hurt, but likely make no real difference.