Well, this lady says that after they're fully cured, they won't be damaged by freezing.
"Karen Gilliam, WSU - Whatcom County Master Gardener
Old Stone sinks and animal troughs have been used to grow plants for many years. Now they are scarce and very expensive. So, we can make them from hypertufa Hypertufa is an artificial medium with absorbent properties similar to tufa rock. It is lighter than regular concrete, and will not be damaged by freezing after it is completely cured."
Here's the link to her site and her hypertufa recipe:
http://whatcom.wsu.edu/ag/home...
And I remember Carol Eads used to make lots of hypertufa pots, and she was in Zone 3 as I recall.