Last year ( 2003 ) we messed around with Hypertufa
It all started with me wanting to make “Rocks” for the pond area.
After a lot of searching the net I came across Hypertufa
Hypertufa is very east to make and use.
Equal parts of:
Portland cement
Sphagnum peat moss
Mason’s sand

Sue made some very cool little Grots
You can see some of last year pics HERE

This little guy was left out all winter and came through with no problem.

There is a ton of info on Google Search