Those that know me will recognize the addiction I have to Hypertufa, a mix of Perlite, Portland Cement and Peat Moss.
One of the characteristics of hypertufa is that it's porous once it's dry - but what happens when you want it to hold water?
I use a similar mixture, except instead of the peat moss, I use some of my soil which is extremely silty and sandy.
Please note; this is sub soil, not garden soil with lots of organic matter in it.
This takes the place of the organic matter (peat moss), and creates a waterproof finished product - just what you need for a bird bath.
I was recently asked for instructions for some different ways to make a pedestal to put the birdbath on.
My first thought on how to do this pedestal was to use a piece of pvc plumbing pipe, which I just happen to have available.
The pipe can be put into a plastic bag, and then the hypertufa mix or soil cement mix can be slathered over the pipe and the bag used to mold the mix into the required rough shape.
Or, if you're like me and impatient, use a big clay or pottery pot - no fuss, no muss.
Other thoughts about this; using soil cement is a different kind of mix. It tends to slump off, rather than stick in place the way hypertufa does.
For this reason, I might use soil cement to make several blocks or tubes, using plastic nursery pots as molds, one gallon size. Then the tubes would be stacked with a piece of rebar holding them in place, and the bird bath placed on top of it.
Oh brother, make a decision and stick to it; the big pot is the final solution and it looks great! Sometimes, simple is best.