I think you should not use any peatmoss because you want the wheel to look like stone without chunks of peat that will rot out. I would use one part cement, and 1 1/2 parts each of sand and vermiculite. I estimate you will need five to six bags of cement. Remember- the finished piece will weigh about six hundred pounds. If the water is continuously running over the wheel, it will erode some in time.

I have a water feature that I made of with hypertufa (millwheel)in 2006..as well a 4' Mill wheel for a home builder for his model homes...I made the large one as well with Tufa

I'm not sure if this will work but here's the pics of the Millstones...thanks

Here is a link that might be useful: flikr photos

myback garden: Love your water feature! Your millstone is EXactly what my husband wants me to create for our garden! Would you be willing to share your process w/ us all? I've found some instructions on how to create a mold... but I would love to hear how you did yours & how you went about creating the "grid" on the top. Did you use something on the bottom & poured the tufa on top, or did you use something to score the top? How much tufa did it take & what recipe did you use?
I am anxious to hear your details!

This is from a previous post when I first made the millstone...........The design was done by my hubby..I asked him to do up something for me and that is what he came up with...lol.. he drew it and I used a saw to make the lines.I would like to do another one with leaf imprints in it..It weighs about 25 to 30 lbs so one person can manage it..but far easier if 2 pick it up..Once it is up on it's side though it rolls just like a wheel so that makes for easy moving by one person.
I used...16 litres of port
4 litres of sand
12 litres of vermiculite
3/4 cup of brown colorant
3/4 of a bottle of acrylic admix(I think it was overkill on the admix) :).. but I just kept pouring and before I knew it most of the bottle was gone.lol
it set up for demolding and carving in about 24 hrs
....I believe it is about 24" round-fountain Mill stone...the larger one is about 4' round..I took a different route with the making of the large millstone and just used a bucket for measuring..I can't remember the ratio...but I just kept repeating it...for every wheel barrel full...It took about 5 barrels...to fill and it is 6.5" in depth.. this one is very heavy...It cannot be lifted without mechanical help..my hubby's guess is about 400 lbs...find a mold the size you want and then just fill it up to the depth you want..I used a large barrel(plastic drum)...took my sawzal and cut it down..Hope this helps I have some more pics i will try to put on flikr..

myback garden: Thank you very much for the more detailed instructions. The additional details really help!
I think this is a project that my DH and I will enjoy doing together once the weather cools down a bit here (100's right now)! I'll be checking back to see if you post more pics of your beautiful finished projects.

Just a shot of what's available commercially for about 89 bucks..includes pump, resevoir, and fibreglass millstone

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I didn't like it, so I "dressed" the stone with 'tufa, and tried again..

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Still didn't like it,(My SIL said it looked like a cow pie) so I took off the "stone" and just piled on river rocks. (I'll show a picture tomorrow..none on file)