2. I want to make a large (32" x 6", maybe 8") millstone to be the top of a bird fountain. Obviously I cannot make it out of concrete. Thinking about Hypertufa.

3. I found a formula that says 1 part cement, 1.5 parts peatmoss, and 1.5 parts vermiculite. If I use 80# bags of cement I estimate it will take approximately 2 bags of cement, 3 bags of peat and 3 more of vermiculite - so this means it should only weigh around 200 pounds.

Questions: Does what I have stated in paragraph 3 seem relatively accurate?

Question: If I do this, how many days must it sit before I can work on it? Know it takes around 28 days to totally cure; but how long before I can remove mold, clean up edges, etc.

Thanks.

Does what I have stated in paragraph 3 seem relatively accurate? I am sure you found this formula on the internet. There are many combinations using various other ingredients. The one you have will work, I like to use perlite in my hypertufa mix.
Unmold after 24- 36 hrs and work the damp hypertufa with a wire brush, then wet and cover with large garbage bag, keeping moist for a few weeks.
My suggestions would be to start with a small project. Learn to mix. Too much water is not good, too dry a mix will cause the hypertufa to crumble.
Also It will be heavy when you unmold, so make this project right where you plan to use it.
Finally, you dont mention where you live. You should not make hypertufa if the temps go below 50o F , it will not cure properly, resulting in cracks. Good Luck

Steve

http://forums2.gardenweb.com/forums/load/gardart/msg102150229927.html

It does not seem to be absorbing water at all and is hard as a rock. I did change my formula slightly: 2cf cement, 3cf vermiculite, 1 cf strained peat moss, 1 gal acrylic fortifier and half bag of polyester reinforcing fibers.

"I just completed my first hypertufa project and it was a big one. A 32x6 inch millstone I'm turning into a fountain. It was both harder and easier than I expected. I wanted a larger piece than the concrete one I just finished. It was 32x3.5 inches and weighs about 250 pounds best I can figure. The hypertufa millstone is still curing, but I estimate it will probably weigh about 250 pounds also.
I made the mold out of a piece of plywood for a base and used a 25' piece of 6" flashing for the circular mold. The center hole was a piece of PVC pipe wrapped with wax paper.

The formula I used was 2 ninety pound bags of Portland cement (2cf), 2cf of medium vermiculite, and 2cf of strained peat moss. For reinforcement I mixed in a good bit of polyester fibers, for color I added dry brown tint, and for additional strength I added a gallon of acrylic fortifier to the water.

I rented a small cement mixer and thoroughly mixed all dry ingredient before adding the water. The hard part was getting it mixed right. When I started adding the water it really started to ball up but I had read someplace that just means you needed to add more water. We played around with it for quit a bit before I decided we had to start putting it in the mold even though it really wasn't runny at all. ( Note: after the fact it was clear I just needed to add a bit more water and it would have been a lot easier. ) We dumped the mix in the mold and I pounded it tight with an old brick and then used a cement trowel to smooth the surface. Let it sit for about 15 minutes and then put some grooves in the top for realism. On the second day I pulled out the PVC pipe in the center. On the third day I took the mold off and it was perfect as you can see in the pic. I kept it covered for three days After four days it already felt hard as a rock. I waited a week and then mounted the fountain on it's base. She came out really nice. I'm playing around with the idea to stain it a little darker, don't know yet."