I am making my own concrete millstone. It is 33" across and 4" thick. Question how long do I need to leave it in the mold. And second, how long before I can work with it without damaging it. thanks.
Is it reinforced?
Wasn't planning too
It will need to be reinforced with either wire or fiber (or both) or you will get cracking. If this is to be functional and not decorative, I'd also want a high cement content as well as being careful about the correct proportions and size of aggregate. You can remove it from the mold in 24-48 hours, but it won't achieve it's full strength for 28 days or so.
Curing is the procedure you use to keep the water in the mix from evaporating so the concrete can reach full hydration and full strength. Since it is cast in a form you would only need to cover the exposed surface with plastic or a wet cloth for 3 or 4 days if it is not in a hot or windy climate. The longer you cure it the stronger it will be so the answer depends on how much stress you intend to put on it. If it will be highly stressed in tension as well as compression, reinforcing will be more effective than extra curing time.