I have build the first dome with 5:1 ratio of vermiculite Portland cement and the dome was quite crumbly. I have decided to redo it and use the normal cement with the same ratio. I have then cover it with a light render of cement, sharp sands vermiculite. I am waiting a couple of weeks before removing the gym ball.
Do you know if normal cement is good enough?
Good enough for concrete not so much for pizza oven