Yes, you can but you will need to make sure it is all level before watering it because it dries so fast.
Postcrete usually takes around 5-10 minutes to set, but in perfect conditions, it can set in just 1-2 minutes.
No, cement and Postcrete are not the same. Postcrete is a ready to use a blend of concrete ingredients that do not need mixing.
Blue Circle recommends that 1 bag will fill an 8 x 30-inch hole with a 4x4 post.
A bag will fill a hole 20cm wide x 75cm deep.