Cement is used to harden a mixture so it can be used for rendering. This means that the mixtures that mixtures need to be made accurately with the correct measurements of each material. If a mixture is too weak, it will crumble. If a mixture is too strong, any slight movement in the building will cause cracks to show.
Cement has to sets. The first is 45 minutes and the second is no more than 10 hours. The hardening starts after the 10 hour set. The cement should stay hydrated so water needs to be available. This is curing.
Curing can be done either by covering the plaster in plastic or you can just spray it with water. The curing process takes between 4 and 7 days to complete.