leaf casting

have started making leaves and have a few questions--am using quickcrete--which is almost too quick! sets up fast and is hard to work with-- it calls for 5 and a half parts cement to one part water which is kind of dry - am afraid to add much water because it might weaken the leaves
so --can i add grout mix to slow down the setting time and to make it easier to work with? has anyone used grout mix to do their leaves with