Although this might be the same question as making ice cubes on Shabbos, is it permitted to make a Slurpee/Slushee on Shabbos using the following method: a specialized ice-pack is left in the freezer for a few hours then inserted into a soft silicone cup. After pouring soda into the ice-pack/silicone cup combination, the soda takes on a slush partial liquid/ice consistency after squeezing the cup.