In a 4 quart container, combine lemonade concentrate, limeade concentrate, powdered sugar, egg whites, and crushed ice; mix well. Cover and freeze at least 30 minutes before serving. Spoon 2 cups slush mixture into blender. Add 1 cup of carbonated water. Blend until frothy. To serve rub rim of glass with lime slice, dip in salt, and fill glass. Decorate with lime slices.