In a large bowl, dissolve sugar and gelatin in boiling water. Place the strawberries and pineapple juice in a blender or food processor; cover and process until smooth. Add to the gelatin mixture. Stir in concentrates and cold water. Cover and freeze for 8 hours or overnight.
Remove from the freezer 45 minutes before serving. For each serving, combine 1/2 cup slush mixture with 1/2 cup lemon-lime soda; stir well.