1. In a wide pot, cook rhubarb, sugar, salt, and 2 1/2 cups water over medium heat, stirring often and mashing with a spoon if needed, until rhubarb turns to mush.
2. Pour mixture through a strainer into a bowl and rub with the back of a ladle to push through as much of the rhubarb as possible.
3. Transfer to a shallow pan and freeze until firm, stirring and scraping with a fork every hour if possible, about 4 hours total. Scrape up granita with fork and spoon into bowls.