Dishwasher salt is a particular grade of granulated, crystalline sodium chloride intended for use in dishwashers. Like water softener salt, dishwasher salt regenerates ion exchange resins, and removes the calcium and magnesium ions that characterize hard water. Dishwater salt granules are larger than those of table salt. The granule size ensures that the salt dissolves slowly, and that fine particles do not block the softener unit.
Dishwasher salt is 95% sodium chloride; an anticaking agent (e.g. sodium ferrocyanide or potassium ferrocyanide) constitutes the remaining 5%.
References
Dishwasher salt Wikipedia(Text) CC BY-SA