Abstract: In a method of treating clay-containing phosphate rock comprising at least mining the phosphate rock, reducing the phosphate rock to a predetermined size range, adding water to the phosphate rock, and storing the admixture containing at least the phosphate rock and water; the improvement characterized by adding to the admixture an amount effective to control problems associated with the swelling of residual clays of a chemical treatment selected from the class consisting of aliphatic hydroxy acids containing 2-6 carbon atoms, inclusive, of which citric acid and tartaric acid are members; the aliphatic hydroxy acid salts, and effective blends of the aliphatic hydroxy acid or the aliphatic hydroxy acid salts with a strong caustic, or base. The chemical treatment prevents a build up of viscosity such that the admixture remains pumpable even with a concentration of solids in excess of 66 percent by weight.