The disclosure relates to a method for effectively and efficiently adding sodium trimetaphosphate to a gypsum slurry. In one exemplary embodiment, the sodium trimetaphosphate is added at the same time as foam is added to the gypsum slurry through the foam waterline. In another exemplary embodiment, the sodium trimetaphosphate is added at the same time as additional water is added to the gypsum slurry through the gauging waterline. The sodium trimetaphosphate can be added to either or both waterlines in its dry form or in a diluted solution.