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.