An improved process calcining a gypsum/cellulosic fiber slurry to produce a composite gypsum/cellulosic fiber product resides in adding a selected crystal modifier to the gypsum/cellulosic fiber slurry prior to the heating step to reduce the time necessary to carry out the calcination process, to reduce the temperature at which the calcination process is run or to increase the aspect ratio of the acicular calcium sulfate alpha hemihydrate crystals formed during the calcination process. Useful crystal modifiers include aluminum sulfate, aluminum chloride, chlorine, zinc sulfate, iron (III) sulfate, aluminum sulfate hexadecahydrate, iron (II) sulfate heptahydrate, iron (III) sulfate pentahydrate, zinc sulfate heptahydrate, copper sulfate pentahydrate, copper chloride dehydrate, manganese sulfate monohydrate and trisodium phosphate.