A process of manufacturing a product comprising alpha calcium sulfate hemihydrate and beta calcium sulfate hemihydrate comprising the steps of: mixing water and gypsum to form the 50 - 75 wt. % gypsum slurry and feeding the 50-75 wt. % gypsum slurry to a first reactor, wherein the first reactor is a continuous stirred tank reactor (CSTR), the slurry comprising calcium sulfate dihydrate, at least one crystal modifier and water; calcining the slurry in the first reactor at conditions sufficient to form a partially calcined slurry comprising water, calcium sulfate dihydrate and alpha calcium sulfate hemihydrate, wherein the slurry is held in the first reactor at conditions for calcining the gypsum to convert 80-95% of the gypsum to alpha calcium sulfate hemihydrate; wherein the first reactor is operating at a pressure of 3,4 to 4,8 bar (35 to 55 psig) during the calcining in the first reactor and wherein the residency time of the slurry in the first reactor ranges from 5 to 15 minutes during the calcining in the first reactor; dewatering the partially calcined slurry to form a water stream and dewatered solids comprising the calcium sulfate dihydrate and alpha calcium sulfate hemihydrate; feeding the dewatered solids to a second reactor, wherein the dewatered solids have a temperature between 66 and 149°C (150 and 300°F) upon dewatering and when fed to the second reactor, wherein the second reactor comprises a kettle containing a fluidized bed comprising the dewatered solids; and calcining the dewatered solids in the second reactor to convert at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate, wherein the calcining of the dewatered solids occurs in the kettle operated at atmospheric pressure and a temperature of from 121 to 343°C (250 to 650°F); and thereby producing the final product comprising 80-90 wt. % alpha calcium sulfate hemihydrate and 20-10 wt. % beta calcium sulfate hemihydrate. mixing water and gypsum to form the 50 - 75 wt. % gypsum slurry and feeding the 50-75 wt. % gypsum slurry to a first reactor, wherein the first reactor is a continuous stirred tank reactor (CSTR), the slurry comprising calcium sulfate dihydrate, at least one crystal modifier and water; calcining the slurry in the first reactor at conditions sufficient to form a partially calcined slurry comprising water, calcium sulfate dihydrate and alpha calcium sulfate hemihydrate, wherein the slurry is held in the first reactor at conditions for calcining the gypsum to convert 80-95% of the gypsum to alpha calcium sulfate hemihydrate; wherein the first reactor is operating at a pressure of 3,4 to 4,8 bar (35 to 55 psig) during the calcining in the first reactor and wherein the residency time of the slurry in the first reactor ranges from 5 to 15 minutes during the calcining in the first reactor; dewatering the partially calcined slurry to form a water stream and dewatered solids comprising the calcium sulfate dihydrate and alpha calcium sulfate hemihydrate; feeding the dewatered solids to a second reactor, wherein the dewatered solids have a temperature between 66 and 149°C (150 and 300°F) upon dewatering and when fed to the second reactor, wherein the second reactor comprises a kettle containing a fluidized bed comprising the dewatered solids; and calcining the dewatered solids in the second reactor to convert at least a portion of the calcium sulfate dihydrate of the dewatered solids into beta calcium sulfate hemihydrate, wherein the calcining of the dewatered solids occurs in the kettle operated at atmospheric pressure and a temperature of from 121 to 343°C (250 to 650°F); and thereby producing the final product comprising 80-90 wt. % alpha calcium sulfate hemihydrate and 20-10 wt. % beta calcium sulfate hemihydrate.