No fiber calcination of gypsum for gypsum fiberboard

A method comprising:

• performing a first calcination stage comprising heating a first gypsum slurry comprising a first portion of gypsum and a first aqueous fluid to a first temperature in a range of about 270° F. to about 295° F. and under pressure for a sufficient time to convert at least a portion of the first portion of gypsum to a first portion of calcium sulfate hemihydrate and nucleate at least a portion of the first portion of calcium sulfate hemihydrate to form nucleated acicular crystals of a first size, thereby producing a first calcination stage calcined gypsum product slurry having the nucleated acicular crystals of the first size;
• combining in any order a second portion of gypsum and a second aqueous fluid with the first calcination stage calcined gypsum product slurry to form a combined gypsum slurry;
• and
• performing a second calcination stage comprising maintaining the combined gypsum slurry under pressure at a second temperature in the range of about 210° F. to about 240° F. for a sufficient time to form acicular crystals of a second size greater than the first size, thereby forming a second stage calcined gypsum product slurry,
• wherein a final added gypsum amount is a total weight of the first portion of gypsum and the second portion of gypsum,
• wherein a total feed weight is a total weight of the first portion of gypsum, the second portion of gypsum, the first aqueous fluid, and the second aqueous fluid,
• wherein the final added gypsum amount is equal to at most about 50% of the total feed weight, and
• wherein the first portion of gypsum is about 10% to about 50% by weight of the final added gypsum amount.