Tricalcium aluminate hexahydrate (TCA) is used within the Bayer industry as a filter aid during purification of sodium aluminate liquors - a role in which the TCA morphology and particle size are particularly important. In spite of this usefulness, no studies have been reported in the open literature examining the influence of reaction variables on the morphology of TCA prepared at 100 degrees C under conditions of interest to the Bayer industry. In this paper, we examine the influence of sodium hydroxide concentration, gibbsite (Al(OH)(3)) charge, CaO charge, reactant addition procedure, added salts (Na2CO3, NaCl and Na2SO4) and reaction time on the formation and morphology of TCA at 100 degrees C. Our results indicate the reactant addition procedure significantly influences the TCA purity and particle size. For example, reaction between CaO (15 or 60 g/L) and a sodium aluminate solution yields TCA crystals with a median particle size of approximate to 6 mu m but with Ca(OH)(2) as an impurity, while reaction between CaO and gibbsite in a sodium hydroxide solution can give pure TCA with a median particle size of approximate to 11 mu m. The CaO loading or liquor alumina/caustic ratio do not significantly affect the particle size for TCA formed by reaction between CaO and a sodium aluminate solution. The morphology of TCA formed by addition of CaO and gibbsite to a sodium hydroxide solution is significantly affected by the CaO/Al2O3 ratio. In particular, a molar CaO/Al2O3 ratio between 1.8 and 3 forms TCA with a morphology similar to that of a rhombic dodecahedron, while a molar CaO/Al2O3 ratio less than 1.8 forms TCA of a less well defined morphology. Conducting the reaction between CaO and gibbsite in a sodium hydroxide solution containing added Na2CO3, NaCl and Na2SO4 forms hexagonal plates of a calcium aluminate carbonate hydrate with the approximate formula 4.1 CaO . Al2O3 . 0.75 CO2 . 11 H2O. This compound subsequently reacts to well defined TCA crystals with a median particle size of approximate to 11 mu m.