Alpha alumina (alpha-Al2O3) powders and alpha-Al2O3/boehmite (gamma-AlOOH) mixtures with controlled gamma-AlOOH contents were synthesized hydrothermally under alkaline or acidic conditions at 380 degrees-435 degrees C for 1-10 days, under 6.9-14.5 MPa pressure, from concentrated precursors without stirring. The precursors were formed by mixing different types of aluminum hydroxides with water, and optionally with alpha-Al2O3 seeds, hydrogen peroxide, sulfuric acid, dopants (i.e., KMnO4), and/or other additives. The experiments were performed on industrial scale in large production autoclaves. The synthesized alpha-Al2O3 powders exhibited up to 100% phase purity, 99.98% chemical purity, equiaxed morphology, low aggregation levels, narrow crystallite size distributions with primary particle sizes ranging between 100 nm and 40 mu m, and high reproducibility. Precursor types, seeds, chemical additives, and temperature/time of the hydrothermal synthesis were found to govern properties of the powders. Different growth mechanisms for nanosized and rough powders are discussed. Results of this study enable the use of hydrothermal alpha-Al2O3 powders in a multitude of applications, and make their hydrothermal production a commercial reality.