Single-phase alpha-CaSO4.0.5H(2)O whiskers were directly synthesized from waste Ca(NO3)(2) solution using a hydrothermal method, and HNO3 was synchronously regenerated. The effects of reaction temperature and Ca2+ concentration on the phase composition and morphology of products were determined by X-ray diffraction and optical microscopy. On the basis of the experimental results, the formation diagram of alpha-CaSO4.0.5H(2)O was plotted within the range of 5-35 g.L-1 Ca2+ and 115 degrees C-150 degrees C. In addition, the conditions of the direct synthesis of alpha-CaSO4.0.5H(2)O were determined. Well-crystallized, single-phase alpha-CaSO4.0.5H(2)O whiskers with high aspect ratios (length, 1785 mu m; diameter, 10.63 mu m; aspect ratio, 168) and HNO3 (70.25 g.L-1) were obtained at the optimal conditions of 25 g.L-1 Ca2+ and 125 degrees C. (C) 2020 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.