Effective utilization of industrial solid wastes is of great benefit for environmental and economic needs. In order to clarify the theoretical issues and pave the way to synthesize high quality Ca-alpha-SiAlON powder from coal gasification slag by carbothermal reduction nitridation (CRN), in this study, the theoretical issues including the network structure of the slag as well as the phase and morphological transformation stages of raw slag during CRN process were comprehensively studied at different temperatures (1350, 1400, 1450 and 1500 degrees C). It was found that the CRN process was dominated by nitridation of the Ca-Si-Al-O glass component in slag. With the increase of processing temperature, spherical Ca-Si-Al-O particles were firstly nitrided into porous Ca-Si-Al-O-N spheres, plate-like O-SiAlON grains, and then nitrogen-rich SiAlONs (beta-SiAlON, 15R polytypoid and alpha-SiAlON). At the end of CRN, Ca-alpha-SiAlON was found to be the only SiAlON phase presenting elongated prismatic morphology. In addition, high concentration Ca-alpha-SiAlON powder was obtained through a two-step purification process with a conversion rate as high as 45 wt%. The obtained results shed light on the preparation of Ca-alpha-SiAlON products from coal gasification slag and related wastes. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.