Experiments were conducted on the selected ashes with different additions of CaCO3 for understanding the effects on slag flow properties including ash fusion temperatures, slag viscosity, temperature of critical viscosity and type of slag. ICP-AES, XRD and FTIR analyses were applied to determine the component and structure of the slags. Factsage was used to calculate liquidus temperatures in the SiO2-Al2O3-CaO-FeO system and to predict formed mineral matters and proportion of solid phase as a function of temperatures. The results show that the liquidus temperature calculated by Factsage well predicts the variation of ash fusion temperatures. Slag viscosity behavior changes with increasing addition of CaCO3 because the formation process of solid phase is different. The Fourier transform infrared (FTIR) spectrum indicates that Ca2+ leads to break polymerized Si-O-Si into Si-O, so the increasing Ca2+ in slag results in the decrease of viscosity above liquidus temperature. Below liquidus temperature, solids content decreases with increasing addition of CaCO3 above the temperature of critical viscosity (T-cv). Meanwhile, it is found that the rate of solid formation is related with T-cv and a new prediction method of T-cv based on that was proposed. Moreover, the type of slag changed with addition of CaCO3 was predictable by XRD analysis. The prediction on ash fusion temperature, T-cv and type of slag is expected to serve as a reference for adding flux to regulate coal/ash properties suitable for slag tapping gasification technology. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.