The effect of BaCO(3) (witherite) addition on the sintering behavior of lignite coal fly ash taken from the Seyitomer power plant of Kutahya/Turkey was examined at temperatures of 1100, 1150 and 1200 degrees C in air atmosphere. Bloating of the fly ash samples sintered at 1150 degrees C was prevented, that is, the decomposition temperature of CaSO(4) in the fly ash is shifted to a higher temperature, and their physico-mechanical properties (porosity, water absorption, bulk density and bending strength) were improved with BaCO(3) addition. Positive effects of BaCO(3), however, were not seen on the fly ash samples sintered at 1100 degrees C. All the fly ash samples sintered at 1200 degrees C were bloated due to the gas evolving and also they melted. During the thermal treatment at 1150 degrees C a phase transformation from CaSO(4) (anhydrite) to BaSO(4) (Barite) occurred in the fly ash with BaCO(3) addition as seen from the X-ray diffraction (XRD) patterns and the bar shaped fly ash samples with BaCO(3) saved their structural integrity up to 1150 degrees C. (C) 2008 Elsevier Ltd. All rights reserved.