The paper presents a comparison between Pulverized Coal Fly Ashes (PCFA) and Spreader Stoker Coal Fly Ashes (SSCFA) by analyzing their chemical, mineralogical and physical characteristics. PCFA have been recognized as being valuable industrial by-products and many research studies have been published on their characteristics, properties and utilizations. On the contrary, relatively little is known about SSCFA due to a lack of research work and their valorisation appears to be a difficult task, mainly due to their high unburned carbon content. Three fly ashes are studied here, two resulting from pulverized coal power plants and one from spreader stoker power plant. The results show that the tested fly ashes have a similar chemical and mineralogical composition whatever the combustion process used. SSCFA presents some specific characteristics approaching those of normalized PCFA. However, the combustion system seems to have an impact on the physical properties and performance of fly ashes as supplementary cementitious materials when blended with Portland cement. SSCFA has indeed a high unburned carbon content, which could be explained by a combustion process not adapted for coal. Rheological and mechanical tests results show that SSCFA/cement Portland-based mortars have interesting properties and could be used in some specific industrial applications. (C) 2015 Elsevier Ltd. All rights reserved.