Conventional coal fly-ash (CFA) and two coal-biomass fly-ashes (CBFAs) were obtained at a thermoelectric power station (Atikokan, Ontario) from combustion of undiluted lignite coal and co-combustion of lignite coal with up to 66% wood pellets (on a thermal basis). Fly-ashes were characterized and analyzed for use as cement admixtures. Co-combustion did not markedly change the fly-ash composition, owing to an extremely low ash content of wood pellets compared to lignite coal; toxic metals and minor elements were within ranges reported for other coal fly-ashes. All fly-ashes had losses on ignition (LOI) <1 wt% and therefore complied with ASTM LOI regulations for use in concrete. All fly-ashes contained major amorphous phases, along with quartz and periclase. Partial substitution of cement with fly-ash (up to 40 wt%) had a moderate effect on the entrained air content of mortars (up to 2.5%), but this difference vanished upon addition of air entraining agent (0.6 mL/kg of cementitious material). Substituted mortars exceeded 75% of the strength of ash-free mortar after 28 days of curing (therefore meeting ASTM requirements for strength development), and by 90 days, met or surpassed 100% of the strength of ash-free mortar. Amending mortar with 20 wt% CFA or CBFA had no effect on its durability following repeated freeze-thaw cycles when air content was kept constant. Also, no micromineralogical differences were observed between hydrated CFA- and CBFA-amended mortars, with fly-ash particles reacting with Ca ions originating from dissolution of cement clinker or calcium hydroxide. (c) 2010 Elsevier Ltd. All rights reserved.