Characterization and utilization of fly ash derived from the co-combustion of biomass and coal as a mineral admixture for ordinary portland cement

Co-firing coal with renewable/waste biomass for power generation can mitigate the atmospheric discharge of pollutants (e.g., SO2) and green-house gases. Wide-spread application of this technology is impeded, however, by current standards which prohibit fly ash derived from biomass as a partial substitute for portland cement in concrete.All the fly ashes had the same density within experimental uncertainty. Particle size distribution analyses, however, showed significant differences between CFA and 66CBFA on one hand and 15CBFA on the other. They were attributed to the 15CBFA formed during combustion when the fuel input into the boiler was at its maximum and soot blowers causing an upward flow of steam entrained larger particles (compared to CFA and 66CBFA which were collected when soot blowers were not in use). The microstructure of each fly ash was analysed by scanning electron microscopy and energy dispersive spectrometry. All of the fly ashes were primarily composed of spherical particles which were either completely solid or porous. Although the composition of individual fly ash particles varied widely, each fly ash exhibited the same range of compositions. The majority of particles were aluminosilicates, with varied amounts of calcium and iron and to a lesser extent magnesium, sodium and trace elements. Some particles have concentrated amounts of iron, or calcium, but such particles were found in all fly ashes.In the second phase of this research the effect of fly ash-amendment of mortars was investigated. In no instance did the type of fly ash make any difference. Increasing the fly ash content decreased the water requirement independently of the type of fly ash. 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, again independently of fly ash type. Amending mortars with up to 40 wt% fly ash retarded the early strength development of mortars, but increased the later strength, with mortars containing CFA or 66CBFA consistently exhibiting higher compressive strengths than those containing 15CBFA. Mortars containing up to 40 wt% CFA, 15CBFA, or 66CBFA met required strength specifications by 28 days according to ASTM C618 (2003). Addition of 20 wt% CFA or 15CBFA was found to have little effect on resilience to rapid freeze-thaw of mortars after 140 cycles.Some fly ash particles reacted to form calcium silica hydrate (CSH) gel apparently contributing to the strength of the mortars.In the first stage of this research, fly ashes from the combustion of coal (CFA) and the co-combustion of coal and biomass (CBFA) were characterized according to chemical and physical properties, as well as microstructure. The fly ashes were obtained from full scale combustion tests which took place at the Atikokan Ontario thermoelectric power station. The fly ashes were derived from the combustion of undiluted lignite coal (CFA) and 15:85 and 66:34 (on a thermal basis) wood pellet/lignite mixture, termed 15CBFA and 66CBFA, respectively. All fly ashes were found to be very similar in composition and meet requirements put forth by ASTM and CSA on the allowable amount of carbon for use as a partial cement substitute.