Autoclaved Aerated Concrete Produced with Low NO_x Burner/Selective Catalytic Reduction Fly Ash

Electric utility coal combustion processes employing low NO_x burner and high-dust selective catalytic reduction (SCR) emission control technologies produce fly ash that is high in carbon and ammonium salt content. Such ash is considered undesirable for use as admixture in standard concrete because of its decreased compressive strength and ammonia odor. This research investigates the use of low NO_x burner/SCR fly ash for production of autoclaved aerated concrete (AAC). For this study, samples were obtained from the Orlando Utility Commission's Stanton Power Generation Plant Unit 2, which contained 6% carbon and 28 mg of ammonium ion/kg fly ash. A number of AAC recipes were developed with this ash, producing blocks with compressive strength values ranging from 2.268 to 4.435 kPa and densities ranging from 560 to 812 kg/m³. During block production, carbon particles in the ash exhibited hydrophobic properties and separated from water at the AAC slurry/gas bubbles boundary that is produced in the rising cake. Separated carbon was unable to physically interfere with calcium silica hydrate gel formation occurring in the slurry. In addition, the alkaline environment inside the reactive AAC slurry was sufficiently high to cause the release of ammonia gas. Consequently, ammonia odor was not observed during the later setting of an undisturbed block or during curing stages of the finished block. The toxicity characteristic leaching procedure and synthetic precipitation leaching procedure were found to be below the appropriate toxicity thresholds. In general, toxicity characteristic leaching procedure concentrations were higher than synthetic precipitation leaching procedure concentrations for all samples and all analytes (As, Ba, B, Cd, Cr, Ni, and Pb). It is concluded that the high carbon, ammonia bearing fly ash from the Stanton Unit 2 Low NO_x burner/SCR pulverized coal combustion process is a suitable source of siliceous material for the production of AAC prototype block, from physical, environmental, and aesthetic perspectives.