Dry-processing of long-term wet-stored fly ash for use as an addition in concrete

The paper describes a study carried out to explore the potential for recovery of fly ash from wet-storage areas for use as an addition in concrete, and involved drying and processing material to achieve finer fractions/reduce particle size. Three relatively coarse fly ashes with medium/high carbon contents (loss-on-ignition; LOI) were obtained from two UK lagoons and a stockpile. Initially, drying (105 °C) and screening (600 μm) were carried out, giving acceptable handling properties. Thereafter, processing using (63 μm) sieving, air classifying or grinding was investigated. The methods gave increased fineness levels (greatest with grinding), with some reductions in LOI also found after sieving and air classifying. Assessment of fly ash reactivity indicated improvements with processing, and most noticeable effects by grinding. Tests on concrete found that both consistence (slump) and compressive (cube) strength increased with processing and tended to follow fly ash fineness. Further analysis showed that strength could be related to the sub 10 μm particle level in fly ash. This appeared to be irrespective of processing method, with progressive increases (in strength) up to sub 10 μm contents of about 60% (maximum in the study). Concrete tests for water absorption and intrinsic (air) permeability, carbonation and chloride diffusion showed that these were also influenced by fly ash sub 10 μm content. The factors affecting behaviour are discussed and practical implications of the research considered.