Influence of wet storage on fly ash reactivity and processing for use in concrete

Wet stored fly ash is increasingly being considered as a cement component in concrete. However, the effect of these conditions on the material's reactivity is uncertain. The research described here investigated this property for wet laboratory-stored (10% moisture) and site stockpile fly ashes, using lime consumption (BS EN 196-5, Frattini) and activity index (BS EN 450-1) tests. Progressive reactivity losses occurred with laboratory storage up to 730 days. This was influenced by dry fly ash fineness and holding period, suggesting that the formation of agglomerates/products (assessed by scanning electron microscopy) affects lime's access to particle surfaces, with similar type behaviour for stockpile materials. Compressive (cube) strength reductions were also found between dry and wet stored fly ash concretes. Stockpile fly ash reactivity following laboratory- (drying/ball milling) and pilot-scale (flash drying/de-agglomerating, air classifying, micronising and carbon removal) processing was then investigated. Exposure of reactive material using these methods appears to be important, with greater improvements generally noted as the fly ash particle size is reduced and at later test ages. To meet activity index requirements, fly ash sub-10 μm contents, with the Portland cement used, needed to exceed about 30%, irrespective of the storage conditions/processing used. Minor benefits to concrete strength were obtained with increasing sub-10 μm contents, particularly beyond 28 days.