Mechanical processing of wet stored fly ash for use as a cement component in concrete

Wet storage effects on fly ash mean that processing may be necessary to achieve the physical properties required for use in concrete. This paper considers drying, de-agglomeration and milling of various wet stored fly ashes at laboratory and pilot/benchtop scales, towards meeting these. In the laboratory, different batch quantities and milling times with as-received/pre-screened materials were examined using a ball mill. Greater particle size reductions were obtained with increased milling time but at gradually reducing rates. Pre-screening and batch quantity had relatively minor effects on particle size reductions, with small differences also found between wet and dry stored fly ash. Extended milling time resulted in a darkening of colour; slight increases in loss-on-ignition, the main oxides content and crystalline components; reductions in water requirement (to a point); and greater reactivity. Similar effects were generally noted in concrete for the superplasticising admixture dose to achieve a target slump and compressive (cube) strength. At pilot/benchtop scale, a dryer-pulveriser and spiral jet mill were used, which gave general agreement with the behaviour noted in the laboratory, but with the effects tending to be less. Fineness levels in Standards were achievable, with subsequent performance in concrete, appearing to depend on the milling process used.