Wet storage effects on fly ash properties relevant to its use in air-entrained concrete

Abstract

With the retirement of coal-fired power stations, wet-stored stockpile fly ash is increasingly receiving attention for use in concrete. An important consideration for dry fly ash in this application is its impact on air entrainment, which can be affected by unburnt carbon present, influencing bubble stability and air-entraining admixture (AEA) dose requirements. Preliminary tests suggest wet storage can influence the process and the present study examined this for laboratory-moistened and stockpile fly ashes. The research indicated that chemical processes lead to product formation on particle surfaces and increases in loss-on-ignition in the material. Agglomeration of fly ash also occurred with wet storage, developing with time. While median particle size and specific surface area (by nitrogen (N2) adsorption) of fly ash increased, both foam index and Acid Blue 80 adsorption were less with wet storage. Mortar tests mainly gave increased air content at fixed AEA dose for material held under these conditions. The data suggest admixture accessibility to wet-stored fly ash particles influenced behaviour and this increased with the breakdown of particle agglomerates (by grinding). Some influences of particle surface chemistry on the process were also found. A pilot-scale processing trial with stockpile fly ash gave general agreement with effects observed in the laboratory.