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This paper considers the influence of moisture addition (conditioning) to PFA of up to 40% by mass on its physical and chemical properties and performance as a cement component in concrete. The work examined conditioned PFA, covering a range of fineness levels and lime contents, stored for up to 18 months, either in the laboratory or in stockpiles established at four UK power stations. Data for laboratory-conditioned PFA indicated progressive particle agglomeration (mainly in the finer fractions) with storage period, which was greatest at moisture levels of 10–20% and for material with total and free lime contents in excess of 3·0% and 0·1%, respectively. Corresponding increases in water demand and reductions in strength factor were also noted with conditioned material. Other changes, including increased LOI, reduced free lime content and formation of hydration products such as bassanite and gypsum, indicated that the effects were due to chemical activity. However, no significant changes to bulk oxide, modal composition and glass contents of PFA were obtained, suggesting that conditioning effects are mainly localized at particle surfaces. In general, similar behaviour to that in the laboratory was noted for PFA stored in power station stockpiles. The workability and strength development to 28 days of conditioned PFA concrete were comparable to that containing dry PFA, with only slight reductions in both properties with long-term conditioned PFA storage.

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