Beneficiated Fly Ash Versus Normal Fly Ash or Silica Fume

摘要

The performance of a beneficiated fly ash (BFA), with very low carbon and very fine particle size (5 mun as mean size) was compared with silica fume in superplasticized high strength concretes. The silica fume content of concrete was 40 kg/m~3 and the amount of BFA was 80 kg/m~3 to obtain approximately the same cost as that of silica fume (SF). When the two concrete mixtures are compared at the same water-binder ratio (0.39) and at a given slump (about 165 mm), the dosage of superplasticizer was much higher for the SF concrete (2.6 kg/m~3) than for the concrete with BFA (1.2 Kg/m~3). The compressive strength of the SF-concrete was higher than that of the BFA-concrete, particularly at early ages, and this effect could be related to the better space filling capability of SF, as compared to BFA. However, when the same dosage of superplasticizerwas used, the water-binder ratio of the BFA-concrete turned out to be lower (0.31) than with SF concrete (0.39). The strength increase in the BFA-concrete with respect to the SF-concrete has been recorded in specimens cured at room temperature as well as with steam-cured. In a second set of tests, BFA was used to manufacture high strength self-compacting concrete (SCC) in comparison with a corresponding SCC where untreated fly ash (FA) was used. In such a case the most important advantage in using BFA rather than FA was the self-leveling properties needed particularly in placing slabs or ground- floors. Due to the higher cost of BFA with respect to FA, there is no significant advantage in using the former in manufacturing SCC when the above mentioned self-leveling properties are not needed.