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These days, large and complicated concrete structures are mushrooming. Performance requirements include not only functionality and strength but also aesthetics. From a material viewpoint, it can be said that the requirements for high performance concrete include : adequate flowability, strength and durability. This study shows that combining fly ash and silica fume as mineral admixture is an effective way to improve the properties of concrete. The influence of fly ash and silica fume on fluidity and strength of mortar and concrete was experimentally investigated. Five fly ashes, classified into 3 types obtaining from each hopper of precipitator, were used and constituted 60, 70, 80 and 100% by weight of the total amount of mineral admixture. In this study, flowability was investigated by measuring the flow/slump flow while keeping the chemical admixture-binder ratio within a pre-determined range. Strength was evaluated by compressive strength tests at 14, 28 and 91 days. The relationship between mineral admixture properties and mortar/concrete properties were studied. As a result of the tests, higher strength and adequate flowability were obtained by combining fly ash and silica fume.

The development of a high performance, high volume fly ash (HVFA) concrete incorporating a small amount of silica fume, and part replacement of both cement and sand with fly ash (FA) is reported. This paper presents the results on the engineering properties such as strength, dynamic modulus and swelling/shrinkage of such high volume fly ash concrete. The mixtures were proportioned to give 30 to 40 MPa cube strength at 28 days. Two basic mixtures with total binder contents of 350 kg/m3 and 450 kg/m3, and, with a minimum portland cement content of 150 and 200 kg/m3 respectively, were investigated. In each mixture, about 60 per cent of the cement was replaced by fly ash. In addition, in some mixtures, a nominal amount of silica fume was incorporated, and in some others, additional FA was incorporated as replacement for sand. The results show that the total binder content had little effect on strength, swelling strain and drying shrinkage, but had a significant effect on the dynamic modulus of elasticity implying a clear densification of the microstructure by fly ash and silica fume. On the whole, HVFA concrete with a nominal amount of SF, and FA as part replacement of both cement and sand showed better overall performance. The engineering properties of the HVFA concretes investigated show good potential for use in structural and mass concrete applications.

Seven air-entrained concrete mixtures including reference concrete without fly ash, concrete incorporating 25 and 35 per cent fly ash, and a high-volume fly ash concrete mixture with 58 percent fly ash were made in this study. The water-to- cementitious materials ratio of the mixtures ranged from 0.32 to 0.45, and the fly ash used was an ASTM Class F, low calcium fly ash. Concrete slabs were cast and used for the determination of the resistance of concrete to the de-icing salts scaling. Most slabs were cast in horizontal moulds and finished using a wood trowel but a number of slabs were cast in vertical moulds. The slabs were either moist cured or cured using a curing compound before being subjected to drying prior to the scaling test. Different moist-curing and air-drying periods were used. Also, the water absorption of the surface of the slabs was determined immediately before the scaling test. For the same water-to-cementitious materials ratio, the fly ash concrete showed more scaling than the reference concrete. However, concretes incorporating up to 35 percent fly ash by mass and having a W/(C+FA) of 0.40 or less performed well in the scaling test when tested using the standard 14-day moist-curing and 14- day air-drying periods. Extended moist-curing periods beyond 14 days do not insure increased resistance to de-icing salt scaling for concrete, and this is particularly so for fly ash concrete. The drying affects significantly the surface of the slabs and makes them more vulnerable to scaling, possibly through the development of microcracking; this effect seems to be more severe for the fly ash concrete. The performance of the slabs cast vertically was not significantly different from that of the slabs cast horizontally. The use of the curing compounds greatly improved the scaling resistance of all concretes tested but was more beneficial for the fly ash concretes.

cement; fly ash; pore size distribution; pozzolans.

Recently, the amount of fly ash generated from electric power plants and other industries in Japan was about 6.4 million tones per year, and it is expected to increase to 12 million tones per year in 2010. Hence, there is a great need to investigate solutions on how to utilize fly ash more effectively. One of the promising solutions is to use high amounts of fly ash in concrete as replacement of the fine aggregate, instead of replacing portland cement. When replacing a part of the cement by a high amount of fly ash, changes in strength development and resistance to carbonation may cause problems in the applications of the concrete to actual building construction in respect to the structural and durability requirement. Though the replacement of a part of fine aggregate by fly ash reduces low strength development, higher rates of carbonation of the concrete may remain as a problem, as calcium hydroxide will be consumed by pozzolanic reaction with the fly ash.This study aims to clarify the effects of high level of replacement of fine aggregate by fly ash on the properties of concrete in the fresh state as well as in the hardened state, and investigate the limit of the replacement in view of the practical application of this type of concrete, Rheological properties, compressive strength and rate of carbonation of mortars of water to portland cement ratio of 0.3, 0.4 and 0.5 in which the fine aggregate was replaced with fly ash at 25 and 50 percent levels were evaluated experimentally. It was found that the rheological constants increased with higher replacement level of fly ash and that, when water to portland cement ratio was maintained the strength development and carbonation properties were improved.