International Concrete Abstracts Portal

The performance of a beneficiated fly ash (BFA), with very low carbon and very fine particle size (5 µm as mean size) was compared with silica fume in superplasticized high strength concretes. The silica fume content of concrete was 40 kg/m3 and the amount of BFA was 80 kg/m3 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/m3) than for the concrete with BFA (1.2 Kg/m3). 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 superplasticizer was 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.

Alkaline activation of granulated blast furnace slags by highly concentrated solutions of sodium or potassium ions has been a subject of numerous investigations for many decades. Irrespectively of the type of activator used, the so-called C-S-H phase formed is very compact, of low C/S ratio, rich in sodium, aluminium and magnesium and is predominant hydration product. Properties of AAS pastes, mortars and concretes strongly depend on the chemical and phase composition of the slag. This paper presents the properties of the alkali activated pastes and mortars produced on the base of synthetic alumino-silicate glasses of gehlenite type. Setting time, mechanical properties and heat of hydration of the gehlenite-glass pastes are presented. Detailed studies of phase composition, microstructure and structure of alkali-activated gehlenite glasses are presented in the paper. Alkaline activation of gehlenite slag glasses is influenced by molar ratio Al2O3/SiO2 of the slag and concentration of NaOH. The hydration process is much quicker in the case of gehlenite type glasses than for typical industrial melilite granulated blast furnace slags. The results of XRD and SEM/EDS examinations show that in gehlenite type pastes amorphous C-A-S-H phase containing high amount of sodium, silicon and aluminium are the dominating hydration product.

This publication contains the proceedings from the Ninth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, held in Warsaw, Poland, in May 2007. The 36 papers include topics related to silica fume in high-quality concrete, temperature attack and freezing-and-thawing cycles on durability of high-strength concrete with silica fume, measuring shrinkage of self-consolidating concrete incorporating fly ash and silica fume, and development of alkali-activated slag concrete for practical use, among others. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-242

Ten reinforced concrete blocks (1.5 x 1.5 x 0.5 m) were cast during the autumn of 1982 and exposed in the tidal zone in the Trondheim fjord in March 1983. The blocks were made of concrete with two different strength grades and with 0, 10 and 20 % silica fume addition. The blocks have been subject to several investigations during the years, especially with respect to chloride intrusion. The last examination was executed at approximately 21 years of exposure. The results indicate that the activity factor of silica fume regarding resistance against chloride intrusion is in the same order as for compressive strength (approximately 3). The chloride diffusion coefficients have been calculated for several exposure periods. This makes a calculation of the aging effect, and long term development of the effective chloride diffusion coefficient, possible. The concrete was composed according to the common practice in Norway in the early 1980s, and the concrete proportions are therefore hardly relevant to day. The effect from addition of silica fume with respect to durability and chloride intrusion for well documented, long term field exposed concrete is however considered very valuable.

This paper presents possibilities of use of fly ashes from co-burning bituminous coal and other fuels in cement production process. Both fly ashes coming from co-burning bituminous coal and biomass and the ones from coal combustion were analysed. The physical and chemical properties of the fly ashes were examined by determination of fineness, chemical and phase composition, pozzolanic activity and structure of the glassy phase. Cement samples with different content of the fly ashes were prepared. The following properties of the samples were tested: porosity, compressive strength as well as heat of hydration. The results show that cement samples containing fly ashes from co-burning bituminous coal and biomass had demonstrated adverse features like higher porosity, lower compressive strength after specified ages, than the ones containing fly ashes from bituminous coal combustion. The investigations of microstructure of the cements were also carried out by SEM.