International Concrete Abstracts Portal

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.

This paper reports high-strength concrete behavior subjected to temperatures up to 200 °C and 100 freezing and thawing cycles in regime of 8 hours in water at + 20 °C and 16 hours at - 20 °C (weekends in frost). The concrete is composed of 425 kg/m3 of portland cement of CEM I 42.5 type, 32 kg/m3 of silica fume, 5.6 L/m3 of super plasticizer Melment and has a W/C of 0.32. Compressive strength is 78.5 MPa at 28-day curing on cubes for temperature resistance tests and 63.1 MPa on prisms for freezing and thawing tests, both after 28-basic curing in 20 °C/100 % R. H. - air. Evident C-S-H dewaterization of the cement paste is observed between 100 °C and 200 °C. Initial shrinkage within 24-hour period due to rapid cooling is more detrimental on the cement paste strength than shrinkage due to C-S-H dewatering at temperature elevation from 100 °C to 200 °C. The strength, elastic modulus and volume deformation of concrete are irreversibly influenced either by temperature elevation or rapid cooling to 20 °C. Differences in strength, elastic modulus and shrinkage or expansion after 100 freezing and thawing cycles relative to those in water are negligible. The compressive strength of prisms subjected to 118-day freezing and thawing was 62.9 MPa, compared to 65.2 MPa for those kept in water.

The mixtures of normal portland cement with fly ash from black coal combustion in classic installation and fly ash from brown coal combustion in fluidized bed were examined; the silica fume additive was used as a hydration process modifier. The hydration kinetics was followed by means of calorimetry. Then the calcium hydroxide content was determined by TG method and the electric conductivity of hydrating suspensions was measured. The course of so-called pozzolanic reaction between the additives and calcium ions was investigated and a selective consumption of calcium ions by silica fume was deduced from the data thus obtained. The use of silica fume as an additive brings about the acceleration of hydrolysis of cement components and acceleration of the formation of hydration products, usually by intensive consumption of Ca ions from the liquid phase. It has been found that silica fume, by quick consumption of calcium ions has a retarding effect in the beginning, but soon after, within a few hours, an intensive hydration, with the formation of dense C-S-H, takes place.