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

This paper investigates the influence of two silica rich fine materials - a fine glass powder used as an inert filler, and nano-silica used as a supplementary cementing material - on the behavior of cement pastes at early ages using electrical impedance. The effective electrical conductivities of the pastes as a function of time, and their derivatives are used to identify the different stages in the microstructure development of the pastes. The incorporation of fine glass powder in the cement pastes in the proportions used in this study does not significantly influence the setting and hardening behavior of the pastes. The increased normalized conductivity of glass powder modified pastes at very early ages even when the local pore volume is reduced by the addition of glass powder filler, and the near equivalence with plain pastes at relatively later times is explained using the pore connectivity factor. The nano-silica modified pastes show significantly different conductivity-time response. It is seen that the setting and hardening phases of cement hydration are accelerated by the presence of nano-silica in the mixture. The nano-silica modified pastes exhibit a significant degree of matrix densification at early ages as could be observed from their conductivity responses as well as compressive strength results.

This paper presents a long-term concrete shrinkage test on self-consolidating concrete (SCC) conducted at The University of Hong Kong. In this study, one normal concrete mixture with only portland cement and five SCC mixtures incorporating fly ash or both fly ash and silica fume were produced and tested for their shrinkage characteristics in the form of prismatic specimens. Fiber-optic sensors, which give stable and reliable measurements, were embedded into the prismatic concrete specimens to measure shrinkage strains. Compared with the normal concrete mixture, the autogenous shrinkage of the SCC mixtures included in this study is larger while the one-year drying shrinkage is smaller. Besides, lowering the water/cementitious materials ratio of a SCC mixture would increase its autogenous shrinkage but reduce its one-year shrinkage. Experimental results also reveal that replacement of cement by fly ash would reduce both autogenous and one-year shrinkage strains of SCC whereas replacement of cement by silica fume would increase both. Lastly, the shrinkage half-time of SCC is found to be longer than that of the normal concrete.

The reactive particles in fly ash are glassy with the major constituents: SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, and K2O. From glass science, it is known that the reactivity would vary with these constituents. Therefore, it may be possible to develop a predictive model of reactivity based on chemical composition. However, it is difficult to study this in real fly ashes because the composition varies from one particle to another. It is not practical to separate the individual particles to isolate specific glass compositions. An alternative approach is to prepare synthetic glass samples with appropriate compositions. Thus calcium aluminum silicate glasses with specified CaO/Na2O+K2O ratios have been prepared as model systems. The compositions were selected to span the ranges measured in representative fly ashes using automated single particle elemental analysis. After characterization the silicate speciation of the glasses using Raman spectroscopy, the reactivity of the synthetic fly ashes was measured using a , inelastic neutron scattering and nuclear resonance reaction analysis. The results showed agreement with glass durability theory and demonstrated the advantages of using a synthetic glass model for fly ashes.

Silica fume (SF) and high reactivity metakaolin (HRM) are two highly reactive pozzolans that offer excellent potential for use in high-performance concrete since concrete mixtures containing them demonstrate superior performance in terms of strength, and durability. High-performance concrete applications, such as pavements and bridge decks, are also required to demonstrate superior performance against early age shrinkage cracking. This paper describes a comparative study of the effects of SF and HRM on the early age stress development and cracking in restrained mortar mixtures due to shrinkage. The restrained ring test was used to assess early age residual stress development in mortar ring specimens. In addition, free shrinkage strains and splitting tensile strength measurements were performed to assess the cracking potential. It was found that the addition of SF and HRM increased the shrinkage level in the mixtures which resulted in increases in residual tensile stress development due to restraint. In addition, their addition in the mixtures increased the cracking potential and resulted in early cracking in the ring specimens.