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To assess the risk of corrosion due to high silica fume or fly ash content, hardened cement paste and concrete tests were performed at the Institute for Building Materials Research at the Aachen University of Technology to determine the influence of these concrete admixtures on the alkalinity of the pore solution and on chloride-induced corrosion of the reinforcing steel in the concrete. The fly ash content in the tests was up to 60 percent by mass and the silica fume content up to 25 percent by mass of total binder content. The mixtures were made up with a portland cement and a portland blast furnace slag cement (50 percent by mass blast furnace slag) at varying water-binder ratio. A combination of 45 percent by mass portland cement, 15 percent by mass silica fume, and 40 percent by mass fly ash was also included in the program. Reducing the portland cement clinker content in mixtures with high silica fume contents by the use of blast furnace slag or by the substitution of high amounts of fly ash leads to a rapid exhaustion of calcium hydroxide. Substantial quantities of alkalies are bound to reaction products, resulting in a dramatic drop of pH value in pore solution (below pH = 12.0) and, hence, increasing the risk of depassivation of the steel surface. The reduced alkalinity must be weighed against a significant refinement of pore structure through the rapid pozzolanic reaction of silica fume, clearly increasing the electrolytic resistance of concrete and reducing the corrosion rates to possibly negligible values.

The moisture diffusivity is of considerable importance for quantitative assessments of creep and shrinkage, as well as durability of cementitious material. For this reason, the influence of the composition of repair mortars on their effective moisture diffusivity as a function of the relative humidity of the surrounding air was investigated. Silica fume, superplasticizer, and polypropylene fibers were added to reduce permeability and to control cracking induced by drying shrinkage. It was shown that the moisture transport in cementitious materials can be realistically described by a nonlinear diffusion process governed by Fick's law. A computer program based on the finite volume method was used to get the best effective moisture diffusivity by comparing experimental results (moisture losses of drying mortar cylinders) with the numerical solution. The applicability of a combined experimental-numerical approach to characterize repair mortars regarding their moisture diffusivity was demonstrated. The material properties necessary for the characterization and qualification of new materials can be found numerically. Moreover, the diffusivities obtained provide useful input data for further numerical calculations. The positive effect of the addition of silica fume on moisture diffusivity was clearly shown. The positive combined effect of polypropylene fibers and silica fume with increasing entrained air content was observed. Finally, no significant detrimental effect on the addition of fibers (even at relatively high volumes) has been observed for materials cast under shrinkage free conditions.

Sulfate resistance of two types of silica fumes from ferrosilicon (FeSi) and silicoferrochromium (SiFeCr) furnaces has been evaluated using ASTM C452 and ASTM C1012 test procedures. Cubic mortar specimens have also been immersed separately in 10 percent N

Hgh Reactivity Metakaolin (HRM) is produced by controlled thermal activation of purified kaolinite, an aluminosilicate mineral, to a reactive, amorphous state. HRM, being pozzolanic, reacts with free lime (Ca(OH) 2), a byproduct of portland cement hydration. In this investigation, two high- performance concrete mixtures containing HRM were studied. In the first mixture proportion, HRM was formulated as an addition to the cement. In the second mixture, HRM was used as a cement replacement. The compressive strength and rapid chloride permeability of the HRM concretes was compared to nonpozzolanic concrete controls and concretes that contained equal amounts of silica fume. The results of this study show that the strength and impermeability of HRM concrete is significantly higher than nonpozzolanic concrete. The HRM concrete showed properties equivalent to similar silica fume (SF) concretes, while using significantly less superplasticizer to reach an equivalent consistency.

Exothermal reactions take place during cement hydration and heat the cement mass. This temperature increase, from the initial moment of setting until the hardening of the cement, causes shrinkage which contributes to the cracks that can be seen in some constructions that are made of large masses of concrete or cement rich mixtures. The use of pozzolans reduces the heat given off by the cements during hydration. However, the reactions of the pozzolanic materials with the lime also produce heat; the decrease should not be proportional to the level of clinker substitution in the mixed cements. The first hours show that most of the additions, which present higher activity at early stages, increase the hydration heat of the mixed cements in relation to the control cement. The effect of different silica fumes on the hydration heat with respect to a control cement and other pozzolanic materials was studied and is presented in this paper.