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The efficiency of finely ground blast furnace slags (BFS) was studied in relation to the fundamental physico-mechanical and structural properties of cement mortars. A positive effect of BFS fineness on the workability of fresh mortars was proved. Due to a small water content (20 percent by mass of the cementitious material) when combined with a high-range, water-reducing admixture (HRWRA), the compressive strengths of mortars ranging between 80 and 100 MPa are ascertained at normal curing conditions. Special curing conditions, such as autoclaving or hot water curing, produce specimens with compressive strengths in the range of 100 to 130 MPa, depending on the grading of BFS and the composition of the binder. With additional heat curing, the compressive strength of the mortars increase, in general, by about 10 to 50 percent over that of either autoclaved or hot water cured mortars. In this paper, some other properties of high-strength mortars incorporating finely ground BFS are discussed, including porosity and durability investigations. The efficiency of BFS addition is compared with other fine mineral powders, such as silica fume and fine silica powder, with special attention paid to binder compositions and curing conditions.

The sensitivity of strength and deformation of high-strength concrete incorporating silica fume to variations in strain rates were studied experimentally and compared with those of normal strength concrete (without silica fume). The observed phenomena in the experiments were qualitatively interpreted according to an assumed mechanism of strain rate sensitivity of concrete. The differences of the material structure between high-strength concrete with silica fume and normal strength concrete without silica fume are discussed in this paper; emphasis is placed on the change of pore structure and moisture content due to the incorporation of silica fume for high-strength concrete and its influence on the rate sensitivities to strength and deformation of concrete. In particular, the Stefan Effect is believed to play a very important role in the case of rate sensitivity. In general, it was found that high-strength silica fume concrete is more sensitive to the variation of strain rate than normal strength concrete as far as strength and deformation in compression are concerned. However, in tension, this rate sensitivity is less pronounced.

To better understand the behavior of silica fumes (SF) in fresh mortar and concrete, four French silica fumes, with different chemical and textural properties, were characterized with respect to their surface properties and their behavior in aqueous dispersion. Zeta potential measurements were performed as a function of pH and ionic strength. Below about pH=4, all materials behave similarly, whereas at higher pH, the electrochemical potential was found to be related to the Ca content. In suspension, the decrease of the average particle size (bimodal distribution) is directly related to the power input (ultrasonic treatment) and to the intragranular compactness, mass-fractal dimension, and densification treatment. With Ca-rich SF, sedimentation volumes and velocities were found to increase as dispersion proceeds, which suggests that the elementary sub-micrometric silica spheres re- agglomerate after the dispersion treatment, probably due to bridging by calcium silicate hydrates.

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.