Showing papers on "Silica fume published in 1986"

Hydraulic cement slurry

Abstract: The cement slurry of the present invention is gas tight, very low strength retrogression at high temperature, and substantially no tendency to settle or segregate The cement slurry is made with a hydraulic cement, 5-85% microsilica based on the weight of the cement, 5-250% of a high density filler material based on the weight of the cement, 0-5% of a retarder (dry weight) based on the weight of the cement, 0-12% of a thinner (dry weight) based on the weight of the cement, 0-8% of a fluid loss additive (dry weight) based on the weight of the cement, 0-30% silica flour and/or silica sand based on the weight of the cement and water in such an amount that the cement slurry has a density between 195 and 240 g/cm 3

Permeability and Pore Structure of Cement Pastes Containing Fly Ash, Slag, and Silica Fume

Abstract: As part of research to develop a highly durable concrete container for radioactive waste disposal in chloride and sulfate bearing granite groundwaters, a variety of cement pastes were studied. A sulfate resisting portlandcement was used with various replacement levels of Class F fly ash and pelletized blast furnace slag at a water to solids ratio (W/S) = 0.36. Blends with fly ash, slag, and silica fume were also combined with a super water reducer at W/S = 0.25. Results are presented for strength development, permeability to water, and pore size distribution after 7, 28, 91, and 182 days moist curing. As a direct measure of durability, after 91 days moist curing, paste prisms were immersed in both de-ionized water and a synthetic chloride and sulfate bearing groundwater at 70°C. While all three supplementary cementing materials (mineral admixtures) reduced ultimate permeabilities, silica fume was more effective in reducing permeability at early ages. Silica fume was also the most effective in reducing calcium hydroxide contents of the pastes while slag was the least effective; only reducing calcium hydroxide levels by dilution of the portland cement. From preliminary analysis, there does not appear to be a way of accurately predicting permeability from porosity or pore size parameters alone.

Hydration Reactions in Cement Pastes Incorporating Fly Ash and Other Pozzolanic Materials

Abstract: A model for reactions that occur in hydrating portland cement is now generally well developed. Incorporation of various by-products to form blended cements modifies both the hydration reactions and the physical properties of the resulting pastes. A review of recent progress in understanding the effects of blending agents on these reactions is presented. The blending agents considered are low-calcium (Class F) fly ash, high calcium (Class C) fly ash, blast furnace slag, silica fume, biosilica and natural pozzolans. Effects of the blending agents on physical properties such as rheology are also considered. Particular attention is given to the essential role of alkalies in pore solutions and the beneficial reactions that occur with high silica content blending agents.

Freeze-Thaw Durability of Concrete With and Without Silica Fume in ASTM C 666 (Procedure A) Test Method: Internal Cracking Versus Scaling

Abstract: The freeze-thaw durability of concrete with and without silica fume was investigated in accordance with the requirements of ASTM Test Method for Resistance of Concrete to Rapid Freezing and Thawing (C 666 Procedure A). The water-cement ratio of all mixes was 0.5, and the silica-cement ratio of the silica fume mixes 0.1. The test results show that the critical value of the air-void spacing factor in these ASTM C 666 tests is significantly lower for the silica fume concretes. These concretes are therefore more susceptible to internal cracking caused by rapid freeze-thaw cycles in water, even though the use of silica fume decreased the surface scaling of the test specimens. This confirms that scaling and internal cracking are two different forms of frost damage caused by rapid freeze-thaw cycles in water. The use of silica fume also decreased the internal cracking of the nonair-entrained mixes damaged during the tests.

Retardation of Cs+ and Cl− Diffusion Using Blended Cement Admixtures

Abstract: Diffusion of cesium chloride through thin plates of hardened cement pastes was studied. Blast-furnace slag and condensed silica fume were used as blending admixtures in an attempt to retard the diffusion of cesium and chloride ions. The curing and diffusion temperatures were varied from 27° to 60°C, and the water/solid ratio was varied from 0.30 to 0.40. Results indicate that the cesium ion diffuses more slowly than the chloride ion in hardened cement paste systems. Blending admixtures caused a further reduction in diffusivity for both ions, which is important for preventing corrosion or restricting radionuclide transport.

Microstructure of Cement Blends Containing Fly Ash, Silica Fume, Slag and Fillers

Abstract: The hydration of a blended cement through hydraulic or pozzolanic reactions results in heterogeneous polyphase materials. Because portland cement clinker is the major component in most cement blends, the microstructural development of portland cement hydrates, including C-S-H and pore structures, is first discussed. Slag, fly ash, silica fume and limestone filler cements are then compared to portland cement with regards to C-S-H morphology and composition, aluminate crystallization, cement paste interfaces and pore size distribution.

Microstructural Study of Different Types of Very High Strength Concretes

Abstract: Very high strength concretes with water-cement ratios ranging from 0.21 to 0.27, having compressive strengths varying between 73 and 118 MPa, were prepared. One series was made with only high early strength cement (Type III), and the other series contained 6% to 11% silica fume. In general, the microstructure of very high strength concrete is very dense and is composed mainly of C-S-H in the gel and crystalline phases. Mg, Al, S, Cl, K and Fe were detected in a number of C-S-H locales. The Ca/Si ratio was variable. In concretes without silica fume, the CH content is much lower than in normal concrete, and in the silica fume concretes it is still lower and not well crystallized. A few large, partly reacted and unreacted silica fume particles with surface cracks were present. Strong cement-aggregate bonding is seen in concretes with silica fume containing limestone aggregates, whereas the gravel concretes show microcracks and a weaker bonding.

Silica fillers from silicon powder

Abstract: A process for formation of a silica filler product comprising the steps of subjecting fluidized silicon powder to combustion at a temperature of about 3000° K. and quenching the combustion products to form a silica powder characterized by a large proportion of discrete, non-agglomerated primary particles.

Reinforcing materials treated with silica fume

Abstract: Fibrous reinforcing materials ordinarily used in the matrix of cementitious, plastic or rubber products are treated with silica fume preferably in aqueous slurry, whereby particles of silica fume are deposited on surfaces and into the small spaces between adjacent fibres, which may be fibres of glass, organic material, carbon or metal. Best results are achieved by applying the silica fume in aqueous slurry, which contains a dispersant, to fibres such as glass rovings for use in cementitious materials but, if desired, the silica fume may be applied to a web or mat of reinforcing material which may be used to advantage in general industrial applications.

Bond of Lightweight Aggregate Concrete Incorporating Condensed Silica Fume

Abstract: Synopsis: Condensed silica fume, at up to 30% by weight, was used as a partial cement replacement in lightweight aggregate concrete. The results of round and deformed bar cube pull-out tests, with and without applied lateral stress, show that condensed silica fume increases ultimate bond strength and affects the mechanism of failure. The influence of condensed silica fume on bond stress of round bars was similar at all lateral stresses, producing a 50% increase at 20% by weight replacement of cement. For deformed bars the increase in bond strength was more pronounced at higher levels of lateral stress, producing increases approaching 70% at 20% silica fume content. The improvements in ultimate bond strength with condensed silica fume are shown to only partly result from the associated increases in compressive strength, the greater part resulting from the modified properties of the concrete matrix.

Opportunities with Alkalies in Concrete Testing, Research, and Engineering Practice

Abstract: From about 1940 alkalies have been known as a potentially important species in concrete, despite being a minor cement constituent. The hydration reaction of silica in aggregates in concrete in the presence of alkalies, which may cause deleterious expansion, is chemically similar to the process which makes finely ground cementitious components, such as blast-furnace slag, and mineral admixtures, such as fly ashes, natural pozzolana, and silica fume, able to prevent expansive reactions. The chemistry of this process is briefly reviewed, and the impact of elevated temperatures, which occur more often in field concrete than in laboratory concrete and mortars, is discussed. The advantageous effects of alkalies on the rheology of concrete containing mineral admixtures are discussed with reference to recent research regarding the electrochemistry of cement paste. The corresponding effect in densifying the microstructure of the cement paste in hardened concrete is also discussed. Updating of the application of silicate chemistry is proposed as a basis for development of testing tailored to serve effectively the increased uses of the mineral admixtures.