Showing papers on "Silica fume published in 2000"

The nature of the hydration products in hardened cement pastes

Abstract: An understanding of the performance of portland cement-based materials requires knowledge at the microstructural level. Developments in the instrumentation of several techniques have led to improved understanding of the composition, morphology, and spatial distribution of the various products of cement hydration. In particular, our understanding of the nature of the nearly amorphous calcium silicate hydrate (C–S–H) phases – which are the principal binding phases in all portland cement-based systems – has been advanced by developments in solid-state NMR spectroscopy and analytical TEM. This paper presents an overview of the nature of the hydration products formed in hardened portland cement-based systems. It starts with the most straightforward cementitious calcium silicate systems, C3S and β-C2S, and then considers ordinary portland cement and blends of portland cement with silica fume, ground granulated iron blast-furnace slag, and finally alkali hydroxide-activated slag cements.

High strength concrete containing natural pozzolan and silica fume

Abstract: Various combinations of a local natural pozzolan and silica fume were used to produce workable high to very high strength mortars and concretes with a compressive strength in the range of 69–110 MPa. The mixtures were tested for workability, density, compressive strength, splitting tensile strength, and modulus of elasticity. The results of this study suggest that certain natural pozzolan–silica fume combinations can improve the compressive and splitting tensile strengths, workability, and elastic modulus of concretes, more than natural pozzolan and silica fume alone. Furthermore, the use of silica fume at 15% of the weight of cement was able to produce relatively the highest strength increase in the presence of about 15% pozzolan than without pozzolan. This study recommends the use of natural pozzolan in combination with silica fume in the production of high strength concrete, and for providing technical and economical advantages in specific local uses in the concrete industry.

Effect of admixtures on the setting times of high-strength concrete

Abstract: The effect of silica fume (SF), metakaolin (MK), fly ash (FA) and ground granulated blast-furnace slag (GGBS) on the setting times of high-strength concrete has been investigated using the penetration resistance method (ASTM C 403). In addition, the effect of a shrinkage-reducing admixture (SRA) on the setting times of normal and high-strength concrete was also studied. The setting times of the high-strength concrete were generally retarded when the mineral admixtures replaced part of the cement. While the SRA was found to have negligible effect on the setting times of normal strength concrete, it exhibited a rather significant retarding effect when used in combination with superplasticiser in high-strength concrete. The inclusion of GGBS at replacement levels of 40% and greater resulted in significant retardation in setting times. In general, as replacement levels of the mineral admixtures were increased, there was greater retardation in setting times. However, for the concrete containing MK, this was only observed up to a replacement level of 10%.

The effect of mineral admixtures on the properties of high-performance concrete

Abstract: The paper presents a laboratory study on the influence of two mineral admixtures, silica fume (SF) and fly ash (FA), on the properties of superplasticised high-performance concrete. Assessment of the concrete mixes was based on short- and long-term testing techniques used for the purpose of designing and controlling the quality of high-performance concrete. These include compressive strength, porosity, oxygen permeability, oxygen diffusion and chloride migration. Measurements were carried out after curing at 20% and 65% relative humidity up to the age of 1 yr. The results, in general, showed that mineral admixtures improved the properties of high-performance concretes, but at different rates depending on the binder type. While SF contributed to both short- and long-term properties of concrete, FA required a relatively longer time to get its beneficial effect. In the long term, both mineral admixtures slightly increased compressive strength by about 10%, but contributed more to the improvement of transport properties of concretes.

Flame retardant mechanism of silica gel/silica

Abstract: Various types of silica, silica gel, fumed silicas and fused silica were added to polypropylene and polyethylene oxide to determine their flame retardant effectiveness and mechanisms. Polypropylene was chosen as a non-char-forming thermoplastic and polyethylene oxide was chosen as a polar char-forming (slight) thermoplastic. Flammability properties were measured in the cone calorimeter and the mass loss rate was measured in our radiative gasification device in nitrogen to exclude any gas phase oxidation reactions. The addition of low density, large surface area silicas, such as fumed silicas and silica gel to polypropylene and polyethylene oxide significantly reduced the heat release rate and mass loss rate. However, the addition of fused silica did not reduce the flammability properties as much as other silicas. The mechanism of reduction in flammability properties is based on the physical processes in the condensed phase instead of chemical reactions. The balance between the density and the surface area of the additive and polymer melt viscosity determines whether the additive accumulates near the sample surface or sinks through the polymer melt layer. Fumed silicas and silica gel used in this study accumulated near the surface to act as a thermal insulation layer and also to reduce the polymer concentration near the surface. However, fused silica used in this study mainly sank through the polymer melt layer and did not accumulate near the surface. The heat release and the mass loss rate of polypropylene decreased nearly proportionally with an increase in mass loading level of silica gel up to 20% mass fraction. Polyethylene oxide samples with fumed silicas and silica gel formed physically strong char/silica surface layers. This layer acted not only as thermal insulation to protect virgin polymer but also acted as a barrier against the migration of the thermal degradation products to the surface.

Evaluation of rapid chloride permeability test (rcpt) results for concrete containing mineral admixtures

Abstract: The correlation between the charge passed data derived from rapid chloride permeability test (RCPT) and the chloride penetration coefficient K derived through 90-day soaking test for the concrete containing mineral admixture is elucidated. For this purpose, concrete specimens containing varying proportions and fineness of mineral admixtures, such as ground granulated blast-furnace slag (GGBFS) and silica fume (SF), and moist-cured for different periods, were subjected to a 90-day soaking test and a RCPT. To supplement the results, the electrical resistivity and compressive strengths of concrete mixtures were also evaluated. For the majority of mixtures containing SF and GGBFS, the chloride penetration coefficient K and the charge passed data were following different trends. This suggests that the charge passed and K are the independent properties of concrete controlled by different factors. The charge passed depends on the microstructure and the pore fluid conductivity (especially OH-negative ions) of the concrete, while K depends primarily on the microstructure of the concrete. Furthermore, the charge passed through SF and GGBFS concrete was shown decreasing exponentially with increasing electrical resistivity of the mixture. Thus, the charge passed through the concrete mixture containing SF and GGBFS can be used as a measure of the overall conductivity of concrete, rather than as a measure of the resistance of chloride penetration. The charge passed was shown to be related to the compressive strength of SF and GGBFS concrete; the compressive strength increases linearly with the charge passed decreasing.

Optimization and performance of air-entrained, self-consolidating concrete

Abstract: The use of self-consolidating concrete (SCC) can enable the reduction of labor demand for vibration and surface finishing, accelerate placement rate of concrete, and secure superior surface quality. Despite the low yield value required for deformability, SCC is characterized by a moderate viscosity to enhance cohesiveness and stability of the fresh concrete. The air entrainment of SCC for frost durability can reduce viscosity, leading to greater risk of segregation and blockage of concrete flow upon spreading between closely spaced obstacles. This paper investigates the mixture proportioning of air-entrained SCC suitable for filling congested sections, such as in the case of repair of the underside of bridge deck girders and conventional nonrestricted elements. The results of a laboratory study undertaken to optimize and evaluate properties of air-entrained SCC are presented. The mixtures were proportioned with 370 kg/cu m, 450 kg/cu m, and 550 kg/cu m of cementitious materials and water-cementitious material ratios of 0.45 to 0.50. Ternary binders containing 20% Class C fly ash or 40% ground blast-furnace slag with 3% silica fume were used. The mixtures were evaluated for slump flow consistency, restricted deformability and surface settlement, strength development, elastic modulus, temperature rise, shrinkage, permeability, and frost durability. Examples of the use of such concrete for repair of a densely reinforced beam in a parking structure and a moderately reinforced beam-wall element with restricted access in a powerhouse are also discussed. Test results clearly indicate the feasibility of proportioning air-entrained SCC of high stability and resistance to blockage. Optimized mixtures exhibited adequate engineering properties and durability. Field studies demonstrated the effectiveness of such high-performance concrete to repair damaged sections presenting difficulties for placement and consolidation.

Long-Term Mechanical Properties and Durability Characteristics of High-Strength/High-Performance Concrete Incorporating Supplementary Cementing Materials under Outdoor Exposure Conditions

Abstract: This paper presents the results of tests performed on the compressive strength of high-strength/high-performance concrete after up to 10 years; the modulus of elasticity (E-modulus) after 2 years, 4 years, and 10 years; and the carbonation depth and the resistance of concrete to chloride-ion penetration after 10 years. The tests were performed on drilled cores taken from structural test elements simulating concrete columns. In addition, the compressive strength of cylinders cured in a moist room and in the field, as well as the compressive strength of drilled cores taken from the structural elements after up to 4 years, was determined. After 10 years, the compressive strength of the cores drilled from the column elements of the control portland cement concrete (PCC) and concrete incorporating various supplementary cementing materials ranged from 86.4 MPa to 110.3 MPa. The highest strength was obtained for the high-volume fly ash concrete followed by the control PCC, slag concrete, silica fume concrete, and concrete incorporating a combination of slag and silica fume, in that order. Even though the high-volume fly ash concrete at ages up to 28 days had lower strength than the other concretes, it attained the highest strength gain of more than 120% between 28 days and 10 years. On the contrary, the concrete incorporating 12% silica fume had the highest compressive strength at ages up to 28 days, but it had a strength gain of only 18% beyond that age. In general, the E-moduli of the moist- and field-cured cylinders and the cores taken from the column elements were similar. For the cores drilled from the column elements, the fly ash concrete had the highest E-modulus at all three ages of 2 years, 4 years, and 10 years. The experimentally determined E-moduli ranged from 83% to 116% of the values calculated according to American Concrete Institute Code 318. In tests performed in accordance with American Society for Testing and Materials C 1202, the charge passed through all the concretes at 10 years was less than 1,000 coulombs, indicating very high resistance of the concretes to the chloride-ion penetration. After 10 years of outdoor exposure, the depth of carbonation in all the concretes was negligible.

Cement-matrix composites for smart structures

Abstract: Cement-matrix composites for smart structures are reviewed. The functions include strain sensing, damage sensing, temperature sensing, thermoelectricity, vibration reduction and radio wave reflection. The functions are rendered by the use of admixtures, such as short carbon fibers, short steel fibers and silica fume.

Effect of Silica Fume and Rice Husk Ash on Alkali-Silica Reaction

Abstract: The alkali-silica reaction (ASR), which occurs between the alkaline hydroxides in cement paste and reactive minerals in the aggregate, produces an expansive gel that may cause cracking and displacement in concrete structures. Currently, pozzolanic materials are used to prevent or minimize this cracking. An experimental program was undertaken to study the effect of different levels of cement replacement by silica fume (SF) or rice husk ash (RHA) on the expansion of mortar bars containing two types of reactive aggregates. The morphology and composition of the ASR gel were determined using scanning electron microscopy and energy-dispersive x-ray. The results indicate that it is possible to significantly reduce the mortar bar expansion for both types of aggregate using either SF or RHA. As predicted by double-layer models, the expansion of the mortar bars was highly correlated to the gel composition. The experimental results were also analyzed using the Kruskal-Wallis nonparametric method to validate the effect of these pozzolanic replacements on ASR gel composition.