Showing papers on "Silica fume published in 1987"

Condensed Silica Fume in Concrete

Abstract: A balanced view is presented of the effect of condensed silica fume on the physical, chemical, mechanical, and durability aspects with respect to cement pase, mortar and concrete It discusses the nature and types of condensed silica fume, physical characteristics, product variation and problems involved in its handling and transportation The book covers the hydration and microstructural changes occuring in cement paste, the effect of silica fume on the physical properties of mortar, and its influence on the aggregate-cement paste interface The properties of fresh and hardened concrete containing superplasticizers are covered, as well as fly ash and blast furnace slag, and the durability of silica fume concrete The applications and curent standards, and the health and environmental hazards are also considered

Microstructural Gradients in Cement Paste Around Aggregate Particles

Abstract: The effectiveness of condensed silica fume as a strength enhancing additive in concrete has been attributed to its ability to modify the interfacial zone between paste and aggregate. This paper describes a microstructural investigation of this interface using backscattered electron (bse) imaging combined with quantitative image analysis. Composite specimens were made in which a single piece of aggregate was embedded in cement paste. Granite, dolomite and garnet aggregates were used. After curing, the specimens were sectioned perpendicular to the surface of the aggregate particles and polished. The variation in porosity, amount of anhydrous material and calcium hydroxide (CH), with distance from the aggregate surface was measured. It was found that the porosity increases in the paste close to the interface, while the content of anhydrous grains decreases. No significant increase in CH content was found near the interface. The results confirm the applicability of the bse - image analysis technique, but indicate that the interfaces in specimens prepared in this manner may not be representative of aggregate paste interfaces in concrete.

Effect of Condensed Silica Fume on the Microstructure of the Interfacial Zone in Portland Cement Mortars

Abstract: The development of the microstructure at the cement paste-sand grain interfacial zone was studied, and the effect of condensed silica fume was evaluated. In portland cement mortars, the microstructure of the interfacial zone, extending to about 20 to 50 μm from the sand grain surface, was significantly different from that of the bulk paste, and it was characterized by a massive Ca(OH)2 layer engulfing the sand grain and by some channel-type gaps. When 15% condensed silica fume was added, the interfacial zone had a homogeneous and dense microstructure, similar to that of the bulk paste, without the presence of a massive Ca(OH)2 layer or gaps. On the basis of observations at early ages, it is suggested that these characteristics were the result of differences in the nature of the fresh paste, which was more prone to bleeding in the portland cement mortar.

Cementitious composite material containing metal fiber

Abstract: Cementitious composite materials having high strength, vacuum integrity, good thermal properties and low coefficient of thermal expansion are prepared from a mixture of (1) a high strength cement matrix, and (2) a filler component, comprising a metal fiber. These composites are useful in the manufacture of molds and tools for forming metals and plastics. A preferred composite mixture comprising Portland cement, chemically reactive silica particles, inorganic oxide particles, a cement superplasticizer, an irregularly shaped aggregate, metal fibers, and water. The mixtures preferably contain chemically reactive silica fume particles and reactive silica particles such as crystalline silica or quartz particles or a vitreous/glassy form of silicon. The mixtures preferably comprise stainless steel fibers and stainless steel aggregate. High nickel steel and silicon carbide can be used as aggregate in applications requiring low coefficient of thermal expansion.

Effect of Aggregate, Cement, and Mineral Admixtures on the Microstructure of the Transition Zone

Abstract: This paper contains a review of the results from the studies at the University of California at Berkeley on various factors influencing the microstructure of the transition zone in concrete. Two types of aggregate, two different cement, and three mineral admixtures were investigated. Using cement paste-polished aggregate composite specimens cured up to three years, X-ray diffraction, scanning electron microscopy, and microhardness testing techniques were used for characterization of the transition zone. Compared to the transition zone between a quartz aggregate and an ASTM Type I portland cement, transition zones with smaller and less preferentially oriented crystals of calcium hydroxide were obtained when using a Type K expansive cement, or limestone aggregate, or mineral additives, such as condensed silica fume, granulated blast-furnace slag, and fly ash.

The Contribution of the Transition Zone to the Strength of High Quality Silica Fume Concretes

Abstract: The strength of high strength silica fume concretes is usually attributed to the reduction in w/c ratio and the refinement of the pore structure. A study of concretes and pastes, with and without silica fume, suggests that the contribution of the silica fume to strength is also the result of the densification of the transition zone. It is argued here that this influence is as important as the one due to the reduction in w/c ratio. It is suggested that the densification of the transition zone is the result of the effect of the silica fume on the nature of the fresh concrete.

Protective systems for new prestressed and substructure concrete. final report

Abstract: A three-year corrosion research project on 11 corrosion protection systems was undertaken in two laboratory studies. A total of 124 small reinforced concrete slabs were subjected to a 48-week, cyclic wet and dry saltwater exposure in the first study. The second year-long study dealt with cyclic saltwater exposure on 19 full-size sections of reinforced concrete columns and beams and precast, prestressed piles and stay-in-place bridge deck panels. The slab tests evaluated concrete having w/c ratios of 0.51, 0.40, and 0.28 with clear cover of 1, 2, and 3 in. The reinforcing steels evaluated were normal gray bars, normal prestressing strands, galvanized bars, and fusion-bonded epoxy-coated bars and prestressing strands. A calcium nitrite corrosion-inhibiting admixture for fresh concrete was evaluated. A penetrating silane sealer and methacrylate coating system were evaluated as surface treatments for hardened concrete. The full-size member tests included most of the above materials in concrete at a constant w/c ratio of 0.44, generally with 1-in. cover. A concrete containing a silica fume admixture was also evaluated in the full-size member tests. The full-size columns and beams were moist cured while the precast, prestressed piles and bridge deck panels were heat cured overnight at 130 to 140 deg F. Corrosion-related measurements included monitoring macrocell corrosion current and instant-off voltage between corroding and noncorroding reinforcement, half-cell potentials, chloride contents at the initiation of corrosion and at the conclusion of the test cycle, and measurement of the corroded areas on the reinforcement. The measured corrosion activity of these numerous specimens is presented and comparisons are made in their corrosion protection performance. Of particular significance were the beneficial influence of low w/c ratios and adequate concrete cover in reducing corrosion and chloride penetration. The silane sealer and the silica fume pozzolanic admixture concrete both dramatically reduced chloride penetration and the reinforcement did not develop corrosion. Epoxy-coated reinforcing bars and prestressing strands were not corroded in these tests, even though surrounding concrete had high chloride contents. Galvanized reinforcement embedded in conventional concrete and bare reinforcement embedded in concrete made with the calcuim nitrite corrosion inhibitor both developed a low level of corrosion when surrounding concrete had high chloride contents. Design considerations are suggested for these different corrosion protection materials to provide for greater corrosion protection for new cast-in-place reinforced concrete and precast, prestressed concrete bridge members.

Laboratory investigation of concrete containing silica fume for use in overlays

Abstract: Hydraulic cement concretes containing silica fume were batched and tested in the laboratory to assess their suitability for use in overlays having a minimum thickness of 1/4 in. (32 mm). Tests were made for strength, permeability, and freeze-thaw resistance, and the characteristics of air voids in the hardened concrete were determined by petrographic examination. Concretes made with silica fume from two sources at a cement replacement rate of 5 percent by weight and with a water-cement ratio of 0.40 or lower yielded the properties desired for thin overlays. It is thus expected that concretes made with silica fume can provide a cost-effective protective system for bridge decks when placed in overlays having a minimum thickness of 1 1/4 in. (32 mm).

Comparative study of the microstructures of normal and very high-strength concretes

Abstract: The microstructure of a normal concrete with water/cement ratio=0.56 was compared with two series of very high-strength concretes (up to 118 MPa or 17 120 psi), one made with Type III cement and the other with the same Type III cement plus 6 to 10% silica fume. These concretes were water cured for 91 days. Their microstructural examination was undertaken using mercury porosimetry and scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDXA). Similarities and differences are described in detail. Basically, both are composed of calcium silicate hydrates (C-S-H) and calcium hydroxide (C-H), but their proportions and morphology are quite different. The composition and types of C-S-H also vary. Significant differences in porosity and aggregate-cement bonding are present. Chloride ion permeability tests indicate the very high-strength concretes to be as impervious as polymer-impregnated types.

Silica fume slurry

Abstract: A stable, aqueous dispersion of silica fume is formed using a 70-80% ferrosilicon production fume by-product by the inclusion of very small amounts of a stabilizing agent selected from a phosphoric acid, citric acid, hydrofluoric, fluorosilicic acid or their sodium or potassium salts or mixtures thereof

Mechanical properties and freezing and thawing resistance of high-strength concrete incorporating silica fume

Abstract: This report presents results of an investigation dealing with the mechanical properties and freezing and thawing resistance of high-strength, silica fume concrete using ASTM Test Method for Resistance of Concrete to Rapid Freezing and Thawing (C 666-84, Procedure A). Eighteen nonair-entrained and 6 air-entrained concrete mixtures, 0.06 m cubed in size, were made. The water-to-cement (cement + silica fume) ratio (W/C + S) of the mixtures ranged from 0.25 to 0.36, and the percentages of cement replacement by silica fume were 0, 10, and 20% on a weight basis. Any loss in slump due to the use of silica fume was compensated for by the use of a superplasticizer. A number of test cylinders were made for testing in compression at various ages, and test prisms were cast for determining their resistance to repeated cycles of freezing and thawing in accordance with ASTM C 666, Procedure A. Sawn sections of some of the prisms were used for determining the air void parameters of the hardened concrete. Nonair-entrained, high-strength concrete with a compressive strength of up to 87 MPa at 28 days, regardless of the W/C + S and irrespective of the silica fume content used, had durability factors less than 12 when tested in accordance with ASTM 666, Procedure A. Also, air-entrained concrete prisms incorporating 10 and 20 % silica fume as replacement for cement failed to complete 300 cycles of freezing and thawing.

High-strength hydraulic material composition

Abstract: PURPOSE:To provide the title fluid composition having precise transferability, especially excellent in flexural strength, comprising a hydraulic material consisting mainly of alkaline earth metal oxide and Al2O3, ultrafine powder, metallic aggregate and dispersant. CONSTITUTION:(A) A mixture of (i) 95-50wt.% of a hydraulic material with a particle size of 10-30mu consisting mainly of alkaline earth metal oxide and Al2O3 (e.g. alumina cement consisting mainly of calcium aluminate with a molar ratio: CaO/Al2O3<0.5) and (ii) 5-50wt.% of ultrafine powder with its average particle size smaller than the component (i) by one order or more (e.g. silica fume) is incorporated with (B) <=5wt.% times the mixture A, of metallic aggregate with a granular size of 0.1-5mm (e.g. iron powder), (C) 1-5wt.% of a dispersant (beta-naphthalensulfonate formaldehyde condensate), (D) <=35wt.% of water, and, if needed, (E) curing regulator (e.g. Na2SO4), inert inorganic powder, metallic powder etc. followed by kneading, defoaming treatment under a vacuum and vibration treatment, thus obtaining the title composition.

Interaction Between Naphthalene Sulfonate and Silica Fume in Portland Cement Pastes

Abstract: Portland cement pastes were mixed with predissolved naphthalene sulfonate superplasticizer at normal water: cement ratios. Solutions were separated from the fresh pastes at intervals and the residual concentration of the superplasticizer determined by UV spectrophotometry. At low dosage levels essentially all of the superplasticizer was found to be removed from solution within a few minutes; at high dosage levels a substantial concentration was maintained in solution at least to approximately the time of set. In pastes in which silica fume replaced 10% by weight of the cement, it was found that the incorporation of silica fume significantly increased the uptake of superplasticizer. In separate trials it was found that the silica fume by itself adsorbed little superplasticizer, even from high pH solution simulating that of cement paste.

Mechanism of Orientation of Ca(OH)2 Crystals in Interface Layer Between Paste and Aggregate in Systems Containing Silica Fume

Abstract: Factors such as the type of aggregate, the addition of silica fume, and the roughness of the aggregate surface, which influence the orientation index of Ca(OH)2 crystals in the interface layer, are discussed, based on the experimental results of Maso [3] and the author. Then, a hypothesis about the mechanism of this orientation is proposed, involving the Ca(OH)2 crystal structure and the principles of physical chemistry of surfaces. From this hypothesis, it is suggested that the reduction of the orientation index of Ca(OH)2 crystals when silica fume is added is due to physical aspects other than the large water requirement of silica fume. The same is also true when the aggregate surface is rough.

Alkali-aggregate reaction in concrete containing fly ash --proceedings of the 7th international conference on concrete alkali-aggregate reactions, ottawa, canada, 1986

Abstract: This paper depicts the first steps taken to answer the urgent need of developing guidelines for proper, economical and efficient use of fly ash to reduce alkali-aggregate reaction damage in concrete. More than 1300 mortar-bars were cast and tested according to the mortar-bar test method, ASTM c 227, with 0, 17, 26, 34, 45 and 62 percent replacement of the volume of cement in the mixture with fly ash. The effect of silica fume in the mixture was compared to that of fly ash at 17, 34 and 45 percent cement replacement. The variables studied included: type of aggregate, alkali content of the cement, type of pozzolan, percent of cement being replaced and blending of the cement with the fly ash. Both ASTM class c and class f fly ashes were investigated. The test results indicate that the replacement of a portion of the volume of cement with fly ash may reduce the alkali-aggregate expansions, regardless of whether the fly ash is blended or not with the cement at the time of mixing, depending on the proper combination of the following factors: fly ash alkali content and cement replacement with fly ash. The test results also suggest that the replacement of a portion of the volume of cement with silica fume is equally or more effective in reducing the alkali-aggregate expansions. For the covering abstract of the conference see IRRD 811982.

Silica Fume, Bond Strength, and the Compressive Strength of Mortar

Abstract: The strength and strain–rate sensitivity of cement paste and mortar is studied as a function of water–cementitious material ratio (W/C) and silica fume content. W/C’s of 0.30 and 0.35 are used for materials without silica fume, while W/C’s ranging from 0.336 to 0.436 are used for material containing silica fume. The volume fractions of cement paste matrix and sand are held at 63 and 37 percent, respectively, for all mortars. Strain rates of 30, 3000, and 300,000 microstrain per second are used. The results indicate that materials with silica fume are less strain–rate sensitive of than materials without silica fume. The replacement of cement by silica fume appears to (1) reduce rather than increase the bond strength between cement paste and sand and (2) increase the compressive strength of mortar primarily by increasing the strength of the cement paste matrix.

Comparative Study of the Air-Void Stability in a Normal and a Condensed Silica Fume Field Concrete

Abstract: Very little reliable data exist concerning the durability of field-condensed silica fume (CSF) concrete exposed in service to freezing and thawing in the presence of deicing salts. The data available indicate that CSF concrete without the right bubble-spacing factor performs very poorly, but with the correct spacing factor, it performs satisfactorily in similar climatic conditions. This paper reports the findings of a research program studying freeze-thaw durability and the stability of the air void system in a specific CSF mix from batching through placement. Additional test data show shrinkage and compressive strengths. With proper mixing and placing techniques, the air-void system of VSF concrete was found to be as stable as that of normal concrete. The proper spacing factor was obtained without any problem, and freeze-thaw tests confirmed the durability of the CSF concrete.

Effects of microsilica and class c fly ash on resistance of concrete to rapid freezing and thawing and scaling in the presence of deicing agents. concrete durability. katherine and bryant mather international conference, held at atlanta, georgia, usa, 27 april-may 1987

Abstract: The freeze-thaw durability of concretes containing up to 42% class c fly ash or 15% microsilica by weight of cement with ratios of water to cementitious material of 0.53-0.88 is examined to establish the extent to which such relatively lean mixtures can yield good durability along with the satisfactory levels of strength already known to be achieveable, particularly with microsilica. The results show that, while the inclusion of fly ash or microsilica does not detract from performance in rapid freezing and thawing (procedure a of ASTM c666), performance with respect to scaling (ASTM c672) may not be satisfactory, even with an apparently adequate air void spacing factor. When spacing factors are adequate, the 300-cycle durability factors consistently exceed 85%, even for the leanest mixtures, while scaling of some of the mixtures with fly ash or microsilica is severe. Concretes with microsilica can be durable in both ASTM c666 and c672 at w/c+s ratios up to a limit of 0.70, where strengths of 25 mpa at 28 days can be reached with as little as 225 kg/cu m of cement. For concretes with class c fly ash, the corresponding limit for w/c+f appears to be no higher than the 0.45 recommended for normal concrete when exposed to deicing agents. (Author/TRRL)