Showing papers on "Silica fume published in 1985"

Electrochemical monitoring of corrosion of steel in microsilica cement pastes

Abstract: The corrosion behaviour of steel in hardened cement pastes containing various proportions of microsilica and sodium chloride has been studied by corrosion potential and linear polarisation measurements. Replacement of increasing proportions of Portland cement with microsilica in pastes of constant total chloride content and water content caused the ratio of free chloride ion concentration to hydroxyl ion concentration in the pore water to increase. This tended to destabilise the passivity of steel during the early stages of hydration but the effect became less significant as curing progressed. It is suggested that the growth of anodic pits may be restrained in microsilica cement pastes owing to the fine pore structure of the material which limits the mobility of chloride ions near the surface of embedded steel.

Lightweight concrete roof tiles.

Abstract: Lightweight concrete roof tiles are produced by extruding a cement/lightweight aggregate/water mix containing a non-thixotropic silica fume in a weight ratio of silica fume (calculated as dry weight) to cement of about 2:98 to about 25:75, preferably about 8:92 to about 12:88. The presence of non-thixotropic silica fume in the mix enables lightweight roof tiles of adequate flexural strength to be obtained from the mix by extrusion. Novel cement/lightweight aggregate/water mixes containing non-thixotropic silica fume and novel roof tiles prepared therefrom are also described.

The feasibility of using silica fume to control concrete expansion due to alkali-aggregate reactions

Abstract: Resultats d'un programme d'essais de 600 jours visant a controler l'adequation de la cendre de silice a prevenir l'expansion du beton due a la reaction alcali-granulate (opale). Les eprouvettes ont ete conservees a des temperatures de 23°, 38° et 50°C; on a fait varier les pourcentages de cendre de silice et les types d'adjuvants

Asbestos-free, hydraulic inorganic material-based sheet products and process for their production

Abstract: An aqueous dispersion containing an inorganic sheet-forming component preferably silica fume pulp, reinforcing fiber, fibrous inorganic material, flocculant and hydraulic inorganic material in specified proportions is excellent in dispersibility of the reinforcing fiber, retention of the hydraulic inorganic material and other aspects in spite of the absence of asbestos therein. Sheets formed from the aqueous dispersion in the manner of papermaking are resistant to layer separation and to cracking in the step of shaping such as corrugation. Products derived from the sheets are excellent in physical properties, such as strength.

Calcium silicate base materials

Abstract: of EP0078119A calcium silicate based material is disclosed comprising a matrix of calcium silicate consisting predominantly of xonotlite and/or tobermorite to which silica, preferably finely divided silica such as silica fume, has been added to occupy the interstices of the matrix. Calcium ions, residual from the crystal forming reaction may be present or added as calcium oxide, calcium hydroxide or a calcium salt which dissociates in aqueous solutions excluding CaCl2 and other anions which may deletrously affect the material. These then undergo a superficial reaction with the silica particles, providing a cementitious bond between the particles without significant loss of the thermal properties of the silica. The material has improved properties as regards to thermal insulation, shrinkage on drying, and compressibility, after heating at elevated temperatures.

The Development of Durable Cementitious Materials for Use in a Nuclear Fuel Waste Disposal Facility

Abstract: This paper describes the work on cement paste development and short-term leaching tests in Standard Canadian Shield Saline Solution (SCSSS) in the presence of bentonite at 150/sup 0/C. It has been found that: (1) supplementary cementing materials such as silica fume or fly ash could significantly improve the properties of sulfate resistant portland cement (SRPC), in particular, permeability to water and pore size distribution; (2) the addition of bentonite suppressed the normal tendency of the pH of groundwater to increase rapidly in the presence of cement; (3) the presence of bentonite increased the release of potassium ions from the cements; and (4) SRPC blended with 20% silica fume resulted in a groundwater pH lower than that of SRPC, with and without bentonite. Moreover, its cumulative fraction of release of potassium was significantly lower than that of SRPC.

Thermal Stability of Certain Hydrated Phases in Systems Made Using Portland Cement.

Abstract: : As part of the study of hydraulic-cement system for use in possible underground isolation of nuclear wastes, this study was made to determine the temperature stability of ettringite and chloroaluminate. Either or both of these phases may be expected in a hydraulic cement system depending on the presence of salt. (NaCl). The study of ettringite was made using 15 mixtures that contained portland cement, plaster, 2 levels of water, and in some mixtures, 1 of 6 pozzolans (3 fly ashes, 1 slag, a silica fume, a natural pozzolan), plus a 16th mixture with anhydrous sodium sulfate replacing plaster (CaSO4 . 1/2H20). Specimens were made and stored at 23, 50, and 75 C or 23, 75, and 100 C (all four temperatures in one case) for periodic examination by x-ray diffraction for phase compositiion and ettringite stability, and testing for compressive strength and restrained expansion. A more limited study of the stability of chloroaluminate was made along the same lines using fewer mixtures, salt instead of plaster, and higher temperatures plus some pressure. It was found that while some ettringette was decomposed at 75 C, depending on the composition of the mixture, all ettringite was undetectable by X-ray diffraction at 100 C, usually within a few days. The evidence indicates that the ettringite became amorphous and no significant test phases formed in its place. Since there was no corresponding loss in strength or reduction in volume, this loss of ettringite crystallinity was considered to be damaging. Based on much more limited data, chloroaluminate was found to decompose between 130 C at 25 psi and 170 C at 100 psi; no significant phases replaced it.

Biological Effects of Fume Silica (Amorphous Type)

Abstract: The studies done on workers exposed to the inhalation of amorphous silica have not been unanimous about the identification of their respiratory aggression, many authors do not accept it, or only with doubt (Volks 1960; Roberts 1965; Swensson 1971; Jahr 1980; Parkes 1982) However, other authors have admitted this occupational disease in the handling of amorphous silica (Smart 1952; Vaillant 1983) as in ferroalloy industries (Glomme 1965; Johnson 1973; Davies 1974; Taylor 1977; Vitums 1977; Dowie 1978; Brambilla 1980)

Tunnel shotcreting--strength in fibers

Abstract: Steel fiber reinforced shotcrete (SFRS) costs less, has a lower rate of rebound and allows modern excavating equipment to advance with fewer interruptions than conventional shotcrete. SFRS has become popular in the U.S. and Canada in the last five to seven years. Conventional concrete design, equipment and laboratory tests have heavily influenced shotcrete mix design. SFRS can be made by either the dry or wet mix methods. Adding microsilica, also known as silica fume, to the reinforced shotcrete mix enhances the SFRS properties.

A Strength Model for Concretes Containing Fly Ash, Blast-Furnace Slag and Silica Fume

Abstract: This paper describes preliminary a model for the strength of concretes containing industrial by-products such as fly ash, blast-furnace slag, and silica fume. A formula that describes the various strength production mechanisms of these by-products is developed. These materials produce strength by pozzolanic reaction, by latently hydraulic reactions and by self-cementing reactions similar to Portland cement. A method of separating the effect of each mechanism is proposed. A parameter for each of these strength producing mechanisms can be determined from concrete strength data.

Durability of Lightweight Concrete for Arctic Concrete Structures

Abstract: The experimental results regarding characteristics of lightweight concrete incorporating condensed silica fume are discussed. Many factors such as type and moisture content of lightweight aggregates, dosage of condensed silica fume, air content, water to cement ratio are selected as experimental factors. It was concluded that high strength and durable lightweight concrete satisfying requirements for arctic concrete structures could be obtained by using lightweight aggregate with low moisture content and utilizing condensed silica fume as a mineral admixture.