Showing papers on "Silica fume published in 2008"

Effects of silica fume addition and water to cement ratio on the properties of high-strength concrete after exposure to high temperatures

Abstract: This paper presents the results of an experimental investigation on the effect of different amounts of silica fume (SF) and water to cement ratios (w/c) on the residual compressive strength of high-strength concrete after exposure to high temperatures. Based on the results obtained the rates of strength loss for concrete specimens containing 6% and 10% SF at 600 °C were 6.7% and 14.1% lower than those of the ordinary concrete. The dosage of SF had no significant effect on the relative residual compressive strength at 100 and 200 °C, whereas the amount of SF had considerable influences on the residual compressive strength above 300 °C similar to the response at 600 °C. The optimum dosage of SF and w/c was found to be 6% and 0.35, respectively.

Preparation of C200 green reactive powder concrete and its static–dynamic behaviors

Abstract: In this paper, a new type of green reactive powder concrete (GRPC) with compressive strength of 200 MPa (C200 GRPC) is prepared by utilizing composite mineral admixtures, natural fine aggregates, short and fine steel fibers. The quasi-static mechanical properties (mechanical strength, fracture energy and fiber–matrix interfacial bonding strength) of GRPC specimens, cured in three different types of regimes (standard curing, steam curing and autoclave curing), are investigated. The experimental results show that the mechanical properties of the C200 GRPC made with the cementitious materials consisting of 40% of Portland cement, 25% of ultra fine slag, 25% of ultra fine fly ash and 10% of silica fume, 4% volume fraction of steel fiber are higher than the others. The corresponding compressive strength, flexural strength, fracture energy and fiber–matrix interfacial bonding strength are more than 200 MPa, 60 MPa, 30,000 J/m2 and 14 MPa, respectively. The dynamic tensile behavior of the C200 GRPC is also investigated through the Split Hopkinson Pressure Bar (SHPB) according to the spalling phenomena. The dynamic testing results demonstrate that strain rate has an important effect on the dynamic tensile behavior of C200 GRPC. With an increase of strain rate, the peak stress rapidly increases in the dynamic tensile stress–time curves. The C200 GRPC exhibits an obvious strain rate stiffening effect in the case of high strain rate. Finally, the mechanism of excellent static and dynamic properties gains of C200 GRPC is also discussed.

Thermal and dynamic mechanical behavior of bionanocomposites: Fumed silica nanoparticles dispersed in poly(vinyl pyrrolidone), chitosan, and poly(vinyl alcohol)

Abstract: Various bionanocomposites were prepared by dispersing fumed silica (SiO2) nanoparticles in bio- compatible polymers like poly(vinyl pyrrolidone) (PVP), chitosan (Chi), or poly(vinyl alcohol) (PVA). For the biona- nocomposites preparation, a solvent evaporation method was followed. SEM micrographs verified fine dispersion of silica nanoparticles in all used polymer matrices of compo- sites with low silica content. Sufficient interactions between the functional groups of the polymers and the surface hydroxyl groups of SiO2 were revealed by FTIR measurements. These interactions favored fine dispersion of silica. Mechanical properties such as tensile strength and Young's modulus substantially increased with increas- ing the silica content in the bionanocomposites. Thermog- ravimetric analysis (TGA) showed that the polymer matrices were stabilized against thermal decomposition with the addition of fumed silica due to shielding effect, because for all bionanocomposites the temperature, corre- sponding to the maximum decomposition rate, progres- sively shifted to higher values with increasing the silica content. Finally, dynamic thermomechanical analysis (DMA) tests showed that for Chi/SiO2 and PVA/SiO2 nanocomposites the temperature of b-relaxation observed in tand curves, corresponding to the glass transition tem- perature Tg, shifted to higher values with increasing the SiO2 content. This fact indicates that because of the reported interactions, a nanoparticle/matrix interphase was formed in the surroundings of the filler, where the macromolecules showed limited segmental mobility. V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 1739-1749, 2008

The influence of aggregate type on the strength and abrasion resistance of high strength concrete

Abstract: This paper examines the influence of aggregate type on the strength characteristics and abrasion resistance of high strength silica fume concrete. Five different aggregate types (gabbro, basalt, quartsite, limestone and sandstone) were used to produce high strength concrete containing silica fume. Silica fume replacement ratio with cement was 15% on a mass basis. Water-binder ratio was 0.35. The amount of hyperplasticizer was 4% of the binder content by mass. Gabbro concrete showed the highest compressive and flexural tensile strength and abrasion resistance, while sandstone showed the lowest compressive and flexural tensile strength and abrasion resistance. High abrasion resistant aggregate produced a concrete with high abrasion resistance. Three-month compressive strengths of concretes made with basalt, limestone and sandstone were found to be equivalent to the uniaxial compressive strengths of their aggregate rocks. However, the concretes made with quartsite and gabbro aggregate showed lower compressive strength than the uniaxial compressive strength of their aggregate rocks.

Effects of elevated temperature on compressive strength and weight loss of the light-weight concrete with silica fume and superplasticizer

Abstract: In this study, structural light-weight concretes produced by Pumice (LWC) and concretes with normal-weight aggregate (NWC) were investigated. Compressive strength and weight loss of the concretes were determined after being exposed to high temperatures (20, 100, 400, 800, 1000 °C). To achieve these objectives, 12 different types of concrete mixtures were produced. In producing the mixtures, silica fume (SF) was used to replace the Portland cement in the ratios of 0%, 5% and 10% by weight. Half of the mixtures were obtained by adding superplasticizers (SP) to the above mixtures in the ratio of 2% by weight. In conclusion; unit weight of LWC was 23% lower than that of NWC. The LWC containing 2% SP could retain 38% of the initial compressive strength. Rate of deterioration was higher in NWC when compared to LWC. The loss of compressive strengths increased depending on the ratio of using SF at about 800 °C and over.

Dynamic mechanical analysis of fumed silica/cyanate ester nanocomposites

Abstract: Fumed silica particles with average primary particle diameters of 12 and 40 nm were combined with a low viscosity bisphenol E cyanate ester resin to form composite materials with enhanced storage modulus and reduced damping behavior, as evidenced by dynamic mechanical analysis (DMA). The storage modulus increased with volume fraction of fumed silica in both the glassy and rubbery regions, but the increase was more pronounced in the rubbery region. The maximum increase in storage modulus in the glassy region was 75% for 20.7 vol% of 40 nm fumed silica, while the same composition showed a 231% increase in the rubbery storage modulus. Furthermore, decreases in damping behavior were used to estimate the effective polymer-particle interphase thickness. The glass transition temperature of the nanocomposites was not changed significantly with increasing volume fraction.

Rheology of Cement Paste: Role of Excess Water to Solid Surface Area Ratio

Abstract: Although many attempts have been made in previous research to identify the various parameters governing the rheology of cement paste, there has been little progress in evaluating the combined effects of these parameters. In this paper, a new parameter, the excess water to solid surface area ratio, is proposed to evaluate the combined effects of water content, packing density, and solid surface area on the rheological properties of cement paste. For the purpose of determining the value of this new parameter, a new wet packing method has been developed to measure the packing density of cementitious materials so that the voids content of the cementitious materials and the amount of excess water in the cement paste can be quantified. A number of cement paste samples containing different proportions of cement, pulverized fuel ash and condensed silica fume, and different water contents have been tested and their rheological properties, as measured by a rheometer, correlated to the new parameter. The correlation revealed that the excess water to solid surface area ratio, which serves as an indicative measure of the average water film thickness, is the single most important factor governing the rheology of cement paste.

Effects of RHA on autogenous shrinkage of Portland cement pastes

Abstract: In this work, the effects of partial replacements of Portland cement by rice-husk ash (RHA) on the autogenous shrinkage were investigated. Pastes with water/binder ratio 0.30 and substitutions of 5% and 10% cement by RHA were tested. Two RHAs, both amorphous and partially crystalline, were studied. Comparisons between pastes with silica fume, and control pastes (without RHA or silica fume) are presented. Autogenous deformations, internal relative humidity and compressive strength were recorded. The RHA, amorphous or partially crystalline, when used in an appropriate way, reduces autogenous shrinkage.

Using lithium nitrate and pozzolanic glass powder in concrete as ASR suppressors

Abstract: This investigation studies the influence of two different mineral admixtures, lithium nitrate (Li) and pozzolanic glass powder (PGP) on the expansion induced by alkali-silica reaction (ASR). Four numbers of concrete prisms were produced for each concrete mix to measure the expansion resulted from the ASR according to the test method of the BS 812-123:1999. Chemical analysis was performed using X-ray spectra. Test results confirmed that Li and PGP have significantly reduced the ASR expansion. Lower calcium to silica ratio (Ca/Si) was found in concrete mix contains (PGP) because of the high amorphous reactive silica and low calcium content in PGP compared to ordinary Portland cement CEM1. Similar components and minerals phases were obtained in different concrete mixes by using XRD.

Fracture behaviour of fumed silica/epoxy nanocomposites

Abstract: The paper shows the results of an experimental programme oriented to the manufacturing and characterisation of nanocomposites based on epoxy resin modified with fumed silica nanoparticles (0.1, 0.3 and 0.5 vol.%) with different surface treatments (unmodified, AMEO and GLYMO). The resulting degree of dispersion and the interfacial adhesion were investigated by TEM and SEM. Previous tensile and fracture toughness properties are discussed together with new results on the fatigue behaviour under mode I loading. The fracture toughness K Ic turned out to be significantly increased and a positive trend towards the improvement in the crack propagation threshold under fatigue was observed.

Chloride penetration in binary and ternary blended cement concretes as measured by two different rapid methods

Abstract: In this study, the two rapid chloride permeability tests; the AASHTO’s rapid chloride permeability test (RCPT) and the University of Cape Town (UCT) chloride conduction test were employed and compared using concrete specimens cast with effective w/b ratio of 0.48 and applying seven days of curing. Fly ash and blast furnace slag were used in a systematic replacement of cement at the levels of 25%, 50%, and 70%. In addition, silica fume was added at 10% cement replacement. The matrix therefore, was either a binary or ternary blend of cementitious materials. The experimental results were tested for significance using standard statistical methods. The results indicate that ternary blends of 25% fly ash and 10% silica fume exhibited significant decrease in charge passed compared to 25% fly ash. Similarly, the charge passed in the ternary blends of 25% BFS and 50% BFS with addition of 10% silica fume showed lower charge compared to their respective binary blends. The UCT test has been faster and more convenient to use than the RCPT. However, when binary and ternary blends were compared, the RCPT proved to be more sensitive in showing appreciable differences in the values obtained while such differences were insignificant when the UCT test was used.

Microstructure engineering of Portland cement pastes and mortars through addition of ultrafine layer silicates

Abstract: Pozzolanic submicron-sized silica fume and the non-pozzolanic micron- and nano-sized layer silicates (clay minerals) kaolinite, smectite and palygorskite have been used as additives in Portland cement pastes and mortars. These layer silicates have different particle shape (needles and plates), surface charge, and size (micron and nano). The structure of the resulting cement pastes and mortars has been investigated by atomic force microscopy (AFM), helium porosimetry, nitrogen adsorption (specific surface area and porosity), low-temperature calorimetry (LTC) and thermal analysis. The main result is that the cement paste structure and porosity can be engineered by addition of selected layer silicates having specific particle shapes and surface properties (e.g., charge and specific surface area). This seems to be due to the growth of calcium-silicate-hydrates (C–S–H) on the clay particle surfaces, and the nano-structure of the C–S–H depends on type of layer silicate. The effect of layer silicate addition is most pronounced for palygorskite and smectite having the largest surface area and negative charges on the particle surfaces. The cement pastes containing palygorskite and bentonite have, in comparison to the pure cement paste and the paste containing kaolinite, a more open pore structure consisting of fine pores. Silica fume paste contains a significant amount of closed pores. As a secondary result, it is demonstrated that both the degree and duration of sample drying strongly modifies the structure of the cementitious materials under investigation.