Showing papers on "Silica fume published in 2002"

Influence of recycled aggregates on mechanical properties of HS/HPC

Abstract: Recently, aggregates derived from demolished concrete structures were of relatively low strength, and applications were of secondary importance. Since a short time, the necessity of demolition of structures with strong concrete, like building frames or bridge beams, has appeared and created the source of recycled aggregate of quite new generation. Besides of obvious environmental aims recycling of concrete has gained new economical aspects. Concrete in responsible structures, with strength of 40–70 MPa for instance, was originally mixed from aggregate of good quality, e.g. granite or basalt, and with large amount of cement. The aggregate obtained from crushing of such structures retained some binding abilities as may be activated by means of silica fume or fly ash admixtures. Generally, such aggregates are different from natural and, consequently, concrete made with use of them has specific properties. The results of research on mechanical properties of structural recycled high performance concrete (RHPC) are presented.

Effects of Test Conditions and Mixture Proportions on Behavior of High-Strength Concrete Exposed to High Temperatures

Abstract: In this research, mechanical properties of high-strength concrete exposed to elevated temperatures were measured by heating 100 x 200 mm cylinders at 5 deg C/min to temperatures of up to 600 deg C. Heating was conducted with/without sustained stress, and properties were measured at elevated temperatures and after cooled to room temperature (RT). 4 mixes with water-cementitious materials (w/cm) ratios ranging from 0.22-0.57 and RT strengths ranging from 51-98 MPa were used. 2 of the mixtures contained silica fume. Measured compressive strengths and elastic moduli were normalized with respect to RT values, and analysis of variance was used to determine whether the test condition, the value of w/cm, or the presence of silica fume affected results. The influence of these variables on the tendency for explosive spalling was also examined. Results show that losses in relative strength due to high-temperature exposure were affected by the test condition and w/cm, but there were significant interactions among the main factors that resulted in complex behaviors. The presence of silica fume does not appear to have a significant effect. Measurements of temperature histories in the cylinders revealed complex behaviors believed to be linked to heat-induced transformations and transport of free/chemically combined water.

Durability of ternary blend concrete with silica fume and blast-furnace slag: laboratory and outdoor exposure site studies

Abstract: In September 1998, a site investigation assessing the durability performance of ternary blend concretes was initiated by an academic-government-industry consortium. Different concretes were cast in the field to assess durability performance in an outdoor exposure setting as well as with standard lab tests. Resistances to the following deterioration mechanisms were assessed: alkali-silica reactivity, chloride ion ingress, and deicer salt scaling. Compressive strength was also measured at various ages. This paper describes this project in detail and presents field observations and lab findings up to 2 years later.

Review: Improving cement-based materials by using silica fume

Abstract: The effects of silica fume as an admixture in cement-based materials are reviewed in terms of the mechanical properties, vibration damping capacity, freeze-thaw durability, abrasion resistance, shrinkage, air void content, density, permeability, steel rebar corrosion resistance, alkali-silica reactivity reduction, chemical attack resistance, bond strength to steel rebar, creep rate, coefficient of thermal expansion, specific heat, thermal conductivity, fiber dispersion, defect dynamics, dielectric constant and workability. The effects of silane treatment of the silica fume and of the use of silane as an additional admixture are also addressed.

Effect of Nano-and Micro-Silica Fillers on Polyurethane Foam Properties:

Abstract: Two series of rigid and flexible polyurethane foams were prepared with two types of silica fillers. The density of the flexible foams was 60 kg/m 3 and that of rigid 30 kg/m3. The fillers were micro-silica of the average particle size of 1.5 mm and nano-silica of the average particle size of 12 nm. The concentration of fillers varied from 0–20%. The micro-silica filler did not show any significant effect on density of either rigid or flexible foams. Nano-silica increased the density of both types of foams only at concentration above 20%. Nano-silica lowered the compression strength of both types of foams at all concentrations while micro-silica exhibited the same effect at concentrations above 10%.The hardness and compression strength in flexible polyurethane foams with nano-silica was increased and the rebound resilience decreased. Reduced density of foams was not changed by nano-silica concentrations up to 20%. It is assumed that the nano-filler, as an additional physical crosslinker, increased modulus ...

Monitoring electrical resistance of concretes containing alternative cementitious materials to assess their resistance to chloride penetration

Abstract: Chloride ion penetration into concrete and the resulting deterioration (cracking and spalling due to the corrosion of reinforcement) is a major concern of engineers and owners of bridges and marine structures. Several publications have reported the excellent performance of concrete containing alternative cementitious materials (ACMs), such as pulverised fuel ash (PFA), ground granulated blast furnace slag (GGBS), microsilica (MS) and metakaolin (MK) in marine environment and highway structures. The resistance offered by these concretes has been related to the low mobility of chloride ions due to either the reduction in the number of interconnected pores as a result of the pozzolanic reaction of the ACMs or the chemical binding with the cement hydrates. However, the secondary reaction products are formed slowly in Portland cement concrete containing ACMs and as a result it is likely that the resistance offered to the penetration of chloride ions also increases slowly with time. In order to monitor the continuous behaviour of concretes containing these ACMs in a chloride exposure regime, an investigation was carried out, the results of which are reported in this paper. Ten different concrete mixes were subjected to a cyclic ponding regime with 0.55 M sodium chloride solution and the changes in concrete were monitored by measuring the changes in resistance between pairs of stainless steel electrodes embedded in the concrete at different depths from the exposed surface. The test was continued for nearly one year. The results indicated that, although the resistance of concrete decreased initially due to the penetration of chlorides, in the longer term the resistance of concretes containing ACMs outperformed the control concrete made with ordinary Portland cement (OPC). Drilled dust samples extracted after different durations of ponding were tested for the chloride content, which confirmed that the increase in resistance of the ACMs was due to the combined effects of the reduction in the penetration of chlorides and the continuous hydration activity of the ACMs.

Processing and characterization of a lightweight concrete using cenospheres

Abstract: A study has been conducted in which a lightweight concrete was processed using ceramic microspheres, known as cenospheres, as a primary aggregate. The mechanical properties, including compressive strength, tensile strength, flexural strength and fracture toughness, were tested and cataloged. It was determined that the addition of high volumes of cenospheres significantly lowered the density of concrete but was also responsible for some strength loss. This strength loss was recovered by improving the interfacial strength between the cenospheres and the cement. The interfacial properties were quantified using interfacial fracture mechanics techniques. These techniques were also employed to find a suitable surface modifier with which to improve this interface. The admixture silica fume and the coupling agent Silane™ were found to be suitable candidates and both performed well in small-scale compression testing. Silica fume was eventually isolated as a prime candidate. The concrete produced with this admixture was tested and compared to a concrete with an equal volume fraction of cenospheres. The addition of silica fume improved the compressive strength of cenosphere concrete by 80%, tensile strength by 35%, flexural strength by 60% and fracture toughness by 41%.

Carbon containing castables: current status and future prospects

Abstract: Measures to overcome the main technical di fficulties hindering the development and application of carbon containing castables are discussed The aqueous wettability and dispersion properties of graphite can be improved by coating with materials such as carbides (SiC) and oxides (Al2O3, TiO2, SiO2, MgO, ZrO2) or by forming micropellets / briquettes Thick and dense crack free coatings are needed not only to improve the aqueous wettability and dispersion of graphite, but also its oxidation resistance Small and dense micropellets or briquettes enable a homogeneous distribution of graphite in the matrix to be achieved, along with acceptable mechanical strength and corrosion resistance Coating techniques have also been used to improve the hydration resistance of aluminium based antioxidants, but detailed studies in this area are still needed The main binder systems are currently based on superfine silica fume and hydratable alumina as these do not form low melting phases in castables As well as de

Influence of chemical and phase composition of mineral admixtures on their pozzolanic activity

Abstract: The influence of the chemical and phase composition of different mineral admixtures on pozzolanic activity was studied. Local kaolin clay, activated kaolin clay, porcellanite, activated porcellanite, pumice, fly ash, silica fume and ground quartz were used as components of blended binders for various mortars. Thermal treatments were performed as a means for activation of the minerals. The phase transformations during heat treatment were studied by X-ray diffraction services (XRD) and differential thermal analysis/thermogravimetric analyses (DTA/TGA). The pozzolanic activity of these materials as a function of heat treatment parameters was investigated.

Supplementary Cementing Materials

Abstract: There are many types of cements varying in their mineralogical composition. Many of them are mixed with mineral admixtures. The addition of a mineral admixture such as silica fume produces cement with high strength. Cembinders are aqueous silicic acid suspensions containing silicic acid particles expressed as silica in the range of about 8% to 60% by weight of solution. The specific surface area of the particles is in the range of 50 to about 200 m 2 /g. Superplasticizers are surface active agents and have water-reducing characteristics, and these are also known as high range water reducers. Superplasticizers behave differently with different cements, and different types of superplasticizers behave differently with the same cement. The difference with the cements mainly depends upon the sulfate and aluminate components in the cement. The type of sulfate in the cement has a major effect on the viscosity and yield.

Advantages in the use of fly ashes in cements containing pozzolanic combustion residues: silica fume, sewage sludge ash, spent fluidized bed catalyst and rice husk ash†

Abstract: The enhancement in the workability of Portland cement concrete when coal fly ashes are added to the mix is well-known. This behaviour has been attributed to the spherical shape and the smooth surface of the fly ash particles. On the other hand, when other combustion residues with pozzolanic properties are added to concrete mix formulations, a loss in workability is observed, and an increase in water content may be necessary for obtaining a satisfactory plastic consistency of the concrete, or alternatively chemical additives may be used (plasticizers and superplasticizers). The effect of the combination of fly ash with other mineral admixtures in respect of the effect on the workability of mortars has been studied. The pozzolanic combustion residues examined were: silica fume, sewage sludge ash, spent fluid catalytic cracking catalyst and rice husk ash. Additionally, and due to the pozzolanic properties of coal fly ash and these residues, the strength developments of Portland cement mortars containing fly ash and the selected admixtures have also been measured.

Early age creep and stress relaxation of concrete containing blended cements

Abstract: The main objective is to investigate key properties that influence the stress development in concrete at early ages and the effect of using blended cements. Mineral additives and amount by weight of total binder used in the blended cements are fly ash (25%), ground granulated blast furnace slag (25%), and silica fume (10%). The properties investigated include tensile creep, elastic modulus, split tensile strength, and autogenous shrinkage. The relaxation modulus used for stress prediction was obtained from the creep data fitted using a log-power creep function. These findings show that tensile creep and stress relaxation are important properties of Portland cement concrete. These properties however are reduced in concretes containing blended cements. Blended cements affect the early age strength and elastic modulus moderately but significantly alter the autogenous deformation. Water/cement ratio (w/c), type and dosage of mineral additives were found to influence the magnitude of autogenous deformation. This deformation was found to be significant in low water-cement ratio concretes and should be included in early age stress calculations.