Showing papers on "Silica fume published in 2005"

Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibres

Abstract: Many investigations are realized to establish the basic mechanical properties of vegetable fibre reinforced composites (VFRC) but not their shrinkage and creep behaviour. Some works have been realized to establish the shrinkage of cement mortar matrices reinforced with cellulose fibres, but very few results has been published with regards to shrinkage of VFRC with short sisal and coconut fibres. In this paper a concise summary of several investigations is presented to establish the influence of sisal and coconut fibres on the free and restrained plastic shrinkage, early drying shrinkage cracking, crack self-healing and long-term drying shrinkage of mortar matrices. The free and restrained shrinkage were studied by subjecting the specimens to wind speed of 0.4–0.5 m/s at 40 °C temperature for up to 280 min. The self healing of cracks of the VFRC was studied by using the same specimens as for the study of restrained shrinkage which were kept further in a controlled environment with 100% relative humidity and temperature of 21 °C for up to 40 days. Drying shrinkage tests were carried out at room temperature with about 41% relative humidity for 320 days. The influence of curing method, mix proportions and partial replacement of ordinary Portland cement (OPC) by ground granulated blast-furnace slag and silica fume on the drying shrinkage of VFRC was also investigated. Finally, based on the obtained results on drying shrinkage an equation using the recommendation of ACI model B3 was adjusted and compared well with the obtained experimental data.

Sulfate attack and role of silica fume in resisting strength loss

Abstract: This paper presents a detailed experimental study on the sulfate attack of Portland cement mortars, and the effectiveness of silica fume in controlling the damage arising from such attack. The test solutions used to supply the sulfate ions and cations were 5% sodium sulfate solution and 5% magnesium sulfate solution. Tap water was used as the reference solution. The main variables investigated in the study were the water/cementitious materials ratio, and the level of cement replacement. Compressive strength measured on 50 mm cubes was used to assess the changes in the mechanical properties of mortar specimens exposed to sulfate attack for 510 days. X-ray diffraction and differential scanning calorimetry were used to evaluate the microstructural nature of the sulfate attack. The test results showed that the presence of silica fume had a beneficial effect on the strength loss due to sodium sulfate attack. The best resistance to sodium sulfate attack was obtained with a SF replacement of 5–10%, but even then, a strength loss of 15–20% can be expected. On the other hand, mortars with silica fume were severely damaged in the magnesium sulfate environment. Further, the compressive strength loss actually increased with increasing SF content. The test results thus showed clearly that the use of SF in concrete exposed to magnesium sulfate solution is not recommended. The test results also showed that the w/cm ratio is the most critical parameter influencing the resistance of concrete to sulfate attack. All the tests reported in the study were carried out at 20 ± 1 °C.

Dispersion of Short Fibers in Cement

Abstract: The degree of dispersion of short microfibers in cement, as assessed by electrical resistivity measurement for the case of electrically conductive fibers at a volume fraction below the percolation threshold, is improved by the use of admixtures (namely, silica fume, acrylic particle dispersion, methylcellulose solution, and silane) and fiber surface treatment (such as ozone treatment). Acrylic particle dispersion is more effective than latex particle dispersion.

Effect of pozzolans on the performance of fiber-reinforced mortars

Abstract: Randomly oriented short fibers have been shown to increase tensile strength and retard crack propagation of cement based materials such as fiber-reinforced mortars for diverse applications, especially in aggressive environments. In the case of reinforced concrete, it is very important to produce a “high quality” cover in order to prevent corrosion of the rebars. In order to obtain a high performance material the use of a pozzolan is advisable because low permeability is achieved. The objective of this research was to determine the effect of pozzolans such as silica fume (SF), fly ash (FA), and metakaolin (MK) on the properties of fiber-reinforced mortars. Different types of natural and synthetic fibers were used. A superplasticizer was used to keep the same workability as that of the control mortar. Results of the mechanical and durability properties of the fiber-reinforced mortars are reported. The results show that a loss of resistance due to embedding fibers in mortar is compensated for by the increase in strength caused by silica fume or metakaolin additions to the mortar. The addition of 15% of SF or MK produces an improvement of up to 20% and 68%, respectively, when compared with those mortars without addition. There is a significant decrease in the coefficient of capillary absorption and chloride penetration when a highly pozzolanic material is incorporated into the matrix. In general, these materials, especially SF and MK, improve the mechanical performance and the durability of fiber-reinforced materials, especially those reinforced with steel, glass or sisal fibers. The fly ash addition had a different performance, which could be attributed to its low degree of pozzolanicity.

Environmental-friendly durable concrete made with recycled materials for sustainable concrete construction

Abstract: Concrete is one of the most widely used construction materials in the world. However, the production of portland cement, an essential constituent of concrete, leads to the release of significant amount of CO2, a greenhouse gas; one ton of portland cement clinker production is said to creates approximately one ton of CO2 and other greenhouse gases (GHGs). Environmental issues are playing an important role in the sustainable development of the cement and concrete industry. For example, if we run out of limestone, as it is predicted to happen in some places, then we cannot produce portland cement; and, therefore, we cannot produce concrete and all the employment associated with the concrete industry goes out-of-business. Limestone powder is sometimes interground with clinker to produce cement, reducing the needs for clinker making and calcinations. This reduces energy use in the kiln and CO2 emissions from calcinations. A sustainable concrete structure is one that is constructed so that the total environmental impact during its entire life cycle, including during its use, is minimum. Concrete is a sustainable material because it has a very low inherent energy requirement, is produced to order as needed with very little waste, is made from some of the most plentiful resources on earth, has very high thermal mass, can be made with recycled materials, and is completely recyclable. Sustainable design and construction of structures have a small impact on the environment. Use of “green” materials embodies low energy costs. Their use must have high durability and low maintenance leading to sustainable construction materials. High performance cements and concrete can reduce the amount of cementitious materials and total volume of concrete required. Concrete must keep evolving to satisfy the increasing demands of all its users. Reuse of post-consumer wastes and industrial byproducts in concrete is necessary to produce even “greener” concrete. Use of coal ash, rice-husk ash, wood ash, natural pozzolans, GGBFS, silica fume, and other similar pozzolanic materials can reduce the use of manufactured portland cement clinker; and, at the same time, produce concrete that is more durable. “Greener” concrete also improves air quality, minimizes solid wastes, and leads to sustainable cement and

Attack of cement pastes exposed to organic acids in manure

Abstract: Manure such as silage effluents and liquid manure contains organic acids which constitute a severe chemical threat toward the concrete of agricultural structures. The purposes of this study were to identify the chemical composition parameters that influence durability by analysing the behaviour of the chemical elements of the cement paste (Ca, Si, Al, Fe and Mg) in organic acid solutions and to compare the intensity of the chemical attack by the different acids found in liquid manure. This study was carried out on cement pastes made from four binders (ordinary Portland cement, slag cement, OPC blended with silica fume and OPC blended with fly ash). The hardened cement pastes were first crushed, then immersed in solutions made of five organic acids with an initial pH of 4 and constantly stirred. The pH and the concentrations of major elements were monitored over time. The results show that Si, Al, and Fe appear to be favourable elements for the chemical resistance of binders whereas the amount of Ca should be limited. Moreover, it is shown that the four acids found in liquid manure (acetic, propionic, butyric, iso-butyric) are equally aggressive. Lactic acid, present with acetic acid in silage effluent, is more aggressive according to the value of its pKa. � 2005 Elsevier Ltd. All rights reserved.

Effect of pozzolanic materials and curing methods on the elastic modulus of HPC

Abstract: The modulus of elasticity of a material is a fundamental property required for the proper modeling of its constitutive behavior and for its proper use in various structural applications. This paper discusses experimental evaluation of the elastic modulus of high-performance concrete made from mixes using various percentages of fly ash, silica fume, and granulated blast furnace slag. Results are compared to those from control specimens at various ages between 1 and 90 days. The results presented are part of a study for the New Jersey Department of Transportation (NJDOT) to develop and implement High-Performance Concrete (HPC) mix design and technical specifications for transportation structures. The study also investigates the effect of curing on the elastic modulus. Three methods of curing were evaluated: (1) air-dry curing, (2) curing compound, and (3) wet curing with burlap. The results showed that adding silica fume resulted in an increase in strength and modulus at early ages, however, there was no change in the modulus at 28 and 56 days. In addition, adding 20% fly ash with various percentage of silica fume had an adverse effect on both strength and modulus values at all ages to 90 days. It is also shown that dry curing and curing compound reduce the modulus of elasticity compared to wet curing with burlap. Results showed the elastic modulus of HPC is proportional to the compressive strength, but the prediction equations of ACI-318 and ACI-363 may not accurately predict the modulus values for high-performance concrete with pozzolans.

Evaluation of capillary pore size characteristics in high-strength concrete at early ages

Abstract: The quantitative scanning electron microscope-backscattered electron (SEM-BSE) image analysis was used to evaluate capillary porosity and pore size distributions in high-strength concretes at early ages. The Powers model for the hydration of cement was applied to the interpretation of the results of image analysis. The image analysis revealed that pore size distributions in concretes with an extremely low water/binder ratio of 0.25 at early ages were discontinuous in the range of finer capillary pores. However, silica-fume-containing concretes with a water/binder ratio of 0.25 had larger amounts of fine pores than did concretes without silica fume. The presence of larger amounts of fine capillary pores in the concretes with silica fume may be responsible for greater autogenous shrinkage in the silica-fume-containing concretes at early ages.

Durability of mixtures containing calcium nitrite based corrosion inhibitor

Abstract: The influence of calcium nitrite based corrosion inhibitor on the corrosion of reinforcing steel embedded in 14 different mortars is experimentally investigated in this work. Two Portland cements, NPC and SR (type I and V according to ASTM Standards) and 12 blended cements were used. The pozzolanic materials used were three lignite fly ashes, silica fume and one natural pozzolan (Milos’ Earth). All blended cements were produced in the laboratory by grinding Portland clinker, gypsum and the appropriate pozzolanic material. One commercially available blended cement (CEM II/A-M 32.5N) was also used in this research. Mortar specimens (cylinders 100 · 40 mm) were prepared, according to DIN 1164 with and without calcium nitrite and used for measurements of carbonation and chloride-induced corrosion for a time period of 2 years. Chloride resistance was monitored according to ASTM C876 on specimens immersed in a 5% NaCl solution after an initial curing of 28 days. The carbonation depth was measured on cylinders cured in a severe environment using a spray indicator enabling the estimation of different pH values. Results show that calcium nitrite has a beneficial effect in shifting the corrosion potential towards electropositive direction especially in the case of NPC and SR cements. The corrosion potential of blended mixtures was also shifted towards electropositive direction, but since the pozzolanic materials had a beneficial effect by themselves, the reduction was comparative smaller. The beneficial effect of calcium nitrite was also confirmed by the gravimetric weight loss measurements performed after 2 years of immersion in the 5% NaCl solution. Carbonation depth of all mixtures was reduced or remained the same when calcium nitrite was used. Chloride permeability was not seriously influenced by the addition of calcium nitrite, as it is indicated by the total chloride measurements performed after 2 years of immersion in the 5% NaCl solution. � 2004 Elsevier Ltd. All rights reserved.

Influence of Ultrafine Fly Ash on the Early Age Response and the Shrinkage Cracking Potential of Concrete

Abstract: In this paper, the influence of ultrafine fly ash on the early age property development, shrinkage, and shrinkage cracking potential of concrete is investigated. In addition, the performance of ultrafine fly ash as cement replacement is compared with that of silica fume. The mechanisms responsible for an increase of the early age stress due to restrained shrinkage were assessed; free shrinkage and elastic modulus were measured from an early age. In addition, the materials resistance to tensile fracture and increase in strength were also determined as a function of age. Results of the experimental study indicate that the increase in elastic modulus and fracture resistance with age are comparable for the control, ultrafine fly ash, and silica fume concretes. Autogenous shrinkage is shown to play a significant role in determining the age of cracking in restrained shrinkage tests. A significant reduction in the autogenous shrinkage and an increase in the age of restrained shrinkage cracking were observed in the ultrafine fly ash concrete when compared with the control and the silica fume concrete. Increasing the volume of ultrafine fly ash and decreasing the ratio of water-to-cementitous materials resulted in further increase in the age of restrained shrinkage cracking and a significant increase in the compressive strength.

Ultra-high performance fibre-reinforced concrete

Abstract: This paper describes the use of ultra-high performance fibre-reinforced concrete (UHPFRC) for the strengthening of structures using a mat of fibres and the infiltration of these with a slurry in-situ. The two main products are compact-reinforced concrete (CRC) and reactive powder concrete (Ductal). Both products use 12mm steel fibres. The materials can either be supplied as a complete prebagged mix, or for CRC as a bagged cement, silica fume and plasticiser that is then mixed with local aggregate. The properties of UHPFRC are described: strength, creep and shrinkage, durability, and fire-resistance. The structural design of UHPFRC is covered in the interim recommendations of the Association Francaise de Genie Civil. The applications of UHPFRC include beams, columns, arch footbridges, truss footbridges, street furniture, canopy roofs, and drain covers. The cost of these materials is discussed.

Investigating the role of reactive silica in the hydration mechanisms of high-calcium fly ash/cement systems

Abstract: High-calcium fly ashes (ASTM Class C) are being widely used as a replacement of cement in normal and high strength concrete. In Greece such fly ashes represent the majority of the industrial by-products that possess pozzolanic properties. Even thought the contribution of factors, such as fineness and water/binder ratio, on the performance of fly ash/cement (FC) systems has been a common research topic, little work has been done on examining whether and to what extent reactive silica of fly ashes affects the mechanisms occurring during their hydration. The work presented herein describes a laboratory scale study on the influence of active silica of two high-lime fly ashes on their behavior during hydration. Volumes up to 30% of Greek high-calcium fly ashes, diversified both on their reactive silica content and silicon/calcium oxides ratio, were used to prepare mixes with Portland cement. The new blends were examined in terms of compressive strength, remaining calcium hydroxide, generation of hydration products and microstructural development. It was found that soluble silica of fly ashes holds a predominant role especially after the first month of the hardening process. At this stage, silica is increasingly dissolved in the matrix forming additional cementitious compounds with binding properties, principally a second generation C–S–H. The rate however, that fly ashes react in FC systems seems to be independent of their active silica content, indicating that additional factors such as glass content and fineness should be taken into account for predicting the contribution of fly ashes in the final performance of pozzolanic cementitious systems.