Showing papers on "Silica fume published in 2019"

3D printable concrete: Mixture design and test methods

Abstract: The current study deals with a yield stress based mixture design approach for 3D printable concretes. The mixtures were evaluated based on buildability, extrudability, robustness and tests for structural build-up. For the print parameters (such as pump type, nozzle size and extrusion velocity) used in the study, it was found that both extrudability and buildability could be achieved only when the material yield stress was within a range of 1.5–2.5 kPa. Below this range, the material lacked enough strength to achieve shape stability, while above this range, the extrudabilty of the material was difficult. The robustness of the mixtures was quantified in terms of a variability factor defined in terms of the variation in yield stress with small changes in the superplasticizer dosage. Inclusion of 10% of silica fume, 0.1% of viscosity modifying agent (VMA) and 0.1–0.3% addition of nanoclay resulted in decreasing the variability factor, hence improving the robustness. The structural changes due to thixotropy and cement hydration increased the yield stress with time. This structural build-up was assessed by measuring the yield stress with increasing rest duration. The mixture with silica fume showed the maximum structural build-up while the mixture with VMA showed the least. Heat curves from semi-adiabatic calorimetry and penetration curves were also used to assess the structural build-up. They showed a similar trend to that of the yield stress vs time plots.

Fillers to improve passing ability of concrete

Abstract: Concrete possessing high-passing ability needs to be flowable and cohesive. Hence, passing ability cannot be improved by solely adding superplasticizer, which increases both flowability and segregation of concrete simultaneously. Decreasing the maximum size of aggregates so that concrete segregates at lower cohesiveness is a possible but undesirable way as it narrows the aggregates' grading and decrease dimensional stability of concrete. With the same maximum size of aggregates, passing ability can be improved by raising the concurrent flowability-segregation envelope of concrete. In this paper, fly ash and silica fume (cementitious fillers) and limestone (inert filler) were selected to replace cement partially and subsequently the passing ability of concrete was studied. From the results, it was evident that when either type of fillers were used, the passing ability and maximum limits of flowability and segregation achieved simultaneously increase. It is because these fillers are finer than cement that provides better filling effect to increase packing density and excess water leading to better flowability. Concurrently, the cohesiveness of concrete also increases as the content of fine particles increases. These allow concrete to hold the coarse aggregates more firmly when passing through narrow gaps, after which the concrete will keep flowing rapidly.

Impact of silica fume, fly ash, and metakaolin on the thickness and strength of the ITZ in concrete

Abstract: The interfacial transition zone (ITZ) has a major detrimental impact on the structural performance of concrete. This negative impact can be modulated by introducing mineral admixtures to a concrete mix, which fill the excessive voids within ITZ and react with portlandite to form more compact products. The approach described here, consisting of characterization of phases and micromechanical modeling, enabled assessment of the effect of silica fume, fly ash, and metakaolin on ITZ thickness and strength. The proposed model was based on the Mori-Tanaka scheme coupled with an estimation of deviatoric stress within ITZ. This study suggests that silica fume is efficient in reducing ITZ thickness, while the addition of fly ash more significantly contributes to ITZ strength. Moderate replacements of Portland cement for silica fume or fly ash, up to 20%, can positively influence concrete performance; in case of metakaolin, replacement up to 10% is recommended.

Influence of silica fume on properties of fresh and hardened ultra-high performance concrete based on alkali-activated slag

Abstract: Based on the principles of ultra-high performance concrete (UHPC) a Portland cement free, alkali-activated material was optimized in order to enhance strength and durability. The formulation is based on ground granulated blast furnace slag and as an activator a combination of potassium water-glass and potassium hydroxide was used. Furthermore, silica fume and metakaolin as inorganic fines were used to increase the packing density of the mixture. Quartz sand (0–2 mm) and quartz powder were added as aggregates. The results comprise an enhancement of the rheological properties by the stepwise increase of silica fume. A w/b ratio of less than 0.25 was realised using a certain mixing procedure with a high intensity mixer. The compressive strength reaches values comparable with the strength range of an UHPC. The already low capillary porosity could further be decreased by the substitution of slag with a small amount of metakaolin, which is leading to a higher polymerisation degree due to an incorporation of aluminium in the reaction products. The enhanced polymerisation degree was estimated by FTIR and XRD. Although no effective superplasticizer can be used in this high alkaline material, the low w/b ratio and a good workability can be achieved by using certain amounts of silica fume.

Estimating the optimal mix design of silica fume concrete using biogeography-based programming

Abstract: The use of silica fume in concrete mixtures has been dramatically increased in concrete industry, especially for achieving high strength concrete. An accurate model of estimating the compressive strength and optimal mix design of silica fume concrete can save in time and cost. In this study, the biogeography-based programming (BBP) was used as a symbolic regression method to predict the compressive strength of silica fume concrete, while the constrained biogeography-based optimization (CBBO) was used to estimate its optimal mix design. For this purpose, a comprehensive database was gathered from various published documents. From the collected data, about 75% of all data was employed to train the model, while the rest was used to verify the developed model. The amounts of cement, water, silica fume, coarse aggregate, fine aggregate, superplasticizer, as well as the maximum size of aggregate and concrete age were selected as the effective input variables of the model. The compressive strength of silica fume concrete was considered as the only output variable. The results show that the BBP model can be successfully used for the prediction of the compressive strength of silica fume concrete with acceptable accuracy. In addition, a graphical user interface was designed which allows the users to estimate the optimal mix design of silica fume concrete.

New insights from reactivity testing of supplementary cementitious materials

Abstract: Tests to determine the reactivity of supplementary cementitious materials (SCMs) by using isothermal calorimetry and thermogravimetric analysis have been proposed. In one such test, the heat release and calcium hydroxide consumption of SCMs mixed with calcium hydroxide (3:1 ratio of calcium hydroxide and SCM) at 50 °C in a 0.5 M potassium hydroxide environment are measured. In this study, we show the results of such testing for a large variety of SCMs and fillers, ranging from conventional materials such as fly ash, slag, silica fume, quartz, and limestone, to alternative materials such as calcined clays, municipal solid waste incineration fly ash, basic oxygen furnace slag, ground lightweight aggregates, ground pumice, ground glass pozzolan, and basalt fines. A total of 54 SCMs are tested using this approach. Results show that even among SCMs of the same type, there is considerable difference in the heat release and calcium hydroxide consumption, likely due to differences in amorphous content, chemical composition, and fineness, leading to different reactivities. Based on the response in the test, SCMs are classified into inert, pozzolanic, and latent hydraulic; the pozzolanic and latent hydraulic materials are further classified into less reactive and more reactive. The relationship between heat release and calcium hydroxide consumption depends on the chemical composition of the SCMs, and SCMs with high calcium, high alumina, and high silica contents show different relationships (determined by the slope of the heat release vs. calcium hydroxide plot).

Mechanical, fracture and durability properties of self-compacting high strength concrete containing recycled polypropylene plastic particles

Abstract: This study focuses on the mechanical, fracture and durability characteristics of self-compacting high-strength concrete (SCHSC) containing recycled polypropylene plastic particles (RPPP) with and without silica fume (SF). The designation of the two different sets of SCHSC containing plastic particles were used on the basis of a constant water–cementitious substance (w/cm) ratio of 0.32 and a total cementitious materials content of 550 kg/m3. The first set of mixtures included binary cementitious blends of 20% fly ash (FA) and 80% Portland cement (PC). However, the second series of the mixtures incorporated ternary cementitious blends of 20% FA, 10% SF and 70% PC. To produce the concretes, medium size aggregate was replaced with RPPP at five designated percentages of 0%, 10%, 20%, 30% and 40% by volume in both sets of concretes. Totally, 10 mixtures were produced and tested for mechanical, fracture and durability properties such as elastic modulus, compressive and splitting tensile strength, flexural strength, sorptivity, chloride ion permeability, gas permeability and fracture energy. The tests were carried out 28 and 90 days after casting. The test results showed that the use of RPPP significantly improved the fracture and ductility properties, whereas aggravated other measured properties of SCHSCs. However, with the addition of SF all mechanical and durability characteristics remarkably enhanced. The results also demonstrated that SCHSC with compressive strength higher than 70 MPa at 90 days was produced by using RPPP content up to 40% replacement level by total medium aggregate volume, and 10% SF.

Effect of multi-walled carbon nanotubes (MWCNTs) on the strength development of cementitious materials

Abstract: The proposed study aimed to investigate the potential use of pristine multi-walled carbon nanotubes (MWCNTs) as nano reinforcement in enhancing mechanical properties of hybrid MWCNT/silica fume cement composites. Dispersion of MWCNTs was facilitated utilizing very fine particles of silica fume which also helped in an improved interfacial bond between MWCNTs and the cement matrix. The MWCNTs dispersion within the hardened cement matrix was qualitatively assessed by Field Emission Scanning Electron Microscope (FESEM) analysis. It was also observed that addition of MWCNTs accelerated the hydration process. The test results showed an increment in compressive strength by 12.4% and reduction in autogenous shrinkage by 8.5% for hybrid MWCNT/silica fume cement composites containing 0.01% MWCNTs (by wt. of binder). However, higher additions (greater than 0.03%) of MWCNTs appeared to have adverse effects on specimens. It was found that properly dispersed MWCNTs filled the fine pores in the cement matrix by providing an additional nucleation site for the formation of calcium silicate hydrate (C-S-H) that results in a denser microstructure, which in turn enhanced the strengths and limited the autogenous shrinkage.

The engineering properties and microstructure of sodium carbonate activated fly ash/ slag blended mortars with silica fume

Abstract: Highly concentrated and corrosive alkaline activators are often used for activating polycondensation reactions of aluminosilicate minerals in geopolymers at elevated temperatures. This study investigated the interaction between Pulverised Fuel Ash (PFA), Ground Granulated Blast Furnace Slag (GGBS) and Silica Fume (SF) activated with small dosages of low alkalinity sodium carbonate and sodium silicate. Tests were inclusive of assessments on the consistency and setting behaviours, mechanical strengths and microstructure properties. Results were compared with Ordinary Portland Cement (OPC) mortars and sodium hydroxide activated geopolymer mortars which were established through an existing study. The mechanical strength of sodium carbonate activated geopolymer was found to be comparable with that of the sodium hydroxide activated geopolymer. Geopolymer mortars with 4% of silica fume exhibited the highest mechanical strength. The sodium carbonate activated binder system comprised largely of an interconnected calcium carbonate crystals framework and C-A-S H gels.

Green Concrete: By-Products Utilization and Advanced Approaches

Abstract: The popularity of concrete has been accompanied with dreadful consumptions that have led to huge carbon footprint in our environment. The exhaustion of natural resources is not yet the problem, but also the energy that is needed for the fabrication of the natural materials, in which this process releases significant amount of carbon dioxide (CO2) emissions into the air. Ordinary Portland Cement (OPC) and natural aggregates, which are the key constituents of concrete, are suggested to be recycled or substituted in order to address the sustainability concern. Here, by-products have been targeted to reduce the carbon footprint, including, but not limited to, fly ash, rice husk ash, silica fume, recycled coarse aggregates, ground granular blast-furnace slag, waste glass, and plastic. Moreover, advanced approaches with an emphasis on sustainability are highlighted, which include the enhancement of the hydration process in cement (calcium-silicate hydrate) and the development of new materials that can be used in concrete (e.g., carbon nanotube). This review paper provides a comprehensive discussion upon the utilization of the reviewed materials, as well as the challenges and the knowledge gaps in producing green and sustainable concrete.