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At least 703 records · Page 39

Enhanced Techniques for Concrete Decontamination During Decommissioning - 19005

As the primary material of construction for nuclear power plant structures, concrete naturally represents the largest fraction of contaminated material by surface area. As such, decontamination of concrete is a major activity during decommissioning projects. As reported in EPRI 3002005412, Characterization and Remediation of Contaminated Concrete, currently used techniques for decontamination of contaminated concrete are generally slow and labor-intensive to apply. This report notes that little change to the techniques used for concrete decontamination have been made during the past 20 years. There is a clear need for improvement to the techniques available for decontamination of concrete. Needed enhancements include substantial improvements in efficiency, waste management, industrial safety, and worker radiation exposure. Potential areas for development include increased automation of existing techniques, improvement in delivery systems for decontamination equipment, and further development of alternative techniques such as microwave and laser scabbling, and nitrogen blasting. To address this need, EPRI is conducting a project to develop enhanced techniques for concrete decontamination. The project focuses on two primary areas: 1. Adaption of existing techniques to improve efficiency through automation and/or altering the delivery systems for the decontamination equipment (for example, through use of exiting robotic systems); and 2. Field demonstration of technologies currently in the engineering development stage, such as nitrogen blasting and laser scabbling. This paper will report the initial results of this EPRI project, which includes identification of promising candidate technologies that will be demonstrated during the next project phase. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

A comparison of explosively driven shock wave radius versus time scaling approaches

Abstract Explosively driven shock wave radius versus time profiles are frequently used to document energy release and relative explosive performance. Recently, two universal shock wave radius versus time profiles have been presented in the literature, which demonstrate the ability to represent explosively driven shock wave profiles for all explosive sources in any fluid environment. These two universal shock wave profiles are examined here relative to each other and relative to a commonly used nonlinear shock wave profile, which is fit to experimental data for individual explosive materials. The nonlinear profile, originally developed by Dewey, is examined here, and a universal non-dimensional form of the equation is proposed. The universal shock wave profiles are all found to be relatively similar, but with slight variations in a transition region of non-dimensional radii $$0.15\lesssim R^*\lesssim 2$$ 0.15 ≲ R ∗ ≲ 2 . The variations in this region result in different estimations of energy release or blast strength between the curve fits.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Effect of Aluminum Dissolved in Hematite on Formation of Calcium Ferrites at 1473 K

Silico-ferrite of calcium and aluminum (SFCA) is a primary bonding phase, which determines the quality of high basicity sinter used widely as raw materials in blast furnace ironmaking. Calcium ferrite containing aluminum (CFA) is a key precursor during the formation process of SFCA and is influenced by Al{sub 2}O{sub 3} content in iron ore fines. In this work, the experiment was conducted to investigate the effect of the aluminum occurrence state on the formation of calcium ferrite by the simulating sintering process of different types of aluminum-bearing hematite at 1473 K. X-ray diffraction, optical microscope, scanning electron microscopy, and energy-dispersive spectroscopy were used to characterize the mineral change in sintered samples. The result shows that the formation of CFA was promoted by both granular Al{sub 2}O{sub 3} wrapped in Fe{sub 2}O{sub 3} and aluminum dissolved in Fe{sub 2}O{sub 3} solid solution, especially for the latter, due to the formation of the CFA directly from the reaction between Fe{sub 2}O{sub 3}-Al{sub 2}O{sub 3} solid solution and CaO, and the aluminum content in CFA formed was influenced obviously by the solid solution amount of alumina into Fe{sub 2}O{sub 3} as raw materials.

36 MATERIALS SCIENCE↗

Effects of Particle Diameter and Coke Layer Thickness on Solid Flow and Stress Distribution in BF by 3D Discrete Element Method

In order to reduce ironmaking-related CO{sub 2} emissions, hydrogen-enriched blast furnace (BF) operation is currently under development. In hydrogen-enriched BF operation, coke layer thickness can be decreased to reduce CO{sub 2} emissions. However, BFs operating with thin coke layers may experience instability or discontinuous phenomena such as particle slip and gas channeling problems, so it is important to optimize the particle diameter and coke layer thickness for optimal BF operation. In this study, the effects of particle diameter and coke layer thickness on the solid flow and stress distribution in a BF were analyzed using a three-dimensional discrete element method. Furthermore, the effects of particle diameter and coke layer thickness on the burden layer stabilities, particle velocities, and particle stress distributions have been investigated. The results show that decreasing the coke layer thickness caused instability owing to the mixing of the coke and ore layers in the BF-cohesive zone and slight increases in both the average particle velocities and the average normal particle stress magnitudes. In addition, the average particle velocities and average normal particle stresses were higher for the smaller particles than for the larger ones during the simulations.

36 MATERIALS SCIENCE↗

Influence of Iron Ore Addition on Metallurgical Reaction Behavior of Iron Coke Hot Briquette

Utilization of iron coke hot briquette (ICHB) has been considered to be one effective countermeasure to achieve low-carbon blast furnace. Before its actual application, a series of extensive fundamental investigations on the ICHB are required. In this study, the influence of iron ore addition on the metallurgical reaction behavior of ICHB made from carbonizing coal–iron ore composite agglomeration were investigated based on Chinese raw material conditions, and the corresponding mechanisms were revealed. The results show that the addition of iron ore concentrate leads to the degradation of the compressive strength and drum strength of ICHB, but they are maintained at a relatively high level. Furthermore, by increasing the addition ratio of iron ore from 0 to 20 pct, the density of ICHB is enhanced from 1413 to 1637 kg/m{sup 3}, and the porosity is increased from 8.03 to 12.84 pct, and the optical texture index (OTI) is reduced from 58.1 to 25.3. The phase composition of ICHB mainly consists of C, Fe, SiO{sub 2}, Fe{sub 3}O{sub 4}, and FeO after the addition of the iron ore concentrate. Moreover, the reactivity of ICHB is accelerated from 46.74 to 69.54 pct due to the catalytic effects of metallic iron and iron oxide, which worked as oxygen transfer carriers, while the post-reaction strength is reduced from 74.08 to 36.81 pct. In addition, the starting temperature of the ICHB gasification reaction is decreased remarkably from 919 °C to 839 °C. The activation energy (E{sub a}) for the gasification reaction under nonisothermal conditions is reduced from 312.78 to 249.83 kJ/mol, and the preexponential factor (A) shows a decreasing trend as well. Simultaneously, E{sub a} and A display the kinetic compensation effect and present a good linear relationship.

36 MATERIALS SCIENCE↗

Novel Approach to Studying Influences of Na{sub 2}O and K{sub 2}O Additions on Viscosity and Thermodynamic Properties of BF Slags

Abstract:: The present work calculated the heat capacity, enthalpy change, and slag temperature of CaO–MgO–Al{sub 2}O{sub 3}–SiO{sub 2} slags after adding Na{sub 2}O or K{sub 2}O, and investigated the influences of Na{sub 2}O and K{sub 2}O on the slag viscosity under given temperatures and heat quantities. It was found that the slag viscosity decreases with the addition of Na{sub 2}O and tends to increase with K{sub 2}O additions at the same temperature. The heat capacity of the slag increases, while the enthalpy change decreases obviously with the increasing addition of Na{sub 2}O or K{sub 2}O. Under the constant heat quantity, an increase in content of Na{sub 2}O or K{sub 2}O of the slag leads to an appreciable increase in slag temperature, whereas the viscosity decreases significantly. Besides, the Na{sub 2}O or K{sub 2}O additions also help to stabilize the slag fluidity and lower energy consumption of blast furnace.

36 MATERIALS SCIENCE↗

TTT Diagram of 10 pct CaO-90 pct Fe{sub 2}O{sub 3} in Low-Temperature Region Below 1499 K

The important property of a sinter is its ability to withstand the load in a blast furnace (BF). During the cooling process, many compounds among CaO(C), FeO(W), and Fe{sub 2}O{sub 3}(F) precipitate, thereby affecting the quality of the sinter. In a previous study, the compounds that precipitated at a high quenching rate (7600 K/s) were clarified. In this study, a 10 pct CaO-90 pct Fe{sub 2}O{sub 3} sample was melted and held at the desired temperatures from 1 minute to 168 hours using the hot thermocouple (HTC) method. The precipitated compounds were clarified by X-ray diffraction (XRD), while a time-temperature-transformation (TTT) diagram for the 10 pct CaO sample was determined below 1499 K. Unstable compounds (C{sub 2}W{sub 4}F{sub 9} and WF{sub SS}) were precipitated together with CF and F in the low-temperature region below 1428 K (1155 °C). However, the unstable compounds disappeared when the holding time was long. In the temperature region from 1428 K to 1499 K (1155 °C to 1226 °C), the rate of formation of CF{sub 2} was low; over a period longer than 168 hours, CF disappeared. Therefore, the reaction, CF + F = CF{sub 2}, is very slow because the locations of CF and F are separated during precipitation from the melt.

36 MATERIALS SCIENCE↗

A Thermodynamic Model Toward the Comprehension of Ferrous Burden Softening and Melting Using FactSage Macro-Processing

The softening and melting properties of iron-bearing materials play a decisive role in the formation of the cohesive zone, which greatly affects blast furnace gas flow distribution and heat-transfer efficiency. To improve the understanding regarding the evolution of condensed phases during reduction, softening, and melting, a thermodynamic model has been developed using FactSage™ thermodynamic software and macro-processing. The model was constructed using a series of equilibrium stages and splitters to determine flow directions of streams and to consider kinetic inhibitions. For all iron-bearing materials studied, the methodology proposed for modeling was capable of obtaining reduction degrees in very good agreement to its experimental data. The evolution of solid phases was qualitatively comparable to the available literature, with Fayalite, Kirschsteinite, Melilite, and FeO as the main solid phases in equilibrium before slag formation. The comparison between the profiles of the calculated slag mass fraction and the experimental pressure drop showed a close relation between these properties. Moreover, the level of heterogeneity of each raw material may play a significant role in the interpretation of its results.

36 MATERIALS SCIENCE↗

Micro- and macroscale coefficients of friction of cementitious materials

Millions of metric tons of cementitious materials are produced, transported and used in construction each year. The ease or difficulty of handling cementitious materials is greatly influenced by the material friction properties. In the present study, the coefficients of friction of cementitious materials were measured at the microscale and macroscale. The materials tested were commercially-available Portland cement, Class C fly ash, and ground granulated blast furnace slag. At the microscale, the coefficient of friction was determined from the interaction forces between cementitious particles using an Atomic Force Microscope. At the macroscale, the coefficient of friction was determined from stresses on bulk cementitious materials under direct shear. The study indicated that the microscale coefficient of friction ranged from 0.020 to 0.059, and the macroscale coefficient of friction ranged from 0.56 to 0.75. The fly ash studied had the highest microscale coefficient of friction and the lowest macroscale coefficient of friction. -- Highlights: •Microscale (interparticle) coefficient of friction (COF) was determined with AFM. •Macroscale (bulk) COF was measured under direct shear. •Fly ash had the highest microscale COF and the lowest macroscale COF. •Portland cement against GGBFS had the lowest microscale COF. •Portland cement against Portland cement had the highest macroscale COF.

36 MATERIALS SCIENCE↗

Distinctive microstructural features of aged sodium silicate-activated slag concretes

Electron microscopic characterisation of 7-year old alkali-activated blast-furnace slag concretes enabled the identification of distinct microstructural features, providing insight into the mechanisms by which these materials evolve over time. Backscattered electron images show the formation of Liesegang-type ring formations, suggesting that the reaction at advanced age is likely to follow an Oswald supersaturation–nucleation–depletion cycle. Segregation of Ca-rich veins, related to the formation of Ca(OH){sub 2}, is observed in microcracked regions due to the ongoing reaction between the pore solution and available calcium from remnant slag grains. A highly dense and uniform interfacial transition zone is identified between siliceous aggregate particles and the alkali activated slag binders, across the concretes assessed. Alkali-activated slag concretes retain a highly dense and stable microstructure at advanced ages, where any microcracks induced at early ages seem to be partially closing, and the remnant slag grains continue reacting.

36 MATERIALS SCIENCE↗

Applying high resolution SyXRD analysis on sulfate attacked concrete field samples

High resolution synchrotron X-ray diffraction (SyXRD) was applied for a microstructural profile analysis of concrete deterioration after sulfate attack. The cement matrices consist of ordinary Portland cement and different amounts of supplementary cementitious materials, such as fly ash, natural pozzolana and granulated blast furnace slag. The changes of the phase composition were determined along the direction of sulfate ingress. This approach allows the identification of reaction fronts and zones of different phase compositions and conclusions about the mechanisms of sulfate attack. Two reaction fronts were localized in the initial 4 mm from the sample surface. The mechanism of deterioration caused by the exposition in the sulfate-bearing soil is discussed. SyXRD is shown to be a reliable method for investigation of cementitious materials with aggregates embedded in natural environments.

36 MATERIALS SCIENCE↗

Autogenous healing of sea-water exposed mortar: Quantification through a simple and rapid permeability test

Concrete has an autogenous ability to heal cracks potentially contributing to its functional water tightness and durability. Here, we quantify the crack-healing capacity of sea-water submerged mortar specimens through a simple and rapid permeability test. Defined crack width geometries were created in blast furnace slag cement specimens allowing healed specimens to be quantified against unhealed specimens. Specimens with 0.2 mm wide cracks were not permeable after 28 days submersion. Specimens with 0.4 mm cracks had decreases in permeability of 66% after 28 days submersion, and 50–53% after 56 days submersion. Precipitation of aragonite and brucite in the cracks was the main cause of crack healing. Healing potential was dependent on the initial crack width, thermodynamic considerations and the amount of ions available in the crack. To our knowledge, this is the first study to quantify the functional autogenous healing capacity of cracked sea-water exposed cementitious specimens.

36 MATERIALS SCIENCE↗

Phase evolution of C-(N)-A-S-H/N-A-S-H gel blends investigated via alkali-activation of synthetic calcium aluminosilicate precursors

Stoichiometrically-controlled alkali-activated pastes containing calcium-(sodium) aluminosilicate hydrate (C-(N)-A-S-H) and sodium aluminosilicate hydrate (N-A-S-H) gels are produced by alkali-activation of high-purity synthetic calcium aluminosilicate powders. These powders are chemically comparable to the glass in granulated blast furnace slag, but without interference from minor constituents. The physiochemical characteristics of these gels depend on precursor chemical composition. Increased Ca content of the precursor promotes formation of low-Al, high-Ca C-(N)-A-S-H with lower mean chain length as determined by quantification of solid state nuclear magnetic resonance spectra, and less formation of calcium carboaluminate ‘Alumino-ferrite mono’ (AFm) phases. Increased Al content promotes Al inclusion and reduced crosslinking within C-(N)-A-S-H, increased formation of calcium carboaluminate AFm phases, and formation of an additional N-A-S-H gel. Small changes in precursor composition can induce significant changes in phase evolution, nanostructure and physical properties, providing a novel route to understand microstructural development in alkali-activated binders and address key related durability issues.

36 MATERIALS SCIENCE↗

Utilisation of steel furnace slag coarse aggregate in a low calcium fly ash geopolymer concrete

This paper evaluates the performance of steel furnace slag (SFS) coarse aggregate in blended slag and low calcium fly ash geopolymer concrete (GPC). The geopolymer binder is composed of 90% of low calcium fly ash and 10% of ground granulated blast furnace slag (GGBFS). Mechanical and physical properties, shrinkage, and detailed microstructure analysis were carried out. The results showed that geopolymer concrete with SFS aggregate offered higher compressive strength, surface resistivity and pulse velocity than that of GPC with traditional aggregate. The shrinkage results showed no expansion or swelling due to delayed calcium oxide (CaO) hydration after 320 days. No traditional porous interfacial transition zone (ITZ) was detected using scanning electron microscopy, indicating a better bond between SFS aggregate and geopolymer matrix. Energy dispersive spectroscopy results further revealed calcium (Ca) diffusion at the vicinity of ITZ. Raman spectroscopy results showed no new crystalline phase formed due to Ca diffusion. X-ray fluorescence result showed Mg diffusion from SFS aggregate towards geopolymer matrix. The incorporation of Ca and Mg into the geopolymer structure and better bond between SFS aggregate and geopolymer matrix are the most likely reasons for the higher compressive strength observed in GPC with SFS aggregate.

36 MATERIALS SCIENCE↗

Effect of the inherent alkalis of alkali activated slag on the risk of alkali silica reaction

Alkali silica reaction (ASR) occurred in the alkali activated slag (AAS) concrete was evaluated by a revised mortar prism method. The mortars were made of blast furnace slags activated by alkali silicate solution possessing different modulus (0.5, 0.75, 1 and 2) as well as cations (sodium and potassium) and standard sand with a volume replacement of 25 vol% by borosilicate glass beads. Their changes in length and dynamic elastic modulus were monitored up to 7 months. The results showed that ASR has taken place among some mortars, which were evident under both light microscopy and electron microscopy with EDX analysis. The results also revealed that high alkali content did promote the risk of ASR but the degradation due to ASR was affected in a much more complex way.

36 MATERIALS SCIENCE↗

Isothermal calorimetry and in-situ XRD study of the NaOH activated fly ash, metakaolin and slag

Much is unknown about the reaction processes responsible for the formation and polycondensation of geopolymers and other alkali activated materials. In this work, isothermal calorimeter and in-situ XRD were adopted to study the heat and mineral evolution of NaOH activated fly ash, metakaolin and ground granulated blast furnace slag. Both activator concentration and temperature have profound influences on duration of exothermal geopolymerization peaks. NaOH activated fly ash is more temperature dependable, with much higher activation energy than metakaolin and slag. The dissolution of source precursor is rapid and the formation of new phases can be detected by the end of the initial dissolution period. The in-situ XRD measurement together with the PONKCS analysis method promotes quantitative estimation of amorphous evolution during alkali activation.

36 MATERIALS SCIENCE↗

Sulfuric acid resistance of one-part alkali-activated mortars

One-part alkali-activated (geopolymer) mortars based on three different silica-rich starting materials and sodium aluminate, with and without ground granulated blast furnace slag (GGBFS) addition, were tested regarding sulfuric acid resistance according to DIN 19573:2016-03 (70 days at pH = 1). Corresponding pastes were characterized by XRD, SEM, chemical analysis, {sup 29}Si MAS NMR and {sup 1}H-{sup 29}Si CPMAS NMR after water storage and after acid exposure. The mortars exhibited a high resistance against sulfuric acid attack, with the best ones conforming to the requirements of DIN 19573:2016-03. The analytical results showed that this was due to precipitation of silica gel at the acid-mortar interface, which formed a mechanically stable layer that protected the subjacent mortar and thus inhibited further degradation. The addition of GGBFS decreased the acid resistance via formation of expansive calcium sulfate phases.

36 MATERIALS SCIENCE↗

Temperature transformation of blended magnesium potassium phosphate cement binders

In this study, a multi-technique approach was utilised to determine the high temperature performance of magnesium potassium phosphate cement (MKPC) blended with fly ash (FA) or ground granulated blast furnace slag (GBFS) with respect to nuclear waste immobilisation applications. Conceptual fire conditions were employed (up to 1200 °C, 30 min) to simulate scenarios that could occur during interim storage, transportation or within a final geological disposal facility. After exposure up to 400 °C, the main crystalline phase, struvite-K (MgKPO{sub 4}·6H{sub 2}O), was dehydrated to poorly crystalline MgKPO{sub 4} (with corresponding volumetric and mass changes), with MgKPO{sub 4} recrystallisation achieved by 800 °C. XRD and SEM/EDX analysis revealed reaction occurred between the MgKPO{sub 4} and FA/GBFS components after exposure to 1000–1200 °C, with the formation of potassium aluminosilicate phases, leucite and kalsilite (KAlSi{sub 2}O{sub 6} and KAlSiO{sub 4}), commensurate with a reduced relative intensity (or complete elimination) of the dehydrated struvite-K phase, MgKPO{sub 4.} This was further supported by solid-state NMR ({sup 27}Al and {sup 29}Si MAS), where only residual features associated with the raw FA/GBFS components were observable at 1200 °C. The high temperature phase transformation of blended MKPC binders resulted in the development of a glass/ceramic matrix with all existing porosity infilled via sintering and the formation of a vitreous phase, whilst the physical integrity was retained (no cracking or spalling). This study demonstrates that, based on small-scaled specimens, blended MKPC binders should perform satisfactorily under fire performance parameters relevant to the operation of a geological disposal facility, up to at least 1200 °C.

36 MATERIALS SCIENCE↗