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In the semiconductor industry, the long "seasoning" time required after the installation of new ceramic components in etching equipment restricts the wafer production efficiency. This study proposes a novel and practical method for the fabrication of pre-seasoned ceramics through surface modification of Y 2 O 3 ceramics. Fluorine-containing reactive vapors were produced by utilizing NH 4 F as the source. Further, the formation of yttrium oxyfluoride layers on the surfaces of Y 2 O 3 ceramics was demonstrated. The modified surface comprises a single-crystalline phase near the interface with the Y 2 O 3 substrate. The change in the composition, crystalline nature of the modified layer, and orientation relationship with the substrate were examined via transmission electron microscopy. The kinetics of the surface modification were studied for the sintered Y 2 O 3 bulk as well as for practical coatings such as plasma-sprayed or PVD coatings. The proposed surface modification approach can be used to fabricate pre-seasoned Y 2 O 3 coatings, which can significantly benefit the semiconductor industry. [ABSTRACT FROM AUTHOR]

Facile sintering of metal oxides and ceramics is essential for energy-efficient manufacturing processes of various engineering systems. It should satisfy the increasing density of the resulting layer packed on the substrate in a mild environment and improve the specific performance through structural changes in the internal grain boundaries. However, conventional sintering inevitably involves maintaining high-temperature conditions for a long duration, resulting in energy inefficiency and unintended element diffusion. Herein, we report a novel composite-fuel-assisted sintering strategy as an alternative to conventional sintering for enhanced densification of manganese cobalt oxide (MCO) spinel. MCO spinels, which are commonly used to form a protective layer for metallic interconnects, are electrophoretically deposited on Cr-containing stainless steel, while the initial porosity and thickness are controlled by regulating the deposition parameters. Then, 2,4,6-trinitrophenol combined with sodium azide as a composite fuel is dropped onto the pre-deposited MCO layer. During air drying, this composite fuel tends to self-assemble into an elongated microwire-shaped structure featuring π-stacked aromatic rings and provides a serial connection between the dried fuel particles, facilitating their penetration into the coated material. The composite fuel enables the implementation of a combustion wave passing through the MCO layer in a mild sintering environment, thereby providing instantaneous thermal energy for densifying the initial MCO layer. The densification efficiency achieved using the optimized composite-fuel-assisted sintering strategy is 93 % higher than that achieved conventional air sintering. This study will pave the way for the development of new hybrid sintering strategies based on versatile composite fuels to achieve high efficiencies under mild environments. [ABSTRACT FROM AUTHOR]

A method is proposed to determine the effective binary diffusivities in a dissolution of dense ceramics in liquid slags. This is achieved by combining computational fluid dynamics with a boundary layer approach to overcome the resolution problem that is associated with high Schmidt numbers. The rotating finger test experiment was performed with a dense alumina fine grain ceramic in a calcium aluminosilicate (CAS) slag. During the experiment, the sample dissolved, and the sample's shape deviated substantially from the initial cylindrical shape. The simulations were conducted in a middle step whereat the sample was already corroded, and the geometry was obtained from the laser measurements of the corroded surface. The diffusivities were evaluated via the average mass flux density and by fitting the experimental and simulation curves. Good agreement between the simulated and experimental corrosion profiles existed for the diffusivities evaluated by using the two methods. [ABSTRACT FROM AUTHOR]

This paper investigates the moisture dependency of ion diffusion in binary cementitious binders with Portland cement (OPC) in combination with slag or silica fume. Resistivity measurements and calculations with a thermodynamic model, GEMS, were used to assess the unsaturated chloride diffusion coefficient. The results show that the relation between the relative diffusion coefficient and the degree of saturation is not significantly affected by w/b (in the investigated range of w/b from 0.38 to 0.53), while the investigated binder compositions seem to affect the relation. These results are of value for service-life models, where they potentially represent important input data. The results also highlight the need for considering the pore solution conductivity when using resistivity measurements for evaluation of ion transport properties. [ABSTRACT FROM AUTHOR]

This paper reports an assessment of the performance of calcium sulfoaluminate-based cementitious materials in chloride-rich environments while addressing the importance that insightful mix proportioning can have on chloride ingress performance. Belite-calcium sulfoaluminate pastes and mortars were prepared at 4 different calcium sulfate to ye'elimite ratios (ranging from 0 to 2) and the properties of the matured matrix (i.e. porosity and binding capacity) were assessed. A comparative assessment was made of the effective chloride diffusion coefficient in belite-calcium sulfoaluminate mortars through commonly used migration (NT Build 492) and diffusion (NT Build 443) accelerated testing protocols. The influence of paste porosity was apparent in NT Build 492, where lower performance was noted for the low sulfate mixes. These results were in contrast with NT Build 443 observations and are attributed to the short testing period of the migration method in which the potential binding affinity of the hydrates is restricted. [ABSTRACT FROM AUTHOR]

Water vapor diffusion coefficients have been determined with a new set-up of the cup method for mortars with three binders (OPC, OPC + 70% slag, OPC + 10% silica fume) and two water/binder-ratios (0.4, 0.5). The measurements were made as a function of relative humidity with samples on both the absorption and desorption limbs of the sorption isotherm. For OPC samples the results show a clear difference between absorption and desorption. For the samples with supplementary cementitious materials the dependence on RH is small. [ABSTRACT FROM AUTHOR]

The rapid chloride migration test (RCM) is a commonly used accelerated test for the determination of the chloride diffusion coefficient in concrete. Nevertheless, the initial development and further experience with the RCM test concern mainly the ordinary Portland cement system. Therefore, the objective of this work is to analyse the application of this test method for other types of binders, by performing and analysing the RCM test results for mortars prepared with additions of supplementary cementitious materials. A comparison is given between the total chloride concentration profiles measured in concrete after the RCM test and the colourimetric test results. The presented results show that the accuracy of the silver nitrate colourimetric technique is sufficient for the determination of the chloride penetration front and that the RCM test is a suitable method also for the determination of chloride diffusion coefficient in mortars with blended cements. [ABSTRACT FROM AUTHOR]

The transport properties of cement mortar with graphene nanoplatelet (GNP) are investigated experimentally in this study. GNP, a low cost carbon-based nano-sheet, was added to mortar at contents of 0, 2.5, 5.0 and 7.5%, by weight of cement. The water penetration depth, chloride diffusion coefficient and chloride migration were determined for cement mortar with GNP and compared with plain cement mortar specimens. Test results showed that the addition of 2.5% GNP can cause significant decrease of 64%, 70% and 31% for water penetration depth, chloride diffusion coefficient and chloride migration coefficients respectively. The reduced water and ions ingress can be partially attributed to a reduction in the critical pore diameter of about 30%. This refinement of the microstructure by the GNP is validated by the mercury intrusion porosimetry (MIP) results. The impermeable GNP also contributes to the reduced permeability due to the increased tortuosity against water and aggressive ions ingress. [ABSTRACT FROM AUTHOR]

We used dynamic sorption balance measurements to evaluate the diffusivity for cement pastes with three different binders (OPC, OPC + 70% slag, OPC + 10% silica fume). The diffusion of water vapor in cement based materials is normally assumed to follow Fick's law of diffusion, but our results clearly show that Fick's law cannot completely describe the sorption process in our materials. In this paper we report the evidence for this anomalous sorption behavior and discuss a possible method to evaluate diffusivities from such measurements. [ABSTRACT FROM AUTHOR]

From conductivity theory, general models for relative molecular diffusivity and fluid permeability are first derived with unknown tortuosity function and modification coefficient, which can be respectively deduced from hydraulic diffusivity and water retention curve (WRC). Based on empirical laws for hydraulic diffusivity and WRC of cement-based material, unified models for relative molecular diffusivity and fluid permeability are further formulated with only two measurable parameters. Because of practical difficulty for measuring water permeability and pure gaseous molecular diffusivity, only relative gas permeability and relative chloride diffusivity models are verified by the reported data. It is found that the predicted relative gas permeability agrees with measured values and exponential law is a little more preferable than power law for quantifying hydraulic diffusivity. Moreover, relative chloride diffusivity from the unified model also agrees well with experimental data derived via Nernst–Einstein Equation. However, the unified model doesn't capture the possibly overestimated relative chloride diffusivity from Fick's law. [ABSTRACT FROM AUTHOR]

Chloride ion transport driven by electrical potential gradients is discussed in concretes wherein OPC is partially replaced by limestone or a combination of limestone and fly ash/metakaolin at replacement levels of 20% or 35% (volume-basis). The ternary formulations demonstrate non-steady state Cl − migration (NSSM) coefficients that are comparable to or lower than those of the control OPC concretes, with metakaolin blends showing markedly better performance. A pore structure factor extracted through electrical conductivity measurements before the NSSM test is correlated with Cl − penetration depths after the migration test. The transport of all ionic species (Cl − , OH − , Na + , K + ) is modeled using an explicit finite element framework via the coupled Poisson–Nernst–Planck (PNP) equation with suitable consideration of: (a) concentration (depth)-dependent diffusion coefficients, (b) pore-structure factor, and (c) Cl − binding. With informed inputs of material properties, the simulations are able to reliably capture Cl − penetration behaviors in plain and blended binder formulations. [ABSTRACT FROM AUTHOR]

This article is dedicated to the development of a method for acquiring experimental data on ionic diffusion in partially saturated cement-based materials. A modified “half-cell” method is adopted in order to introduce the diffusing solute without disturbing the initial water content of the material. After a certain diffusion time (5 to 10 months), the concentration profiles of the tracer within the samples are measured by elemental mapping using the Laser Induced Breakdown Spectroscopy technique (LIBS). Analyses of the diffusion profiles obtained show a net decrease of the liquid-phase diffusivity with desaturation of the material, in accordance with the results in the literature. The innovation provided by this study consists in taking into account the adsorption of the tracer (Li) in the analytical processing of the diffusion profiles. This adsorption was quantified by measuring the interaction constant. [ABSTRACT FROM AUTHOR]

The relative humidity (RH) dependence of the water permeability of cement is calculated from the water concentration profile of a paste exposed to an RH gradient and the desorption isotherm. The profile is measured using GARField, standing for Gradient at Right Angles to Field, NMR. The isotherm is derived from other earlier NMR measurements. The Darcy equation gives the intrinsic permeability as 4.6×10−21 m2. The apparent intrinsic permeability to water flow shows a broad “U” shape dependence on RH, with a minimum of 7×10−22 m2 at RH 55%. The “U” shape is attributed to the fact that the transport mechanism involves a coupling of liquid and vapour modes. The data is further analysed in terms of a model of coupled liquid and vapour diffusion and Darcy flow due to Baroghel-Bouny et al. (Cem. Concr. Res. 2011 41 828) from which the relative liquid water and vapour permeabilities are calculated. They are strongly RH dependent. The former increases with increasing RH; the latter decreases. [ABSTRACT FROM AUTHOR]

Abstract: Knowledge of the transport properties of damaged concrete in marine environments is essential for predicting its durability. The objective of this study was to fill this gap by correlating the change in permeability and chloride diffusivity with an increasing uniaxial load on ordinary concrete (OC) and high performance concrete (HPC). Concrete cylinders were induced microcracks by mechanical uniaxial compression between 60% and 90% of the ultimate strength to get diffuse damage. The damage variable of specimens was evaluated by elastic stiffness degradation and ultrasound pulse velocity. After unloading intrinsic gas permeability was measured using a constant head permeameter, the chloride migration coefficient was evaluated by migration test in steady state conditions, with the same concrete specimen. The damage variable of specimens showed relationship with gas permeability and chloride diffusion of concrete in this experiment. A correlation was obtained between intrinsic permeability coefficient and chloride diffusion coefficient depending on the damage variable, specific for each concrete type (OC and HPC). [Copyright &y& Elsevier]

Abstract: A flow-through method for measuring dissolution rates of cement minerals is described. The quantity of dissolved material is measured after the dissolution experiment from SEM-BSE images. This makes it possible to measure dissolution rates of individual phases in multiphase cements. The SEM images also directly show if precipitation (hydration) occurred and how the surface roughness changed during dissolution. Furthermore, the method is designed for the purpose of determining if dissolution rate is reaction- or diffusion-controlled, by changing water flow rate or by calculating expected diffusion fluxes. Dissolution rates of flat-ground Portland cement clinker and synthesized alite in deionized water were measured to demonstrate the potential of the method. Initial dissolution rates at lower flow rates were transport-controlled as indicated by the flow rate dependency and predicted diffusion fluxes. The highest alite dissolution rate measured in this study was 93μmol/m2/s. [Copyright &y& Elsevier]

Abstract: The apparent (Dapp ) and effective (Deff ) migration coefficients obtained in chloride migration tests are investigated in this study. The presented Dapp profiles in concrete show that the apparent migration coefficient is strongly concentration-dependent. As demonstrated, the binding of chlorides during the migration tests is very low at low free-chloride concentrations and therefore the chloride penetration front progresses throughout the concrete only slightly retarded by the binding. The diffusion flux during migration tests is shown to be insignificant compared to the migration flux. The DRCM obtained from the Rapid Chloride Migration (RCM) tests are found to be equal to the computed Dapp at the locations of the chloride penetration fronts, which gives an indication that the DRCM represents only the migration coefficient at the front. A linear correlation is found between the DRCM obtained from the traditional RCM model and the Deff obtained from the chloride transport model which includes non-linear chloride binding and concentrations in non-equilibrium. [Copyright &y& Elsevier]

The combination of chloride deicer exposure and temperatures near the freezing point of water makes for an aggressive environment for many concrete pavements and bridge decks. In laboratory-simulated conditions, the formation of calcium oxychloride hydrate (3CaO·CaCl2·15H2O or 3CaO·CaCl2·12H2O) phases has been associated with the expansion and deterioration of concrete and mortar, but calcium oxychloride phases have yet to be observed in the field. A decomposition path from calcium oxychloride hydrate to CaCl2·Ca(OH)2·2H2O to calcite CaCO3 is proposed here, and offered as an explanation for the lack to date of convincing field evidence for a calcium oxychloride hydrate deterioration mechanism. [ABSTRACT FROM AUTHOR]

Abstract: A robust numerical solution of the nonlinear Poisson–Boltzmann equation for asymmetric polyelectrolyte solutions in discrete pore geometries is presented. Comparisons to the linearized approximation of the Poisson–Boltzmann equation reveal that the assumptions leading to linearization may not be appropriate for the electrochemical regime in many cementitious materials. Implications of the electric double layer on both partitioning of species and on diffusive release are discussed. The influence of the electric double layer on anion diffusion relative to cation diffusion is examined. [Copyright &y& Elsevier]

Abstract: The ionic diffusivity in non-saturated cement-based materials is quantitatively studied using lattice Boltzmann method. The Shan–Chen multi-phase lattice Boltzmann model modified by incorporating the Carnahan–Starling equation of state into the model is used to simulate the equilibrium distribution of water and gas phases with density ratio as high as 775 in the three-dimensional microstructure of cement paste at various degrees of water saturation. The lattice Boltzmann diffusion model is subsequently applied to simulate the diffusion process of the ionic species through the partially saturated cement paste. Before the application of the modified Shan–Chen model, the benchmark tests including bubble test and contact angle test are carried out to validate it. It is shown that the ionic diffusivity is greatly influenced by the degree of water saturation. The simulated relative ionic diffusivity as a function degree of water saturation is in good agreement with the experimental data obtained from literature. In addition, the effect of w/c ratio on the ionic diffusivity through cement paste under non-saturated conditions is investigated. It indicates that there is a significant difference between the relative diffusivity for cement paste with w/c ratios of 0.4 and 0.5 over the whole range of water saturation degree. However, the difference of relative diffusivity for cement paste with w/c ratios of 0.5 and 0.6 is not obvious. [Copyright &y& Elsevier]

Abstract: This study addresses the influence of the applied voltage (electrical field) on the value of the chloride migration coefficient, as determined with the Rapid Chloride Migration (RCM) test, and on other properties of cement based mortars. It is shown that in the investigated ranges of applied voltages, the chloride migration coefficients, computed from two different chloride transport models, are relatively constant. However, other properties of mortars are changing due to the application of the electrical field. It is shown that the resistance of the test samples increases during the migration test (therefore the DC current during the test decreases). Moreover, the mass of the samples increases and this increase is found to be proportional to the chloride penetration depth. The pH of the catholyte solution (10% NaCl water solution) increases significantly during the migration test, thus the test conditions change as the OH− to Cl− proportion changes. Furthermore, the measured values of the polarization of the electrodes confirm the value of 2V, assumed in the guidelines for the RCM test. Also, a dark coloration is observed on samples split after the test, prior to spraying with the colorimetric chlorides indicator. This coloration is attributed to a liquid-saturation of the samples only in the colored region and not in the entire volume of the sample, as is assumed after performing the vacuum-saturation prior to the migration tests. [Copyright &y& Elsevier]

Abstract: Diffusional gas transport of a H2/CO2 mixture versus N2 in the pore system of hardened cement pastes was studied at four temperatures up to 350°C in a Wicke-Kallenbach cell. The pastes possessed separation factors α H2,CO2 from 1.42 to 3.43, i.e. the diffusion of hydrogen took place considerably faster than the diffusion of carbon dioxide. The separation factors depended on the threshold radii of the pastes, smaller threshold radii leading to higher separation factors. The Knudsen numbers of the controlling constrictions of the pore system and the temperature dependence of the effective diffusion coefficients of the gases show that gas transport in these constrictions takes place in the transient regime between Knudsen diffusion and bulk diffusion, smaller constriction widths leading to predominating Knudsen diffusion. It is therefore possible to use cement paste membranes to separate gas components of low molecular weight from higher weight components. [Copyright &y& Elsevier]

Abstract: A material model for calcium dissolution/diffusion in cement-based materials is presented, taking the increase of porous space due to calcium leaching into account. The so-called differential-scheme is employed to link the underlying effective diffusive properties to the pore-space volume. The so-obtained material description is used within an analysis scheme based on the finite element method for the simulation of calcium dissolution/diffusion processes at the macroscopic scale. The re-analysis of leaching tests on cement-paste specimens in deionized water and ammonium nitrate solution, respectively, in terms of the rise in calcium concentration in the sample-enclosing aggressive solution, is used for assessment of underlying model assumptions, such as the material function relating calcium concentration bound in solid material skeleton to calcium concentration in the pore fluid. Moreover, the diffusion coefficient of the electrolyte solution in the pore space of cement-based materials, serving as input for determination of the effective diffusion properties, is investigated. [Copyright &y& Elsevier]

Abstract: To study the corrosion of reinforced concrete members subjected to various exposure conditions, a finite-element framework is developed. This framework evaluates the effects of the most critical parameters that may expedite or slow down the corrosion process. Some of these parameters, such as concrete properties and diffusion characteristics, are categorized as internal parameters. In contrast, the environmental parameters, such as ambient temperature, relative humidity, and concentration of carbon dioxide or chloride ions, are considered as external parameters. Using detailed three-dimensional finite-element models, the influential parameters are examined as individual physical environments. The analyses of these environments are based on the concept of transient thermal analysis with appropriate modifications. The novelty of the proposed framework is to consider the nonlinear time-dependent characteristics of the involved parameters along with their mutual interactions. This will result in a more realistic estimation of the extent of degradation over the service life of a structure. [Copyright &y& Elsevier]

Abstract: This paper reports the results of a study on concretes containing lightweight aggregate (LWA) retrieved from the tidal zone of a marine exposure site. In terms of chloride resistance, the LWA concrete performed equivalently to similar concretes of the same age produced with normal density aggregate that were retrieved from the same site 2years earlier. The partial replacement of Portland cement with slag led to substantial reductions in chloride penetration and the chloride diffusion coefficient. However, at w/cm≥0.50, the incorporation of slag resulted in increased surface deterioration (scaling) attributed to freezing and thawing. Concrete with LWA, silica fume and w/cm=0.33, showed better-than-expected performance with regard to resistance to chloride-ion penetration and it is speculated that this may be partly attributed to “internal curing” provided by the LWA which reduces the impact of self desiccation. Further studies are needed to confirm this phenomenon. [Copyright &y& Elsevier]

Abstract: This paper presents the results of a C3S paste characterization study. The objective was to determine the parameters needed to model the process of degradation. The experimental study focused on determining the portlandite content and the ionic diffusion coefficients of C3S paste. The molar C/S ratio of C–S–H in hydrated C3S pastes was also investigated. The portlandite content was determined with an experimental method based on an electron microprobe analysis. This method leads to a portlandite mass content of 24.4±2.3%. The diffusion coefficient of each ionic species was determined by inverse analysis of diffusion test data performed on hydrated C3S samples using a multiionic transport model. [Copyright &y& Elsevier]

Abstract: Red mud, the main waste generated in aluminum and alumina production by the Bayer process, is considered “hazardous” due to its high pH. The characteristic of high alkalinity associated with the presence of aluminosilicates facilitates the assimilation and formation of compounds by reaction with chloride ions. The high pH also provides greater protection of rebars, which is reflected in the low corrosion potential and high electrical resistivity (filler effect) of concrete. In this study, the chloride concentration was monitored by measuring the conductivity of the anolyte. Red mud proved to be a promising additive for concrete to inhibit the corrosion process. The corrosion potential was monitored by electrochemical measurements and the electrical resistivity was evaluated using sensors embedded in concrete test specimens. The results showed that the addition of red mud is beneficial to concrete, reducing its chloride migration rate (diffusion coefficients) and corrosion potential and increasing its electrical resistivity. [Copyright &y& Elsevier]

Abstract: Life management of electric hydro or nuclear power plants requires to estimate long-term concrete properties on facilities, for obvious safety and serviceability reasons. Decades-old structures are foreseen to be operational for several more decades. As a large number of different concrete formulations are found in EDF facilities, empirical models based on many experiments cannot be an option for a large fleet of power plant buildings. To build predictive models, homogenization techniques offer an appealing alternative. To properly upscale creep, especially at long term, a rather precise description of the microstructure is required. However, the complexity of the morphology of concrete poses several challenges. In particular, concrete is formulated to maximize the packing density of the granular skeleton, leading to aggregates spanning several length scales with small inter particle spacings. Thus, explicit meshing of realistic concrete microstructures is either out of reach of current meshing algorithms or would produce a number of degrees of freedom far higher than the current generic FEM codes capabilities. This paper proposes a method to deal with complex matrix-inclusions microstructures such as the ones encountered at the mortar or concrete scales, without explicitly meshing them. The microstructure is superimposed to an independent mesh, which is a regular Cartesian grid. This inevitably yields so called “gray elements”, spanning across multiple phases. As the reliability of the estimate of the effective properties highly depends on the behavior affected to these gray elements, special attention is paid to them. As far as the question of the solvers is concerned, generic FEM codes are found to lack efficiency: they cannot reach high enough levels of discretization with classical free meshes, and they do not take advantage of the regular structure of the mesh. Thus, a specific finite differences/finite volumes solver has been developed. At first, generic off-the-shelf linear system solvers were used. To further improve the efficiency in terms of memory requirements, specific variants of the preconditioned conjugate gradient were implemented. This allowed to homogenize the conductivity of a concrete-like microstructure using more than 109 degrees of freedom on a rather common hardware for 2010 (a PC embedding 48GB of RAM). Taking benefit of the properties of the regular Cartesian grid we have also investigated a multi-level method to improve the CPU efficiency of the code. [Copyright &y& Elsevier]

Abstract: The effects of entrained air on microstructure and transport properties of concrete with up to 11.5% air at different w/c ratios, curing and conditioning regimes were investigated. It was found that air voids disrupt the packing of cement and increase the heterogeneity of the microstructure. The width of the affected interface is around 30μm. Gaseous diffusivity and permeability are increased by up to a factor of 2–3 at the highest air contents. This effect is similar to that due to increasing w/c ratio from 0.35 to 0.50 when samples are conditioned at 52% r.h or 50°C. The effect on sorptivity is less consistent, while the effect on electrical conductivity is influenced by the moisture condition of the air voids. It is estimated that every 1% increase in air content increases transport by 10% or decreases it by 4%, depending on whether the air voids act as conductors or insulators. [Copyright &y& Elsevier]

Abstract: Cracking of concrete cover due to corrosion induced expansion of steel rebar is one of the major causes of the deterioration of reinforced concrete (RC) structures exposed to marine environments and de-icing salts. This paper presents two models that deal with the chloride-induced corrosion and subsequent cracking of concrete cover in RC structures. The former analyses the chloride diffusion within partially saturated concrete. A comprehensive model is developed through the governing equations of moisture, heat and chloride-ion flow. Nonlinearity of diffusion coefficients, chloride binding isotherms and convection phenomena are also highlighted. The latter describes the internal cracking around the bar due to expansive pressures as corrosion of the reinforcing bar progresses. Once a certain chloride concentration threshold is reached in the area surrounding the bar, oxidation of steel begins and oxide products are generated, which occupy much greater volume than the original steel consumed by corrosion. An embedded cohesive crack model is applied for cracking simulation. Both models are incorporated in the same finite element program. The models are chained, though not explicitly coupled, at first instance. Comparisons with experimental results are carried out, with reasonably good agreements being obtained. The work is a step forward for the integration of the two traditional phases (initiation and propagation) widely used in the literature and usually analysed separately. The estimation of the service life of the structure needs to evaluate the associated time for each one. [Copyright &y& Elsevier]

Abstract: The microporous material Ionsiv is used for 137Cs removal from aqueous nuclear waste streams. In the UK, Cs-loaded Ionsiv is classed as an intermediate-level waste; no sentencing and disposal route is yet defined for this material and it is currently held in safe interim storage on several nuclear sites. In this study, the suitability of fly ash and blast furnace slag blended cements for encapsulation of Cs-Ionsiv in a monolithic wasteform was investigated. No evidence of reaction or dissolution of the Cs-Ionsiv in the cementitious environment was found by scanning electron microscopy and X-ray diffraction. However, a small fraction (≤1.6wt.%) of the Cs inventory was released from the encapsulated Ionsiv during leaching experiments carried out on hydrated samples. Furthermore, it was evident that K and Na present in the cementitious pore water exchanged with Cs and H in the Ionsiv. Therefore, cement systems lower in K and Na, such as slag based cements, showed lower Cs release than the fly ash based cements. [Copyright &y& Elsevier]

Abstract: The interaction between concrete/cement and swelling clay (bentonite) has been modeled in the context of engineered barrier systems for deep geological disposal of high-level radioactive waste. The geochemical transformations observed in laboratory diffusion experiments at 60 and 90°C between bentonite and different high-pH solutions (K–Na–OH and Ca(OH)2-saturated) were reconciled with the reactive transport code CrunchFlow. For K–Na–OH solutions (pH=13.5 at 25°C) partial dissolution of montmorillonite and precipitation of Mg-silicates (talc-like), hydrotalcite and brucite at the interface are predicted at 60°C, while at 90°C the alteration is wider. Alkaline cations diffused beyond the mineralogical alteration zone by means of exchange with Mg2+ in the interlayer region of montmorillonite. Very slow reactivity and minor alteration of the clay are predicted in the Ca(OH)2-bentonite system. The model is a reasonable description of the experiments but also demonstrates the difficulties in modeling processes operating at a small scale under a diffusive regime. [Copyright &y& Elsevier]

Abstract: The assessment of the durability of cement-based materials, which could be employed in underground structures for nuclear waste disposal, requires accounting for deterioration factors, such as chemical attacks and damage, and for the interactions between these phenomena. The objective of the present paper consists in investigating the long-term behaviour of cementitious materials by simulating their response to chemical and mechanical solicitations. In a companion paper (Stora et al., submitted to Cem. Concr. Res. 2008), the implementation of a multi-scale homogenization model into an integration platform has allowed for evaluating the evolution of the mineral composition, diffusive and elastic properties inside a concrete material subjected to leaching. To complete this previous work, an orthotropic micromechanical damage model is presently developed and incorporated in this numerical platform to estimate the mechanical and diffusive properties of damaged cement-based materials. Simulations of the chemo-mechanical behaviour of leached cementitious materials are performed with the tool thus obtained and compared with available experiments. The numerical results are insightful about the interactions between damage and chemical deteriorations. [Copyright &y& Elsevier]

Abstract: Based on the gathered experimental data concerning adsorption/desorption processes in cement mortar, it has been stated that the rate of these processes changes in time even if they proceed in stable conditions. In this paper an attempt is made to describe such processes by applying linear and non-linear diffusion theories for comparison. The main aim of these studies is to determine the diffusion coefficient by correlating the theoretically determined desorption isotherms with the experimental ones. The validation of the diffusion coefficient was accomplished through comparison of the theoretical desorption curves with the experimental data for narrow and broad ranges of the air humidity changes. The final conclusion is that the moisture transfer in hygroscopic porous materials for broad ranges of the air humidity changes should be modeled by the non-linear diffusion theory, in which the diffusion coefficient is a function of moisture content. The new material in this paper concerns very long time measurements in desiccators, and evaluation of the diffusion coefficient by an advanced optimization algorithm. [Copyright &y& Elsevier]

Abstract: A computer model has been written to simulate the Rapid Chloride Permeability Test (RCPT) described in ASTM C1202. The model represents the key processes of diffusion and electromigration using standard equations but then maintains charge neutrality by modelling changes to the voltage distribution. This method enables the model to predict current–time transients similar to those recorded in experiments and it can then be used to obtain basic parameters such as diffusion coefficients for tested samples by optimising to the observed data. Experimental data showing a non-linear voltage distribution is presented together with model results which show that non-linearity has a significant effect on the current. Other predictions from the model are compared with published data and shown to give good agreement. [Copyright &y& Elsevier]

Abstract: A survey of 11 concrete bridges located in a marine environment at 0.65–48.65 years was examined in terms of chloride transport. As a result, the apparent diffusion coefficient (D) and the surface chloride concentration (C S) are time dependent; the D exponentially decreased with time and the C S increased in the form of a logarithm function to time. Using these data, governing equations for the D and C S were derived to predict the chloride transport in a long term. The time dependent model indicated the higher chloride ingresses in ordinary Portland cement (OPC) concrete than the time independent model, due to a build-up of the C S with time, but ground granulated blast furnace slag (GGBS) concrete indicated a similar range of the chloride ingresses, due to the rapid decrease in the D. To ensure the accuracy of the model that the present study suggested, the fitted model was compared to the well known model of the LIFE 365 together with a chloride profile obtained from an in-situ examination. Then it was found that the model in the present study well predicted the rate of chloride transport, while the LIFE 365 indicated a poor description of the chloride ingress in a long term, due to a constant C S and an overwhelming rapid decrease in the D. [Copyright &y& Elsevier]

Abstract: This article presents a comprehensive modeling of temperature, carbonation, water and chloride ions transport in cover concrete using the transport model “TransChlor”. The TransChlor transport model employs weather data and chloride ion concentrations present on the concrete surface to predict the temporal and spatial evolution of the presence of chloride ion concentrations in the cover concrete pores. The main features of the TransChlor model are presented and validated. The TransChlor model has been calibrated using experimental data on liquid water movement in concrete of different permeabilities under realistic microclimatic conditions. Chloride ion transport is validated by means of experimental results obtained from a newly developed chloride ion optical fiber based sensor. [Copyright &y& Elsevier]

Abstract: In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity. [Copyright &y& Elsevier]

Abstract: The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The obtained results show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work. [Copyright &y& Elsevier]

Abstract: Cut-off walls for the containment of polluted sites are vertical in-ground barriers of low hydraulic conductivity. To construct these barriers, self-hardening watery suspensions of a special cement-based hydraulic binder and a cement-stable bentonite are used. The formation of the pore structure during hardening of suspensions with different solid contents and the water self-diffusion in the resulting cut-off wall materials were studied by non-destructive 1H NMR techniques. It was found that an increased amount of hydrating solids in the suspension leads to a decrease in NMR relaxation times and self-diffusion coefficients of the pore water, indicating a reduction of the pore sizes and an enhancement of the diffusion resistance. The self-diffusion coefficients of the water in the hardened cut-off wall materials were determined to be about four orders of magnitude smaller than in bulk liquid water and two orders of magnitude smaller than in pure bentonite–water suspensions confirming the excellent diffusive resistance of the cut-off wall materials. [Copyright &y& Elsevier]

Abstract: This paper provides an improved mathematical analysis of chloride penetration into concrete employing a time-dependent diffusion coefficient for the solution of Fick''s second law of diffusion. In the paper the possible errors caused by the application of oversimplified mathematical expressions used in some models for the evaluation of service life of reinforced concrete structures are discussed. The results from this mathematical analysis demonstrate that some models based on the oversimplified error function complement (ERFC) solutions may easily overestimate the service life by orders of magnitude, especially when the age factor is high. Some chloride profiles after up to 10 years'' field exposure were used to compare the oversimplified with the improved models. The results show that both the oversimplified and the improved models fairly well predict the 10 years'' chloride ingress in Portland cement concrete, but the oversimplified ERFC model significantly underestimates the chloride ingress in concrete with fly ash. [Copyright &y& Elsevier]

Abstract: Sub-cooling of cryogenic propellant by helium injection is one of the most effective methods for suppressing bulk boiling and keeping sub-cooled liquid oxygen before rocket launch. Compared with the helium injection cooling under atmospheric condition, helium injection cooling under pressurized condition has advantage that it can greatly reduce re-warming time of the sub-cooled liquid oxygen. Helium injection cooling under pressurized condition is characterized by cooling of initially sub-cooled cryogenic liquid, which is significantly different from that of the atmospheric condition where liquid oxygen usually exists at saturated condition. In this paper, we discuss the characteristics of helium injection cooling under pressurized condition, with the associated physical understanding of the process. Experimental results are presented along the simulations of variously combined system parameters based on the finite heat transfer and instantaneous diffusion mass transfer model. A non-dimensional parameter for identifying the cooling regime is conceived. The critical values of the non-dimensional parameters and injected helium temperatures are also estimated. [Copyright &y& Elsevier]

Abstract: Sub-cooling of cryogenic propellant by helium injection is one of the most effective methods for suppressing bulk boiling and keeping sub-cooled liquid oxygen before rocket launch. In order to design the cooling system, understanding of the limitations of heat and mass transfer is required. In this paper, an analytical model for the helium injection system is presented. This model’s main feature is the representation of bubbling system using finite-rate heat transfer and instantaneous mass transfer concept. With this simplified approach, the effect of helium injection to liquid oxygen system under several circumstances is examined. Experimental results along with simulations of single bubble rising in liquid oxygen and bubbling system are presented with various helium injection flow rates, helium temperatures, and injection methods. The overall cooling effect for rocket application is also discussed. [Copyright &y& Elsevier]

• "In the present experimental conditions, corrosion of T91 and Fe are characterized by a pure dissolution phenomenon". • "Pure dissolution rates of T91, and iron are similar". • Modelling of the corrosion process shows that for liquid natural convection, corrosion is controlled by iron diffusion in the diffusion boundary layer. • For flowing liquid alloy, corrosion is controlled by a mixed process combining both the interfacial reaction rate of iron dissolution and the iron diffusion rate in the diffusion boundary layer. T91 (Fe-9Cr steel) and pure iron were corroded in pure dissolution domain in liquid lead bismuth eutectic (LBE) from 450 to 540 °C in static and in dynamic LBE flow. The modelling showed that corrosion rates were controlled: (i) for static condition (LBE natural convection), by the iron diffusion in the diffusion boundary layer; (ii) for flowing LBE, by a mixed process combining both the interfacial reaction rate of iron dissolution and the iron diffusion rate in the diffusion boundary layer. By fitting the modelling on the experimental points, the constant of interfacial reaction rate was determined. [ABSTRACT FROM AUTHOR]

The service-life of steel-reinforced concrete structures is primarily determined by the ability of the concrete cover to resist chloride ingress. With limited literature available on the ingress of chloride into alkali-activated slags (AAS) under service conditions, it is critical that this is described by appropriate models. This paper describes an interactive software framework to relate chloride ingress into AAS with the chemistry of the concrete cover, by considering the chloride binding capacity and porosity of the binder as a function of time, based on thermodynamic calculations of the phase assemblage as a function of slag and activator composition. This provides a major step towards developing the ability to predict the ingress of chlorides in alkali-activated concretes from a sound theoretical basis, which is essential in providing confidence in the durability of these materials in essential infrastructure applications. [ABSTRACT FROM AUTHOR]

This study investigated the formation of different phases and their influence on the microstructural and mechanical development of carbonated reactive MgO cement formulations. Hydrated magnesium carbonates (HMCs) were identified and quantified via XRD, TG-DTG and SEM. Visual observations of the 3D internal structure at different depths were performed by X-ray computed tomography and confocal Raman microscopy. Elastic modulus of different components was obtained by nano-indentation. The expansive formation of HMCs led to dense microstructures composed of well-connected carbonate networks. The transition of artinite to hydromagnesite/nesquehonite was observed over time. The initial formation of a dense carbonate layer on sample surface inhibited continuous diffusion of CO 2 , resulting in a slower strength development at later ages. Despite this limitation, the continuation of hydration and evolution of carbonate morphology led to 28-day strengths of 60 MPa in concrete samples, supported by the higher elastic modulus of HMCs than brucite (19.4 vs. 2.5 GPa). [ABSTRACT FROM AUTHOR]

The influence of atmospheric carbonation on gas diffusion was investigated using four hardened cement pastes (CEM I, CEM III/A, CEM V/A, and a low-alkalinity binder) with common water-to-binder ratio (0.4). The diffusivity of the non‑carbonated and carbonated pastes with respect to helium and nitrogen was measured at different relative humidities. Carbonation decreased the diffusivity of the CEM I paste, whereas that of the other binders significantly increased after carbonation. These results show the competition between porosity clogging and cracking induced by carbonation. The consequences of carbonation are therefore believed to depend on the considered binder. Clogging dominates in ordinary Portland cement (OPC), leading to a decrease in its transport properties after carbonation; cracking dominates in blended cements, leading to a significant increase in its transport properties after carbonation. [ABSTRACT FROM AUTHOR]