The formation,stability and microstructure of calcium sulpoaluminate hydrates present in hydrated cement pastes,usingin situ synchrotron energy-dispersive diffraction

Synchrotron radiation-energy dispersive diffraction has been used for the first time to study the formation and stability of the calcium sulphoaluminate hydrates in hydrated Portland cement pastes. By using this technique it has also been possible to investigate microstructural and compositional characteristics of the ettringite (AFt) phase. The longer term slow development of the monosulphate (AFm) phase has also been monitored, although the characterized content is quite low. Differences were detected between the microstructural characteristics of the AFt phase formed in the high ferrite sulphate-resisting-type cement pastes, as compared with the equivalent phase formed from the ordinary Portland cements. These differences were especially significant at later hydration times and have been ascribed to compositional differences between the ettringite (AFt) formed from the two different types of cement.     

Durability of reactive powder composites: influence of silica fume on the leaching properties of very low water/binder pastes

Reactive Powder Composites are new cement-based materials which could be used for the storage of nuclear wastes thanks to their excellent microstructural properties. This paper studies their durability when submitted in the laboratory to a water leaching attack. In order to understand the behaviour of the hydrates (CSH), the study was carried out on a pure cement paste and on a cement + silica fume paste. The beneficial effect of silica fume is demonstrated from considerations on the calcium leaching, from XRD analysis, from SEM observations and from the tritium diffusion and pore distribution analysis. It was found that the leaching greatly affects the microstructure, especially that of the anhydrous cement grains remaining in the paste.     

Leaching resistance of hazardous waste cement solidification after accelerated carbonation

When cement-based materials are carbonated, some of their physicochemical properties are changed, which includes reductions of porosity by 20% and pH from 12 to 13 to 8–9. These changes can enhance the retention ability of cementitious solids containing hazard waste. This research studied the effect of carbonation on the leaching resistance of hazardous waste cement solidification. The finite element software COMSOL Multiphysics was used to simulate the process of accelerated carbonation and the effect of carbonation on leaching. Laboratory tests were conducted to validate the numerical models. Parametric studies from the numerical simulations revealed that carbonation could significantly improve leaching retention capabilities of cementitious solids containing hazardous wastes.     

Modelling calcium leaching kinetics of cement asphalt paste

Calcium leaching is an important durability problem for cement asphalt (CA) mortar used in ballastless slab track which is under the coupled action of train-induced dynamic load and rainwater erosion. In this paper, a model based on the calcium mass conservation and the thermodynamic equilibrium of calcium was developed to describe the calcium leaching of CA paste in both deionized water and ammonium nitrate solution. In this model, the spatial–temporal distribution of solid calcium content in skeleton, porosity and diffusivity, which are directly or indirectly relevant to the calcium concentration in pore solution, were considered. Finite difference method was utilized to solve this model, and a three layer Crank–Nicolson differential scheme was applied to improve the stability. The model was experimentally verified using the Ca/Si molar ratio in solid and the calcium leaching rate characterized by SEM–EDX and potentiometric titration respectively. It was found that asphalt content does not change the spatial distribution of solid calcium in CA paste. Asphalt affects the leaching rate by reducing solid calcium concentration and changing the activation energy of micro-pore wall. Accelerating factors by ammonium nitrate solution were experimentally determined and predicted by the model.     

Performance simulation and quantitative analysis of cement-based materials subjected to leaching

A 3D numerical modelling platform (MuMoCC) developed in a previous work by the authors is applied in this paper to investigate the effect of leaching of some solid phases of cement paste (portlandite and hydrated aluminates or sulfoaluminate phases) on the mechanical and diffusivity performances of cement paste and mortar. The platform is based on a multi-scale approach and uses two numerical tools. First, NIST’s CEMHYD3D code is used to simulate 3D Representative Volume Elements of cement paste and mortar. Then mechanical and diffusivity behaviour of the numerical materials are simulated using ABAQUS software. The proposed three-dimensional heterogonous model presents at least two advantages. Firstly, it is able to capture the complexity of the random microstructure of cement-based materials. Secondly, only a few parameters have to be fitted compared to the other existing models, which indicates the relevance of the model. The numerical simulations of leached cement paste and mortar performance highlight and quantify the significant effect of portlandite and hydrated aluminate and sulfoaluminate phases’ dissolution on the decrease of elastic modulus and compressive strength and on the increase of ductility and diffusivity. The numerical results show that the leaching of portlandite decreases the compressive strength of a w/c = 0.4 cement paste by a factor of 1.33. The dissolution of portlandite and hydrated aluminates or sulfoaluminate phases involves a decrease by a more important factor (1.86).The leaching of portlandite phase involves an important increase, by a factor of 31, of the effective diffusion coefficient.Using the developed multi-scale modelling and knowing the leaching kinetics values, the mechanical and diffusion performances of cement-based materials can be estimated correctly according to leaching duration.     

Analysis of calcium leaching behavior of plain and modified cement pastes in pure water

The calcium ion leaching behavior of cement pastes modified with a high-alkali fine glass powder, silica fume, and fly ash, exposed to deionized water, is reported in this paper. Porosity enhancement in pastes subjected to leaching is attributed both to the dissolution of calcium hydroxide (CH) as well as decalcification of C–S–H gel. A methodology that combines the measured porosity increase along with the CH and C–S–H contents remaining after leaching for a particular duration is developed to separate the porosities created due to CH and C–S–H leaching. In order to quantify the influence of leaching on the amounts of Ca ions remaining in the CH and C–S–H phases, solid–liquid equilibrium curves for calcium are developed for the unleached and leached pastes. Leaching depths are also calculated using the CH contents of the leached and unleached specimens. All the modified pastes show better leaching resistance than the plain paste. In addition to the microstructure densification, the lower Ca–Si molar ratio in modified pastes that reduces the equilibrium liquid Ca ion concentration contributes to this observation. For the glass powder modified paste, the presence of higher alkali content in the pore solution further reduces the dissolution of CH due to common ion effect, thus providing it with the highest leaching resistance. Fly ash and silica fume modified pastes demonstrate leaching resistance in between those of the plain and glass powder modified mixtures.     

Micro-mechanical properties of calcium sulfoaluminate cement and the correlation with microstructures

To investigate the micro-mechanical properties of calcium sulfoaluminate cement and the correlation with the microstructures, we apply a variety of advanced techniques of microstructural and micro-mechanical characterization, including scanning electron microscopy with backscattered electron and energy-dispersive X-ray spectroscopy detectors, X-ray fluorescence, X-ray diffraction and nanoindentation. For the first time, the micro-mechanical properties of material microstructures present in a calcium sulfoaluminate cement are estimated. In the calcium sulfoaluminate cement used in this research, two type of hydration product microstructures with the differentiable microstructural morphologies, compositions and micro-mechanical properties are identified and investigated. The correlation of the micro-mechanical properties with the microstructures shows that the hydration product microstructure containing more ettringite has lower indentation modulus and hardness than that containing more aluminum hydroxide.     

Decalcification of activated paper sludge – Fly ash-Portland cement blended pastes in pure water

Decalcification in pure water of ternary Portland cement (TPC) pastes, containing thermally activated paper sludge and fly ash, has been evaluated from the leaching of Ca²⁺ in pure water at the temperature of 20 °C during 90 days. Monitoring of calcium loss showed that the leaching kinetics are controlled by diffusion. The degradation of the material over time is estimated from the calcium effective diffusivity. A similar study of plain ordinary Portland cement (OPC) pastes was carried out for comparison. The results showed lower effective diffusion coefficients of calcium in the case of the TPC pastes. This behaviour is related to its microstructure, which is denser than that of OPC as a result of the pozzolanic activity of both additions.     

耦合离子与水化产物间相互作用的水泥基材料氯离子传输模型

钢筋混凝土结构为土木工程领域应用最广泛的结构形式,而氯离子诱发的钢筋锈蚀为降低混凝土结构耐久性的主要原因之一。在以往的研究中,基于Nernst-Planck方程的数值模型通常被用来模拟混凝土中离子的传输,进一步预测混凝土结构的服役寿命,然而这些数值模型并未考虑传输过程中离子与水化产物之间的热力学作用。因此,本文基于离子传输的物理化学作用的本质过程,建立了饱水状态下水泥基材料中多离子传输的数值模型。首先,采用表面络合模型和相平衡模型,建立了孔溶液中离子与水化产物间的热力学数值模型;然后,借助于算子分裂算法,求解了耦合热力学作用的Nernst-Planck方程多离子传输的有限元模型,得到了水化产物中各相成分、孔隙率及孔溶液中各自由离子浓度的演化规律。最后,通过已有文献的试验研究验证了本文建立的数值模型的准确性。     

Microstructure-based simulation of thermomechanical behavior of composite materials by object-oriented finite element analysis

While it is well recognized that microstructure controls the physical and mechanical properties of a material, the complexity of the microstructure often makes it difficult to simulate by analytical or numerical techniques. In this paper we present a relatively new approach to incorporate microstructures into finite element modeling using an object-oriented finite element technique. This technique combines microstructural data in the form of experimental or simulated microstructures, with fundamental material data (such as elastic modulus or coefficient of thermal expansion of the constituent phases) as a basis for understanding material behavior. The object-oriented technique is a radical departure from conventional finite element analysis, where a “unit-cell” model is used as the basis for predicting material behavior. Instead, the starting point of object-oriented finite element analysis is the actual microstructure of the material being investigated. In this paper, an introduction to the object-oriented finite element approach to microstructure-based modeling is provided with two examples: SiC particle-reinforced Al matrix composites and double-cemented WC particle-reinforced Co matrix composites. It will be shown that object-oriented finite element analysis is a unique tool that can be used to predict elastic and thermal constants of the composites, as well as salient effects of the microstructure on local stress state.     

Thermodynamic equilibrium calculations in cementitious systems

This review paper aims at giving an overview of the different applications of thermodynamic equilibrium calculations in cementitious systems. They can help us to understand on a chemical level the consequences of different factors such as cement composition, hydration, leaching, or temperature on the composition and the properties of a hydrated cementitious system. Equilibrium calculations have been used successfully to compute the stable phase assemblages based on the solution composition as well as to model the stable phase assemblage in completely hydrated cements and thus to asses the influence of the chemical composition on the hydrate assemblage. Thermodynamic calculations can also, in combination with a dissolution model, be used to follow the changes during hydration or, in combination with transport models, to calculate the interactions of cementitious systems with the environment. In all these quite different applications, thermodynamic equilibrium calculations have been a valuable addition to experimental studies deepening our understanding of the processes that govern cementitious systems and interpreting experimental observations. It should be carried in mind that precipitation and dissolution processes can be slow so that thermodynamic equilibrium may not be reached; an approach that couples thermodynamics and kinetics would be preferable. However, as many of the kinetic data are not (yet) available, it is important to verify the results of thermodynamic calculations with appropriate experiments. Thermodynamic equilibrium calculations in its different forms have been applied mainly to Portland cement systems. The approach, however, is equally valid for blended systems or for cementitious systems based on supplementary cementitious materials and is expected to further the development of new cementitious materials and blends.     

Digital image-based modeling applied to the homogenization analysis of composite materials

Absract The systematic methodologies to derive accurate microstructural models are developed for studying the mechanical behaviors of composite materials. Since the geometric information of a microstructure is often given by an image or a set of images, the direct interpretation of the geometry is possibly by digitizing it. By identifying each pixel or voxel with a finite element (FE) and accompanying appropriate image processing, an FE model can be automatically generated. It is also emphasized that the digitized models can be suitable for solving the FE equations by utilizing the uniformity of the FE mesh. The finite element analysis (FEA) with the homogenization method enables the prediction the thermo-mechanical behavior of the periodic microstructure (unit cell) as well as the global mechanical response of a structural component, while we are taking into account the specific effect of the geometric structural configuration of the microstructure through digitization. Several kinds of the digitizing techniques are presented to illustrate the potential of digital image-based (DIB) FE modeling of the unit cell. Keeping the microstructural design in mind, the modification of the plane image is introduced and the virtual realization of the unit cell geometry is presented so that a microstructural analysis utilizing the homogenization method would be realistic.     

Effect of long time service exposure on microstructure and mechanical properties of gas turbine vanes made of IN939 alloy

In the present study, the effects of long-term service exposure have been investigated on microstructure and mechanical properties of gas turbine vanes made of IN939 superalloy. The major microstructural changes for the investigated service-exposed vanes include the formation of continuous grain boundary carbides and the transformation (degeneration) of MC carbides located at the grain boundaries. The brittle σ phase, which is predicted to be stable on the basis of thermodynamic calculations, is not observed in the microstructure of service-exposed vanes. The microstructural changes during service lead to a loss in room temperature ductility as well as in creep properties of the alloy.     

Microstructure and properties of α-tricalcium phosphate-based bone cement

This paper examines the physicochemical properties and microstructure of brushite calcium phosphate cements possessing strength acceptable for application in surgery (15–20 MPa) and ensuring an optimal acidity (pH 6.5–7.5) of solutions in contact with them. Holding in a physiological saline produces significant changes in the microstructure of the cement relative to that before immersion in the solution: it causes a transformation of the most soluble components into platelike hydroxyapatite crystals.     

Supercritical carbonation of calcareous composites: Influence of curing

This paper reports the effect of curing on the susceptibility of cementitious composites to carbonation using supercritical carbon dioxide. Samples made using a compression moulding technique were cured in water before and/or after carbonation and the effect on porosity, microstructure, solid phase assemblage and flexural strength was determined. In terms of development of mechanical strength, no benefit was gained from any period of pre- or post-carbonation curing regime. Yet samples cured prior to carbonation underwent minimal chemical reaction between supercritical carbon dioxide and calcium hydroxide, unhydrated cement or C–S–H. Thus there was no correlation between chemical degree of reaction and strength development. The effects responsible for the marked strength gain in supercritically carbonated samples must involve subtle changes in the microstructure of the C–S–H gel, not simple pore filling by calcium carbonate as is often postulated.     

Microstructural changes of Pt/Ti bilayer during annealing in different atmospheres — an XRD study

The high temperature behaviour of Pt/Ti base electrode bilayers is crucial for the deposition of ferro-electric thin films on top of such metal films. Therefore the microstructural development during annealing has to be considered. These parameters are studied by X-ray diffraction (XRD). Comparing the microstructure before and after thermal treatment significant differences could be found. Pt₃Ti compound formation was observed in all coatings. The stress in the Pt phase changes from compressive to tensile during annealing. In contradiction the stress in the Pt₃Ti phase is compressive after thermal treatment. From measurements of the total reflected signal an amount of TiO₂ on the surface of the oxygen treated film could be found. A model based on diffusion of Ti along grain boundaries and considering thermodynamic aspects of stress development is predicted to describe the microstructural changes during annealing.     

Impact of Cement Hydration on Durability of Cellulosic Fiber‐Reinforced Cementitious Composites in the Presence of Metakaolin

Degradation of cellulosic fiber in the alkaline environment of concrete generated in the process of cement hydration is the primary reason for the low durability of such composites. However, the impact of cement hydration on cellulosic fiber's degradation in cementitious systems has not been thoroughly understood. This paper presents the dependence of deterioration behavior of cellulosic fiber‐cement systems on cement hydration in the presence of metakaolin. Experimental investigations, such as isothermal calorimetry, thermogravimetric analysis and energy dispersive X‐ray spectroscopy, and thermodynamic simulations are carried out to investigate cement hydration kinetics and hydration products. Durability of cellulosic fiber‐reinforced cement composite is assessed based on the degradation in flexural properties. The results indicate that, in the presence of metakaolin, the hydration of cement is enhanced accompanied by consumption of calcium hydroxide, low release of hydration heat, decreased Ca/Al and Ca/Si ratios of C–S–H phase, and reduced OH^‐ and Ca²⁺ amounts in pore solution. A cement substitution by 30 wt% metakaolin results in an improvement of flexural toughness and durability of cellulosic fiber‐reinforced cement composites by 42 and 269%, respectively. The correlations between composite durability and hydration of Portland cement are established.

     

Synthesis and properties of bone cement materials in the calcium phosphate–calcium sulfate system

We have studied the influence of the cement liquid composition and the relationship between the components of the calcium sulfate–precipitated calcium phosphate system in a wide concentration range on the setting time, phase composition, microstructure, and mechanical properties of cement materials. The results demonstrate that the greatest promise is held by a magnesium phosphate-based cement liquid which, when mixed with powder, forms a high-strength phase, leading to a considerable increase in the strength of the cements. The addition of 20 wt % calcium sulfate to the starting mixture ensures dispersion hardening of the cements. We have obtained new cement materials offering a strength of up to 60 MPa, which are expected to find medical applications.     

Changes in the phase assemblage of concrete exposed to sea water

In the present study the phase changes in an ordinary concrete standing for 10 years in a tidal zone were investigated with a range of techniques. From the exposed surface and inwards different zones had formed. SEM–EDS analysis of a polished section of the surface near region, showed a Mg rich layer with a thickness of 10–20 μm, as well as the filling of cracks leading from the surface with a Mg rich phase, most likely brucite. In the outermost 2 mm, an increase in the calcium carbonate content was identified by XRD and TGA. In the same zone SEM–EDS analyses indicated enrichment in ettringite and thaumasite. In the first 20 mm calcium hydroxide leaching was observed using XRD and TGA. Chlorides appeared to have penetrated up to approx. 70 mm from the surface. Part of the chlorides were found to be bound in alumina containing phases and in the C–S–H by SEM–EDS. These experimentally observed phase changes generally agreed with the predictions of a thermodynamic model.     

Modelling the leaching of calcium hydroxide from cement paste: effects on pore space percolation and diffusivity

As concrete is exposed to the elements, its underlying microstructure can be attacked by a variety of aggressive agents. For example, rainwater and groundwater can degrade the concrete by dissolving soluble constituents such as calcium hydroxide. Using computer simulation, this paper examines the effects of calcium hydroxide dissolution on two material properties: the percolation properties or connectivity of the capillary pore space, and the relative ionic diffusivity. A microstructural model for cement paste is used to produce a hydrated specimen which is subsequently subjected to the leaching process. Pore space percolation characteristics and relative ionic diffusivity are computed throughout the leaching process as a function of total capillary porosity. Material variables examined are water: solids ratio and silica fume content. Percolation theory is used to develop the concept of a critical volume fraction of calcium hydroxide plus capillary pore space. It is shown that this critical combined volume fraction determines the magnitude of the effect of leaching on relative ionic diffusivity.

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Resume Quand le béton est exposé aux éléments, sa microstructure sous-jacente peut être agressée par divers agents. Ainsi, la pluie et l'eau souterraine peuvent détériorer le béton en dissolvant les composants solubles tels que l'hydroxyde de calcium. A l'aide d'une simulation par ordinateur, on examine ici les effects de la dissolution d'hydroxyde de calcium sur deux propriétés du matériau: la percolation de l'espace interstitiel et la diffusivité ionique relative. On utilise un modèle microstructural de la pate de ciment pour produire une éprouvette hydratée qu'on soumet ensuite au lessivage. On calcule les caractéristiques de la percolation interstitielle et la diffusivité ionique relative tout au long du phénomène de lessivage en fonction de la porosité capillaire totale. Les variables du matériau considérées sont le rapport eau/solides et la teneur en micro-silice. On se sert de la théorie de la percolation pour développer le concept de fraction volumique critique d'hydroxyde de calcium en combinaison avec l'espace interstitiel capillaire. On montre que cette combinaison critique détermine l'importance de l'effet de lessivage sur la diffusivité ionique relative.