Electrical properties of cement paste obtained from impedance spectroscopy

The impedance spectra of hydrating Portland cement paste showed a small part of a large depressed arc and a single depressed arc at low- and high-frequency regions, respectively. A new equivalent circuit was proposed for hydrating cement paste, which consists of a serial combination of a resistor and a constant phase element (CPE) with a resistor in parallel for the bulk effects, and a parallel combination of a resistor and a CPE for the electrode effects. The microstructural evolution in hydrating cement paste was associated with the changes in the resistances, capacitance and the critical relaxation time.     

Interpretation of impedance spectroscopy of cement paste via computer modelling

Computer simulation of impedance spectroscopy (IS) of hydrating cement paste, using a three-dimensional, four-phase model, is described. Two puzzling features of experimental IS results, the possible offset resistance in the Nyquist plot and the sharp decrease in normalized conductivity within the first 50 h of reaction, have been studied using the computer simulation model. Insight is provided into these features using the ability of the model to compare quantitatively microstructure and properties. It is concluded that the offset resistance is an experimental artefact, and does not directly relate to microstructure. The drop in conductivity during the first 50 h is shown to be a consequence of a gradual shift from parallel-dominated to series-dominated behaviour of the electrical conductivity, as microstructural modifications take place during hydration, causing the capillary pore structure to become more tortuous. This tortuousity can also explain the high-frequency impedance behaviour in terms of a two-arc response.Part II of this has been previously published in volume 29 issue 19, pages 4984–4992 of this journal.     

Interpretation of the impedance spectroscopy of cement paste via computer modelling

The d.c. conductivity, , and low-frequency relative dielectric constant, k, of Portland cement paste were monitored, using impedance spectroscopy, during cooling from room temperature down to -50 °C. Dramatic decreases in the values of and k, as great as two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and larger capillary pores. This result provides strong experimental support for the dielectric amplification mechanism, proposed in Part II of this series, to explain the high measured low-frequency relative dielectric constant of hydrating Portland cement paste. Only gradual changes in the electrical properties were observed below this sudden drop, as the temperature continued to decrease. The values of and k of frozen cement paste, at a constant temperature of -40 °C, were dominated by properties of calcium-silicate-hydrate (C-S-H) and so increased with the degree of hydration of the paste, indicating a C-S-H gel percolation threshold at a volume fraction of approximately 15%–20%, in good agreement with previous predictions. Good agreement was found between experimental results and digital-image-based model computations of at -40 °C. Freeze-thaw cycling caused a drop in the dielectric constant of paste in the unfrozen state, indicating that measurements of k could be useful for monitoring microstructural changes during freeze-thaw cycling and other processes that gradually damage parts of the cement paste microstructure.     

Interpretation of the impedance spectroscopy of cement paste via computer modelling

Dielectric properties of cement pastes are measured using impedance spectroscopy, and the effective dielectric constants of the low frequency bulk arcs are reported. The unusually high values thereby obtained, and their dependence on reaction time and water:cement ratio, are explained by the presence of microstructural features that serve to amplify the dielectric constants of the individual material phases. The dielectric properties of three-dimensional cement paste models and of simple two-dimensional models of the hypothesized microstructural features are analysed. The model results provide insight into the proposed dielectric amplification mechanism in real cement paste.     

A simple method for predicting the chloride diffusivity of cement paste

A simple method for predicting the chloride diffusivity of cement paste is presented. In this method, cement paste is modeled as a two-phase composite material, consisting of solid matrix and pore space. By incorporating the classical percolation theory into the effective medium approach, an explicit solution is formulated for the chloride diffusivity of cement paste. Two parameters involved in the solution are determined by fitting the solution to experimental data. After the validity of the simple method is verified with experimental results obtained from the literature, the effects of the water/cement ratio and the degree of hydration on the chloride diffusivity of cement paste are evaluated in a quantitative manner. The paper concludes that the proposed simple method can predict the chloride diffusivity of cement paste with reasonable accuracy.     

电学方法研究掺有窑灰水泥的水化特性

利用非接触式电阻率新方法,研究普通硅酸盐水泥及粉煤灰和粉磨时间分别为1h、3h和4h的窑灰混掺后胶凝材料的水化特性.试验结果表明,窑灰分别粉磨1h、3h和4h后,在同样掺量情况下,对水泥浆体早期电阻率随时间的变化有明显的影响.高碱度和高Cl-、高SO3含量将直接影响到窑灰作为混合材或直接回窑后水泥的早期水化特性.不同掺量的粉煤灰在一定程度上可以平衡窑灰水泥浆体系中的碱,同时窑灰中的碱也可激发粉煤灰的早期活性,在熟料掺量小于50%时仍可获得较高的早期强度.     

Study on electrical properties of Ni-doped SrTiO3 ceramics using impedance spectroscopy

The ceramics, SrTiO₃ (ST) and 0.4, 0.8 atom % Ni doped SrTiO₃, were prepared by solid state reaction route. The average grain size of undoped and doped samples was measured and found to be 1.2, 1.9 and 3.7 μm, respectively. The impedance measurements were conducted at 400–600°C to separate grain and grain boundary contributions. The grain and grain boundaries relaxation frequencies were shifted to higher frequency with temperature. Bulk resistance of doped and undoped ST ceramics was more or less the same. Single grain boundary resistance of doped sample was higher than that of undoped one, indicating that GB resistance increases with acceptor doping. Activation energies were calculated to confirm the same.     

Penetration resistance and electrical resistivity of cement paste with superplasticizer

The purpose of this paper is to investigate the setting process and evolution of electrical resistivity of Portland cement pastes with constant water to cement ratio (w/c) of 0.3 and with different dosages of naphthalene superplasticizer (SP) from 0 to 1.2 %. The setting process of cement paste was monitored by the Vicat needle test. The depth of penetration was recorded and used to calculate the shear resistance generated by the cement paste. Electrical resistivity was measured by a non-contacting electrical resistivity apparatus. The hyperbolic curve of electrical resistivity versus time was plotted to determine the ultimate electrical resistivity. The results show that the addition of SP to the pastes with a fixed w/c can cause longer setting time and delay the evolution of electrical resistivity. The final setting time (t _f) and the occurring time of maximum rate of electrical resistivity (t _r) were both delayed when the dosage of SP was increased. This may indicates that the electrical resistivity measurement can be used to monitor the setting process of cement. The compressive strength at 28 days and the ultimate electrical resistivity show a same tendency for the cement pastes with different dosages of SP. Thus, it would be possible to predict the compressive strength of hardened cement paste by its ultimate electrical resistivity.     

The early-age short-term creep of hardening cement paste: AC impedance modeling

Impedance spectra were monitored at early ages on hydrating Portland cement pastes subjected to a sustained load. The pastes were prepared with two different water-cement ratios (0.35 and 0.50). The experiments were conducted in a controlled chamber maintained at (96±2)% relative humidity. The three ages at loading investigated were 18, 24 and 30 hrs. Real-time changes in paste microstructure due to sustained load were followed through the coupling of an AC impedance frequency analyzer with a miniature loading system. Cement paste specimens were in the form of T-shaped columns with a minimum thickness value (for the web and flanges) less than 1.25 mm. The impedance analysis included an assessment of the relevance of the high frequency arc depression angle to an understanding of the creep and shrinkage behavior of cement paste. Electrical models were developed in order to predict the creep coefficient of normal (w/c = 0.50) and high strength (w/c = 0.35) cement pastes from early age data.     

Coupled AC impedance and thermomechanical analysis of freezing phenomena in cement paste

Freezing and thawing properties of cement paste were studied using an alternating current impedance spectroscopy (ACIS) technique coupled with the thermomechanical analysis (TMA). The measurements from the ACIS were analyzed in association with the length change of the cement paste determined by TMA. The micro-scale behavior of cement paste observed by the ACIS technique was well correlated with the residual expansion of cement paste after the freezing and thawing cycle. The efficacy of the ACIS technique for assessing the durability of cement paste to frost action was confirmed.     

High-pressure electrical properties of CdS studied by impedance spectroscopy

The electrical properties of cadmium sulfide have been studied by impedance spectroscopy at room temperature, pressures from 20 to 50 GPa, and frequencies from 50 Hz to 100 kHz. The anomalies in impedance observed at ～30–35 GPa correlate with earlier thermopower and conductivity data.     

A coupled AC impedance—creep and shrinkage investigation of hardened cement paste

The effect of three pre-drying treatments (prior to resaturation with synthetic pore fluid) on time dependent deformation was investigated. The pre-drying treaments comprised methanol and isopropanol exchange (followed by vacuum heating at 37°C) and vacuum drying at 37°C alone. Real-time changes in microstructure due to sustained load were followed through the coupling of an A.C. impedance frequency analyzer with a miniature loading system. Cement paste specimens were in the form of T-shaped columns with minimum thickness value around 1 mm. The impedance analysis included an assessment of the relevance of the high frequency arc depression angle to creep and shrinkage behavior of cement paste.

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Résumé L’effet de trois traitements de pré-séchage (suivi d’une resaturation avec une solution interstitielle synthétique) sur les déformations différées a été investigué. Les traitements de pré-séchage consistent en un séchage sous vide à 37°C d’éprouvettes avant et après immersion dans un solvant (méthanol ou isopropanol). Les modifications causées par la charge appliquée sur la microstructure étaient suivies en temps réel par le biais du couplage d’un analyseur de fréquences d’impédance avec un système de chargement miniature. Les éprouvettes de pate de ciment étaient de petites colonnes en forme de T ayant une épaisseur minimale d’environ 1 mm. L’analyse des spectres d’impédance inclue une évaluation de la pertinence de l’angle de dépression sur le fluage et le retrait de la pate de ciment.

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Editorial Note Laval University (Canada) is a RILEM Titular Member. Prof. J. Marchand was awarded the 2000 Robert L’Hermite Medal. He is Editor in Chief forConcrete Science and Engineering and Associate Editor forMaterials and Structures. Jacques Marchand participates in RILEM TC 186-ISA ‘Internal Sulfate attack’.     

Near-infrared spectroscopy investigation on the hydration degree of a cement paste

Near-infrared spectroscopy is a fast and easy-to-perform technique characterized by high sensitivity regarding water containing systems and for this reason it is a suitable tool for investigating structural modifications of hydrating cementing materials, even if the lack of knowledge in this field makes the interpretation of NIR vibrational bands very difficult. In this paper, the NIR spectrum of a hydrated ordinary Portland cement is extensively investigated and an interpretation of the different bands is proposed on the basis of both experimental evidence and reference to literature. The obtained results were applied to the investigation of cement hydration lasting up to 28 days, analyzing the variations detectable in the shape of the spectrum as the reaction went on. The degree of this variation, quantified through area calculations of the main significant bands, revealed a broad agreement with the degree of hydration of the paste measured by thermal methods. The findings of this research supply a solid foundation for future in-field application of NIR spectroscopy, for example, for investigations on cements with different hydration behavior, or to evaluate the effect of additives on cement pastes and, at a further level, for cement quality control.     

Heat deformations of cement paste phases and the microstructure of cement paste

The paper presents the results of an experimental investigation of the influence of thermal deformations of phases present in hardened cement paste on its microstructure. For the observation of the thermal deformation course in the range of temperature20–800°C, a method of complex thermal analysis (DTA, DTD, TD) was employed. The microstructure of heated cement pastes was observed by both optic and electron microscopes. From an analysis of the results of the investigation it has been found that chemical processes stimulated by temperature in particular phases of cement paste have a significant influence on the thermal deformation course. It is shown that micracracks caused by different thermal deformations, appear first in the aregas of Ca (OH)₂ concentration (ca.300°C), and next (ca.400°C) as well as in the areas of occurrence of unhydrated large clinker grains.

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Résumé On rend compte d'une étude expérimentale sur les causes de la destruction de la microstructure de la pate de ciment portland exposée à la chaleur dans une gamme de températures allant de20 à800°C. On a prêté une attention particulière aux déformations thermiques de chacune des phases reconnues de la pate de ciment durci. On a utilisé une méthode d'analyse thermique complexe (DTA, TD, DTD) pour l'étude de l'évolution de déformations thermiques. On a observé les microstructures de la pate de ciment chauffée au microscope optique et électronique. On a étudié les phases suivantes de la pate de ciment: clinkers (C₃S, β-C₂S, C₃A, C₄AF), minéraux hydratés de ciment (hydrates C₃S, β-C₂S, C₃A, C₄AF), et produits d'hydratation [Ca(OH)₂, ettringite]. Les résultats de l'étude ont permis à l'auteur de déterminer l'allure de la déformation thermique des phases que présente la pate de ciment durcie sous températures croissantes. On a constaté que les processus chimiques que l'élévation de température induit dans chacune des phases de la pate de ciment exercent une influence notable sur l'évolution des déformations thermiques. Les microfissures causées par différentes déformations thermiques des phases apparaissent dans les zones de concentralisation de Ca(OH)₂ à environ300 et400°C aussi bien que dans les zones où se rencontrent des grains de clinker non hydraté.

     

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.

     

Microwave compressive strength estimation of cement paste using monopole probes

In-situ evaluation of the compressive strength of existing concrete structures using a direct approach is the objective of this investigation. The principle factor affecting the compressive strength of a concrete structure is its water/cement (w/c) ratio. The reflection properties of four cement paste samples with w/c ratios of 0.35, 0.40, 0.50, and 0.55 were measured using several monopole probes at microwave frequencies of 5, 9, and 12 GHz. The effect of several parameters, such as the operating frequency, the probe length (h), and the properties of the cement paste (w/c ratio) on the measurement sensitivity, were studied theoretically as well. For a given monopole probe (fixed diameter),h and the operating frequency can be optimized such that any slight change in the w/c ratio causes a large change in the reflection coefficient. Lower microwave frequencies are shown to be more sensitive in detecting variation in the reflection coefficient of these samples as a function of the w/c ratio. After the reflection coefficient measurements for these samples were conducted, they were tested to measure their compressive strength. Subsequently, a correlation between the compressive strength and the reflection coefficient of these samples was shown. The effect of an air gap around the monopole was investigated as well.     

Real-time monitoring primary cardiomyocyte adhesion based on electrochemical impedance spectroscopy and electrical cell-substrate impedance sensing

The cell-substrate distance is a direct indicator of cell adhesion to extracellular matrix which is indispensable in cell culture. A real-time monitoring approach can provide a detailed profile of cell adhesion, so that enables the detecting of adhesion-related cell behavior. In this work, we report a novel real-time impedance-based method to record the adhesion profile of cardiomyocyte, overcoming its inscrutability due to the primary culture. Microfabricated biosensors are applied in cardiomyocyte culture after characterizing the cell-free system. Cyclic frequency scanning data of cell-related impedance are generated and automatically fit into the equivalent circuit model, which is established using electrochemical impedance spectroscopy. The data are displayed as the alteration of normalized cell-substrate distance and the essential parameters for manual electric cell-substrate impedance sensing calibration of absolute distance. The time course displays a significant decline in the equivalent cell-substrate distance, from 155.8 to 60.2 nm in the first 20 h of cardiomyocyte culture. Furthermore, the cardiomyocytes cultured in long-term medium and short-term medium (ACCT) for 10 h exhibit distinct difference in adhesion rate as well as cell-substrate distance (72 vs 68 nm).     

Validity of using methanol for studying the microstructure of cement paste

Experiments were conducted to assess the extent of interaction of methanol with calcium hydroxide at 22°C. The following techniques were used: X-ray diffraction, infrared spectroscopy, surface-area determination, length change, calorimetry, differential thermal analysis and thermogravimetric analysis. It was concluded that either calcium methoxide or a methylated complex is formed when calcium hydroxide is mixed with methanol.

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Resume On a exécuté des essais afin de déterminer l'importance de l'interaction du méthanol et de l'hydroxide de calcium à 22°C. On a utilisé les techniques suivantes: diffraction X, spectroscopie infrarouge, détermination de l'aire superficielle, variation de longueur, analyse thermique différentielle et thermogravimétrie. Les résultats de ces essais ont mis en évidence la formation d'un produit nouveau. On a observé la présence de méthylate de calcium dans le diagramme X d'hydroxide de calcium traité au méthanol. Pour ce système, les pics IR correspondaient aussi à ceux qui se trouvaient dans le spectre IR du méthylate de calcium. On a observé des augmentations importantes de l'aire superficielle mesurée par absorption d'azote et la variation de longueur d'échantillons d'hydroxide de calcium après traitement au méthanol. En outre, on a observé une interaction chimique entre le méthanol et l'hydroxide de calcium mise en évidence par les courbes évolution thermique en fonction du temps obtenues par calorimétrie différentielle. Des pics exothermiques de 330 et 450°C dans les courbes ATD d'hydroxide de calcium traité au méthanol indiquait la formation d'un nouveau composé. Les données ATG ont montré une augmentation en rapport avec le temps de la perte de poids (600 à 800°C) correspondant à la décomposition du carbonate de calcium qui résulte de la formation et de la décomposition du méthylate de calcium. Il faut considérer les implications de l'interaction méthanol-hydroxide de calcium dans le développement des méthodes d'essai pour le béton qui utilisent le méthanol.