Electrically induced chloride ion transport in alkali activated slag concretes and the influence of microstructure

Chloride ion transport in alkali silicate powder and liquid activated slag concretes, and the influence of the material microstructure are discussed. Increasing the Na₂O-to-slag ratio (n) results in a reduction in the rapid chloride permeability (RCP) and non-steady state migration (NSSM) coefficients (D_nssm) of solid sodium silicate activated slag concretes. Increasing the SiO₂-to-Na₂O ratio (M_s) of the activator beneficially influences the transport parameters of liquid sodium silicate activated concretes. The chloride transport parameters are related to the reaction product microstructure and composition evaluated using mercury intrusion porosimetry (MIP) and Fourier Transform Infrared (FTIR) spectroscopy respectively. A reduction in the critical pore sizes reduces the transport coefficients to a larger extent than porosity reduction, which is responsible for the better performance of solid sodium silicate activated concretes. The total chloride concentration determined from the charge passed and the ionic transference numbers is used to predict an apparent non-steady state transport coefficient.     

Characterizing Enhanced Porosity Concrete using electrical impedance to predict acoustic and hydraulic performance

Enhanced Porosity Concrete (EPC) is manufactured by gap grading coarse aggregates to create interconnected porosity in the system. The porosity and physical features of the pore network are characterized in this paper using Electrical Impedance Spectroscopy (EIS). Porosity alone was found to be an inaccurate indicator of the electrical conductivity of the sample. While several studies have shown that a conventional form of Archie's law can describe porous systems, it was observed that Archie's law did not completely describe the electrical conductivity of the EPC system. Therefore, a modified version of Archie's law was used that incorporated the matrix conductivity, which described the system more accurately than the conventional form. The pore connectivity factor determined using EIS is found to be linearly related to the acoustic absorption of the material. Similarly, conductivity results determined from EIS were used with total porosity to compute the hydraulic connectivity factor. This factor was related to intrinsic permeability calculated from hydraulic conductivity (measured using a falling head permeameter). Based on these studies, it appears that a single electrical impedance test could provide information for the design, quality control/quality assurance, and utilization of EPC.     

Microstructural analysis of plain and reinforced mortars under chloride-induced deterioration

This paper reports the results of microstructural analysis of plain and steel-reinforced mortar specimens deteriorated by chlorides that were admixed or introduced through chloride ingress. The electrical properties of mortars were measured and their microstructural characteristics were investigated using quantitative image analysis techniques. The influence of chloride ions on mortar microstructure are discussed in terms of hydration and corrosion products. The research reveals that chlorides will induce changes in the chemical compositions and morphology of cement hydration products, and thereby exert influence on ion transport in the mortar specimens. The electrical properties of plain and reinforced mortars are not only related to the presence of chlorides in the pore system but are also influenced by the pore structure characteristics. The cementitious matrix undergoes certain alterations in conditions of the combined effects of: cement hydration, chloride ion transport and chemical binding mechanisms. To this end the pore structure characteristics appeared to be a significantly contributing factor in the process of chloride-induced corrosion in reinforced cement-based materials.     

Development of an Equivalent Circuit Model for Cement Pastes from Microstructural Considerations

The general approach to equivalent circuit modeling of cement pastes has been to interpret spectra obtained from ac impedance measurements, assigning electrical responses to microstructural features such as hydration products and fluid-filled porosity. In this paper, we take an alternative approach. Starting with a consideration of paste microstructures and the potential conduction pathways that such microstructures could support, we propose an equivalent circuit model. Ac impedance data from pore-reduced cements have been measured for the first time and are used to test the model. This approach has permitted estimation of conductivities of in situ pore fluid, which we believe to be influenced by hydrogen bonding to varying extents depending on pore size, and hydration product. Preliminary results are in broad agreement with data obtained by alternative methods.     

Extracting the performance predictors of Enhanced Porosity Concretes from electrical conductivity spectra

This paper discusses the application of an analytical model of the dielectric spectrum of Enhanced Porosity Concretes (EPC, or pervious concretes) to predict its porosity and effective conductivity. While porosity is the dominant material parameter that dictates the performance characteristics (acoustic absorption and hydraulic conductivity) of EPC, effective conductivity has been shown to be an important parameter in the non-destructive estimation of performance. The porosities of the EPC specimens were measured using a volumetric method and an image analysis method. The effective conductivities were calculated from the bulk resistances of the samples obtained from Nyquist plots. The fits of the model to the conductivity spectra were used to extract the values of porosity and effective conductivity. The measured and predicted values of porosities and effective conductivities show good correlation. The frequency dependence of the dielectric spectra as well as the advantages of using a conductivity spectrum as opposed to a dielectric constant spectrum is outlined. The need to use effective conductivity rather than conductivity at any frequency is also brought out. It is shown that a single electrical conductivity spectrum could be used to predict the parameters that best describe the performance characteristics of EPC.     

矿物掺合料对混凝土中氯离子渗透性的影响

采用可蒸发水含量法、氯离子渗透快速实验法,研究了粉煤灰、硅灰、粉煤灰与硅灰复合掺入及不同龄期等条件制备的混凝土的孔结构、结合氯离子性能及渗透性的变化规律,探讨了掺粉煤灰、硅灰混凝土的孔结构、结合氯离子性能对其氯离子渗透性的影响.结果表明:粉煤灰、硅灰对混凝土的孔结构、结合氯离子性能及氯离子渗透性均存在不同程度的影响.对于掺粉煤灰、硅灰的混凝土,在胶凝材料水化前期,主要是混凝土的孔结构变化引起其6h库仑电量下降;而在胶凝材料水化中后期,主要是混凝土孔结构变化与混凝土对氯离子的结合共同作用导致其6h库仑电量降低.混凝土的孔结构改善及其对氯离子的结合是导致混凝土中氯离子渗透性降低的重要原因.     

Three‐dimensional pore structure and ion conductivity of porous ceramic diaphragms

The ion conductivity of two series of porous ceramic diaphragms impregnated with caustic potash was investigated by electrochemical impedance spectroscopy. To understand the impact of the pore structure on ion conductivity, the three‐dimensional (3‐D) pore geometry of the diaphragms was characterized with synchrotron x‐ray absorption tomography. Ion migration was calculated based on an extended pore structure model, which includes the electrolyte conductivity and geometric pore parameters, for example, tortuosity (τ) and constriction factor (β), but no fitting parameters. The calculated ion conductivities are in agreement with the data obtained from electrochemical measurements on the diaphragms. The geometric tortuosity was found to be nearly independent of porosity. Pore path constrictions diminish with increasing porosity. The lower constrictivity provides more pore space that can effectively be used for mass transport. Direct measurements from tomographs of tortuosity and constrictivity opens new possibilities to study pore structures and transport properties of porous materials.     

Transport properties study of supplementary cementitious materials: case of tuff,limestone filler and granodiorite

The aim of this research is to determine the effects of replacing cement with tuffs on the transport properties and service life of cement-based materials. Specimens were prepared replacing 5% and 15% by weight of cement with both white and brown tuffs. Additional specimens were prepared replacing cement with 5% and 15% limestone filler and granodiorite to compare the effects of these types of materials. For these four types of specimen as well as a control mortar with no cement replacement transport properties (gas permeability and chloride ingress) were measured as well as porosity and pore diameter. The transport properties were compared the changes in these porosity and pore diameter. Based on the results, white tuff significantly improves the transport properties of the mortars due to its pozzolanic reaction and that white tuff may be an economically and functionally viable replacement for cement where concrete durability is important. Pore diameter and porosity did not correlate well with transport properties.     

Effect of binder content on the performance of alkali-activated slag concretes

This paper assesses the mechanical and durability performance of concretes produced using alkali silicate-activated ground granulated blast furnace slag as sole binder. Alkali-activated concretes are formulated with 300, 400 and 500 kg slag per m³ of fresh concrete, and their performance is compared with reference concretes produced using Portland cement (OPCC). Regardless of the binder content, the alkali-activated slag concretes (AASC) develop higher compressive strength than the comparable reference concretes. A higher binder content leads to increased strength in both AASC and OPCC at 28 days. However, at 90 days, the performance penalty for low binder content is more significant in the OPCC than AASC samples. Permeability, water sorption and carbonation resistance properties are also improved at higher binder contents. By controlling mix design parameters, it is possible to produce AASC with mechanical strength and durability comparable to conventional Portland cement concretes.     

Effect of mixing proportions of concrete on its electrical conductivity and the rapid chloride permeability test (ASTM C1202 or ASSHTO T277) results

The rapid chloride permeability test (RCPT)—American Society of Testing and Materials (ASTM) test method C1202 or American Association of States Highway and Transportation Officials (AASHTO) test method T277—is virtually a measurement of electrical conductivity of concrete, which depends on both the pore structure characteristics and pore solution chemistry of concrete. This paper discusses the effects of several factors, such as cement composition, replacement of cement with supplementary cementing materials and inclusion of aggregate, on the electrical conductivity or RCPT results of hardened cement mortars and concrete. Analyses based on published results have indicated that all the three factors may have significant effects on the chemistry and specific conductivity of concrete pore solution, which has little to do with the transport of ions in the solution. Thus, RCPT is not a valid test for evaluation of permeability of concretes made with different materials or different proportions.     

The patch microstructure in concrete: The effect of superplasticizer

It has been previously shown that laboratory- and field-mixed concretes exhibit dense areas or patches of hardened cement paste (hcp) which are sharply delineated from adjacent, highly porous areas. Direct experiment with long-continued concrete mixing showed that this microstructural pattern is not due to inadequate mixing. An experiment was conducted to determine whether this distinctive microstructure was associated with the flocculation inherent in most fresh concretes. A conventional laboratory concrete was batched and after preliminary mixing a polycarboxylate-type superplasticizer was incorporated. The superplasticizer was added in two successive stages, a modest dose causing some increase in slump, and then a heavy additional dose sufficient to cause complete collapse of slump. SEM examination indicated that the patchy microstructure existed in the ‘base’ concrete, and was retained in the superplasticized concretes as well. Thus the patchy microstructural pattern is not generated as a consequence of the flocculated state that exists in most fresh concretes.     

Capillary porosity depercolation in cement-based materials: Measurement techniques and factors which influence their interpretation

The connectivity of the capillary porosity in cement-based materials impacts fluid-and-ion transport and thus material durability, the interpretation of experimental measurements such as chemical shrinkage, and the timing and duration of curing operations. While several methods have been used to assess the connectivity of the capillary pores, the interpretation of some experimental procedures can be complicated by the addition of certain chemical admixtures. This paper assesses capillary porosity depercolation in cement pastes using measurements of chemical shrinkage, low temperature calorimetry (LTC), and electrical impedance spectroscopy. The experimental results are analyzed to identify the time of capillary porosity depercolation. In addition, the factors that influence the interpretation of each technique are discussed. Experimental evidence suggests that capillary porosity depercolation, as defined by Powers, occurs after hydration has reduced the capillary porosity to around 20% in cement paste systems. The influence of capillary porosity depercolation on the transport properties is demonstrated in terms of a reduction in the electrical conductivity of the cementitious material. Special attention is paid to understand and interpret the influence of shrinkage-reducing admixtures (SRAs) on the freezing behavior of cementitious systems, particularly in regard to the inapplicability of using LTC to detect porosity depercolation in cement pastes containing such organic admixtures.     

Influence of porosity on the electrical properties of La9.33(SiO4)6O2 oxyapatite

Oxyapatites are very promising materials in terms of ionic conductivity. They can be considered as a potential electrolyte for fuel cells as SOFC. The influence of porosity on the electrical properties of La_9.33(SiO₄)₆O₂ oxyapatite is reported here. Hot pressed pellets with various densification ratios have been characterized by the complex impedance method. The high frequency response associated with the bulk contribution is much more affected by the porosity than the grain boundaries contribution: as a consequence, the electrical behaviour of the samples has been considered in assimilating the porous ceramics to composite materials made of apatite with various amounts of air inclusions. Thus, the porosity dependence of conductivity, activation energy and permittivity are reported here. A percolation threshold has been highlighted for porosity values greater than 30%, involving great lowering of the electrical performances.     

混凝土孔结构及其分形维数与氯离子扩散性能的关系

氯离子扩散系数是研究海洋环境下混凝土结构耐久性的重要参数之一。通过开展不同水胶比混凝土的压汞试验和盐雾扩散试验,研究了混凝土内部孔隙率、孔径分布及临界孔径对氯离子扩散系数的影响规律。结合Menger海绵体模型,建立孔体积分形维数与氯离子扩散系数的关系。结果表明:孔隙率和临界孔径与无量纲化氯离子扩散系数的相关性很高,可作为反映混凝土氯离子扩散性能的重要参数;通过数学分析计算得到的孔表面分形维数分布在2.56~3.86之间,孔体积分形维数分布在2.85~2.98之间;基于压汞法和分形理论计算得到的孔体积分形维数可以作为评价氯离子扩散系数的指标,在孔径小于10 nm、10～100 nm、100～1 000 nm以及大于1 000 nm四类区间,氯离子扩散系数随孔体积分形维数的增加而下降。     

Characterization of the pore labyrinth geometry of sea sponges imaged by micro-CT

The internal pore labyrinths of sea sponges were characterized via the analysis of 3D micro-CT images. Methods were developed to isolate and segment the pores and to extract the pore ‘skeleton’, which facilitated the measurement of local pore dimensions and connectivity. These methods were also used to characterize the bulk pore properties such as porosity and structural surface-to-volume ratios, as well as individual pathway analysis in terms of lengths, diameters, and tortuosity. Also, the role that ciliated cells (lining the pores) might play in the transport of fluids throughout the pore labyrinth was explored. It was deduced that cilia may play a larger role in the transport of fluids through smaller diameter pathways and the highly interconnected pathways of the sponge results in a robust network that can maintain nutrient delivery/waste removal in the case of obstruction of some of the pore pathways. Finally, it is discussed how the information gained from this study might be applied to design synthetic porous tissue scaffolds.     

Cellular zirconia ceramics processed by direct emulsification

Highly porous zirconia ceramics for prospective use as separators in alkaline electrolysis cells have been processed by emulsification of paraffin in concentrated zirconia suspensions. Effects of processing parameters on porosity, cell size distribution, and pore interconnectivity are studied through Taguchi statistical design. The porosity of zirconia ceramics has shown a strong correlation with paraffin-to-suspension ratio employed in emulsification. High paraffin-to-suspension ratios combined with high gelatin content in the emulsion are beneficial for creating highly porous cellular materials with interconnected cell sizes. The attained microstructural features allow high percolation of ionic species in these highly porous ceramics after impregnation with aqueous electrolytes, facilitating charge transport within the liquid medium inside the ceramic bodies, as required for lower ohmic losses. This work provides guidelines for developing efficient ceramic separators for future use in alkaline electrolysis cells and other applications requiring thermally and chemically-stable ceramics with high and well-interconnected porosity.     

Micromechanics analysis of thermal expansion and thermal pressurization of a hardened cement paste

The results of a macro-scale experimental study of the effect of heating on a fluid-saturated hardened cement paste are analysed using a multi-scale homogenization model. The analysis of the experimental results revealed that the thermal expansion coefficient of the cement paste pore fluid is anomalously higher than the one of pure bulk water. The micromechanics model is calibrated using the results of drained and undrained heating tests and permits the extrapolation of the experimentally evaluated thermal expansion and thermal pressurization parameters to cement pastes with different water-to-cement ratios. It permits also to calculate the pore volume thermal expansion coefficient α_? which is difficult to evaluate experimentally. The anomalous pore fluid thermal expansion is also analysed using the micromechanics model.     

An investigation into the influence of inter-aggregate spacing and the extent of the ITZ on properties of Portland cement concretes

Conventionally-designed concretes were prepared with different sand particle size distributions, so as to systematically vary the extent of aggregate-cement paste interface and the mean spacing between sand grains. The range of fineness modulus of the sands fully encompassed the range of sands normally used in concretes. The concretes were batched at w/c ratios of 0.30 and 0.50 and cured for various periods before carrying out determinations of mechanical properties and of “rapid chloride permeability”. The conventional notions of the effect of the ITZ on concrete properties would predict that a reduction in strength and an increase in chloride permeability would accompany increased ITZ interfacial area and closer spacing between sand grains. In general, no such influence was found. It appears from this research that the traditional notions of the adverse influence of the ITZ on the properties of conventional concretes may not be accurate, within the realms of conventional concrete and typical inter-aggregate spacings.     

Evaluation of sub-micrometer parylene C films as an insulation layer using electrochemical impedance spectroscopy

We investigated the insulation performance of sub-micrometer parylene C films over time using electrochemical impedance spectroscopy (EIS). For this, interdigitated electrodes were fabricated and completely encapsulated with parylene C in thicknesses of 50, 100, 200, and 500 nm. The EIS was measured in phosphate buffered saline (PBS) solution under an accelerated aging condition at 90 °C over 45 days. To analyze the EIS data, the equivalent circuit models of coating at different stages of coating degradation were used and the lumped circuit parameters of the best fitted equivalent circuit model were extracted by curve fitting. The analysis of impedance using the equivalent circuit model and the FTIR measurements suggest that sub-micrometer parylene C coatings exhibited delamination resulting from water diffusion from the top surface as soon as being immersed in PBS solution, although the degree of delamination varied depending on the film thickness. The penetration of water through sub-micrometers thick parylene C films can occur as quickly as the film is in contact with solution, unlike for thicker coatings in several micrometers where water diffusion would be saturated before water reaches the bottom surface of the coating.     

Relationship between resistivity, diffusivity and microstructural descriptors for mortars with silica fume

Mortars with a sand-to-cement ratio of 3 and water-to-cement ratio of 0.5 were made with 0% and 10% silica fume (SF). Resistivities were measured with alternating current impedance spectroscopy (ACIS). Diffusivities were determined with the propan-2-ol counterdiffusion method. Microstructure was investigated with mercury intrusion porosimetry. It was found that there is a relationship relating hydration time to the product of resistivity and diffusivity. Furthermore, the product of resistivity and diffusivity was related to porosity, mean and threshold pore diameters. The influence of silica fume in refining the pore structure was marked.