Conductivity Of Pore Solution Research Articles

Chloride Permeability of Alkali-Activated Slag Concretes after Exposure to High Temperatures.

The number of fires in buildings and on bridges has increased worldwide in recent years. As a structural material, the strength of alkali-activated slag (AAS) concrete after exposure to high temperatures has been given much attention. However, research of its durability is still lacking, which limits the application of this type of concrete on a larger scale. In this context, as one of the most important aspects of durability, the chloride permeability of AAS concretes after exposure to high temperatures was examined in this study. The influence of the alkali concentration (Na2O%) and the modulus (Ms) of the activator, as well as the influence of heating regimes, including the heating rate, duration of exposure to the target temperature, and cooling method, was also discussed. The results show that the chloride permeability of the AAS concretes increased with temperature elevation. Due to the interference of pore solution conductivity, the influence of the Na2O% and the Ms of the activator on the chloride permeability of the AAS concretes was not made clear by using the ASTM C 1202 charge passed method; however, after exposure to high temperatures, AAS with a lower Na2O% and lower Ms has lower porosity and may have lower chloride permeability, which needs further investigation. Faster heating for a longer duration at the target temperature and water cooling reduced the resistance of the AAS concretes to chloride permeability as a result of their increased porosity.

Durability, workability, and setting time of cementitious systems containing chloride-rich oil and gas production wastewater

The application of alternative non-potable water sources can be leveraged to build infrastructure components that support sustainable industrial operations in regions that experience high frequency of freshwater shortages. This study examines how the use of highly saline produced water (as mixing water) in portland cement mortar impacts workability, setting time, electrical resistivity, pore formation, and microstructure. The results show that produced water decreases both mortars setting time by 30% and workability by 20%. Alkali solubility and pore solution conductivity are also influenced by the produced water-increasing pore solution conductivity by approximately 100 mS/cm. The produced water reacts with the cementitious material to form small deposits of Friedel's salts and calcite that primarily fill the smaller pores in the matrix-thus primarily influencing the pore structure of low w/c mortar systems. In the low water/cement (w/c) produced water mortar systems (0.4), both the pore formation factor (∼2X) and resistivity (∼1.5X) are significantly higher than the control (tap water mortar systems), but in the high w/c systems (0.6) both are slightly lower. The produced water mortar systems also tend to retain more moisture during dry cycling which leads to significantly lower resistivity than the control in air-dried conditions. Diluting the produced water with tap water will reduce moisture retention during dry cycling and increase resistivity.

Preparation and mechanism analysis of nanocrystalline cellulose-tricalcium silicate paste composite with high electromagnetic transmission performance

The tricalcium silicate (C3S) paste with high electromagnetic transmission performance is prepared by addition of nanocrystalline cellulose (NCC), and the enhancement mechanism of electromagnetic transmission performance is revealed. The water to cement ratio (W/C) and the contents of NCC are 0.5 and 1 wt%-4wt%, respectively. The results show that the electromagnetic transmissivity of the C3S paste improves with an increase in NCC content. At the frequency of 3.94 GHz, when the NCC contents are 3 wt% and 4 wt%, the electromagnetic transmission coefficient increases by 113.85% and 153.6%, respectively. The enhancement mechanisms of electromagnetic transmission performance of NCC-C3S pastes are as follows: (1) the polymerization degree of C–S–H and the Van der Waals force between the globules increase; (2) the total volume proportions of low electromagnetic loss C3S and Ca(OH)2 increase; (3) the conductivity of C3S pore solution and the volume proportions of water decrease; (4) the pore volume proportion of the non-harmful pores for electromagnetic energy loss increases.

Experimental study and mechanism analysis of functional nanocrystalline cellulose to improve the electromagnetic transmission performance of ordinary Portland cement

5G signal transmission can be seriously blocked by cement-based materials, so it is very important to improve the electromagnetic transmission performance (ETP) of cement-based materials. Carboxylated nanocrystalline cellulose (NCC) is used to prepare Ordinary Portland cement paste with high ETP. The water-cement ratio and NCC content of the cement paste are 0.5 and 0–4% respectively. The test results show that the ETP of cement paste increases by 13.9%–90.6%, but the compressive strength of the sample decreases by 7.3%–87.6%. The 2% of NCC content causes the ETP of cement paste increases by 75.6%, while the compressive strength only decreases by 14.6%. The reasons for improvement of ETP are that the electrostatic force between the globule, the polymerization degree and mean chain length of C–S–H, the total volume proportion of phases (calcium hydroxide, ettringite and unhydrated cement) with low dielectric loss and the volume proportion of the no harmful pore for electromagnetic energy loss are increased, the conductivity of cement pore solution and the content of water are decreased.

Comparison on early hydration of Portland cement and sulphoaluminate cement in the presence of nano ettringite

This paper proposed adopting ettringite-based nucleation seeds to induce ettringite to precipitate in the pore solution, and thus facilitated early hydration reaction. In present work, sulphoaluminate cement (SAC) and Portland cement (PC) paste were considered, and 0 ∼ 5 % nano-ettringite prepared by hydration-wet grinding method was extra incorporated. The hydration kinetic of the blends was studied by hydration heat, chemical shrinkage and setting time. The effects of nano-ettringite on the dissolution behavior were also examined by dynamic conductivity of cement slurry, ion concentration and conductivity of pore solution. Meanwhile, the hydrate assemblage was determined by XRD, TGA, FTIR and 27Al NMR, and the compressive strength was also tested. On this basis, the hydration mechanism was discussed. Results indicated that nano-ettringite significantly hastened ions dissolution and ettringite precipitation, whether it was SAC or PC paste. The initial hydration heat release and chemical shrinkage were obviously promoted, and even cracks were generated in SAC paste; besides, the setting time was also shortened dramatically. In the presence of 1 ∼ 5 % nano-ettringite, the compressive strength of PC pastes at 8 h was increased by 54 ∼ 180 %. However, the strength of SAC paste at all ages was degraded by adding nano-ettringite, due to the appearance of cracks and reduced amount of AH3.

Real-time monitoring the electrical properties of pastes to map the hydration induced microstructure change in cement-based materials

The effect of the supplementary materials (SCMs) on the moisture content and ion diffusivity at different hydration time is important for the service life modelling of modern concrete. This study designed a simple but valid method to monitor the microstructure change in pastes during hydration. A procedure easy to implement was proposed to detect the water content in pastes. The electrical conductivity of pore solution was evaluated by the evaporable water content in pastes and chemical composition in the binders. Results show that the electrical properties of pastes (conductivity, formation factor and its growth rate) can effectively indicate the hydration reactivity of binder, pore connectivity and volume of pore solution in the hardened pastes. The effect of water-binder ratio and SCMs on the structure of pastes are effectively indexed by the formation factor which is the conductivity of pore solution divided by that of paste. The inflection point of average growth rate of formation factor is a good index for the final setting of pastes. The relation between volume of evaporable water and formation factor is well demonstrated by the extended percolation theory. The real-time monitored electrical conductivity and formation factor of pastes can be used to calculate the chloride migration coefficient in hardened cement pastes.

Mechanisms underlying the relationship between electrical resistivity and corrosion rate of steel in mortars

The linear relationship between electrical resistivity of concrete and corrosion rate of steel in logarithmic coordinates (referred as R-C relationship) has been widely reported. However, R-C relationships are not universal but highly dependent on binder chemistry and corrosion environment, and there has been no consensus on the reasons behind the discrepancy of R-C relationships reported in the literature. In this work, the R-C relationships of alkali-activated slag (AAS) mortars are investigated and compared with that of reference ordinary Portland cement (OPC) mortars, aiming at uncovering the fundamental compositional parameters that cause differences in R-C relationships of various cementitious systems. The results show that the hydroxide ion concentration (pH value) of pore solution has a significant and decisive effect on the R-C relationship. With the decrease of pH, the material resistivity increases because of decreased pore solution conductivity while corrosion rate of steel increases due to the drop of [Cl−]/[OH−], leading to a shift of R-C relationship lines. A new mechanism is proposed to explain the differences in the R-C relationship caused by binder chemistry and corrosion type (chloride- versus carbonation-induced corrosion), which is further verified using the published experimental results in the literature.

Experimental Study of Slag Changes during the Very Early Stages of Its Alkaline Activation.

The very early stages of alkaline activation of slag control its rheology and setting, but also affect its hydration, which occurs later. Simultaneously, these parameters are dictated by the nature and dose of the alkaline activator. Therefore, we investigated and compared the changes in slag particles (SEM, BET, laser diffraction), as well as in the pore solution composition (ICP–OES), pH, and conductivity, of alkali-activated slag (AAS) pastes containing the three most common sodium activators (waterglass, hydroxide, and carbonate) and water during the first 24 h of its activation. To ensure the best possible comparability of the pastes, a fairly nontraditional mixture design was adopted, based on the same concentration of Na+ (4 mol/dm3) and the same volume fraction of slag in the paste (0.50). The results were correlated with the pastes’ hydration kinetics (isothermal calorimetry), structural build-up (oscillatory rheology), and setting times (Vicat). Great differences were observed in most of these properties, in the formation of hydration products, and in the composition of the pore solution for each activator. The results emphasize the role of the anionic groups in the activators and of the pH, which help predict the sample’s behavior based on its calorimetric curve, and offer data for further comparisons and for the modelling of AAS hydration for specific activators.

Electrical resistivity of partially-saturated alkali-activated slag containing sodium nitrite admixture

Electrical resistivity is a key durability indicator for evaluating the mass transport properties of cementitious materials particularly in partially-saturated states. In this work, the electrical resistivity of alkali-activated slag (AAS) with and without incorporation of sodium nitrite admixture, over a broad range of relative humidity (RH), is analyzed and compared to ordinary Portland cement (OPC) pastes. The van Genuchten equation and Archie equation are used to model the dependency of electrical resistivity of AAS on RH, degree of saturation, and pore structure parameters. The results show that at relatively high RH conditions (>58%), the electrical conductivity of AAS is significantly affected by the pore structure and conductivity of pore solution. At relatively low RH (<58%) conditions, the electrical conductivity of AAS is dominantly governed by the amount of adsorption water of reacted phases. Besides, the incorporation of sodium nitrite has multiple effects on the resistivity of AAS and OPC pastes, including provision of extra conductive ions in pore solution, pore structure refinement, and improvement of the water-binding capacity of reacted phases.

Electrical resistivity and piezoresistivity of cement mortar containing ground granulated blast furnace slag

Ground granulated blast furnace slag (GGBFS) is a popular used supplementary cementitious material which can increase the resistivity of cement-based composites, contribute to the cement hydration, and dense the composite structure. The piezoresistivity of cement mortar containing GGBFS was studied in this paper. Three GGBFS/cement ratio, 1.2, 1.5, and 1.8, were designed to study the effect of GGBFS content on the piezoresistivity behaviour of cement mortar. Samples were measured before and after drying treatment, and three water/binder ratios were designed to study the influence of water content on the electrical properties. Furthermore, mechanical properties and microscopy analysis were conducted. Results showed that compared to pure cement mortar, samples containing GGBFS showed an improvement in piezoresistivity behaviour. Sample with GGBFS/cement ratio at 1.2 performed the largest fractional change in resistivity of 5.57, 9.23 and 16.73 with water/binder ratio at 0.3, 0.4 and 0.5, respectively, as the concentration of pore solution was higher for samples with GGBFS/cement ratio at 1.2 compared to pure cement mortar. However, with the increasing GGBFS/cement ratio, the piezoresistivity performance was adversely affected due to the reduced pore solution concentration and conductivity caused by the increasing assumption of free calcium hydroxide. Through the investigation, piezoresistivity performance of the cement-based composite was found to be related to the mobile ions transportation with the pore solution.

Studying the effect of alkali dosage on microstructure development of alkali-activated slag pastes by electrical impedance spectroscopy (EIS)

In this study, electrical impedance spectroscopy (EIS) is applied to investigate the effect of alkali dosage on the microstructural evolution of the alkali-activated slag (AAS) system during the curing process. Pore solution chemistry and pore structure of the paste samples were also determined to get auxiliary interpretation of the EIS results. It is shown by equivalent circuit analysis that the resistance attributed to continuous (R1) and discontinuous pores (R2) increases with curing age due to pore structure development. The value of R1 is more affected by pore solution chemistry while the value of R2 is mainly affected by pore structure. With the increase of alkali dosage, the value of R1 decreases while R2 gradually increases. The exponent n of the constant phase element (CPE), which relates to the capillary pores (n1), decreases with the increase of alkali dosage while the exponent relating to gel pores (n2) is independent of the alkali dosage of AAS paste. The bulk conductivity of AAS paste obtained from the bulk resistance gradually increases with the alkali dosage, partially because of the enhanced conductivity of pore solution. The improvement of microstructure at early age is mainly attributed to the decreased porosity, which is more affected by the pore connectivity with the increase of curing age. These results confirm that EIS is an effective technique to determine the hydration properties and pore structure features of AAS paste.

Characterisation of temporal variations of alkali-activated slag cement property using microstructure features and electrical responses

Relatively few studies currently exist concerning temporal variation of the overall performance of alkali-activated slag (AAS) cement; a topic essential for quality control in practice. This study uses microstructural characteristics and electrical responses to reflect performance variation of AAS cement. To achieve this, four key performance parameters were assessed, pore formation factor, chloride transport coefficient, permeability coefficient and compressive strength. Classical theories used for PC-based materials were applied to estimate the performance of AAS paste. It is found that the formation factor of AAS can be assessed by combining bulk conductivity and pore solution conductivity and the permeation properties of AAS cement cannot be reliably assessed by the Katz-Thomoson (KT) model, while the Millington-Quirk (MQ) model works better. To assess the relationship between compressive strength and permeable porosity, the Balshin and Ryshkevitch equations performed best in terms of describing the relationship between these two parameters. Meanwhile, AAS cement is more sensitive to the mix proportions and additional care is needed to ensure its quality. It is the first time to use electrical response to predict the chloride transport coefficients of AAS cement, which would provide a powerful tool for its quality control and service life prediction.

Electrical resistivity model for quantifying concrete chloride diffusion coefficient

The effects of chloride salts on concrete durability have been financially and logistically overwhelming. Current test methods for quantifying diffusivity of chlorides in concrete are time consuming and demanding. A comprehensive electrical resistivity model has been developed to quantify the chloride diffusion coefficient of concrete. The model employs the modified Nernst-Einstein equation along with an ionic concentration sub-model, a pore solution conductivity sub-model, and a binder hydration sub-model. The sub-models, which account for the concentrations of Na+, K+, Ca2+, and OH− ions in the pore solution derived from the binder degree of hydration, binder chemical composition, and alkalis binding to the cement hydration products, are included to accurately quantify the conductivity of the pore solution. Moreover, an experimental program was developed to verify and validate the accuracy and completeness of the electrical resistivity model. The experimental variables are volume fraction of coarse aggregate, water to binder ratio, total binder content, and silica fume (SF) and ground granulated blast furnace slag (GGBFS) as supplementary cementing materials (SCMs). The model results are found statistically equal to experimentally quantified chloride diffusion coefficient values and possess high accuracy and precision for a wide range of concrete mixture, binder composition and age.

May self-diffusion of ions computed from molecular dynamics explain the electrical conductivity of pore solutions in cement-based materials?

Understanding the physical origins of the electrical response of cement-based materials is crucial to enhance the capabilities of non-destructive techniques, especially those based on resistivity or electrochemical measurements deployed in durability assessment and monitoring study of concrete structures. In this article, we show that using the information on the composition-dependent dynamics of ions obtained from molecular dynamics simulations improves the estimates of the electrical conductivity of the pore solutions. The link between ion dynamics and electrical conductivity in aqueous solutions is discussed from the fundamentals of ionic transport at the molecular scale. Also, we quantify the variability of pore solution conductivity. For validation, modeling results are extensively compared to experimental measurements on various cement systems. We show that a dilution effect explains the w/c-dependency of the electrical conductivity of the pore solutions. Also, accounting for the age-dependency of ionic diffusion in the pore solution is crucial to capture the age-dependency of the electrical conductivity of the pore solutions. These results are significant because they allow the prediction of the conductivity for various compositions of interest that may be encountered in cement systems.

Use of sugarcane bagasse ash sand (SBAS) as corrosion retardant for reinforced Portland slag cement concrete

Despite their growing popularity in the construction industry, cement-based materials containing blast-furnace slag may be subject to various deteriorative processes including corrosion susceptibility owing to the reduced alkalinity of the cementitious matrix. Sugarcane bagasse ash sand (SBAS), the main agro-industrial waste in sugar and ethanol production, has the potential to enhance concrete durability. This study aims to evaluate the effect of SBAS as a fine aggregate on the corrosion of reinforced slag cement concrete. Mixes with different proportions were analysed in terms of mechanical characteristics, chloride penetration, electrolytic conductivity of pore solution, corrosion potential (by open circuit potential), and corrosion rate (by polarisation curves and Tafel plot technique), as well as the analysis of the corroded steel bars (by scanning electron microscopy technique). This paper argues the efficiency of SBAS in retarding corrosion up to an optimal content of approximately 30%. SBAS can refine micropores and act as an alkaline activator of slag cements, leading to increased compressive strength, reduced chloride penetration depth, and delayed corrosion initiation. Above 30%, the corrosion rate increases, which could be owing to the modified microstructure and higher electrolytic conductivity of SBAS-concrete pore solutions, that control the susceptibility of chloride attack on the steel bars.

Activation Energy of Conduction for Use in Temperature Corrections on Electrical Measurements of Concrete

Abstract The formation factor obtained through electrical resistivity measurements is becoming a popular method to determine transport properties of concrete. Resistivity measurements are dependent on multiple factors, including degree of saturation, pore solution conductivity, and temperature. The Arrhenius equation is used to correct electrical resistivity for temperature effects using an activation energy of conduction (Ea-cond). This parameter has been measured on a wide variety of materials, including pore solutions, pastes, mortars, and concretes (with a variety of saturation states). The reported values of Ea-cond typically range from 9 to 39 kJ/mol. This article examines the factors affecting Ea-cond in order to select an appropriate temperature correction. In this study, Ea-cond was determined from data measured on various concrete mixtures used in transportation infrastructure applications as well as extracted and simulated pore solutions. The Ea-cond of pore solutions remains relatively constant (an average value of 13.9 ± 1.5 kJ/mol) for typical pore solutions and was slightly lower than the Ea-cond of saturated specimens (an average value of 15.8 kJ/mol). It was found that Ea-cond increases as the degree of saturation of the specimen is reduced. Drying increases the ionic concentration of the fluid in the pores; however, this does not explain the changes in Ea-cond. The effects of drying were determined to be primarily due to a change in the volume of the conductive fluid film in the concrete and in the connectivity of the fluid-filled pores. While it is better to directly measure the Ea-cond of a concrete mixture, this is not always feasible or practical. In such cases, for pore solutions, a value of 13.9 kJ/mol can be used, and for saturated concretes, a value of 15.8 kJ/mol can be used. For concretes with a varying degree of saturation, the Ea-cond can be estimated using the developed equation.

Effect of mineral additions incorporated in concrete on thermodynamic and kinetic parameters of chloride-induced reinforcement corrosion

In this article, the beneficial effects of pozzolanic mineral additions incorporated in concrete under the attack of chlorides are evaluated. Three different mineral additions incorporated in binary combinations with Portland cement (silica fume, metakaolin and fly ash) were studied. By monitoring and analyzing thermodynamic and kinetic parameters, the corrosion performances of reinforced test specimens were evaluated over an extended period of chloride attack (44 weekly cycles of wetting in a solution of NaCl and air drying, equivalent to 308 days of attack). The electrochemical techniques (Ecorr, Rp and EIS)1Electrical resistivity (ER), corrosion potential (Ecorr), polarization resistance (Rp), electrochemical impedance spectroscopy (EIS), corrosion rate (icorr), control concrete (C), concrete with silica fume (SF), concrete with fly ash (FA), concrete with metakaolin (MK).1, added to the electrical resistivity technique (ER), are allowed to evaluate the protection capacity of the concretes under study, in relation to this strong attack on the reinforcement. They also allowed the production of subsidies capable of generating scientific discussion about the mechanisms of protection, imparted to concrete by mineral additions, in this specific aggressive medium rich in chlorides. The results showed that the used mineral additions produced significant increases in the resistivity of concretes, which is probably due to physical changes in the pore structure of cement paste, besides the improvement of paste-aggregate interface and the changes in the conductivity of pore solution. This higher resistivity certainly delayed the transport of chlorides and their attack on the reinforcement, seen by small changes in the values of Ecorr, Rp and icorr in concretes with pozzolans. The EIS results showed that there was little change in the impedance values at the steel-concrete interface in these systems with mineral additions, in spite of the strong and prolonged attack.

Unsaturated ion diffusion in cementitious materials – The effect of slag and silica fume

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.

Mechanical properties and rapid chloride permeability of carbonated concrete containing reactive MgO

Concrete containing reactive MgO can obtain superior mechanical properties to conventional concrete when subjected to accelerated carbonation curing. This study aims to evaluate the influence of r-MgO replacement levels and accelerated carbonation (CO2 concentration of 99%) curing on the mechanical properties and rapid chloride permeability of concrete containing reactive MgO. Concrete cylinders with r-MgO replacement levels of 10%, 30%, 50% 70% and 90% were studied. Experimental results reveal that carbonation curing improves the mechanical properties of reactive MgO concrete compared to uncarbonated specimens, especially during the first three days of carbonation exposure. Influence of accelerated carbonation on concrete specimens with different r-MgO replacement levels, yields the formation of carbonation products, which diminishes the pore volume and affects the rapid chloride permeability. The rapid chloride permeability of concrete is influenced by the decreases in the overall porosity, the conductivity of pore solution and the connectivity of pore structure due to carbonation.

Characterisation of pore structure development of alkali-activated slag cement during early hydration using electrical responses

This paper describes the results of a study investigating early age changes in pore structure of alkali-activated slag cement (AASC)-based paste. Capillary porosity, pore solution electrical conductivity and electrical resistivity of hardened paste samples were examined and the tortuosity determined using Archie's law. X-ray computed micro-tomography (X-ray μCT) and Scanning electron microscope (SEM) analysis were also carried out to explain conclusions based on electrical resistivity measurements. AASC pastes with 0.35 and 0.50 water-binder ratios (w/b) were tested at 3, 7, 14 and 28 days and benchmarked against Portland cement (PC) controls. Results indicated that for a given w/b, the electrical resistivity and capillary porosity of the AASC paste were lower than that of the PC control, whilst an opposite trend was observed for the pore solution conductivity, which is due to AASC paste's significantly higher ionic concentration.Further, capillary pores in AASC paste were found to be less tortuous than that in the PC control according to estimations using Archie's law and from the results of X-ray μCT and SEM analysis. In order to achieve comparable levels of tortuosity, therefore, AASC-based materials are likely to require longer periods of curing. The work confirms that the electrical resistivity measurement offers an effective way to investigate pore structure changes in AASC-based materials, despite threshold values differing significantly from PC controls due to intrinsic differences in pore solution composition and microstructure.