Pore Solution Concentration Research Articles

Passivation behaviour of mild steel embedded in magnesium potassium phosphate cement-calcium sulphoaluminate cement blended paste

• Passivity of mild steel embedded in MKPC-CSA blended paste were investigated. • CSA addition significantly influences the degree of passivity. • Passivity increases with increased CSA addition and 30% CSA addition leads to a much higher passivity. • 30 % CSA addition benefits the precipitation of phosphate ions and formation of Fe 2+ oxide in passive film. This study did a pioneer work focused on the passivation of mild steel embedded in magnesium potassium phosphate cement (MKPC)-calcium sulphoaluminate cement (CSA) blended paste via electrochemical measurements and materials characterization techniques. Results showed that 30 % CSA addition in MKPC leads to a passivity of mild steel much higher than that in Portland cement (PC) paste. The passivity of mild steel increased with an increase in CSA content in MKPC paste due to an increase in pH and phosphate concentration in pore solution. The formation of iron phosphate and Fe 2+ oxide in passive film of MKPC-30 %CSA contributes to its highest passivity.

The effect of paste composition, aggregate mineralogy and temperature on the pore solution composition and the extent of ASR expansion

The reaction kinetics of the alkali silica reaction depends on the composition of the pore solution. The evolution of the pore solution composition in different cement pastes and concretes was studied. Pastes containing silica fume or metakaolin had the lowest amount of alkalis in the pore solution. In addition, metakaolin increased the aluminium concentrations. The lowest expansion was measured for the concretes made of blended cement pastes with low alkali and hydroxide content in their pore solution, for the duration of the present study, no additional aluminium effect was observed due to the already low pH. Addition of 400 mM of Li slowed down expansion rate of concrete prisms at 40 and 60 °C, however, similar expansion was observed for samples with and without Li at 60 °C after 1 year. Temperature, alkali concentration and pH of pore solution all have a major effect on ASR expansion.

Degradation mechanism of cement-based materials under the effects of stray current, chloride and sulfate

When infrastructure is exposed to a complicated environment containing stray current, chloride and sulfate, it may be subjected to severe degradation. In order to analyze the interaction of different factors and the degradation mechanism of cement-based materials in a complicated environment, the blended solution with four molar concentration ratios of Cl-/SO42- (0, 0.5, 1, and 2) were prepared to simulate a range of aggressive medium. Meanwhile, 40 V DC (direct current) electrical field was applied to cement-based materials to simulate stray current. After exposure, the transport properties, macro-properties, and microstructures of specimens were observed and measured. Additionally, the types and amounts of degradation products were tested to further analyze the degradation mechanism of cement-based materials. Experimental results show that chloride in the blended solution could inhibit SO42- transport into cement-based materials, causing the SO42- concentration in a specimen to decrease with the increase of Cl-/SO42- ratio. Therefore, the specimens appeared less degraded in terms of macro-properties. But the results of microstructural analysis showed that degradation products and cracks were generated in the interior of specimens. Friedel’s salt was detected in the specimens exposed to the solutions containing chloride. The formation of Friedel’s salt consumed calcium aluminate hydrate, which caused the amount of Al-phase involved in the formation of ettringite to reduce. Consequently, the amount of ettringite was reduced, and the degradation of compressive strength and appearance was also reduced. Additionally, an electrical field could induce the ion concentration in pore solution to change and even break the ion chemical equilibrium, which resulted in the decomposition of ettringite and Friedel’s salt. However, Friedel’s salt can also transform into ettringite at a specific condition. Therefore, the amounts of ettringite and Friedel’s salt in specimens were changed over the exposure time.

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.

The combined effect of alkalis and aluminum in pore solution on alkali-silica reaction

This study aims to explore the impacts of alkali ions and aqueous Al species in pore solution on alkali-silica reaction (ASR). Wrapped accelerated mortar bar test (100% RH, 38 °C) was carried out on mortars prepared with binders containing various metakaolin (0% and 5%) and alkali content (Na2Oeq = 0.65%, 1.25%, 2.25%, and 2.75%); solid and aqueous compositions of the corresponding hydrated pastes were analyzed using XRD and ICP-OES. A combined effect of alkali ions and aqueous Al species on ASR is identified, which can be characterized by a ratio of alkali (Na + K) to aluminum concentration in pore solution, and ASR expansion increases with this ratio. This finding provides new insights in understanding the relationship between pore solution composition and ASR, and indicates alternative approaches in controlling ASR, other than limiting alkalis content.

Influence of Soil Salinity on Pore Water Phase Composition

In areas of intensive construction on the Arctic coast, river valleys of the Republic of Sakha (Yakutia) non-freezing taliks with a high concentration of pore solution (cryopegs) are found in the strata of permafrost soils. N. P. Anisimova made a great contribution to the study of the cryopegsformation in the cryolithozone [1]. For solutions of any composition, it is established [2] that there is a general dependence of the relative freezing temperature on the relative concentration of the solution. It has also been established that the effect of dissolved salts is additive. The freezing temperature of the solution in the pores of dispersed media is affected, in addition to the concentration and composition of the solution the adsorption capacity of the material in relation to the components of the solution.

Swelling and swelling pressure of a clayey soil: Experimental data, model simulations and effects on slope stability

The behaviour of soils and rocks containing clay minerals is the effect of complex chemo-mechanical processes of interparticle interaction. This paper presents experimental results and model simulations of the influence of mineral composition and pore fluid composition on swelling and swelling pressure of clayey soils frequently affected by instability problems. The soil from a landslide in tectonized clay shales, characterized by wide ranges of plasticity and activity, was analysed. Soil fractions at different liquid limit wL were reconstituted with distilled water or salt solutions at different concentrations and they were compressed in oedometric conditions. The tendency to swell was induced by unloading or by exposure to a fluid different from the pore fluid. Swelling was permitted in a set of specimens and it was prevented in another set by applying increasing pressures. The temporal evolution of swelling and swelling pressure was monitored, and ion concentration in the pore fluid was evaluated by monitoring the cell fluid composition. The chemically induced swelling was very large in the most plastic soil and negligible in the least plastic. Swelling pressure too attained different values, however, it was also very large in the least plastic soil.The observed coupled chemo-mechanical processes were successfully modelled with the FEM method by using a user-defined UEL subroutine that was implemented in the commercial code ABAQUS. The subroutine takes account of the chemo-mechanical interactions in the solid skeleton, through a suitable constitutive model, and of the coupled fluxes in the porous space, in addition to water desorption/adsorption at clay platelets. In the proposed constitutive model, chemical interactions are assumed to affect both the plastic and the elastic response, as it was observed in experiments, thus an extended elasticity law is proposed. A simplified analysis, calibrated on the model results, shows that the role of swelling/swelling pressure on slope stability is significant even in the case of the least active soil. A decrease in pore solution concentration thus provokes two effects that are both negative for slope stability: a decrease of strength parameters and an increase of horizontal stresses.

Effect of the composition and concentration of geopolymer pore solution on the passivation characteristics of reinforcement

The effect of the composition and concentration of the geopolymer pore solution on the passivation characteristics was investigated. At first, the passivation experiments of reinforcement in five solutions with different compositions and concentrations were carried out. Then, the formation mechanism and electrochemical characteristics of the passivation film in different solutions were compared by electrochemical measurement. Finally, the influence of the composition and concentration of the pore solution on the composition and thickness of the passivation film was discussed. The results show that a passivation film with a bilayer structure is formed in all solutions. The passivation film composition in solutions with the same composition is similar, but the passivation film thickness increases as the OH− concentration increases. The results also show that the charge transfer resistance of the reinforcement in cement is significantly higher than that in solutions with the same OH− concentration, which may be caused by the adsorption of calcium silicate hydrate gel on the surface of the reinforcement.

Using electrical resistivity to determine the efficiency of supplementary cementitious materials to prevent alkali-silica reaction in concrete

In this study, the miniature concrete prism test (MCPT) was used to evaluate expansion due to alkali-silica reaction (ASR) in concrete mixtures with very highly reactive fine aggregates and supplementary cementitious materials (SCMs). The concrete pore solution alkalinity and transport properties of concrete measured through bulk electrical resistivity were determined to investigate their correlation to the MCPT expansions. The findings showed that the concrete mixtures with SCM combinations that resulted in higher bulk electrical resistivity and lower pore solution alkalinity had less ASR expansion in the MCPT conditions. A potential new approach to determine the efficiency of SCM combinations to prevent ASR was proposed by combining the hydroxide concentration of pore solution and electrical resistivity of concrete.

Influence of Loading Pressure and Sample Preparation on Ionic Concentration and Resistivity of Pore Solution Expressed from Concrete Samples

AbstractPore solution expression is an established method to obtain samples of the liquid phase from cementitious systems. This experimental method applies pressure to a cementitious sample, forcing its liquid phase out of the pores. By collecting and studying the liquid phase in cementitious systems, it is possible to obtain information on its ionic concentrations. The ionic concentrations can be used for modeling calibrations and to estimate the resistivity of the pore solution. When the bulk resistivity of concrete is normalized by the pore solution resistivity, it is possible to determine the formation factor. The formation factor is related to the transport properties of the concrete and, as such, it can be used to estimate the rates of transport of ionic species within a concrete structure. The formation factor is currently being included in AASHTO PP84, Standard Practice for Developing Performance Engineered Concrete Pavement Mixtures, as an indicator of transport properties for quality control operations. Pore solution expression is included as one of the available procedures of AASHTO PP84-19 to determine the pore solution electrical resistivity. Previous studies on paste and mortar samples have demonstrated that increased loading pressure during the pore solution expression might impact the final ionic concentrations of the expressed solution. This study aims to verify if the pore solutions of concrete specimens are also influenced by the selected loading pressure and whether the potential consequent change in the measured ionic concentrations of the solution also has an impact on its resistivity. No appreciable trend in increased solubility was observed for the range of applied normal pressures between 600 and 985 MPa. Cyclic loading regimes increased the variability of alkali solubility. Sample preparation, in some cases, influenced the water content of the sample and induced unwanted alteration on the ionic concentrations of the mixtures under study.

Leaching of trace metals (Pb) from contaminated tailings amended with iron oxides and manure: New insight from a modelling approach

Reclamation measurements are commonly applied to mitigate the leaching of metal pollutants in order to reduce the risk for humans and the environment. The stabilization of mine tailings can be performed by amending with organic or inorganic materials. In a recent laboratory microcosm experiment (Thouin et al., 2019), the addition of a mining slurry called ochre and manure, either alone or in combination, drastically reduced the leaching of several metal pollutants, notably Pb. Nevertheless, the biogeochemical processes involved in the immobilization of metal pollutants remain unknown, preventing the management of this remediation technique from being optimized and its extension to other sites. To fill this gap, a multicomponent mixing model was developed to simulate and forecast the impact of amendments on the leaching of metal pollutants. This model accounts for the following biogeochemical processes: kinetically-controlled dissolution/precipitation reactions, sorption reactions (i.e. surface complexation reactions), water-gas interactions and microbial respiration with an explicit microbial growth. For all treatments, simulations revealed that Pb reactivity followed dynamic patterns driven by watering steps. The decrease in Pb concentration in the leachates of amended tailings compared to untreated tailings was also accurately reproduced. In untreated tailings, Pb reactivity is mainly controlled by the dissolution of Pb-bearing mineral phases. These reactions were maintained in thermodynamic disequilibrium due to the renewal of pore solution at each watering step. In amended tailings, this pattern was strengthened as the iron oxides contributed by ochre maintained a low Pb concentration in pore solution by sorbing released Pb. Sorption reactions were enhanced by the increase in pH induced by the dissolution of calcium carbonate initially present in ochre. The latter reaction was partially counterbalanced in tailings amended with manure as organic matter provided sufficient energy to fuel microbial aerobic respiration, leading to the release of protons. Pb desorption was promoted by this pH drop. The magnitude of these reactions was not strictly proportional to the amount of manure added. For 0.15% by weight, aerobic respiration did not occur whereas its yield was similar for 1% and 2%. By providing a better understanding of the effect of amendment, this multicomponent mixing model is a powerful tool to optimize the reclamation of tailings, in order to limit contaminant transfer to the environment.

Predicting changes in the mechanical properties of frozen saline soils

This article summarises a laboratory program involving over 100 tests that were performed to study the influence of soil type, temperature, water content and salinity on the strength and creep of frozen soils under uniaxial compression and shear tests. Analyses of experimental data allowed to get equations for predicting strength and creep parameters depending on the temperature and salinity using generalising parameters for frozen soils (freezing point and the concentration of pore solution). These parameters reflect the total impact of all factors (soil type, salinity and water content) in forming a single pattern for all types of soil. The developed theory of searching for generalised dependencies showed a good convergence for predicting the strength and creep characteristics of frozen soils.

Effects of Concentration of Pore Solution on Stability of Ion-Absorbed Rare Earth Ore Aggregate

The continuous change in solution concentration in ore pores during in situ mineral leaching influences the stability of ore aggregate. In this study, influences of the concentration of ammonium sulfate ((NH4)2SO4) solution on the interaction forces between ore particles were calculated. On this basis, the mechanism by which (NH4)2SO4 solution concentration influences the stability of ore aggregate was analyzed. Furthermore, an empirical formula for estimating the critical (NH4)2SO4 solution concentration for aggregation and dispersion of ore body aggregates with different grain composition was proposed. Some major conclusions were drawn. First, for ore bodies with an initial particle size of less than 0.075 mm, the interaction force between particles was net attraction, with the distance range of this force increasing as the concentration of (NH4)2SO4 solution increased from ≤0.001 to 0.16 mol·L−1, aggregation of ore particles occurring within this distance range. Secondly, for ore bodies with initial particle size of less than 0.075 mm, the interaction force between particles was net attraction, but with the distance range of this force decreasing when the (NH4)2SO4 solution concentration increased from 0.16 to 0.28 mol·L−1, dispersion of ore particles occurring beyond this distance range. Thirdly, for ore bodies with particle sizes of less than 0.038, 0.075 and 0.1 mm, the cation exchange capacity (CEC) was 9.13, 8.96, and 8.8 cmol·kg−1, respectively, and the critical (NH4)2SO4 solution concentration affecting the aggregation and dispersion of ore bodies was 0.12, 0.16, and 0.20 mol·L−1, respectively.

Modelling analysis of chloride redistribution in sea-sand concrete exposed to atmospheric environment

The combined effects of external carbonation and internal chloride transport are the main reasons leading to the degradation of concrete structures manufactured with sea sand. To evaluate the degradation process of sea-sand concrete structures, a mathematical model for describing the redistribution of internal chlorides in sea-sand concrete exposed to atmospheric carbonation was proposed in this study. In the proposed model, changes in porosity, saturation degree, and the release of bound chlorides under the carbonation were considered. The numerical simulation of proposed model was executed by COMSOL software in a concrete model generated with random aggregates, which is verified to fit well with the experimental data obtained from a case study. In the modelling results, the chloride concentration in pore solution was decreased with the deeper depth after redistribution. However, on the chloride profile with the unit of wt% of concrete, a ‘pseudo enrichment’ of chlorides content was found after chloride redistribution and verified to be caused by the unit conversion. In conclusion, the proposed model could be used to predict the service life of sea-sand concrete structures exposed to atmospheric environment.

Modeling and simulation on coupled chloride and calcium diffusion in concrete

Permanently exposed to the chloride-contained water environment, concrete structures are subjected to a complicated interaction between chloride attack and calcium leaching, which can be described by the coupled chloride and calcium diffusion in concrete. Based on the mechanism of coupled chloride attack and calcium leaching on concrete, this paper proposes a series of equations for modeling their coupled diffusion, and they were numerically solved by an implicit finite difference scheme. Via verification, this model is used to numerically simulate the coupled chloride and calcium diffusion in a concrete plate, characterized by calcium concentration in pore solution, porosity, and chloride content in concrete, etc. Results show that calcium leaching increases the concrete porosity and chloride diffusivity, and chloride attack can accelerate the leaching process of concrete. Free chloride content in pore solution increases with the leaching time, and it is obviously lower under single chloride attack than that under the coupled chloride attack and calcium leaching. The decalcification of C-S-H gel and decomposition of Kuzel’s and Friedel’s salts caused by leaching can reduce the bounded capacity of concrete on free chlorides, and it leads to a decrease in the bound chloride content in concrete.

A Novel Testing Method for Examining Corrosion Behavior of Reinforcing Steel in Simulated Concrete Pore Solutions.

In this paper, a new mechanical-based experimental method is proposed to determine the corrosion initiation and subsequent corrosion behavior of steel in simulated concrete pore solutions. The proposed experiment is used to investigate the corrosion of the steel wire under various different conditions and to examine the effects of pre-stress level in steel wire, passivation time of steel wire, composition and concentration of simulated concrete pore solution on the corrosion initiation, and subsequent corrosion development in the steel wire. The experimental results show that the reduction rate of the cross-section area of the steel wire increases with the increase of chloride concentration or decrease of pH value in the solution. However, for the case where the chloride concentration is high and the pH value is low, there is a slight decrease in the corrosion rate due to the coating function of the corrosion products surrounding the wire.

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.

The influence of inorganic admixtures on early cement hydration from the point of view of thermodynamics

Calcium salt and other inorganic chemicals have long been used as the accelerators of cement hydration; however, the underlying mechanism remains to be resolved. In this contribution, thermodynamic analysis of cement systems with calcium chloride (0.06 M and 0.12 M), potassium hydroxide (0.28 M and 0.56 M) and hydrochloric acid (0.09 M and 0.18 M) were combined with quantitative analysis of the calorimetric curves of these systems. Calorimetric tests show that calcium chloride and hydrochloric acid would increase hydration rate, whereas lower dosage of potassium hydroxide would decrease hydration rate. Thermodynamic analyses show that in all the systems, the chemical driving force of C3S dissolution keeps increasing whereas that of calcium silicate hydrates (C-S-H) precipitation keeps decreasing, indicating that the dissolution of C3S could be the limiting step of cement hydration. This is further confirmed by the facts that the chemical driving force of tricalcium silicate (C3S) dissolution is positively related to both the intensity of the silicate peak and the occurring time of the silicate peak time, and that the chemical driving force of C3S dissolution is negatively related to the length of induction period. Furthermore, the calcium concentration of pore solution is positively related to the chemical driving force of C3S dissolution, implying that calcium concentration could play a critical role in regulating effect of inorganic admixtures. This work finds the crucial role of C3S dissolution in early cement hydration and also provides new insight into the effect of inorganic chemicals on cement hydration and the mechanism of cement hydration.

Effect of Pore Structure and Ion Concentration of Solution on the Formation of Ettringite in Composite Cement Paste

Under the background of shrinkage cracking of cement and concrete, ettringite is regarded as the most effective expansion source because of its outstanding expansion characteristics. In order to study the expansion mechanism of ettringite, the growth process and formation conditions of ettringite in composite cement system were studied by means of SEM, MIP and ICP. The effects of pore structure and ion concentration of pore solution on the morphology and expansion properties of ettringite were also analysed in this paper. The results show that the pore structure of cement paste directly affects the expansion properties of ettringite. It is verified that there is no obvious linear relationship between the expansion rate of ettringite and its quantity. The concentration of SO42- and Al(OH)4- in pore solution is the determinant of ettringite formation rate and quantity in composite cement system at the initial stage of hydration reaction. The change of crystallinity of ettringite will be directly caused by pH.

Influence of w/s ratio on alite dissolution and C-S-H precipitation rates during hydration

The first 24 h of hydration of tricalcium silicate (C3S) have been studied at varying water/solid (w/s) mass ratios ranging from 0.5 to 10. Gathered data comprise the reaction degree, pore solution concentrations and specific surface area evolution. Combined with interfacial C3S dissolution rates and C-S-H precipitation rates found in the literature, these data are used to calculate theoretical reaction rates from C-S-H precipitation and C3S dissolution. Compared with the measured reaction rates, theoretical rates calculated from C-S-H precipitation show a good match, indicating a correct kinetic description of C-S-H precipitation. Theoretical rates calculated from C3S dissolution are higher than expected during the first hours of hydration which could be explained by the buildup of a metastable hydrate phase barrier. Furthermore, a comparison of the reaction rates shows no effect of the w/s ratio. The Ca concentration evolution indicates a delay of the first portlandite precipitation with increasing w/s ratio.