Chloride Binding Isotherms Research Articles

Modelling chloride diffusion in concrete with carbonated surface layer

Due to the demand for carbon neutrality, concrete carbonation has been reconsidered as an interesting topic because of its potential for capturing carbon dioxide (CO2) from the atmosphere. Concrete carbonation can significantly modify the chemical and microstructure properties of concrete and thus will have important effects on chloride diffusion. This paper presents a chloride diffusion model in which the concrete cover is divided into three different zones, each with their own defined porosity and chloride binding isotherm. The first is the fully carbonated concrete near the surface, where the porosity and chloride binding isotherm can be obtained from the experimental data of fully carbonated concrete. The second is the uncarbonated concrete near the reinforcement, where the porosity and chloride binding isotherm can be obtained from the experimental data of normal concrete. The third is the transition zone between the fully carbonated and uncarbonated concretes, where the porosity and chloride binding isotherm can be assumed to vary continuously from the carbonated concrete to uncarbonated concrete. To validate the present model, a comparison of the present model with published experimental results is provided, which demonstrates the importance of considering different zones in the chloride diffusion model when the concrete has a carbonated layer near the surface.

Time-dependent concrete chloride diffusion coupled with nonlinear chloride binding: An analytical study

Both the temporal dependence of chloride diffusion coefficient and the effect of nonlinear chloride binding substantially affect the behavior of concrete chloride diffusion. This study develops an exact chloride diffusion solution that accommodates the above two factors by introducing a simplified but reasonably realistic nonlinear chloride binding isotherm. The introduction of this isotherm makes it possible to achieve an analytical solution to a highly nonlinear chloride diffusion equation that involves a free chloride concentration dependent dimensionless factor which reflects the effect of chloride binding. First, a time-dependent chloride diffusion model incorporating the introduced isotherm is established with the corresponding exact solution derived. The proposed solution is successfully validated against two existing analytical solutions and a numerical solution. Parametric study is conducted using the proposed solution to investigate how the parameters regarding chloride ingress influence the corrosion initiation time, and results show that the increase of the initial effective chloride diffusivity or the before-ingress age decreases the corrosion initiation time bringing an acceleration of a different level to chloride diffusion while the increase of the age factor prolongs the corrosion initiation time decelerating chloride diffusion to some extent, and as each of the three nonlinear chloride binding associated parameters (i.e., the deflection-point concentration, the former slope and the latter slope) increases, the corrosion initiation time rises and the retardation effect on chloride diffusion is enhanced to a various extent. Lastly, to demonstrate the practical validity of the proposed solution, the solution is applied to reproducing a reported chloride diffusion test, and it turns out that our solution reproduces the diffusion test reasonably well, justifying its practical validity.

Chloride binding by layered double hydroxides (LDH/AFm phases) and alkali-activated slag pastes: an experimental study by RILEM TC 283-CAM

Chloride binding by the hydrate phases of cementitious materials influences the rate of chloride ingress into these materials and, thus, the time at which chloride reaches the steel reinforcement in concrete structures. Chloride binding isotherms of individual hydrate phases would be required to model chloride ingress but are only scarcely available and partly conflicting. The present study by RILEM TC 283-CAM ‘Chloride transport in alkali-activated materials’ significantly extends the available database and resolves some of the apparent contradictions by determining the chloride binding isotherms of layered double hydroxides (LDH), including AFm phases (monosulfate, strätlingite, hydrotalcite, and meixnerite), and of alkali-activated slags (AAS) produced with four different activators (Na2SiO3, Na2O·1.87SiO2, Na2CO3, and Na2SO4), in NaOH/NaCl solutions at various liquid/solid ratios. Selected solids after chloride binding were analysed by X-ray diffraction, and thermodynamic modelling was applied to simulate the phase changes occurring during chloride binding by the AFm phases. The results of the present study show that the chloride binding isotherms of LDH/AFm phases depend strongly on the liquid/solid ratio during the experiments. This is attributed to kinetic restrictions, which are, however, currently poorly understood. Chloride binding by AAS pastes is only moderately influenced by the employed activator. A steep increase of the chloride binding by AAS occurs at free chloride concentrations above approx. 1.0 M, which is possibly related to chloride binding by the C–(N–)A–S–H gel in the AAS.

Chloride penetration in concrete with trilinear binding isotherm

Chloride binding in concrete is an important factor and it can significantly affect the penetration speed of chlorides in concrete and thus the service life of concrete structures. In this paper a new chloride binding isotherm is proposed to be included in the diffusion model of chlorides in concrete. Instead of the use of the traditional Freundlich or Langmuir isotherm, the present chloride binding model uses a trilinear function to describe the chloride binding in concrete. The trilinear binding isotherm not only reflects the nonlinear binding feature of chlorides in concrete when it is exposed to a chloride environment, but also makes it possible to achieve an analytical solution of the chloride diffusion equation. In the present paper, the superior of using trilinear binding isotherm to other binding isotherms is demonstrated. The detailed derivation of the analytical solution of the chloride diffusion equation with nonlinear binding feature is described and illustrated. To demonstrate the rationality and accuracy of the analytical solution derived, comparisons between the analytical solution and the numerical solution and experimental results are also provided. It is shown that the chloride diffusion profiles obtained from the chloride diffusion equation with the trilinear binding isotherm are much closer the experimental results than that obtained from the chloride diffusion equation with linear or bilinear binding isotherm.

Chloride binding behavior of calcium silicate hydrates and Friedel's salt of pastes

Understanding the chloride binding capacity of cement pastes is crucial to investigating the concrete durability. To characterize the chloride binding behavior of calcium silicate hydrates (C–S–H) and Friedel's salt (Fs), this study investigates the chloride binding capacity of four types of cement paste using chloride binding isotherms, analyzes the correlation between phase composition and chloride binding capacity, and discusses the evolution law of the chloride binding capacity of Fs and C–S–H. The results indicate that the influence of mineral admixtures on the chloride binding capacity of C–S–H is basically the same as that of pastes. Slag (SL) has a significant positive effect, while silica fume (SF) generally plays a negative role, and the effect of fly ash (FA) depends on the concentration of NaCl solution. As to the influence of chloride concentration, the chloride combined by per unit mass of C–S–H and Fs presents different evolution trends, and a concentration break-even point exists between C–S–H and Fs combined chloride. Notably, this point provides the insight into which substance occupies the majority of the total bounded chloride. Moreover, the impact of chloride concentration on the chloride binding capacity of pastes cannot be solely determined by the content of C–S–H and Fs, and the chloride combined by per unit mass of C–S–H and Fs at different chloride concentrations offers more relevance.

Chloride binding behaviors and early age hydration of tricalcium aluminate in chloride-containing solutions

Understanding the hydration kinetics and interactions of hydration products of cement components with chlorides is crucial for designing durable cement-based materials. The most reactive component of ordinary Portland cement (OPC), tricalcium aluminate (C3A), was investigated on its hydration kinetics, chloride binding behaviors and hydrated phase assemblages through multiple ex-situ and in-situ methods. The hydrated phases of C3A demonstrated different chloride binding isotherms in NaCl and CaCl2 solutions at different ages. Chloride ions can suppress the formation of calcium aluminate hydrates (OH-AFm and C3AH6) while calcium ions were favorable to increase the content of hydrocalumite (Cl-AFm) and regulate its crystal structure. The carbonation process deteriorated the chloride binding capacity of C3A pastes through producing more calcium mono/hemi-carboaluminate phases (Mc/Hc) or forming a solid solution between Mc and Cl-AFm. These findings can be used as a guide for corrosion protection design of cement-based materials.

Analytical solution for chloride diffusion in concrete with the consideration of nonlinear chloride binding

An analytical study is presented on the diffusion of chloride in concrete coupled with the effect of nonlinear chloride binding. A double-segment piecewise linear chloride binding isotherm (the DS-PWL isotherm) is proposed that can fit well quite a number of experimentally measured chloride binding isotherms which were reported by many published works. The proposal of the DS-PWL isotherm makes feasible the development of analytical solutions for chloride diffusion in concrete that accommodate nonlinear chloride binding, which is not feasible at present. Based on the proposed isotherm, an analytical solution is obtained for one-dimensional diffusion of chloride in a semi-infinite concrete medium in the context of nonlinear chloride binding. The obtained solution is successfully validated against numerical solution for mathematical correctness while the proposed analytical model is shown to be in tolerably good agreements with a reported chloride diffusion experiment. Moreover, the proposed model is also shown to be in good agreements with the Freundlich-based model, exhibiting a desirable applicability. Parametric study is conducted using the proposed solution to examine the influence of each of the three DS-PWL characterizing parameters on chloride diffusion. Finally, the proposed solution is applied to investigating the influence of nanomaterial addition (the dosage of addition and the type of the nanomaterial added) on the service life of an RC structure member subjected to chloride ingress.

Chloride binding in Portland composite cements containing metakaolin and silica fume

This paper investigates how the composition of Portland composite cements affects their chloride-binding properties. Hydrated cement pastes prepared with a reference Portland cement and composite Portland cements containing metakaolin and/or silica fume were exposed to NaCl or CaCl2 solutions. Chloride-binding isotherms were determined and the hydrate assemblage was investigated using TGA, XRD, 27Al NMR, 29Si NMR and thermodynamic modelling. Compared to the reference Portland cement paste, silica fume replacement did not alter the chloride-binding capacity. The metakaolin replacement resulted in the highest chloride-binding capacity. When combining silica fume with metakaolin, the chloride binding is similar to the reference Portland cement. In this study the differences in chloride binding were linked not only to changes in the AFm content, but also to alterations in the Al-uptake and chain length of the C(-A)-S-H.

The “Skin Effect” Assessment of Chloride Ingress into Concrete Based on the Identification of Effective and Apparent Diffusivity

The method of assessment of the “skin effect” for chloride ingress into concrete has been proposed, based on the inverse problem for the identification of at-surface variability of chloride diffusivity under fully saturated conditions. For this purpose, experimental results of 180-day diffusion tests of five types of concrete were used, which allowed the calculation of their chloride apparent diffusivity (taking into account the chloride binding by the cement matrix) and effective diffusivity (relating to the transport of free chloride ions in the pore liquid). The tested concrete samples with a water to cement ratio of 0.5 differed only in the type of cement (high early strength Portland, low-alkali normal early strength Portland, ash Portland, blast furnace, and pozzolanic). In order to effectively describe the chloride binding isotherm, a first-degree non-uniform spline function was used. Finally, the “skin effect” depth at the untreated outer surface of the concrete samples was estimated up to about 5 mm when analyzing space variability of apparent chloride diffusivity for four types of concrete with low-alkali normal early strength Portland, ash Portland, blast furnace, and pozzolanic cement. In this respect, the “skin effect” on the concrete with high early strength Portland cement was not detected.

Influence of non-linear chloride binding on the determination of apparent chloride diffusion coefficient for cement paste with mineral additives

The determination of apparent chloride diffusion coefficients (Da) in standard codes employs the linear chloride binding assumption. However, this assumption is too simplified as chloride binding is actually non-linear. This paper intends to investigate the influence of non-linear chloride binding on the determination of Da values in the bulk diffusion test. To achieve this aim, a semi-analytical method for the semi-infinite diffusion problem was firstly established. This method can be used in data fitting to obtain the Da values considering non-linear chloride binding. Then, bulk diffusion test and mercury intrusion porosimetry were conducted on four different cement pastes with fly ash and blast furnace slag. Based on chloride profiles, the chloride binding isotherms were obtained through data fitting. Meanwhile, the Da values considering the linear and non-linear chloride binding assumption were calculated respectively and compared. The results show that the chloride binding isotherms in the bulk diffusion test fit well with the Freundlich isotherm but are not so non-linear as expected. The proposed method can calculate the effective diffusion coefficient which cannot be obtained in standard codes. Da values will vary along the chloride ion diffusion direction rather than be a constant once the non-linear chloride binding is considered. The comparison between different cement paste mixtures indicates that both the apparent diffusion coefficient calculated from free chloride profiles or total chloride profiles can only be regarded as a qualitative description of resistance to chloride diffusion for a certain mixture. The comparison between different mixtures based on these values is not reliable.

Influence of pH on chloride binding isotherms for cement paste and its components

In this paper, chloride (Cl−) binding isotherms are developed for cement hydration compounds, specifically, calcium-silicate-hydrates (CSH) and AFm phases including mono-sulfate aluminate, hydroxy-AFm, monocarbonate-AFm, and hemicarbonate-AFm, in simulated concrete pore solutions to account for the effect of pH and the presence of other ions. pH and the presence of other ions have a strong influence on the Cl− binding capacity of cement compounds, which have not been taken into consideration in previous research. A novel experimental technique is developed to characterize the binding capacity from very low (1 mM) to very high concentrations (5 M). To overcome the existing challenges of measuring both low and high Cl− concentrations without significant dilution and in the presence of SO42− and OH− ions, a potentiometric method was used. The amorphous CSH in the hydrated cement paste; thus, the heterogeneity of the hydrated components of the cement paste was also quantified and accounted for in this study.

Alkali cation effects on chloride binding of alkali-activated fly ash and metakaolin geopolymers

In this work, the chloride binding capacity and chloride penetration resistance of alkali-activated fly ash and metakaolin geopolymers synthesized with various alkali cation types (i.e., sodium or potassium) and aluminosilicate composition (i.e., combination of metakaolin and low-calcium pulverized fly ash) are studied. The chloride binding isotherms are measured using a modified equilibrium method and the effect of chloride exposure on the mineralogical alteration of geopolymers is studied by means of X-ray diffraction. The results show that the type of alkali cation considerably affects the chloride binding behaviors of metakaolin-fly ash geopolymers, regardless of aluminosilicate composition. In comparison to the potassium-based activator, sodium activator results in higher strength development, stronger chloride binding capacity, and better resistance to chloride penetration. The metakaolin-fly ash geopolymers immobilize chloride ions through both chemisorptions via the formation of chloride-bearing zeolites (e.g., Cl-chabazite) as well as physical adsorption. The dominant role of physical adsorption in binding mechanism suggests that a high proportion of hydroxyl group in the structure of sodium (or potassium)-aluminosilicate-hydrate (i.e., (N, K)-A-S-H) is exchangeable with chloride ions.

Numerical and Experimental Analysis of Self‐Protection in Reinforced Concrete due to Application of Mg–Al–NO2Layered Double Hydroxides

Concrete possesses an intrinsic chloride binding capacity. Chloride ions from the environment bind with the hydrated cementitious phases in the form of bound chlorides. The contribution of chemically bound chlorides toward increasing the service life of concrete structures is vital as they help in slowing down the chloride diffusion in the concrete thereby delaying reinforcement depassivation. The authors attempt to increase the chloride binding capacity of concrete by adding a small amount of Mg–Al–NO2layered double hydroxides (LDHs) with the objective to delay reinforcement corrosion and by this to considerably extend the service life of concrete structures situated in harsh environments. This study presents numerical and experimental analysis of the action of LDH in concrete. Formation factor is used to determine the effective chloride diffusion coefficient. In addition, the chloride binding isotherms together with Poisson–Nernst–Planck equations are used to model the chloride ingress. A comparable chloride binding is observed for concrete with and without Mg–Al–NO2, depicting only a slight chloride uptake by Mg–Al–NO2. Further investigations are conducted to understand this behavior by studying the stability and chloride entrapping capacity Mg–Al–NO2in concrete.

Critical chloride threshold values as a function of cement type and steel surface condition

This paper presents the application of a new test protocol developed for determining chloride threshold values initiating corrosion in reinforced concrete. The experimental set-up was described in previous work and is based on the intrinsic localized aspect of chloride-induced corrosion. The test protocol is also based on the concept of anodic control of corrosion initiation, and therefore independent of the area of passive steel. The test method is applied to mortar and concrete formulations with different binders and steel surface conditions. Chloride-binding isotherms were also determined to allow the transition from total to free chlorides. It is found that the test protocol is rapid and applicable to all formulations. The experimental results provide an understanding of the influence of binder type, w/b ratio and porosity on chloride-induced corrosion initiation. As expected, the steel surface condition is shown to have an important overall effect on corrosion initiation.

Influence of pH on the chloride binding capacity of Limestone Calcined Clay Cements (LC3)

The high solution to solid ratio normally used for the measurement of chloride binding isotherms, leads to a leaching of ions and a decrease of pH. To make binding test more representative of on-site conditions, we investigated the influence of the increase of pH to compensate the leaching. The binding was characterized on plain cement and Limestone Calcined Clay Cements (LC3). The increase of pH leads to a decrease of the amount of bound chloride. This lower binding can be first explained by the lower chloride content in the solid solution of Friedel's salt and hemicarboaluminate hydrate. Moreover, a lower chloride ions adsorption on C-A-S-H is quantified.

Impact of phosphate corrosion inhibitors on chloride binding and release in cement pastes

The impact of three kinds of phosphate corrosion inhibitors (MFP, DHP and TSP) on chloride binding and release in cement pastes was investigated by means of chloride binding isotherms (CBIs), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). Experiments of chloride binding, chloride release in deionized water and phosphate solutions were implemented separately. The regularity and factors of CBIs are well studied and the results of CBIs showed that phosphate inhibitors reduced chloride binding chemically due to reactions between Friedel’s salt and phosphates. Besides, the release of bound chloride can be well-calculated by CBIs. Furthermore, a modified CBI, taking effective phosphate concentration into account, has been established, and chloride penetration predictions have been made consequently.

Role of swelling agent and set-controlling admixtures on chloride binding and diffusion in cement matrix

Chloride binding and diffusion was investigated for cement matrix with swelling agent (SA), early-strength agent (ESA) and set-retarding agent (SRA). Chloride binding isotherms (CBIs), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and thermodynamic modeling were applied to quantitatively reveal the mechanisms of chloride binding. The current results indicate that ESA will reduce chloride binding capacity and enhance chloride diffusivity due to the competition between sulphate and chloride. SRA holds a reverse regularity on account of increasing long-term hydration degree. The role of SA combines with kaolin and gypsum, resulting in a slighter reduction of chloride binding capacity.

Modelling non-isothermal chloride ingress in unsaturated cement-based materials

The realistic environments for engineering practices are diverse, where the temperature, relative humidity, and surface ionic concentrations often experience seasonal variations. The accurate evaluation of the multi-species transportation is crucial in terms of durability assessment of cement-based materials. In this work, an improved moisture transportation model is developed, which takes the influences of the changes in relative humidity, temperature and microstructure into account. The moisture transportation model is coupled with the Poisson-Nernst-Planck model to study the chloride ingress problems. To model the chloride binding effect, both the pre-designed chloride binding isotherm and the thermodynamic modelling method are implemented. In the thermodynamic method, the physical and chemical binding of chloride are modelled in details, and coupled with other simultaneous reactions so as to model the variations of the microstructure. The proposed method is validated against a variety of reported experiments and a long-term field study. The Numerical modelling demonstrates the effectiveness of the improved moisture transportation model and the significance of using the thermodynamic modelling method for long-term durability assessments.

Water Vapour Storage Capacity of Masonry Renovation Plasters Contaminated with Chlorides

The effect of sodium chloride presence on the water vapour storage capacity was studied for six types of plasters designed for application in masonry restoration. Sorption isotherms were measured using a static climate chamber method. The researched materials were characterised by their bulk density, matrix density, and total open porosity. For the studied materials, measurement of chloride binding isotherms by an adsorption method was done in order to reveal their capacity to accumulate salts and relate the adsorbed water vapour to the amount of contained salt. The experimental sorption data measured for salt contaminated samples were analysed using relation expressing the dependence of the mass equilibrium moisture content (EMC) of plasters saturated with salt solution of given molality on relative humidity of the environment. For relative humidity up to 70 %, good agreement between measured and calculated data was obtained. For higher relative humidity, the water vapour adsorption capacity of studied materials was not fully exploited especially for lower chloride concentrations. The type of binder together with physical parameters of examined materials was found to have a decisive role in chloride binding. One of the tested materials can be possibly classified as WTA renovation plaster, considering its total open porosity, bulk density and high binding capacity for chlorides. On the other hand, one must take into account its hygroscopicity, especially in case of a higher salt contamination. DOI: http://dx.doi.org/10.5755/j01.ms.24.4.19434

Modelisation of chloride reactive transport in concrete including thermodynamic equilibrium, kinetic control and surface complexation

In this study, a new physically and chemically based model taking into account thermodynamic equilibrium, kinetics and surface complexation is proposed to predict the ingress of chloride ions into saturated concretes. The numerical results of this multi-ionic model are compared to experimental data such as total and free chloride concentration profiles and chloride binding isotherms. With only one set of parameters, the results show very good agreement for chloride binding isotherm of concretes with different w/b, types of binders (CEM I, CEM III, and CEM I with fly ash) and various exposure conditions (NaCl solution at different concentrations). Such a model overcomes the use of empirical chloride binding isotherms that can be difficult to asses for concretes with supplementary cementitious materials. The very good results underline the need to take into account all the physical and chemical phenomena included in the model.