Chloride Binding Isotherms Research Articles

Prediction of Chloride Ingress in Saturated Concrete Using Formation Factor and Chloride Binding Isotherm

Abstract This article describes an approach to predict chloride ingress in saturated concrete using the formation factor and chloride binding isotherm. The formation factor is calculated as the ratio of the measured electrical resistivity of concrete and the electrical resistivity of the pore solution. The chloride binding isotherms are determined experimentally by placing ground concrete powder in sodium chloride (NaCl) solutions of varying concentrations and using titration to quantify the bound chloride contents. The Nernst-Plank equation is used to predict the chloride ingress with the ionic diffusion coefficients (calculated from the formation factor) and the fitted (Freundlich) chloride binding isotherm. The simulation results correspond well with experimentally obtained chloride profiles of saturated concrete after being ponded in NaCl solution. It is found that 70 % of simulated values for total chloride contents have a relative error of less than 30 % when compared to the experimental data. The approach presented in this article can be considered as an alternative for mixture qualification and quality control, though further work is needed to benchmark this against existing procedures.

Study on chloride binding capability of coal gangue based cementitious materials

The addition of supplementary cementitious materials containing Al2O3 can improve the chloride binding capability of hardened cement paste to varying degrees, thereby enhancing the durability of reinforced concrete structures. This work was designed to investigate the high-aluminum coal gangue based cementitious materials chloride binding capability, including coal gangue content, coal gangue-slag combined addition and coal gangue calcination temperature. The phase composition and Friedel's salt contents in the specimens were evaluated by XRD and TG-DTG analysis. The results showed that the specimens chloride binding isotherms could be described by Freundlich equation. The coal gangue optimal content was between 20% and 30% and the maximum content was 40%. The chloride binding capability of slag was stronger than that of coal gangue, which promoted the formation of Friedel's salt. In the content range of this study, the chloride binding capability of coal gangue specimens increased at first and then decreased with the increase of the molar ratios of Ca/Si and Si/Al. When the coal gangue-slag composite was added, the chloride binding capability increased with the increase of the mole ratios of Ca/Si and Si/Al, presenting a positive linear relationship.

Effect of Carbonation and Sulphate on Chloride Ingress in Cement Pastes and Concretes with Supplementary Cementitious Materials

The main cause of premature deterioration of reinforced concrete structures is the corrosion of steel bars, induced by chloride ions (for example in marine environment) and/or by carbonation (atmospheric CO2). At the same time, environmental-induced degradations of concrete can also affect the structure, such as sulphate attack. This can lead to the formation of ettringite, inducing expansion inside the materials and finally degradation. Carbonation, chloride and sulphate ingress are usually studied separately in the literature. This is not representative of in-situ conditions since they can occur at the same time and can have an influence on each other. In this paper, chloride ingress are studied for concretes and cement pastes partially carbonated or/and in presence of sulphate in chloride contact solution. The mixtures contain OPC alone or with supplementary cementitious materials (SCM). SCMs are here pozzolanic materials (Fly Ash or Metakaolin) or alkali-activated materials such as ground granulated blast furnace slag (GGBS). The materials, partially carbonated (2 months in chamber at 1.5 % of CO2) or not, are put in contact with chloride solutions in presence of sulphate. This study focuses on the apparent chloride diffusion coefficients, as well as chloride binding isotherms which are obtained by the profile method. In addition, some aspects of the microstructure and of the pore structure are investigated, by using Mercury Intrusion Porosimetry and 27Al NMR, in order to better understand the results obtained relatively to the apparent chloride diffusion coefficients and to the chloride binding. Chloride ingress increases when sulphates are present in the contact solution for all cement materials tested (partially carbonated or not). In addition, chloride ingress is faster when the material is partially carbonated before contact with chloride solution. It appears that both carbonation or presence of sulphate decrease chloride binding, thus explaining the results. The results show an evolution of the properties as a function of the cement replacement ratio by SCMs.

Prediction of chloride binding isotherms for blended cements

Prediction of chloride binding isotherms for blended cements

Prediction of chloride binding isotherms for blended cements

Prediction of chloride binding isotherms for blended cements

Distance-associated chloride binding capacity of cement paste subjected to natural diffusion

Chloride binding capacity is usually quantified by chloride binding isotherms (CBIs). However, an important factor influencing chloride binding capacity, namely the internal chemical environment (ICE), cannot be represented in the conventional CBIs. Knowing the drawbacks of the conventional CBIs, this paper straightly put the emphasis on the distance-associated chloride binding capacity. Specimens undergoing natural diffusion were used for measuring the chloride binding capacity at different distance. CBIs, chloride binding profiles (CBPs), chemical profiles and microscopy result were performed to analyze the rules and mechanisms. From the results of CBIs and CBPs, it is found that the conventional laboratory-determined CBIs would overestimate the chloride binding capacity beyond a certain distance from the surface, and underestimate the chloride binding capacity near the surface. Based on the chemical profiles and the microscopy results, it is found that ICE changes greatly along with natural diffusion, and meanwhile the chloride binding capacity at different distance develops discriminatively with the ICE change. Plus, a hydroxyl-dependent chloride binding model is established to quantify the distance-associated chloride binding capacity. The model explains the difference between the conventional laboratory-determined CBIs and the natural-diffusion-determined CBIs, and it fits well with the test results.

Time/Depth Dependent Diffusion-Covection Model of Chloride with Transportation in Concrete Structures with Chloride-Binding Isotherms

Ion transport through a hydrated cement system often becomes a principal factor to degrade concrete in a chlorideladen environments. Based on the transport mechanism of chloride ion, an analytical solution of mathematical model for predicting chloride penetration into concrete structures was herein derived. The model consists of both the diffusion of chloride due to concentration gradient and the convection of chloride due to moisture transport. The model also depends on temperature, age, relative humidity, and chloride binding. Based on both the diffusion coefficient and the mean velocity with time/depth dependence, an analytical solution of the mathematical model developed by previous researchers was solemnly derived and used to predict the properties of concrete structures contaminated by chloride. The chloride penetration decreases as the time-dependence ratio increases. The amount of chloride accumulated in concrete increases as the relative humidity increases. Moreover, the proposed model may be effectively used for the practical evaluation and design for durability of concrete structures.

Impact of the associated cation on chloride binding of Portland cement paste

Well hydrated cement paste was exposed to MgCl2, CaCl2 and NaCl solutions at 20°C. The chloride binding isotherms for free chloride concentrations ranging up to 1.5mol/l were determined experimentally. More chlorides were found to be bound when the associated cation was Mg2+ or Ca2+ compared to Na+. The chloride binding capacity of the paste appeared to be related to the pH of the exposure solution. In order to explain the cation dependency of the chloride binding a selection of samples was investigated in detail using experimental techniques such as TG, XRD and SEM–EDS to identify the phases binding the chlorides. The experimentally obtained data were compared with the calculations of a thermodynamic model, GEMS. It was concluded that the measured change in chloride binding depending on the cation was mainly governed by the pH of the exposure solution and thereby the binding capacity of the C-S-H.

The influence of ion chloride on concretes made with sulfate-resistant cements and mineral admixtures

The aim of this work is to study the influence of chloride ion in the mechanical, microstructural and chemical properties of concrete. For this purpose, four concrete mixes were designed, with two types of sulfate-resistant cement and two mineral admixtures (silica fume and fly ash). Furthermore, a chloride binding isotherm is offered that is compared with Freundlich and Langmuir models. This provides certain advantages in reproducing experimental results. According to such results, the properties do not present significant changes in all cases. The concretes with blast-furnace slag cement and with silica fume are those that present the best performance.

The impact of sulphate and magnesium on chloride binding in Portland cement paste

The effect of magnesium and sulphate present in sea water on chloride binding in Portland cement paste was investigated. Ground well hydrated cement paste was exposed to MgCl2, NaCl, NaCl+MgCl2, MgSO4+MgCl2 and artificial sea water solutions with a range of concentrations at 20°C. Chloride binding isotherms are determined and pH of the solutions were measured. A selection of samples was examined by SEM-EDS to identify phase changes upon exposure. The experimental data were compared with calculations of a thermodynamic model. Chloride binding from sea water was similar to chloride binding for NaCl solutions. The magnesium content in the sea water lead to a slight decrease in pH, but this did not result in a notable increase in chloride binding. The sulphate present in sea water reduces both chloride binding in C–S–H and AFm phases, as the C–S–H incorporates more sulphates instead of chlorides, and part of the AFm phases converts to ettringite.

Chloride binding in sound and carbonated cementitious materials with various types of binder

Carbonation and chloride ingress are the main causes of reinforced concrete structures degradation. Usually, these two phenomena are studied individually. Here, chloride ingress has been investigated on sound and carbonated cementitious materials (cement pastes and concrete). The supplementary cementitious materials (SCM) are fly ash, slag and métakaolin. The materials were carbonated by exposure to accelerated conditions (1.5% CO2 and 65% RH) in laboratory. Chloride binding isotherms of sound and carbonated samples were assessed by the equilibrium method (exposure to various alkaline NaCl solutions). The microstructure was characterized not only by usual techniques such as XRD and TGA–DTA, but also by 29Si and 27Al NMR spectroscopy. The results show that carbonated samples bind less chlorides than sound ones. The use of a combination of various techniques of microstructural characterization reduces the number of the possible explanations of these findings. In carbonated materials, both the quantity of Friedel’s salts and of chlorides bound by C–S–H decrease. The first decrease can be explained by a lack of portlandite and modification of aluminate phases equilibrium. The second decrease seems to be due to a modification of the C–S–H (change of surface charge) during the carbonation process. Moreover, differences according to the nature and the amount of supplementary cementitious materials (SCM) were observed.

The effect of chloride binding on the predicted service life of RC pipe piles exposed to marine environments

Chloride binding capacity has a significant effect on the service life of reinforced concrete (RC) pipe piles exposed to marine environments. Four typical chloride binding isotherms (i.e., null, linear, Langmuir and Freundlich binding isotherms) were reviewed in this paper. The effect of chloride binding on service life predictions of RC pipe piles, in terms of these different binding isotherms, was investigated. An experimental program was conducted to verify the theoretical predictions developed in the present study. The free and total chloride concentration profiles were presented for RC pipe piles exposed to submerged and splash zones for 5, 10 and 30 years. Furthermore, the predicted service life of RC pipe piles exposed to submerged and splash zones, using different binding isotherms, was also presented in this paper. The results of the present study indicated that the chloride binding has a more significant effect on the service life of RC pipe piles in submerged zone than in splash zone, and RC pipe piles in submerged zone display longer service life than those in splash zone.

Chloride binding isotherms of various cements basing on binding capacity of hydrates

This study investigated the chloride binding isotherms of various cement types, especially the contributions of C-S-H and AFm hydrates to the chloride binding isotherms were determined. Ordinary Portland cement (OPC), Modified cement (MC), Rapid-hardening Portland cement (RHC) and Low-heat Portland cement (LHC) were used. The total chloride contents and free chloride contents were analyzed by ASTM. The contents of C-S-H, AFm hydrates and Friedel's salt were determined by X-ray diffraction Rietveld (XRD Rietveld) analysis. The results showed that OPC had the highest chloride binding capacity, and, LHC had the lowest binding capacity of chloride ions. MC and RHC had very similar capacities to bind chloride ions. Experimental equations which distinguish the chemically bound chloride and physically bound chloride were formulated to determine amounts of the bound chloride basing on chloride binding capacity of hydrates.

Modelling of chloride binding related to hydration products in slag-blended cements

An improved hydration model of slag-blended cements, taking into account new insights, is used to estimate and quantify the hydration products of slag-blended cements. Individual chloride binding isotherms are used to correlate these amounts of hydration products with the amount of bound chlorides. A number of parameters are directly or indirectly taken into account when estimating the chloride binding ability of a slag-blended cement paste: cement composition, slag content, water/binder ratio, curing age, and free chloride concentration. The model allows the study of the breakdown of bound chlorides by hydration products and their source – either the OPC or the slag.

Chloride binding isotherm from migration and diffusion tests

Chloride binding is often described by chloride binding isotherm, which is closely related to the service life of concrete structures in chloride environments. Many methods have been proposed to determine chloride binding isotherm. Compared to other methods, chloride binding isotherms obtained directly from nonsteady-state diffusion tests seem closer to the reality. We studied the chloride binding isotherm from both nonsteady-state electrical-accelerated migration and diffusion tests at different temperatures. Twelve concrete mixes with different supplementary cementing materials and water-to-binder ratios of 0.35, 0.48 and 0.6 were cast for study. The specimens after diffusion (or migration) tests were sliced layer by layer, and acid-soluble and free chloride contents of each layer were measured. A chloride binding isotherm was obtained from one specimen. Experimental results indicated that electrical voltage had a slight effect on the chloride binding isotherm of concrete. Temperature had a positive effect on chloride binding. The higher the water-to-binder ratio was, the higher the chloride binding was.

Ageing and durability of concrete in lab and in field conditions: investigation of chloride penetration

Long-term studies under various natural exposure conditions are still needed in order to understand and quantify ageing and durability of concrete structures, in particular in the case of mixtures which incorporate supplementary cementitious materials (e.g. fly ash). With this purpose, a large-scale and long-term experimental program was initiated a lot of years ago by the LPC network within the framework of the French national project “BHP 2000.” From then, further researches have been carried out in order to complement the program. The present paper focuses on chloride penetration in concrete samples in lab conditions and in reinforced concrete (RC) structural elements exposed for 10 years to two different natural environments (tidal zone in marine environment, as well as road and cold environment). Numerous mixtures (28 day cylinder compressive strengths ranging from 20 to 130 MPa) were studied in these various conditions, e.g. high-performance materials with or without silica fume (SF), as well as normal-strength fly-ash (FA) concretes. Carbonation and chloride penetration depths, along with total chloride concentration and degree of saturation profiles, have been measured on the RC structural elements and analyzed as a function of the exposure conditions and of the mix-parameters. In addition, the chloride data have been compared to lab results. Apparent chloride diffusion coefficients, as well as chloride binding isotherms and capacities, have also been assessed on these structural elements and compared to lab results. Good consistence between results in field and in lab conditions has been highlighted, except when carbonation has affected the field results. In addition, the good performance of SF-HPCs and mature FA concretes has been pointed out in the tested outdoor conditions, as well as that of the mature ground granulated blast furnace slag concretes tested in lab.

Prediction of chloride binding isotherms of cementitious materials by analytical model or numerical inverse analysis

In this paper, various methods of prediction of chloride-binding isotherms (CBIs) of cementitious materials at equilibrium and in saturated conditions are proposed. The first group of methods consists in numerical inverse analysis of an experimental total-chloride-concentration profile. In this group, a first method requires as input data the effective-chloride-diffusion coefficient (DCl−), while a second method predicts DCl−. Once DCl− and the CBI are assessed, profiles after other exposure times can be predicted. The second group of methods is an analytical model based on the composition of the material, which includes physical and chemical components. The “numerical” and “analytical” CBIs respectively obtained, as well as DCl− and the profiles predicted, have been compared to experimental data for various mixtures (with and without SCM) and ages. The excellent agreement observed between predicted and experimental results, as well as between “analytical” and “numerical” CBIs, demonstrates the reliability and accuracy of the proposed methods.

Simulation of Chloride Migration in Compression-Induced Damage in Concrete

The ingress of chloride ions in a harsh chloride environment leads to the corrosion of reinforcing steel bars and reduction in the service life of concrete structures. Prediction of the service life of chloride-exposed concrete structures is strongly dependent on chloride diffusion coefficient, which in turn is influenced by several factors including materials and environmental parameters, binding of chloride ions to the solid phase of concrete, and the stresses caused by external load acting on the structure. The diffusion of chloride in concrete is increased significantly by damage associated with load-induced stress, which, if not taken into account, can lead to erroneous prediction of the service life. This paper investigates the impact of compressive stress-induced damage on chloride transport in concrete. An experimental investigation involving non-steady-state migration test as per NT BUILD 492 was carried out on unloaded concrete specimens damaged under axial compressive load. Numerical simulation of the rapid chloride-migration test using multiphysics finite element software is presented in which a phenomenological damage model and a chloride-binding isotherm coupled with Nernst-Planck equation is used to evaluate chloride migration in the damaged concrete specimens.

New approach for coupled chloride/moisture transport in non-saturated concrete with and without slag

Over the paste decades a considerable research effort has been attributed to chloride ingress into reinforced concrete in order to understand the transport mechanisms and to predict its durability.In saturated concrete, chloride ions enter through diffusion (concentration gradient), whereas in partially saturated concrete, a part of the invading chloride is transported along with the salty water that was sucked by capillary absorption into the dried concrete. This study examines the effects of cyclic drying and wetting with chloride sodium on chloride ingress into non-saturated concrete. For this aim, a new automatic experimental setup has been developed to apply cyclic movement of chloride solution in controlled environmental conditions ambiance. Likewise, a new astute test method to control the concrete moisture content has been presented. As results, the first part of our study concerns chloride binding and water vapour adsorption isotherms. Results showed that Blast Furnace Slag (BFS), used as partial substitution of cement, increases the performances of concrete (BFSC60). Furthermore, the interaction chloride/concrete matrix seems to increase the adsorbed water vapour. In the second part of this research work, the influence of the moisture content, the number of cycles, the water-to-binder ratio and the slag’s addition on chloride profiles has been investigated and analysed. The results showed that the decreasing of relative humidity from 90% to 50% enabled to increase by ≈2–9times the apparent chloride diffusion coefficient, which decreased when the number of cycles increased. A correlation between apparent diffusion coefficient and relative humidity has been obtained.

Chloride binding related to hydration products: Part I: Ordinary Portland Cement

This study attempts to correlate the total amount of chloride bound in concrete with the amounts bound by different hydration products. New insights in the quantities of the hydration products formed will be used in order to predict the phase assemblage of hardened cement pastes. The two main chloride binding mechanisms are considered — through physical adsorption and through chemical reactions. Chloride binding isotherms are used, taking into account the external chloride concentration and the quantity of each hydration product formed.A distinction is made between the chloride binding capacities of monosulfate and hydroxy-AFm. Two new models for the maximum chloride binding ability of a hardened cement paste are proposed — a basic one, using only AFm and C-S-H isotherms, and an extended one which also considers the ability of portlandite and Friedel's salt to physically adsorb chlorides. Results obtained using these models are within 10% of collected experimental data.