Free Chloride Concentration Research Articles

Applicability and limitations of using seawater and sea sand in concrete

The concrete Industry is responsible for the large consumption of raw materials, such as limestone, potable water, steel, and sand, knowing that consuming these materials comes with large economic and environmental costs. Millions of tons of concrete are produced annually, requiring a significant quantity of water. Given the scarcity of fresh water (only 3% of the total water on earth), using seawater as an alternative can greatly conserve potable water. Similarly, replacing natural sand with sea sand can also help conserve natural resources. Consequently, using seawater and sea sand in concrete gained significant attention worldwide, particularly in countries with limited freshwater supply. Much research has been conducted on concrete produced with seawater and sea sand. Whereas seawater is beneficial in some aspects, it can enhance the free chloride content in concrete increasing the corrosion vulnerability. In this paper, the beneficial effects of using seawater and sea sand in concrete have been reviewed. The resulting mechanical, hydration, and durability properties have been assessed and compared with ordinary concrete. The focus has been not only on the material properties but also on the structural attributes. Prospects, challenges, and limitations have also been discussed. The study serves as a basis for efficiently using seawater and sea sand in concrete for sustainable development.

High-performance eco-friendly ternary blended green concrete in seawater environment

The use of blended green concrete with improved performance has significantly increased in the industrial sector owing to the CO2 gas emissions and carbon footprint caused by the production of ordinary Portland cement. Although improvements have been continuously made, the deterioration of concrete structures in seawater poses a serious concern. The long term performance of green concretes in seawater environment are scarcely studied. Here we report the long-term strength, durability and anti-fouling properties of a novel blended eco-friendly concrete made with ordinary Portland cement, fly ash, nanoparticles, and corrosion inhibitor in natural seawater for one year. The mechanical strength, alkalinity, free chloride concentrations, biofouling attachment, and total viability count are studied. After a year of exposure to seawater, the ternary blended green concrete showed improved mechanical strength, a significantly lower chloride penetration and water penetration depth, a 70% reduction in free chloride content, a high calcium silicate hydrate content, and a four-order reduction in the total biomass. Our results suggest that the new concrete mix is promising for applications in seawater environments because of its improved mechanical properties, durability, and biofouling resistance.

Chloride diffusion and binding capacity of sustainable cementitious materials with construction waste powder as cement replacement

Reusing construction waste powder (CWP) for sustainable cementitious materials is an effective method to reduce the amount of construction waste, and the CWP blended cementitious materials has been applied in construction engineering at a large scale and is inevitably exposed to a chloride attack environment. Therefore, this work studied the chloride diffusion and chloride binding capacity of cementitious materials incorporating CWP. The addition of CWP enlarged the pore diameter and porosity of blended paste, but mixing active admixture can refine the pore structure of CWP mixed paste. Incorporating CWP reduced the compressive strength and elevated total porosity of blended mortar, while the mix of active admixture improved the compressive strength and reduced the total porosity of CWP mortar. The chloride diffusion of mortar was elevated as CWP replacement rate and particle size increased, and the free chloride concentration increased but chloride diffusion coefficient decreased as chloride exposure time was prolonged. Mixing both CWP and silica fume or metakaolin could prepare sustainable mortar with a similar chloride resistance to plain mortar. The total chloride concentration of mortar was elevated as CWP was incorporated, whereas the chloride binding capacity reduced with the growth of CWP substitution rate. When CWP replacement percentage was identical, the mortar with CWP ground from cement paste waste had better chloride binding capacity than the mortar with CWP ground from mortar and concrete waste. Through optimizing CWP replacement and active admixture content, good strength and chloride resistance can be guaranteed in CWP blended cementitious materials.

Time and Crack Width Dependent Model of Chloride Transportation in Engineered Cementitious Composites (ECC).

This paper aims to develop a chloride transport model of engineered cementitious composites (ECC) that can consider the influence of both exposure time and crack width. ECC specimens with crack widths of 0.1 mm, 0.2 mm and 0.3 mm were soaked into NaCl solution with periods of 30, 60, 90 and 120 days. The free chloride content profile was measured and used for the development of the transport model. Regression analysis was applied to build the time and crack width dependent models of apparent diffusion coefficient and surface chloride content. The results show that the crack width has significant influence on the free chloride concentration profile when it is above 0.2 mm and the time-dependent constant n decreases linearly with the crack width. The chloride transport model was obtained by subscribing the models of apparent diffusion coefficient and surface chloride content into the analytical solution of Fick’s second law. The model was further validated with the experimental results, showing a deviation within 20%. The findings of the presented study can enhance the current understanding on the chloride transportation in ECC.

An experimental study on the influence of continuous ambient humidity conditions on relative humidity changes, chloride diffusion and microstructure in concrete

Most engineering structures are exposed to various harsh environments, such as temperature fluctuations and humidity cycles, simultaneously. Additionally, moisture transfer and corrosive ion transport in concrete are driven by humidity gradients. Therefore, studying Chloride transport in concrete under constant humidity conditions is a meaningful research project. In this work, a new experimental setup was designed, and the Chloride diffusion behavior of concrete under a continuous humidity environment and NaCl immersion was investigated. Meanwhile, to accurately evaluate the internal humidity of concrete, humidity sensors were applied to the concrete. Additionally, the impacts of the water-binder ratio (w/b) and ambient humidity on the humidity diffusion coefficient, free Chloride content and bound Chloride content in concrete were evaluated. The microstructure was characterized by scanning electron microscopy, X-ray diffraction and mercury intrusion porosimetry. The results show that the content of free Chloride increases with an increasing w/b ratio. The humidity diffusion coefficient of concrete during water absorption (continuous high humidity environment) is significantly higher than that during water loss (continuous dry environment). Furthermore, under the drying condition, the Chloride content and the humidity diffusion coefficient on the concrete surface gradually increased, while during the wetting procedure, the Chloride content inside the concrete increased, and the humidity diffusion coefficient gradually decreased. However, the bound Chloride content inside the concrete is not affected by the humidity level in the environment. In the wetting environment, the calcium hydroxide in the matrix is gradually consumed, and the Chloride diffuses to the matrix to form more Friedel's salt and calcium carbonate. Moreover, lowering the w/b ratio or increasing the ambient humidity can enhance the formation of more hydrated compounds (C–S–H gel), which can reduce the total porosity and can also improve the ability of concrete to resist Chloride diffusion. Overall, this study provides a better understanding of and insight into the design and maintenance of seaside RC infrastructures.

Evaluation of corrosion resistance of precast reinforced concrete subjected to early-age ambient pressure carbonation curing by accelerated impressed current method

In this paper, early-age ambient pressure carbonation curing was successfully developed to lower the corrosion risk by avoiding deep CO2 penetration and preserving high pH around the steel surface. The effect of ambient pressure carbonation on the resistance of chloride-induced rebar corrosion was evaluated by applying an accelerated impressed current (IC) as a trigger. Due to improved physical properties, carbonation-cured concrete had a lowered current reading, a reduced rebar mass loss, a lessened total or free chloride content, and a decreased chloride diffusivity. Its time-to-corrosion-initiation and corrosion-propagation stages were also extended, which could strengthen the structural integrity and extend the residual service life of reinforced concrete components after corrosion initiation. Ambient pressure carbonation-cured concrete is corrosion resistant while still having the capacity to sequestrate carbon dioxide, which could contribute to the sustainability and circular carbon economy.

Reduction of SO42− and Cl− migration rates and degradation of silica nanoparticles incorporated cement pastes exposed to co-existence of sulfate, chloride and electric fields

The reinforced concrete structures in subway engineering are generally subjected to combined exposure of chloride, sulfate and electric fields. Thus, it is necessary to reveal a way of enhancing corrosion resistance of concrete in this complicated environment condition. Although improvement of concrete properties by addition of nano silica is well known, how it can lead to the reduced transport rates of aggressive ions in the cementitious systems has not been systematically confirmed. In this work, the strengthening mechanism of nano-SiO2 on the erosion resistance of cement pastes was investigated in terms of water absorption, distribution of sulfate concentration, free chloride concentration and bound chloride concentration, ionic diffusion coefficients, crystalline phase composition, thermal gravimetric and Vickers hardness. The results indicated that the sulfate concentration exhibited a decreased trend with increasing the depth from exposed surface, while both free chloride and bound chloride concentrations first increased and then decreased gradually. The modification of silica nanoparticles on the cement pastes was remarkable, and 3% nano-silica admixed binary mixtures exhibited a significant reduction of the ionic migration rates and the accompany degradation. Moreover, replacement of 1% nano silica to cement due to pozzolanic reaction and nucleation effect dramatically led to the effective chloride capture near the exposed surface as a result of the declined porosity, which can be substantiated by X-ray diffraction and thermal gravimetric results. The results of this work are beneficial for establishment of a finer transport model in further research and provide a reference for monitoring and evaluation of durability of reinforced concrete structures serving in subway engineering.

Experimental and theoretical investigation of chloride ingress into concrete exposed to real marine environment

Chloride-induced corrosion is a critical issue that influences the durability and mechanical performance of reinforced concrete (RC) structures exposed to marine tidal and splash environments. This paper presents an experimental and theoretical investigation on the chloride transport behavior in concrete exposed to an aggressive marine environment for up to 720 days. Field exposure tests were performed in marine tidal and splash zones, and free chloride concentration profiles for different types of concrete after several exposure times, i.e. 90, 180, 360 and 720 days, were obtained. The effects of the water-to-cement ratio, binder type, exposure environment and exposure time on the time dependency of the chloride diffusion coefficient (D) and surface chloride concentration (Cs) were analyzed and discussed. Based on the obtained results, a modified time-dependent prediction model of chloride ingress into concrete was established and validated by considering the time-dependency effect of material and environmental factors. The results indicate that the comparisons of the predicted chloride profiles using the developed time-dependent prediction model were in good agreement with the results of the real marine exposure test.

Effect of air-entrainment and phase transition on chloride diffusion in partially frozen concrete

In some regions concrete is partially frozen for several months each year, yet there is paucity of information about the effect of air entrainment on chloride diffusivity and binding in partially frozen concrete. To address this issue, concrete without and with entrained air were subjected to chloride diffusion under constant exposure temperatures of +5, 0, −5, and −15 °C and their total and free chloride concentrations were determined. The entrained air reduced chloride diffusivity by 18% when the concrete evaporable water was either unfrozen or nearly fully frozen, but when it was partially frozen it reduced it by only 3%. Non-air entrained concrete exhibited higher binding capacity than air entrained concrete under all the test temperatures. After freezing, the effect of temperature on concrete diffusivity cannot be captured by Arrhenius law alone. A procedure is proposed to account for this deviation and the predicted diffusion coefficients agree reasonably with the experimental values.

Exposure duration and sub-zero temperature effects on concrete chloride diffusion decay index and binding

Accurate prediction of corrosion initiation period and service life of reinforced concrete structures requires accurate determination of their chloride diffusion coefficient and binding capacity. Temperature and exposure duration affect these properties, but there is paucity of knowledge regarding the effects of long-term exposure to sub-zero temperatures. Hence, the effects of sub-zero temperature, T, and exposure duration, t, on the chloride diffusion decay index, m, and binding in ordinary Portland cement concrete are investigated. Non-steady natural diffusion tests are conducted at + 5, 0, −5 and −15 °C for 3, 6, 12 and 18 months, and at the end of each period, the total and free chloride concentration profiles are determined. The index m is found to vary over a wide range, it being directly proportional to T and inversely to t. It drastically deviates from the commonly assumed value of 0.2 in the literature. Chloride binding is found to obey the Freundlich isotherm. The coefficient α of the isotherm equals 1.0 ± 0.16 and is only slightly dependent on T or t while its exponent β varies from 0.178 to 0.502, without exhibiting a systematic relationship with either parameter. Binding is noticed to increase with drop in temperature for the first three months of exposure, but over longer exposure durations, no systematic relationship is observed.

Using a geochemical model for predicting chloride ingress into saturated concrete

Chloride ingress into concrete was simulated based on a geochemical model that includes Fick's second law for transport phenomena and thermodynamic equilibrium, kinetics treatment and surface complexation for chemical phenomena. The geochemical model was validated by comparing the numerical results and experimental data (extracted from the literature) for CEM II concrete exposed to sodium chloride, potassium chloride, magnesium chloride and calcium chloride solutions. The geochemical model was then applied to investigate the influences of different chloride solutions. The results showed that exposure to magnesium chloride solution led to the greatest amount of Ca2+ leaching. Relationships between the total chloride content (TCC) and the free chloride content (FCC) of concrete were established. Increasing the exposure time of the concrete was found to lead to a decrease in chloride binding capacity. The proposed correlations allow determination of the FCC or the TCC of concrete when one of the two is known. The geochemical model was also verified by predicting the chloride content of a CEM III concrete bridge submerged in seawater after 17 and 34 years. With numerous applications, the geochemical model seems a reliable tool for the durability design of concrete structures exposed to chloride environments.

Hydration, Microstructure and Compressive Strength Development of Seawater-mixed Calcium Sulphoaluminate Cement-based Systems

In-depth understanding of the calcium sulphoaluminate (CSA) cement, a type of environment-friendly cement, is essential for its wide application in engineering practice. Partial replacement of CSA by alternative alumina-rich powders has significant potential in the production of cost-effective CSA cement. In this work various supplementary cementitious materials (SCMs) including slag, metakaolin and silica fume with different replacement levels were added into CSA cement, and mixing with seawater, to prepare paste and concrete specimens. The development of hydration products in the paste specimens was characterized by X-ray diffraction and Thermogravimetric analysis. The pore structures of the seawater-mixed paste specimens obtained from mercury intrusion porosimetry tests, along with the pH value and free chloride content in the pore solution, were analyzed. Results show that various SCMs have significantly different effects on the pore solution chemistry, hydration and microstructural formation of the seawater-mixed CSA cement, resulting in different mechanical behavior and durability properties. The results of this work are helpful for the development of cost-effective CSA cement-based concrete used in marine constructions.

Determination of free chloride in seawater cement paste with low water-binder ratio

Water extraction method has been widely applied to determine the free chloride content in cement-based materials. This paper studies the effects of different extraction time, curing ages and w/b ratios on the obtained free chloride content of seawater cement paste with low water-binder ratio by water extraction method. An appropriate testing process of water extraction method for pore solution acquisition and measurement of cement paste with low water-binder ratio is determined and the results are compared with that obtained by pore pressing method. The results show that the water extraction test is suitable for free chloride measurement of cement-based materials with low water-binder ratio by controlling the extraction time within a certain range about 3 min. The obtained results by water extraction with 3 min of extraction time are comparable to those obtained by pore pressing method. However, the physically bound chloride content decreases after water extraction and pore pressing tests, which indicates that the water extraction and pore pressing methods overestimates the free chloride content in cement-based materials. The effect of w/b ratio and curing age on the accuracy of this method is not evident.

Influence of carbonation on chloride binding of mortars made with simulated marine sand

This study investigated the effects of carbonation on the chloride binding of mortars containing simulated marine sand. Various admixtures were added, and metakaolin (MK), fly ash (FA), nano-Al2O3 (NA), and aluminum (A) were considered as supplementary cementitious materials (SCMs) for use in mortars. Changes in free chloride content, total chloride content, bound chloride content, chloride binding rates, and pH values as a function of mortar depth were analyzed, and the effects of carbonation on the hydration products were also assessed. The results indicated that in the place of cement, SCMs could significantly increase the carbonation zone depth in mortar. The chloride ions migrated outward and inward under accelerated carbonation conditions in the mortar, generating two peak values for the chloride content in the mortars. Consequently, the incorporation of SCMs was beneficial for chloride ion migration in mortars under carbonation. Additionally, the bound chloride content and chloride binding rates were initially extremely low, then gradually increased, and finally stabilized with increased depth in the mortars under carbonation. Furthermore, the incorporation of SCMs resulted in the decreased chloride binding capacity of the mortars under accelerated carbonation, and Friedel’s salt gradually decomposed with carbonation.

Aerobic Liquor Washing Improves the Quality of Crude Palm Oil by Reducing Free Fatty Acids and Chloride Contents

AbstractThe presence of free fatty acids (FFA) and chloride contents in crude palm oil is not desirable because they have an impact on oil quality and food safety. This work presents a method to reduce these compounds by washing the crude palm oil (CPO) with treated aerobic liquor (AL). The effects of process parameters on the reduction of FFA and chloride in CPO and the resultant 3‐monochloropropane‐1,2 diol ester (3‐MCPDE) formed after refining are investigated. The results show that the AL dosage, initial FFA content, mixing speed, and duration have significant influence on the reduction of FFA in the CPO. Meanwhile, the chloride content is reduced by approximately 50% regardless of the AL dosage used. Consequently, the 3‐MCPDE content in the oil after refining is up to 53% lower than that of the refined oil produced from the untreated CPO. Furthermore, an oil recovery above 97% can be achieved after the AL‐washing step. The implementation strategy of this method in the palm oil mills has also been proposed. In conclusion, an effective and sustainable method for in situ improvements of the quality and food safety of palm oil has been developed without the need for additional water or chemical.Practical Applications: Treated aerobic liquor can be used to wash the CPO in palm oil mill for in situ reductions of FFA and chloride contents in CPO to improve the oxidative stability of CPO. The lower content of chlorides in CPO could mitigate the formation of 3‐MCPDE during refining, thus improving the food safety of palm oil. This method can readily be implemented by the industry and it is sustainable because it does not require the use of additional process water or chemical.

Chloride Binding Capacity and Its Effect on the Microstructure of Mortar Made with Marine Sand

Chloride binding capacity and its effect on the microstructure of mortar made with marine sand (MS), washed MS (WMS) and river sand (RS) were investigated in this study. The chloride contents, hydration products, micromorphology and pore structures of mortars were analyzed. The results showed that there was a diffusion trend for chloride ions from the surface of fine aggregate to cement hydrated products. During the whole curing period, the free chloride content in the mortars made by MS and WMS increased firstly, then decreased and stabilized finally with time. However, the total chloride content of three types of mortar hardly changed. The bound chloride content in the mortars made by MS and WMS slightly increased with time, and the bound chloride content included the MS, the WMS and the RS arranged from high to low. C3A·CaCl2·10H2O (Friedel’s salt) was formed at the early age and existed throughout the curing period. Moreover, the volume of fine capillary pore with a size of 10–100 nm increased in the MS and WMS mortar.

Anodic Dissolution of Copper in the Acidic and Basic Aluminum Chloride 1-Ethyl-3-methylimidazolium Chloride Ionic Liquid

The anodic dissolution of copper was investigated at a copper RDE in the Lewis acidic and basic composition regions of the room-temperature AlCl3-EtMeImCl ionic liquid (IL) to assess the utility of chloroaluminate liquids as solvents for the electrochemical machining and electropolishing of copper. In the Lewis acidic IL (60 mol % AlCl3), the dissolution of Cu0 proceeds under mixed kinetic-mass transport control with an exchange current density of 7.00 mA cm−2 at 306 K and an apparent activation free energy of 19.7 kJ mol−1. A formal potential of 0.843 V was obtained for the Cu+/Cu0 reaction from potentiometric measurements. In the basic IL (< 50 mol % AlCl3), potentiometric measurements showed that the oxidation of Cu0 resulted in the formation of [CuCl2]−. In this case, the formal potential of the [CuCl2]−/Cu0 reaction is −0.412 V. At small positive overpotentials, the reaction exhibited mixed control and was first order in the chloride concentration, indicating that only a single Cl− is involved in the RDS. However, at more positive overpotentials, the reaction transitions to mass transport control, and a well-defined limiting current is observed for the anodization process. This limiting current scales linearly with the free chloride concentration in the IL.

Effect of carbonation in high chloride-binding capacity mortars subjected to a marine environment at early ages

This paper presents an experimental research on the effect of carbonation on the chloride penetration in mortars exhibiting a refined porous network and high chloride-binding capacity. The research was focused on the use of metakaolin combined with nanosilica, evaluating the influence of the surface area of the nanoadditions. It was observed that the addition of nanosilica reduces the binding capacity of metakaolin, this reduction was influenced by the type of nanosilica. The analysis of the carbonated samples following carbonation periods of three and six months allowed observing that the free chloride content as well as the bound chloride content are diminished with carbonation. Thence, it can be concluded that the carbonation process hinders the chloride penetration, which can lead to a reduction with time in the chloride concentration of samples.

Concrete Examination of 100-Year-Old Bridge Structure above the Kłodnica River Flowing through the Agglomeration of Upper Silesia in Gliwice: A Case Study.

The article analyzes the composition of concrete taken from various elements from a 100-year-old bridge in South Poland, so as to analyze its technical condition. The main methods applied during experimental work were: Designation of pH, free chloride content, salinity, XRD and SEM examinations, as well as metals determination using inductively coupled plasma mass spectrometry (ICP­MS), high-performance liquid chromatography (HPLC)-ICP-MS, and cold-vapor atomic absorption spectroscopy (CV-AAS). The concrete of the bridge was strongly carbonated and decalcified with an extremely high content of chlorides. The pH of the concrete was in a range from 10.5 to 12.0. Acid soluble components were between 9.9% and 17.6%. Typical sulfate corrosion phases of concrete were not detected. Friedels’ salt was found only at the extremity of an arch. The crown block was corroded to the greatest extent. Various heavy metals were absorbed into the concrete, likely from previous centuries, when environmental protection policy was poor. The applied research methodology can be used on bridges exposed to specific external influences. The acquired knowledge can be useful in the management processes of the bridge infrastructure. It can help in making decisions about decommissioning or extending the life cycle of the bridge. This work should also sensitize researchers and decision-makers to the context of “bridge safety”.

Corrosion prediction models for steel bars in chloride-contaminated concrete: a review

Concern has been expressed in the recent decades about the corrosion prediction of concrete structures in a chloride environment. This paper summarises the corrosion prediction models developed for chloride-contaminated concrete, including both the empirical and electrochemical corrosion models. The definition methods of the steel de-passivation process for electrochemical models, such as anode–cathode ratio (Aa/Ac), critical chloride concentration (CCl,crit), modification of anodic Tafel slope (βa) or passive film resistance (Rf), are considered essential and discussed in detail. Furthermore, electrochemical parameters regarding the definition and selection in electrochemical models and experimental measurements are discussed. This study confirms that Aa/Acand CCl,crithave been generally used in recent years, and that the modifications of βaand Rfrelated to free chloride concentration are technically feasible. In addition, the discreteness of electrochemical parameters in electrochemical models and experimental measurements are observed, and time-dependent characteristics of electrochemical parameters are scarcely presented in the existing literature.