Chloride Binding Capacity Research Articles

Experiment and simulation on the coupled effects of calcium leaching and chloride transport in concrete under hydraulic pressure

This study investigated the coupled effect of calcium leaching and chloride erosion on concrete subjected to hydraulic pressure by combining experiments and numerical simulations. Several tests including titration, pH, XRD, TG, MIP, and SEM-EDS were employed to analyze chloride concentration, pH value, solid phase compositions, and microstructure of concrete under hydraulic pressure. Concurrently, a model based on the physicochemical interactions between the pore solution and the hydration products was constructed to elucidate the process of calcium leaching and multi-ion transport. The experimental and simulation results reveal that hydraulic pressure accelerates calcium leaching in concrete, leading to a maximum porosity that reaches 1.5 times the initial porosity after a year. In addition, both the pH value and chloride binding capacity in the zone close to the exposure surface decrease. The enrichment of Ca2+ and OH− occurs at a specific depth within concrete during the calcium leaching process, and over time, this enrichment effect grows increasingly significant. Along the depth within the concrete, a transient increase in chloride binding capacity can be observed, which can be attributed to OH− and Ca2+ enrichment.

The coupling effect of calcined layered double hydroxides and impressed current cathodic protection system on chloride ion binding and migration of sea sand mortar

Chloride introduced by sea sand and sea water leads to steel bar corrosion. Calcined Layered double hydroxides (CLDH) and impressed current cathodic protection (ICCP) system were used to adsorb chloride ion and control steel bar corrosion in chloride abundant environment. In this research, chloride binding effect of two kinds of CLDH were investigated in simulated concrete pore solution. Besides, chloride absorption capacity and steel bar corrosion resistance of Mg-Al CLDH was explored in mortar. Further, hydration degree of mortar installed ICCP system and migration of ions around the anode and cathode were detected. Results show that Mg-Al CLDH exhibits excellent chloride binding capacity and corrosion resistance. After installing the ICCP system, corrosion of steel bars in mortar mixed Mg-Al CLDH was controlled. And ions in mortar specimen around anode or cathode exhibits migration phenomenon in different degree. A two stage-four step model was established to explain the degradation of hydration products and ion migration in CLDH-ICCP system. Under the ICCP system, Cl− in pore solution increased at early stage and then decreased both around anode and cathode, the increment process was controlled by desorption of Mg-Al CLDH and decreased process was controlled by degradation of hydration products. Therefore, the CLDH-ICCP system was recommended to solve the problem of steel bar corrosion introduced by marine environment.

Effects of Na2SiO3/NaOH ratio in alkali activator on the microstructure, strength and chloride ingress in fly ash and GGBS based alkali activated concrete

In this study, the impact of alkaline solutions composed of various Na2SiO3/NaOH ratio and NaOH molarity on the strength, chloride ingress and chloride binding capacity of fly ash (FA) and ground granulated blast furnace slag (GGBS) based alkali activated concrete (AAC) was investigated. The microstructural properties of alkali activated binders (AAB) composites was also studied to understand its influence on the chloride permeability of AAC. Chloride permeability of concrete was tested by using the rapid chloride penetration test (RCPT) and by immersing concrete in NaCl solution (ASTM C1556). The compressive strength, chloride ingress and chloride binding capacity of AAC was compared with Portland cement-based concrete (OPC). Results showed that higher Na2O/SiO2 ratio in alkaline solutions resulted in a weak microstructure with, low strength and high chloride ingress. Concrete activated by alkaline solutions with Na2O/SiO2 molar ratio =1 (+3), exhibited greater compressive strength and resistance to chloride ion penetration. The increase of SiO2 concentration in alkaline solution greatly reduces the binding capacity of AAC while the bound chlorides increased with the NaOH molarity. No chemical chloride binding was observed in AAC but for OPC concrete, calcium salts formed in the microstructure after immersion in NaCl solution. Compared to OPC concrete, AAC showed superior performance in terms of strength and chloride diffusion making it a potential candidate for application in marine structures.

Enhancing cementitious materials with polymer dots: Size effects on strength and chloride resistance

Polymer dots (PDs), as emerging carbon-based nanomaterials, show promise for improving cement material properties due to their small size, excellent dispersity, and affordability. However, the relevant research remains uncovered. Especially, the impact of PDs particle size on mechanical performance and chloride binding capacity is still blank. In light of this, this work offers a comprehensive insight into the size effects of PDs on the mechanical properties and chloride binding capacity of cement pastes, along with the corresponding influencing factors. Specifically, PDs with three particle sizes of ca. 13.69, 5.81, and 2.55 nm (namely, LPDs, MPDs, and SPDs) are efficiently synthesized by altering the reaction temperature based on the Schiff base reaction. In comparison with the blank group, the mechanical properties of paste specimens containing LPDs, MPDs, and SPDs are improved by 3.8 %, 12.9 %, and 16.5 % separately, and their chloride binding capacity are significantly enhanced by 53 %, 117 %, and 129 % respectively. More significantly, the relevant influence mechanism is that smaller-sized PDs can more effectively exert their nucleating effect to further facilitate the formation of C–S–H, further resulting in a denser pore structure and enhanced chloride physical adsorption in cement matrices. Meanwhile, the smaller-sized PDs are conducive to promoting Friedel's salt generation, dramatically improving the chloride chemical binding. This research would provide comprehensive guidance on synthesizing PDs suitable for cementitious materials to enhance their strength and anti-chloride-corrosion properties more efficiently, thereby fostering the development of high-performance cement composites.

Insight into the dynamic evolution of competitive interaction between chloride and sulphate ions in cement-based systems: Effects on chloride diffusion and binding

The dual attack of chloride and sulphate ions is accompanied by the dynamic competitive interaction, yielding a non-negligible influence on their penetrations. Treating this interaction as a steady behaviour will underestimate the ionic ingress and then, overestimate the lifespan of concrete structures. In this study, the evolution of competitive interaction and the associated effect on the chloride diffusion and binding capacity have been investigated by comparing the exclusive chloride and the combined chloride-sulphate attacks. The experimental results disclose that the competitive interaction displayed a dynamic feature. Specifically, the competitive-induced depressions on chloride diffusion and binding capacity evolved first and then appeared to decay after reaching a threshold. Moreover, this competitive interaction was positively related to the w/c ratio and the environmental SO2-4/Cl- ratio. As the slag dosage increased from 0 % to 50 %, the competitive interaction-induced depression upon the free chloride content decreased steadily. However, the inhibition efficiency of the binding capacity decreased first as the slag dosage increased to 20 % and then, evolved significantly with the further increase through 20 % to 50 %. Furthermore, multifactor models are established with good applicability, to respectively predict the effect of this competitive interaction on the diffusion and binding of chloride ions.

Study on the microstructure and impermeability of calcium aluminate cement containing metakaolin for development of high-performance marine engineering materials

To provide a high-performance marine engineering material, the microstructure and impermeability of calcium aluminate cement (CAC) containing metakaolin (MK) were studied. The chloride penetration resistance of the CAC-MK and Portland cement (PC) mortars was assessed based on physical and chemical barrier effects. The results show that the alumina in MK is more easily dissolved than silica, which inhibits the transformation of CAH10 and prevents microstructural deterioration. Adding 7.5% and 15.0% MK reduces the volume of large pores in the 3-day-old CAC paste by 49.8% and 69.1% while increasing the volume of pores smaller than 50 nm by 175.6% and 242.5%, respectively. Moreover, the microstructure of MK-containing CAC pastes becomes denser with age when cured at 40 °C, enhancing the mechanical strength and water penetration resistance. The CAC mortar containing MK exhibits better resistance to chloride attack than PC mortar, due to the stronger chloride binding capacity and migration resistance.

Effects of Mg-based admixtures on chloride diffusion in alkali-activated fly ash-slag mortars

This study evaluated the effects of various Mg-based admixtures, including magnesium oxide (MgO), layered double hydroxides (LDH), and calcined layered double hydroxides (CLDH), on the chloride resistance of alkali-activated fly ash-slag (AAFS) mortars. To this end, chloride diffusion tests were conducted on AAFS mortars containing 5 wt% MgO, Mg-Al-CO3 LDH, and CLDH. The results revealed that all Mg-based admixtures investigated in this study enhanced the chloride resistance of AAFS mortars. The inclusion of MgO led to a finer pore structure and promoted the formation of more Friedel's salt, thereby enhancing the chloride resistance performance. Although the inclusion of Mg-Al-CO3 LDH and CLDH resulted in more lager pores in the matrix, leading to slightly lower compressive strength, the overall content of Mg-Al LDH and Friedel's salt formed in the system increased, which led to an improved chloride binding capacity and enhanced chloride resistance.

Analysis of service life and reliability of C50CASC structures in the splash zone of the South China Sea

This paper presents the results of a reliability analysis of the service life of hydraulic structures made of C50 coral aggregate seawater concrete (CASC) in the splash zone of the South China Sea. The analysis was conducted using the ChaDuraLife V1.0 software, which is based on reliability theory and modified chloride diffusion theory. Several parameters such as the surface free chloride concentration (Cs), chloride binding capacity (R), free chloride diffusion coefficient (Df), apparent chloride diffusion coefficient (Da), and time-dependent index (m) were considered in this analysis. The (1-m) boundary condition is adopted in this study. The effect of concrete cover thickness on the service life of CASC structures was discussed. The influence of rebar types and external protective measures on the service life was also investigated. Durability recommendations for C50CASC hydraulic structures in the splash zone were proposed. The minimum concrete cover thickness needed to achieve the service life of 30a to 100a was determined to provide theoretical support for the application of CASC in island and reef engineering. The results found that the best technical measures for extending the service life were the application of 2205 duplex stainless steel bars and silane protective coating on the surface of concrete structures. A concrete cover thickness of 6–7 cm could achieve the service life of 50a, while a thickness of 8.5–10 cm, could achieve the service life of 100a.

Development of high-dispersion CLDH/carbon dot composites to boost chloride binding of cement

Chloride-induced corrosion of steel reinforcement significantly contributes to the durability deterioration for reinforced concrete (RC) structures. Calcined layered double hydroxide (CLDH) possesses a distinctive structure memory effect to adsorb anions and a nucleation effect to facilitate cement hydration, thereby demonstrating a promising potential for chloride binding enhancement of cementitious materials. However, CLDH suffers from severe agglomeration in cement composites, limiting its performance efficacy. To tackle this problem, novel high-dispersity CLDH/carbon dots (CDs) composites (CC) only at small dosages are applied to greatly improve the chloride binding behavior of cement pastes for the first time. In detail, the specific surface area of CC synthesized by simply stirring CDs and CLDH improves by 166.4 % than that of pure CLDH. Remarkably, cement pastes containing CC display a preferable chloride binding capacity; And the chloride binding substantially increases by 52.41 % when the incorporation of CC (by weight of cement) is 0.8 wt%. More importantly, according to the phase composite and microstructure analyses, the chloride binding enhancement mechanism of CC-containing cement pastes is reasonably ascribed to two effects of CC: providing nucleation sites to accelerate cement hydration and exerting the inherent chloride adsorption capacity. This boosts the formation of C–S–H gels and improves the structure memory effect and anions exchange-induced chloride adsorption, thus significantly strengthening the chloride binding of cement. This work provides a novel approach to improve the dispersity of CLDH, effectively strengthening the chloride binding of cement, thereby conductive to prolonging the service life of RC structures.

Experiments on Chloride Binding and Its Release by Sulfates in Cementitious Materials.

The aim of this study was to experimentally investigate the process of chloride binding and its sulfate-induced release in cementitious materials. The cementitious materials were replaced with hardened cement paste particles (HCPs) with water-to-cement ratios (w/c) of 0.35 and 0.45. A long-term immersion experiment of HCPs in 0.1 M sodium chloride solution was performed to investigate its chloride-binding capacity, and then it was immersed in sodium sulfate solutions with concentrations of 0.1 and 0.5 M to explore the release of chloride binding induced by sulfates. Silver nitrate titration and quantitative X-ray diffraction (QXRD) were used to measure the concentration of free chlorides in the solutions and the content of bound chlorides in HCPs, respectively. The results show that there is a higher chloride-binding capacity in HCPs with a w/c ratio of 0.45 compared to 0.35, and the content of chemically bound chlorides is associated with the formation and decomposition of Friedel's and Kuzel's salts in HCPs. The presence of sulfates can easily result in the release of bound chlorides in Friedel's salt, but it cannot cause a complete release of bound chlorides in Kuzel's salt. Physically bound chlorides are more easily released by sulfates than chemically bound chlorides, and a high w/c ratio or sulfate concentration can increase the release rate of bound chlorides in HCPs.

Valorization of industrial wastes for sustainable cement-based materials with enhanced chloride binding

Improving the chloride binding capacity of cement-based material can effectively slow down the corrosion of steel bars in concrete. This study investigated the valorization of two industrial wastes, carbonated sintering red mud (CSRM) and bauxite tailings (BT), as supplementary cementitious materials (SCMs) to enhance the chloride binding capacity of cement-based materials. CSRM and BT were incorporated separately and in combination to prepare cement paste mixtures, replacing 10–30 % of ordinary Portland cement. The bound chloride contents were determined, and the hydration products were characterized using X-ray diffraction and thermogravimetric analysis. Both CSRM and BT promoted the formation of Friedel's salt (FS) through increased aluminum dissolution and conversion of monocarbonate/monosulfate phases. In the early ages, the synergistic effect of CSRM and BT resulted in the highest FS content. At later ages, BT demonstrated superior chloride binding due to its high alumina content facilitating continued FS formation. Furthermore, the adverse effect of CSRM on the composite system was less than that of BT. At 28d age, when the SCMs content was 30 %, the compressive strength of cement-BT system was 34.5 % lower than the control, while the compressive strength of cement-CSRM system was only 20.9 % lower than the control. The findings highlight the potential of valorizing CSRM and BT as sustainable SCMs for developing chloride-resistant cement-based materials.

Chloride Binding Behavior and Pore Structure Characteristics of Low-Calcium High-Strength Cement Pastes.

While Portland cement produces large amounts of carbon dioxide, low-calcium high-strength cements effectively reduce carbon emissions by decreasing the proportion of high-calcium minerals. In order to enhance the practical application value of low-calcium high-strength cement, the effects of mineral admixtures on the chloride binding capacity and pore structure characteristics of low-calcium high-strength cement pastes were investigated by equilibrium method and mercury intrusion method. The results showed that the chloride binding capacity of low-calcium high-strength cement pastes is superior to that of Portland cement. Fly ash and slag enhance this capacity by promoting monosulfoaluminate and C-S-H gel formation, with fly ash being more effective. Ground limestone also boosts chloride binding when incorporated at less than 10 wt%. However, sulfates have a more significant negative impact on chloride binding capacity in low-calcium high-strength cement pastes compared to Portland cement. The porosity of low-calcium high-strength cement pastes exhibits contrasting trends with the addition of fly ash, ground limestone, and slag. Fly ash and limestone initially coarsen the pore structure but later facilitate the transition of larger pores to smaller ones. In contrast, slag initially has little impact but later promotes the conversion of large capillary pores to medium ones, optimizing the pore structure. Notably, above 10 wt% fly ash, the critical pore diameter decreases with additional fly ash except at 10% where it increases for 3 days. Ground limestone enlarges the critical pore diameter, and this effect intensifies with higher content. During early hydration, slag decreases the critical pore diameter, but its impact diminishes in later stages.

Chloride binding behavior and mechanism of phosphoaluminate cement clinker and its hydration products

As one of potential materials for extending the lifetime of reinforced concrete serving in chloride-rich environments, phosphoaluminate cement (PAC) offers excellent chloride binding capacity and superior resistance to chloride penetration. The chloride binding behavior as well as binding routes and mechanisms of the clinker and its hydration products had not been clarified so far. This study revealed that the clinker would preferentially form Friedel's salts through a dissolution-precipitation route in NaCl solutions, thereby binding chloride. This dissolution-precipitation process led to an accumulation of Al3+ in solutions, which negatively affected the subsequent hydration reactivity of the clinker. In the hydration products, the calcium hydroaluminates, mainly as C2(A,P)H8 with minor C(A,P)H10, could bind chloride rapidly via an ion exchange mechanism. Through the ion-exchange route, the chemical binding of calcium hydroaluminates to chloride could be completed in less than 1 hour. In contrast, the unhydrated clinker exhibited a slow chloride binding behavior until 72 hours. All these would provide theoretical guidance for using PAC cementitious materials in reinforced concrete to enhance resistance to chloride attack.

Effect of chloride binding behavior of cement hydration products on chloride critical value of steel corrosion in the presence of Na+ and Ca2+

To accurately predict the service life of concrete structures, the effect of chloride binding behavior of cement hydration products on steel corrosion should be taken into account. In this study, the steel specimen with cement paste coating were prepared to investigate the effect of chloride binding behavior on steel initial corrosion in the presence of Na+ and Ca2+. The results show that the higher pH level and Ca2+ concentration effectively inhibited the leaching C–S–H, while the Ca2+ increased the Ca/Si ratio of C–S–H, thereby enhancing the physical chloride binding capacity of cement paste coating in the CaCl2-Ns system (the addition of CaCl2 in NaOH solution). This enhancement primarily accounted for the higher critical value (Ccrit) of 0.03 mol/L in the CaCl2-Ns system, surpassing that of specimens exposed to other corrosion solutions, which was 0.01 mol/L. Both chloride ions from NaCl and CaCl2 could partially replace carbonate ions in monocarboaluminate (Mc), indicating that the cation type did not exert significant influence on chemical binding in this study.

Synergetic optimizing particle size distributions of aggregate and cementitious materials: Toward lower chloride diffusivity of concrete

Particle size distributions (PSDs) of aggregate and cementitious material dominate the microstructure and properties developments of concrete. Optimizing the proportion of aggregate can mitigate chloride ingress pathways, leading to a significant increase in surface area and augmented generation of the interfacial transition zone (ITZ). Overemphasis on aggregate proportion can compromise workability of fresh concrete due to limited cement paste lubrication. Although gap-graded distribution benefits cementitious material hydration, accurately classifying cementitious components for broad implementation is challenging. This study designed the aggregate proportion and PSD based on a proposed diluted packing region with low estimated chloride diffusivity. The gap-graded distribution at the cementitious material scale was conveniently achieved by incorporating fine ground blast furnace slag and coarse fly ash into commercial Portland cement. Consequently, dual-scale designed concretes, taking into account PSDs at both aggregate and cementitious material scales, were proposed. These concretes exhibited chloride diffusion coefficients ranging in (1.26–1.46)×10−12 m2/s, with aggregate proportions spanning 52 % to 74 %. Their compressive strength was comparable with Portland cement concrete, even with 60 % supplementary cementitious materials (SCMs). The improved properties of the dual-scale designed concrete can be attributed to the high aggregate proportion with rational PSD, which does not result in a significant rise in the ITZ proportion. Efficient hydration of the cementitious material is beneficial to microstructure densification and chloride binding capacity improvement, helps to increase the chloride diffusion distance and hinder the penetration of chloride. This study provides insights into designing chloride-resistant concrete with less Portland cement reliance.

A comparative study on chloride diffusion in concrete exposed to different marine environment conditions

Chloride penetration in concrete exposed to marine environment is a complicated process. In this paper, we present an experimental study on the chloride diffusion in concrete when it is subjected to marine stationary solution, splash, submerged, and pressured environments. The tests were carried out by using a “wash machine” type device to simulate various marine environment conditions. The specimens tested involve both noncarbonated and carbonated concrete specimens to examine the effect of carbonation on chloride penetration in concretes under these exposures. The experimentally obtained data indicates that the exposure condition has a great influence on the surface chloride concentration, chloride diffusion coefficient, and chloride binding capacity of concrete, which have a large impact on the chloride penetration in concretes. The study also found that carbonation can affect the influence of exposure conditions on chloride diffusion in concrete.

A study of chloride binding capacity of concrete containing supplementary cementitious materials

Chloride-induced steel corrosion is known to be a very common kind of deterioration of reinforced concrete. It is beneficial to bind free chloride ions to reduce the corrosion probability of the reinforcement embedded in the concrete. The binding capacity of the concrete varies according to its cementitious system. This paper investigates the chloride binding capacity of different kinds of supplementary cementitious materials (SCMs): Ground granulated blast furnace slag (GGBFS), Fly ash, and Metakaolin as a partial replacement of Ordinary Portland Cement (OPC). Different properties of concrete after chloride binding are assessed by carrying out the following tests: half-cell potential, accelerated corrosion test, compressive strength, rapid chloride penetration test, sorptivity test, measuring pH value of concrete, and XRD. The results showed that utilizing the SCMs in concrete can enhance the chloride binding capacity, especially those materials that have high quantities of aluminate and calcium in their chemical composition like GGBFS. Based on testing results, it’s recommended that the limit of the chloride content in the different codes should be revised regarding the binding capacity according to the type and quantity of the cementitious materials used.

Effects of pore structure and chloride binding capacity on chloride diffusion in limestone cement paste

The limestone powder has been a preferred admixture in cement-based materials due to its abundant availability and potential to reduce carbon emission. The purpose of this study is to clarify the key influencing factors for chloride diffusion in cement paste as limestone is added. The pore structure, chloride binding capacity and chloride diffusivity of cement paste with different limestone addition and fineness were measured. The deconvolution of physically and chemically bound chloride was achieved by combining X-ray diffraction, energy dispersive spectroscopy and the Rietveld method. The results show that the addition of fine limestone increases the number of 50–100 nm pores while the coarse limestone leads to more pores in the size range of 100–1000 nm. As the limestone content increases, less chloride is bound by physical adsorption while more Friedel’s salt is formed. The effective chloride diffusivity is almost unaffected when less than 10 % fine limestone is added because of increased pore tortuosity. The changes of chloride binding capacity and chloride diffusivity are equally important for the chloride diffusion when 20 % fine limestone is added. On the other hand, the reduction of chloride resistance of cement paste containing coarse limestone is mainly due to the increase of the chloride diffusivity.

Development of a low carbon binder system based on metakaolin: Focus on corrosion inhibitor-intercalated layered double hydroxides

In this work, MgO-CaO activated blends were produced by using metakaolin (MK). Roles of sodium nitrite (NaNO2) corrosion inhibitor on phase assemblance, micro-structure, and chloride binding of layered double hydroxides (LDH) in the systems were systematically studied. The results showed that the compressive strength in the middle and late stages were increased for 32.78% and 73.74% with the addition of CaO, and pore structure of the MgO-CaO-MK blends was optimized. After the addition of 5% NaNO2, NO2− anions were preferably intercalated into the layers of self-formed MgAl LDH and CaAl LDH. The chloride binding capacity of the samples was stronger than that of cement paste, which was attributed to the generation of more LDH by accelerating hydration of the system.

Influences of metakaolin and calcined clay blended cement on chloride resistance and electrical resistivity of concrete

There are many different grades of kaolinite clays around the world. Low-grade kaolinite clay is more abundant than high-grade kaolinite clay in various regions. To aim toward the utilization of low-grade kaolinite clay having an original kaolinite content of about 40% to produce calcined clay, this paper investigated the durability properties of concrete incorporating calcined clay produced from high-grade kaolinite clay or high kaolinite content (commercially available metakaolin or CC1) and calcined clay produced from a low-grade kaolinite clay (CC2). Concrete mixtures were designed to have a water-to-binder ratio of 0.60. A fly ash-to-binder ratio of 0.20 and calcined kaolinite clay-to-binder ratios of 0.10 and 0.20 were studied. The chloride penetration resistance and the electrical resistivity of concrete were assessed, while the mercury intrusion porosimetry (MIP) was utilized in evaluating the pore structure of concrete. The test results revealed that concrete with CC1 and CC2 exhibited superior chloride penetration resistance and chloride binding capacity than OPC and FA20 concretes. Moreover, using a higher calcined clay-to-binder ratio resulted in a more refined pore structure, which significantly enhanced the chloride resistance of concrete. Although CC2 revealed less performance in improving chloride resistance than CC1, it had superior performance compared to fly ash.