Bound Chloride Content Research Articles

Enhanced subsurface chloride transport resistance of cement pastes via optimizing CO2 curing time

The chloride erosion of CO2 cured cementitious materials could be a concern for concrete structure application. This study aims to clarify the chloride transport and binding behavior by elucidating the alteration of free and bound chloride content from the outer towards the core (5 layers with 2 mm intervals). The carbonation depth and chloride binding ratio at the carbonation zone were also determined. The results showed that dense carbonated surface significantly reduced the penetration of chloride, especially in the carbonation zone, resulting in a decrease in total chloride content (48 %–83 %) and a slight increase in the chloride binding ratio (up to 22 %). It is found that the benefits of minimizing surface porosity to impede chloride penetration significantly outweigh the drawbacks associated with the consumption of AFm phase and C-S-H gel. However, as prolonged the CO2 curing time to 7 days, the surface areas with the highest carbonation degree exhibited more chloride content than the inward partially carbonated zone. This is because excessive carbonation at early age could increase the surface pore structures, resulting in the promotion of chloride transport. It was found that the content of chloride and CaCO3 are strongly correlated with CO2 curing time, and CO2 at first 24 h curing can achieve greater benefits in reducing chloride penetration. Moreover, amorphous calcium carbonate gradually transformed into crystalline calcium carbonate during the process of chloride attack, conducive to the further improvement of the compactness of the sample.

Chloride penetration resistance and chloride binding capacity under different application environments of ferroaluminate cement

Free chloride is a crucial factor that threatens the long-term performance of marine concrete structures. The resistance of cement to chloride penetration and its capacity to bind chloride are important parameters for assessing the ability of cement-based materials to resist chloride invasion. Ferroaluminate cement (FAC) has considerable application potential in concrete used for marine structures; however, its resistance to chloride invasion and mechanism of chloride binding have not yet been revealed. In this study, the chloride diffusion coefficient and chloride binding capacity of FAC, for both internal and external chloride ions, are experimentally investigated and compared with that of ordinary Portland cement (OPC). The results show that the chloride diffusion coefficient of FAC mortar is approximately 2.5 times lower than that of OPC mortar. FAC exhibits finer overall pore distribution and the number of pores with sizes within 10–100 nm is reduced, which is responsible for its notably lower chloride diffusion coefficient. The chloride binding capacity of FAC to internal chloride ions is slightly lower than that of OPC, and a portion of the chloride ions are chemically bound by reacting with ye'elimite during hydration. The chlorides bound by physical adsorption constitute a dominant portion of the entire bound chloride content. In the case of external chloride ions, the bound chloride content of FAC is lower than that of OPC. The chemical chloride binding capacity of FAC is comparable to that of OPC and the lower physical adsorption capacity for chloride in FAC leads to a weaker chloride binding capacity compared to that of OPC.

Chloride binding behavior of cement paste influenced by metakaolin dosage and chloride concentration

The chloride binding capacity of cement paste is of great significance for mitigating steel corrosion and failure of concrete structures. Metakaolin (MK), as an important mineral admixture, can improve the chloride binding capacity of cement paste effectively and strengthen its durability under chloride attack environments. This study systemically investigated the effects of MK and chloride concentration on the chloride binding capacity of cement paste and of C–S–H, and quantitatively characterized the evolution of chemically bound chloride and physically bound chloride. The results show that the presence of MK contribute to the growth of total bound chloride content, the underlying mechanism for which is the increase of Friedel's salt (Fs) and hydrated calcium silicate gel (C–S–H) caused by MK. Meanwhile, the monotonous growth in C–S–H adsorbed chloride is the major cause for the continuous increasing of total bound chloride with chloride concentration increasing, considering that Fs fixed chloride first increases, and then declines and stabilizes along with chloride concentration. Furthermore, the incorporation of MK slightly decreases the content of chloride adsorbed by unit mass C–S–H, while the increasing of chloride concentration significantly increases the chloride binding capacity of C–S–H. The last but not the least, when the concentration of salt solution is lower than 1.0 mol/L, the chloride contained in Fs dominates in total bound chloride, and when it is beyond 1.0 mol/L, the C–S–H adsorbed chloride starts taking the absolute lead.

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.

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.

Influence of carbonation on the bound chloride concentration in different cementitious systems

Deterioration of reinforced concrete is often studied in idealised scenarios where only one exposure environment is dominant. For example, to study concrete structures exposed to de-icing salt environments, the effect of a cyclic wetting and drying regime is considered, but deterioration by carbonation is not given any emphasis. However, some studies have shown that carbonation could result in the release of bound chlorides and this could lead to an increase in chloride content near steel in reinforced concrete. Through an investigation on the influence of carbonation on the bound chlorides in different cementitious pastes, this paper highlights the positive initial effects that carbonation has on the chloride binding behaviour and later the changes that occur in both the pH of concrete and the bound chloride content for both low and high w/b mixes. Two mathematical expressions are put forward for mapping: (i) the changes in apparent pH as a function of the duration of carbonation and mix ingredients; and (ii) the reduction in the bound chlorides with a proportional reduction in apparent pH. The latter is valuable in quantifying the changes to binding capacity in service life models due to carbonation, with the help of a simple pH measurement.

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.

A chemo-damage-transport model for chloride ions diffusion in cement-based materials: Combined effects of sulfate attack and temperature

Corrosion of reinforcement caused by chloride penetration has become the primary factor of concrete durability in the saline soil environment. A chemo-damage-transport model for chloride diffusion is studied, in which the effects of sulfate attack and temperature on chloride diffusivity in concrete are taken into consideration. Specifically, the volume changes of Friedel’s salt and ettringite can cause the expansion and cracking of cement-based materials. Meanwhile, temperature has a significant influence on the reactions rates, mass transfer and ions diffusivity. The model is validated by published experimental results. The main conclusion is that there exists a peak value of the bound chloride ions. Near the exposed surface of the samples, the bound chloride content decreases with the increase of temperature. For the peak corresponding position and inward part, with the temperature rises, the bound chloride content in cement-based materials increases obviously.

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.

Modeling and simulation on coupled chloride and calcium diffusion in concrete

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

Chloride-bearing characteristics of alkali-activated slag mixed with seawater: Effect of different salinity levels

This paper presents the microstructural properties of KOH-activated slag pastes produced with natural seawater under different salinity levels. The seawater-mixed samples exhibit higher 91-day strength compared to the control sample produced with deionized water. Their reduced total porosity and decrease in average pore size are closely related to the strength improvement. The dense microstructure is composed of reaction products resulting from the inclusion of seawater; these include Cl-hydrocalumite, Cl-hydrotalcite, AlClO, K2SO4, CaCO3, calcium aluminum iron oxide carbonate hydroxide hydrate, and aluminum chloride hydrate. In addition, CO32− in the CO3-hydrotalcite and CO3-hydrocalumite is replaced by chloride ions, and then a Cl-bearing phase is formed, leading to a higher bound chloride content. Lastly, in the single ocean current investigated in this study, the strength of seawater-mixed samples is weakly vulnerable to salinity variation.

Effects of applied voltage on chloride binding and microstructure of cement pastes subjected to chloride solutions

In this paper, the effects of applied voltage on cement paste subjected to chloride solutions were studied by comparing the microstructure and chloride binding of hardened cement pastes after bulk diffusion and rapid chloride migration (RCM) tests. The chloride concentration index, chloride binding, chemical composition and morphology of hydration products, pore structure and zeta potential of cement pastes after the bulk diffusion and RCM tests were measured. The results showed that the microstructure of cement paste was densified by the chloride binding; the applied voltage of 10 V increased the Ca/Si of C–S–H gel and modified the morphology of Friedel’s salt. The chloride content of the expressed pore solution and the C–S–H bound chloride content increased when external voltage was applied, while no difference was observed for Friedel’s salt bound chloride. The applied voltage increased the total chloride content in cement paste mainly by increasing the free chloride and C–S–H bound chloride.

Modification on the chloride binding capacity of cementitious materials by aluminum compound addition

This study investigates the effects of aluminum compound on the binding of admixed chloride in cementitious materials. Three commonly available aluminum compounds, aluminum oxide, aluminum hydroxide and colloidal nano-alumina are applied. The bound chloride content, hydration products and calcium hydroxide content are analyzed. The results show that aluminum compounds transform to AFm phases such as carboaluminate, monosulfoaluminate if there is no available chloride. When the mixing water includes chloride, the above AFms transform to chloroaluminate, e.g. Kuzel’s or Friedel’s salt, depending on the sodium chloride and aluminum compound content. The bound chloride content is significantly improved with aluminum compound addition during the whole curing age, even though in an early age of 3 days. Since aluminum compounds react slower than alumina-containing clinker, the bound chloride content of aluminum compound blended pastes experience a considerable increase after 7 days, whereas that for plain paste is very minor. Generally, admixed chloride promotes the formation of delayed ettringite and this effect can be suppressed by aluminum compound addition due to higher amount of formed AFms. A higher dosage of admixed chloride increases the bound chloride content and reduces the calcium hydroxide as well as free water content in the system.

Chloride binding behaviors of metakaolin-lime hydrated blends: Influence of gypsum and atmospheric carbonation

Metakaolin-lime (1:1 by mass) blends are prepared to simulate the hydration products of metakaolin in cement and to clarify the effects of gypsum and carbonation on their chloride binding behaviors. The bound chloride content, hydration products and mechanisms were investigated by titration, X-ray diffraction, thermogravimetry and thermodynamic analysis. The results show that formation of stratlingite (C2ASH8, St) and carboaluminate are favored in gypsum-free blend, while monosulfoaluminate and ettringite are preferred in the gypsum-containing blends. During chloride exposure, St demonstrates good stability. Kuzel’s salt (C4As0.5ClH12, Ks) is kinetically favored at 3 h in gypsum-containing blends and it converts to Friedel’s salt (C4ACl2H10, Fs) with continuous exposure. Chlorides can replace the sulfates in monosulfoaluminate (C4AsH12, Ms) and Ks to eventually form Fs and release sulfates that can further react with Ms to form delay-ettringite. The thermodynamic calculation and experiments demonstrate consistent results related to the phase conversion. Addition of 4 wt% and 8 wt% gypsum reduce the bound chloride content up to 11%. During carbonation, chlorides bound by Friedel’s salt and C-S-H gels can reverse to free chlorides due to the decrease of pH and reduced calcium ion concentration in exposure solution. In a deeply carbonated system, ettringite does not exist.

Improving the chloride binding capacity of cement paste by adding nano-Al2O3

The chloride binding capacity of cement paste is of critical importance to the durability of reinforced concrete structure. In this study, nano-Al2O3 (NA) was considered as an additive to be used in cement paste. Paste samples were prepared with the NA dosage varying from 0, 0.5, 1.0, 3.0 to 5.0%. The chloride binding capacity was examined through the conventional equilibrium tests, i.e., the paste samples were exposed to a NaCl solution of 0.05, 0.1, 0.3, 0.5, and 1.0 mol/L. Results indicated that the bound chloride content increased with the adding of NA for a low NaCl solution, whereas it decreased firstly and then increased for a high NaCl solution. Supplementary experiments including the X-ray diffraction (XRD)/Rietveld analysis and the Mercury intrusion porosimetry (MIP) were performed to unravel the mechanism in terms of the formation of AFm phase, Friedel’s salt and microstructure. It was believed that an appropriate adding of NA was able to improve the chloride binding of cement paste.

Properties of coral waste-based mortar incorporating metakaolin: Part II. Chloride migration and binding behaviors

This study investigated the mechanical properties, chloride migration and binding mechanisms of marine mortars with coral waste powder (CP) incorporating metakaolin. Due to the synergetic effect of carboaluminate formation and pozzolanic reaction, the compressive strength and chloride resistance of CP mortars are dramatically improved by metakaolin incorporation. When exposed to 0.5 M NaCl solution, more than 75% of the total bound chloride is bound by the hydrated mortars within 24 h. Friedel’s salt (Fs) is the main chloride-uptaking phase and its X-ray diffraction peak area is well consistent with bound chloride content. The thermodynamic analyses indicate that carboaluminate has great capacity to form Fs, and SO4-AFm can transform to Fs and release gypsum in temperatures below 21 °C. The released gypsum further reacts with SO4-AFm to form additional SO4-AFt after exposed to chloride solution. Besides, nearly linear relationship between bound chloride content and active alumina content in binder is found.

Chloride binding capacity of pastes influenced by carbonation under three conditions

This research investigates the influence of carbonation on chloride binding capacity under three conditions varying in the sequence and way of carbonation-chloride contact: I, hardened pastes first carbonated then contacted with chloride; II, hardened pastes first contacted with chloride then carbonated; III, pastes inner-introduced with chloride during casting, hardened and then carbonated. The results indicate that, before carbonation, the bound chloride content of pastes inner-introduced with chloride is slightly higher than that of pastes first hardened then contacted with chloride because more Friedel's salt is formed through the former way. And during the carbonation process, the remaining bound chloride content mainly depends on the content of un-carbonated C-S-H gel. And based on the content of it before total carbonation, the content of residual bound chloride in samples under condition I is higher than that under the other two conditions after the same carbonation time. After complete carbonation, the bound chloride content under the three conditions all approximates to zero, which indicates that carbonation makes paste lose chloride binding capacity completely.

Effects of Chloride Ion Binding on Microstructure of Cement Pastes

AbstractThe corrosion of steel caused by chloride ions is considered to be the main durability problem of reinforced concrete structures. This paper investigates the total chloride ion content, free chloride concentration in expressed pore solution, Friedel’s salt content, and porosity of portland cement (PC) and PC/fly ash (FA) combination pastes after 7 days of water curing followed by 28 or 150 days immersion in water or 0.3, 0.7, 1.0, and 2.0 mol/L sodium chloride solutions. Results showed that the chloride ion binding capacities of the pastes increased with increase of free chloride ion concentration. The relationship between bound and free chloride ion contents could be described by Langmuir’s equation. The addition of fly ash decreased the bound chloride content and showed negative influences on Friedel’s salt formation due to low activity. Porosity of cement paste decreased as the concentration of soaking chloride ion increased, and was inversely proportional to the amount of Friedel’s salt formed.

EFFECTS OF BLAST FURNACE SLAG ON CHLORIDE PERMEABILITY OF CONCRETE

Chloride-induced corrosion of steel reinforcement is the main cause of deterioration of reinforced concrete structures in marine environments. The penetration of chlodride ions into concrete cover that accelerates corrosion process of steel reinforcement, this affects the bearing capacity of structures. This paper investigates on chloride permeability cheracteristic of concrete using blast furnace slag in terms of chloride diffusion coefficient and chloride binding capacity. The concrete used in this research has grade of 45MPa and the slag content replacement of cement PC50 is in range of 0% - 70%. The chloride diffusion coefficient of concrete is determined by ASTM C1202 and NordTest NT Build 492. Results showed that the blast furnace slag replacement increases (from 0% to 50%), the chloride ion diffusion coeffient decreases and bound chloride content in concrete increases. It is clear to conclude that blast furnace slag can be used to replace cement PC50 in range of 30% to 40% in order to increase the resistance of concrete to chloride penetration without affecting concrete strength.