Crystalline Coating and Its Influence on Chloride Ion Diffusion Resistance of Carbonated Concrete

Abstract

Carbonation and chloride ingress are the most important damaging mechanisms for steel-reinforced concrete. The combination of these two corrosion processes accelerates the destruction of concrete, leads to extensive structural repairs, negatively impacts durability, and significantly reduces the service life of the structure. One possible and effective way to reduce chloride diffusion through the concrete pore system is through the use of crystalline materials. An experimental study focused on the ability of an applied crystalline coating to increase the chloride resistance of carbonated concrete is presented in this paper. Carbonated concrete specimens treated with a crystalline coating were exposed to a sodium chloride solution for various periods of time, and a water-soluble chloride ion content analysis was performed on powder samples taken from the tested specimens. Chloride profiles presenting the chloride ion concentrations at selected depths are presented for multiple types of concrete at various ages to show the effect of crystalline technology on the chloride resistance of concrete. The results of this study confirm the impact of carbonation on chloride ion ingress through concrete and show that crystalline coatings can improve the chloride resistance of concrete. Using crystalline coatings on carbonated concrete can, from a long-term perspective, significantly reduce the chloride ion content in concrete placed in an aggressive environment. The crystalline coatings were functional even after 28 days, when the concentration of chloride ions was below the critical concentration. The crystalline coating was able to reduce the concentration of chloride ions by 68% under the surface of the concrete and by 65% at depths of 20–25 mm after 180 days of immersion, compared to the untreated concrete. Crystalline coatings reduce the depth of critical chloride ion concentration, effectively protect the concrete reinforcement against corrosion and extend the service life of the structure.

1. Introduction

Structural concrete is affected by several negative environmental influences, which cause a gradual degradation of the concrete and a shortening of the service life of the entire structure. The most important of these influences is carbonation, which has a natural aging effect on the concrete. Carbonation results in a gradual reduction in the pH of hardened concrete and a reduction in the natural passivation of embedded steel reinforcements. The importance of chloride corrosion arises from chloride ions and their detrimental effect on these steel reinforcements. Gradual concrete degradation leads to an essential reduction in the service life of existing concrete structures [1].

Chloride ions usually originate from seawater or from the de-icing salts used in cold regions to ensure the operational safety of road structures. The application of de-icing salts during winter increases the vulnerability of the external conditions of all affected concrete structures [2]. The resistance of concrete to chloride ingress depends primarily on the permeability of the concrete and the characteristics of its inner porous network, such as its porosity, pore connectivity, or pore size distribution [3,4]. Several mechanisms can affect chloride permeability, such as absorption, intrusion due to alternating wetting and drying, and capillary suction or diffusion [5,6]. Diffusion is assumed to be the fundamental phenomenon for the penetration of chloride ions through a concrete structure [4]. If the concrete is damaged by cracks, the diffusion is accelerated [7]. The thickness of the concrete cover is also an important aspect of the extent of diffusion [8,9]. As the corrosion mechanism depends on the availability of oxygen and chloride ion content on the reinforcement surface and the rate of their diffusion through the concrete, the initiation of corrosion activity can be reduced by using a thicker concrete cover layer [10]. Reinforcement corrosion initiates when the concentration of chlorides at the steel surface equals or exceeds the critical chloride concentration. Once the critical value has been reached, local dissolution of the passive layer is followed by the nucleation of corrosion pits which eventually may grow. The classical approach assumes that corrosion starts when the chloride concentration exceeds 0.6 wt.% of the binder [11].

Concrete carbonation weakens the protection of steel reinforcements in concrete and initializes their corrosion. Corrosion propagation and the accumulation of rust products lead to a tendency of the concrete cover to crack and to a deterioration that affects the concrete’s structural properties. The carbonation process, together with the relative humidity and temperature, also has an impact on the penetrability and activation of chlorides. It leads to changes in the mineralogical composition of concrete, reduces its pH, and has the potential to liberate bound chloride ions, leading to an increase in the chloride ion concentration and deeper chloride penetration. The combination of carbonation and cracks accelerates chloride-induced corrosion by increasing the concrete’s penetrability and significantly reducing the concrete’s service life [12,13,14]. The carbonation process is especially impactful when the carbonation depth reaches the reinforcement. The concrete pH decreases significantly, which leads to reinforcement corrosion and potential structural failure. It is necessary to note the combined effect of carbonation and chloride diffusion. The natural protection of embedded steel rebars is ensured by the higher alkalinity of hardened concrete. However, due to gradual carbonation and consequent reduction in pH of hardened concrete, similar chloride concentrations exhibit a more vulnerable effect [15]. Thus, the gradual carbonation strengthens the negative impact of a given chloride ion concentration [16].

An effective and increasingly popular way to reduce chloride ion diffusion through concrete pore systems is through the use of crystalline materials, also known as permeability-reducing admixtures (PRAs) [17]. PRAs can be classified as admixtures reducing the permeability of concrete either under non-hydrostatic pressure (PRAN) or under hydrostatic pressure (PRAH) [18]. Crystalline materials are commercial products containing mainly Portland cement or other cementitious materials, slaked lime, and so-called active chemical agents, potentially also including fillers, retarders, or other materials. The specifications of these chemicals are kept confidential by the manufacturers and are part of their trade secrets and formulations. The waterproofing effect of crystalline materials on concrete is achieved through the reaction of the various chemical components contained in the formula when it is combined with water and the concrete matrix [19]. Crystalline materials, in essence, work in such a way that their chemical components react with the cement matrix during the hydration process. This is probably followed by the temporary formation of Ca(OH)₂ and the subsequent formation of disilicate and polysilicate anions. This cumulative process is usually accompanied by the formation of 3Ca⋅2SiO₂⋅3H₂O, together with the formation of 3CaO⋅Al₂O₃⋅Ca(OH)₂⋅12H₂O. The product of this chemical reaction is the growth of new precipitants in the form of needle crystals inside the open pore structure of the concrete [20]. The needle-shaped crystals densify the pore structure of concrete, which results in a waterproofing effect. This process only occurs if there is a sufficient amount of water in the structure. If there is a lack of water in the system, the chemical reaction cannot take place in full, and the effect of crystallization is limited [19,21]. These materials are used in two basic ways: as a coating applied on the surface of a concrete structure [22] or as an admixture added directly into the concrete mixture [23,24]. In the past, several laboratory experiments have been performed to measure the waterproofing ability of crystalline waterproofing systems, aiming to verify their ability to waterproof [25,26,27,28,29,30,31,32] and improve the durability [27,32,33,34,35] of concrete.

The water permeability reduction effect of crystalline materials has been proved in several scientific studies. The interaction of such materials with various types of cement has been shown, and a reduction in water permeability is achieved even when the surface-applied material is removed [36]. Experimental studies have also shown that crystalline materials effectively increase the resistance of concrete against chloride ingress, and this effect, as well as the water permeability, improves over time. These materials are able to extend the service life of structures by preventing concrete carbonation. Furthermore, adding crystalline admixtures into concrete leads to a better rate of crack healing and full water termination over time [36,37]. Using crystalline materials could also reduce the dispersion of corrosion initiation times of concrete reinforcement and may have the effect of delaying the onset of steel corrosion. The lifetime of a concrete structure can be significantly extended by using crystalline materials, delaying the corrosion onset time by approximately 55% [38]. Li et al. (2013) [39] tested crystalline materials in a simulated aggressive marine environment by using seawater hot rain testing, which showed that these materials also significantly reduce the corrosion potential of concrete reinforcement and have a significant influence on the penetration resistance of concrete. An important impact on chloride ingress resistance was also observed in the self-healing capacity of crystalline materials. An experimental study performed on mortar samples showed that those materials can fully heal cracks up to 0.4 mm in width. It has been assumed that this self-healing capacity is related to Ca²⁺ dissolution, which leads to the formation of calcium carbonate [40]. Munn et al. (2003) [29] focused on the impact of crystalline admixtures on the chloride resistance of concrete with and without supplementary cementitious materials, such as fly ash and slag, by using various testing methods; they confirmed that crystalline materials can increase the chloride resistance of concrete. Another related study was conducted by Tan et al. (2023) [32]. They studied the effectiveness of crystalline admixtures using a modified rapid chloride ion migration test and showed a chloride ion penetration depth reduction of up to 75%. The incorporation of an appropriate amount of crystalline admixture can effectively promote the hydration reaction of cement, generating a more suitable content of hydration products to fill the micropores and blocking the transmission of chloride ions inside the matrix, effectively inhibiting the erosion effect of chloride ions and enhancing the concrete’s resistance to chloride ion penetration.

Despite the number of scientific studies performed in the past, the literature focuses mainly on crystalline admixtures, and the field of crystalline coatings is still not sufficiently scientifically described. Recent scientific studies for surface-applied crystalline materials show results for thin mortars with crystalline admixtures in their mixtures, but the crystalline coatings have not yet been sufficiently investigated. The contribution of determining the impact of crystalline coatings also consists of designing a refurbishment procedure that can effectively help extend the service life of concrete structures. The presented experimental program aims to determine the chloride ion content of carbonated concrete samples treated with a crystalline coating and exposed to a sodium chloride solution. The objective of the experimental program is to analyze the effect of the crystalline coating on the resistance of carbonated concrete to chloride penetration as a function of time and exposure to an aggressive environment. As shown in previous research, the application of crystalline materials can significantly extend the lifetime of existing concrete structures, reducing the expenses for potential structural repair and reducing the carbon footprint of structures and the construction industry as a whole.

2. Materials and Methods

2.1. Concrete Properties

A series of concrete specimens with dimensions of 150 mm × 150 mm × 150 mm were created. The concrete mix was designed without admixtures affecting its fresh or hardened properties. The consistency of the fresh concrete was determined during the casting of the samples by using the slump test in accordance with EN 12350-2 [41]. The samples were stored in a humid environment at a temperature of 20.0 ± 2.0 °C and tested for their compressive strength at the age of 28 days in accordance with EN 12390-3 [42]. The concrete mix design is shown in

Table 1. Concrete mix design.

Material	Specification	Volume (kg/m3)
Cement	CEM I 42.5 R	300
Water	w/c 0.51	153
Aggregate	0–4 mm	855
Aggregate	4–8 mm	270
Aggregate	8–16 mm	870

.

A water penetration test according to EN 12390-8 [43] was carried out by using the cubic specimens to determine the concrete permeability. After demolding, the cubes were stored in an aqueous environment and removed the day before the water permeability test to dry. They were then placed on the standard test apparatus, a water pressure bench. The water column load was applied to a 75 mm diameter area for 72 h at a pressure of 0.5 MPa (50 m of water column). When the test was completed, the dimensions of the samples were measured, and their weights were recorded. The cubes were broken in half, and the maximum depth of leakage was read with a caliper. The result of the water permeability test was the average value of all maximum depths recorded (see Figure 1).

2.2. Concrete Carbonation

A set of test samples were subjected to an accelerated carbonation process. The test procedure was based on the test standard applied for hardened concrete testing according to EN 12390-12 [44]. The exposure regime was set up for 365 days in a testing chamber with a CO₂ concentration of 3.0% ± 0.5%, 57.0% ± 3.0% RH, and a temperature of 20.0 ± 2.0 °C. After the prescribed time period, the depth of the carbonation front was monitored. Half of the as-carbonated concrete specimens were intended to be treated with the crystalline coating, and the second half were used to analyze untreated concrete. After the aggressive environment exposure, the crystalline coating was applied to the concrete specimens in one layer, following the instructions in the manufacturer’s datasheet (see Figure 1). Xypex Concentrate was used as the crystalline coating for this study. Xypex Concentrate is an organic chloride-free powder consisting of Portland cement, finely graded silica sand and active proprietary chemicals [45].

2.3. Chloride Ion Concentration

The chloride ion concentration was evaluated for three sets of specimens: control concrete stored under laboratory conditions (C), carbonated concrete (CC), and carbonated concrete treated with the crystalline coating (CCX). All concrete samples were submerged in a 3% solution of sodium chloride (NaCl) for prescribed periods of time: 28, 90, and 180 days. The samples were exposed to the solution under normal laboratory conditions of temperature, relative humidity, and pressure until sampling. One reference sample was also evaluated to obtain the natural Cl⁻ concentration of the original concrete.

The sampling was conducted by drilling 5 mm layers from the sides of the cubes to obtain a profile of chloride ion concentrations up to a 25 mm depth. Five layers were taken in every subsequent measurement from the set of three cubes. One set of control concrete cubes, one set of carbonated concrete cubes, and one set of carbonated concrete cubes with the crystalline coating were used for each sampling. Thus, the total number of all cubes used to determine the chloride profiles was 27. An additional three cubes were used for the water penetration test, three for compressive strength determination and one to obtain the value of carbonation front depth. The crystalline coating was removed from the surface of the concrete prior to the sampling to avoid contamination of the extracted powder with particles of the hardened coating. In addition, removing the layer of crystalline coating helped to maintain a uniform sequence of powder extraction. Through repeated sampling from different sides of the test cubes, possible errors in the measurement and evaluation of the results were eliminated. Approximately 25 g of concrete powder was drilled at each layer, which ensured representative sampling to address inhomogeneity. The as-retrieved concrete powder samples were dried in the laboratory oven at a temperature of 105 °C for 24 h. Aqueous leachates were prepared from the dried powder samples at a ratio of 1:10 (10 g of sample to 100 mL of water), and the sample vials were placed in a shaker for the next 24 h. The samples were filtered through a 0.2 µm syringe filter prior to the analysis. The water-soluble chloride ion content of the aqueous effluents was determined using CAPEL 205 capillary electrophoresis. The chloride ion content was recorded as a percentage of the dry weight of the sample in each layer, and graphs of the chloride ion concentration profiles were created for each type of concrete.

3. Results and Discussion

3.1. Concrete Properties

The average compressive strength of the tested concrete at 28 days was 27.5 ± 1.9 MPa, and the concrete class was determined to be C20/25. The initial water permeability test showed an average maximum water penetration of 106 mm (with a standard deviation of 4 mm).

3.2. Concrete Carbonation

The goal of this experimental work was to assess the effect of an additional crystalline coating applied to the surface of carbonated concrete. Hence, the concrete cubes were aged in a carbonation chamber to accelerate the process of carbonation, because concrete of this class usually exhibits a carbonation depth of only 5–10 mm after the first year under natural conditions (~450 ppm). The aged sets of concrete samples exhibited a carbonation depth of 22–25 mm after a year of accelerated carbonation (see Figure 2), which aligns with the conventional thickness of a concrete cover. The purpose of a concrete cover is to protect embedded steel rebars.

3.3. Chloride Ion Concentration

The initial chloride ion concentration in the concrete used for this experimental program was 0.015 wt.% of cement due to the components of the mixture, which was applied as an offset during subsequent measurements. Chloride ion concentration profiles were created for each set of concrete samples by using the measurements obtained from the concrete specimens after various exposure durations (28, 90, and 180 days). The powder samples for analysis were obtained in 5 mm layers from the concrete specimens; thus, the resulting value for each layer is plotted in the graphs at the mean value of the layer depth. The resulting chloride profiles served as the final evaluation. The detrimental effect of chloride ions is highly dependent on their actual concentration level and, in parallel, on the nature of the exposure. The critical chloride ion concentration is about 0.6 wt.% [11] of the binder used, which is indicated in the resulting chloride profiles.

Figure 3 shows the concentration profile of chloride ions in the control concrete samples stored under laboratory conditions (C). The results show that after 180 days, the chloride ion concentration exceeded the critical concentration of 0.6% within the whole analyzed concrete profile, i.e., at depths from 0 to 25 mm. Under the surface of the concrete, the chloride ion concentration reached 3.44% of the weight of the cement after 180 days of immersion in the NaCl solution. After 28 and 90 days of immersion, the chloride ion concentration did not achieve the critical concentration at depths of 5–10 mm. At depths of 20–25 mm, where steel reinforcements are usually located, the chloride ion concentration was 0.89%. The concentration was very low (close to 0) at depths of over 15 mm for the samples submerged in the solution for 28 days and at depths of 20–20 mm for the samples submerged for 90 days. However, the higher diffusivity for chloride ions of used concrete corresponds with the strength class of such concrete, which was documented by the results of the water penetration test. The increased permeability of this type of concrete is determined by the higher water-to-cement ratio and lower cement dosage [46]. Although of lower-durability performance, such concrete’s load-bearing capacity is suitable for bridge abutment construction.

The results of the analysis of carbonated concrete (CC) are shown in Figure 4. The critical concentration was achieved for all observed immersion times in the NaCl solution, except for at depths of 15–25 mm after 28 and 90 days. After 180 days, the chloride ion concentration was 1.21% at depths of 20–25 mm. Thus, it is obvious that carbonation has a detrimental effect on the resistance of concrete to chloride ingress. Such behavior is due to the reduced content of (OH)- ions, which bind a part of the diffused chlorides, preventing their further transport to the inner part of the concrete mass [47]. Simultaneously, secondary products induced by chloride binding reduce the permeability of the concrete [48]. However, the long-term accumulation of chlorides in the concrete cover is responsible for a general loss of natural resistance to environmental effects; this is due to the instability of secondary degradation products, such as Friedel’s salt, and the obvious risks to crack initiation [49].

Carbonation affected the concentration of chloride ions within the analyzed concrete profiles. Compared to those observed for uncarbonated concrete, the concentrations of chlorides increased mainly at depths over 10 mm for all the observed times. The concentrations of chloride ions increased by 28%–35% in the carbonated concrete after 180 days of immersion.

The chloride ion concentration profile of the carbonated concrete samples treated with the crystalline coating can be observed in Figure 5. It is clearly visible that the concentrations significantly decreased compared to those for the control and carbonated concrete at all the observed times. The concentration dropped below the critical concentration for all groups of samples at the most sensitive depths of 15–25 mm, where the steel reinforcements are usually located. For the concrete submerged in NaCl solution for 180 days, the concentration decreased by 68% under the surface of the concrete and by 65% at depths of 20–25 mm, compared to those observed for untreated carbonated concrete. At the earlier times of immersion in the solution (28 and 90 days), the concentrations of chloride ions decreased by 50% at depths of 20–25 mm compared to those observed for untreated carbonated concrete. The concentrations of chloride ions obtained from the treated concrete at 28 and 90 days of immersion at depths of 20–25 mm were close to the values obtained from the uncarbonated concrete at the same depths from the surface. Despite the confidential composition of the used crystalline coating, the presence of main phases such as Portland cement residues, slaked lime, or pozzolanic materials is well known. These materials can increase the pH of the concrete cover and thus recover the higher alkalinity of the pore water, which has a direct impact on the mitigation of chloride diffusion [16]. A similar approach is used during the renovation and restoration of historical renders by using calcium hydroxide solution [50].

The equivalent depths of the critical concentration of chloride ions for the individual chloride profiles were calculated using linear interpolation, taking into account the two-sided values of the measured concentrations. The achieved depths are shown in

Table 2. Depth of critical concentration for each set of concrete samples.

Designation	Specification	Depth of Critical Concentration (mm)
		28 Days	90 Days	180 Days
C	Concrete after 28 days	4.14	6.47	˃25
CC	Carbonated concrete	8.95	12.00	˃25
CCX	Carbonated concrete with crystalline coating	4.41	7.05	13.45

.

From the long-term perspective, the trend shows an increase in the chloride ion concentration in the reference uncarbonated and carbonated concrete samples, whereas, in the samples treated with the crystalline coating, the function of the coating starts to dominate over time, resulting in only a slight increase in chloride concentrations compared to the reference samples. The untreated samples continue accumulating chloride ions over time. It can thus be assumed that the secondary hydration products have the ability to chemically bind the penetrating chlorides to form complex salts, thereby reducing their transport. This theory was confirmed in previous research by Reiterman et al. (2020), which focused on the transport properties of concrete cover after the removal of applied crystalline material [36]. The as-formed carbonates in the penetrated concrete can fix a proportion of the migrated chloride ions, which has a direct influence on the resulting depth of chloride ion penetration.

An experimental study by Li et al. (2013) focused on the performance of concrete treated with crystalline materials in a marine environment; it showed a reduction in the corrosion potential of steel reinforcements and a significant impact on the concrete’s penetration resistance with the use of these materials [39]. Sisomphon et al. (2012) showed the ability of crystalline materials, used as admixtures, to self-heal cracks up to 4 mm in width [40] and described the hypothesis of ettringite formation during the initial phases of healing. Based on the previously obtained facts and the results of this study confirming a reduction in chloride permeability with the use of a crystalline treatment on carbonated concrete, it can be assumed that using these materials can significantly extend the durability and service life of concrete.

Munn et al. (2003) focused on the impact of crystalline admixtures on the chloride resistance of concrete with and without fly ash and slag [29]. Their research showed a reduction in the chloride penetration depth by approximately 38% for concrete with fly ash and 32% for concrete without supplementary cementitious materials after 35 days of immersion—specifically, from 25 mm to 17 mm and from 24 mm to 14.5 mm, respectively. Tan et al. (2023) used a modified rapid chloride penetration test to show the effectiveness of crystalline admixtures. Their results showed a chloride penetration depth reduction from 8.8 to 2.2 mm (75% reduction). Their research also showed that overdosing on the crystalline admixture can negatively affect the performance in terms of chloride resistance, as it leads to the generation of excessive hydration products, resulting in matrix cracking damage [32]. The current study showed that the crystalline coating is able to reduce the chloride ion concentration to levels under the critical concentration at a depth of 13.45 mm after 180 days of immersion, with a significant reduction compared to the untreated concrete. After the same period of time, the steel reinforcement in untreated concrete would be fully in the area of critical chloride ion concentration. Crystalline coating can protect the concrete reinforcement against the corrosion caused by the chloride ingress. It can also be assumed that crystalline materials can be at least as effective as crystalline admixtures in terms of chemical protection.

One of the main reasons for monitoring the effect of chlorides and their possible permeation into reinforced concrete structures is the risk of corrosion damage to the steel reinforcements. Reinforcement corrosion can ultimately compromise the load-bearing capacity and reduce the service life of the entire structure. Zhu et al. (2016) [51] studied the combined effect of carbonation and chloride ingress using numerical simulations. They concluded that the initiation time of embedded steel corrosion is reduced by 40% due to this combined mechanism. Thus, the deterioration of the concrete structure is accelerated. In the case of a precast class S1 structure in an XC2 or XC3 environment, the resulting calculated nominal concrete cover thickness could correspond to the placement of reinforcements in a layer 10 to 15 mm below the surface. Based on the results of this study, a crystalline coating could be used as an additional and effective protection even for precast concrete structures in an aggressive environment, where untreated and unprotected concrete would fail in terms of its chloride resistance. The chloride ion concentrations in untreated and untreated carbonated concrete at 180 days of immersion were above the critical concentration at a depth of 25 mm. Therefore, such concrete could not even be used for a structure cast at the construction site, with a concrete cover depth of 25 mm, as the concrete reinforcement would be fully in the area of critical chloride concentration. On the other hand, concrete treated with a crystalline coating showed chloride ion concentrations below the critical value at depths of 13.45 mm at 180 days of immersion. This corresponds to a concrete cover of precast structure in an XC2 or XC3 environment.

Considering the self-healing capacity of concrete treated with crystalline admixtures and the combined positive impact on the resistance against chloride ion ingress [52], it can be assumed that the impact of an applied crystalline coating would be comparable. However, no experimental study focused on the combined effect of self-healing capacity and increased chloride ion ingress resistance has been performed on crystalline coatings. An additional investigation in this field would help deepen the knowledge about these materials and contribute to the progressive design of more durable structural solutions.

4. Conclusions

This study focused on the ability of an applied crystalline coating to increase the chloride resistance of carbonated concrete. Carbonated concrete specimens treated with a crystalline coating were exposed to a sodium chloride solution for various periods of time (28, 90, and 180 days), and a water-soluble chloride ion content analysis was performed on powder samples taken from the tested specimens. The results were compared with values obtained from uncarbonated and carbonated concrete samples without the crystalline coating treatment.

• This experimental study confirmed the previous research presented in the scientific literature and showed that carbonation and chloride ingress are two interrelated processes. Carbonated concrete is more permeable to chloride ion ingress than uncarbonated concrete due to reduced chloride binding. The depth of the critical concentration of chloride ions in the carbonated concrete was up to 116% higher than that in the uncarbonated concrete.
• The results of this study show that crystalline coatings can significantly increase the chloride resistance of carbonated concrete. The crystalline coatings were functional even after 28 days of immersion in the sodium chloride solution when the concentration of chloride ions at depths over 5 mm was under the critical concentration. The same tendency was observed after 90 days of immersion, as the chloride ion concentration was lower than the critical concentration at a depth of 10 mm. The crystalline coating was able to reduce the concentration of chloride ions by 68% under the surface of the concrete and by 65% at depths of 20–25 mm after 180 days of immersion.
• Carbonated concrete treated with a crystalline coating showed chloride ion concentrations below the critical concentration at a depth of 13.45 mm at 180 days of immersion. At the same time of immersion, steel reinforcement in untreated concrete would be fully located in the critical chloride concentration area.
• The results of this study show that crystalline coatings can be used in various applications as an effective solution to protect concrete against chloride ingress, e.g., in marine structures or structures burdened by winter maintenance.
• It can be assumed that the performance of crystalline coatings, in terms of improving resistance to chloride ingress, is at least the same as that of crystalline admixtures. To confirm this comparison of the effectiveness of crystalline coatings and admixtures, additional experimental research should be performed.

A reduction in chloride permeability leads to a significant increase in concrete durability and can reduce the risk of corrosion of embedded steel rebars, along with potential future repairs. Efforts to prolong the service life of existing concrete structures align with the concept of sustainability in current structural engineering.