1. Introduction
Current concrete technology incorporates a number of admixtures and additions to improve the properties of cement composites and their durability [
1]. The issue of enhancing concrete properties can be approached from different perspectives. Enhancement can be accomplished by changing the chemical composition of the binder through incorporating various additions (fly ash, slag, etc.), modification of the aggregate composition, or admixtures that modify the properties of either the concrete mix or the hardened concrete. However, as cement concrete is produced due to chemical reactions between the concrete mix components, the curing conditions for those reactions also significantly impact the properties of any hardened concrete.
With the introduction of high-performance concretes (HPC) of low water-to-binder ratios, the effect of self-desiccation and the autogenous shrinkage of concrete was identified as a phenomenon that significantly impacted the durability of such composites [
2,
3]. As conventional curing methods are ineffective in mitigating the autogenous shrinkage of concrete with a low water-to-binder ratio due to limited water migration (due to the low porosity and permeability of HPC), new methods of altering water kinetics within concrete mixtures had to be developed—including internal water curing [
4].
Superabsorbent polymers (SAP) can be described as cross-linked hydrogel networks [
5]. SAP, depending on the parameters of the water absorption environment, absorb water due to high osmotic pressure caused by the accumulation of ions present within the structure [
6]. Absorbing water into SAPs polymer network causes SAP grains to increase their volume, pushing apart ions from each other. With its network stretched out, the osmotic pressure reduces [
7].
As SAP absorbs water, the parameters of the water absorption environment also have to be taken into account (its alkalinity and external pressures caused by SAPs volumetric changes) [
8]. Thus, to describe the water absorption capacity of any SAP, its maximal water capacity is tested in the distilled water environment in standard conditions (temperature, air pressure conditions, and a lack of hydrostatic pressure) and this is referred to as a reference value [
9]. Cement paste is an environment characterized by the presence of different ions influencing the SAP absorption parameters.
Cement paste can be described as a suspension of physical cement grains in water. Due to this fact, capillary forces within such a system also influence the water transport parameters through forming a pore network [
10]. This limits the water absorption capacity of SAP during the absorption stage and prolongs the water desorption process. Due to the combination of those parameters influencing water transport within a cementitious composite, SAP can be used as an internal curing agent [
11].
1.1. Superabsorbent Polymers (SAP) as Internal Curing Agent—Methods
SAP effectiveness as an internal curing agent is based on its influence on water transport parameters during the hydration of cementitious materials [
12]. Internal stresses in the cement matrix due to continuous hydration and self-desiccation of the composite force SAP to release water initially stored in its structure [
13]. Due to that mechanism, several parameters of SAP-modified concrete can be altered—the mechanical properties [
14,
15], degree of hydration [
16], autogenous shrinkage [
17], parameters of the pore network [
18], and others that affect the material’s durability [
19]. To control the parameters of water transport in the composite influenced by the presence of SAP, information on the water absorption and water desorption should be gathered from the SAP structure. SAP water absorption and desorption properties are affected by both its chemical composition and its particle size distribution. The influence of the method of SAP introduction into the concrete mix on the water transport within composite should be considered [
9].
SAP can be introduced into the concrete mix in two ways. The most common method involves adding non-saturated (‘dry’) SAP to the concrete mix [
20]. By doing so, the water absorption phase is limited both in time and volume. After introducing SAP to the mix (usually 0.3–0.6% of the cement mass [
20,
21]), SAP particles absorb water only for few minutes until reaching their maximal water absorption capacity in a cementitious environment. For polyacrylic SAP, that capacity is usually approximately 10-times lower than the SAP reference water absorption capacity [
11].
By intentionally lowering the amount of water stored in SAP, the percentage of mixing water intended for internal curing is also limited (to approximately 5–10% [
9]). Suppose the mass amount of SAP is increased even further to increase the amount of mixing water absorbed by SAP. In this case, extensive changes to the pore network of the composite can be observed—the formation of defects, and SAP-induced macropores [
15,
20]. Such severe changes to the pore network negatively influence the mechanical parameters of the materials.
SAP can also be added to the concrete mix in a hydrogel form [
9,
11]. In this method, SAP is saturated with tap water before its addition to the dry components of the mix. In doing so, it achieves the maximal water absorption capacity in the tap water environment. Knowing that there’s a difference in the volume of water absorbed by polyacrylic SAP in different environments, by introducing fully saturated SAP into the mix, the water desorption process is intensified as it is driven by both internal stresses within a hardening material and the difference in the electrochemical activity of water absorption and desorption environments.
As SAP particles absorb a portion of the mixing water, usually an approach from other internal curing methods, by LWA, for example, is adopted. This involves adding an additional volume of water to the concrete mix to compensate for the water stored in SAP during the mixing phase [
9,
15]. Due to this fact, three different water-to-cement ratios can be distinguished—the total water-to-binder ratio (w/b
tot), effective water-to-binder ratio (w
eff/b), and (w
e/b)—a ratio of water entrained in SAP to the mass of the binder [
11].
The amount of water entrained in SAP is experimentally verified either by measuring the amount of water required to keep the consistency of SAP-modified concrete comparable with non-modified variant or based on SAP water absorption tests in an appropriate cementitious environment (in the case of polyacrylic SAPs, the water absorption in cementitious environments is approximately 10-times lower than in a tap water environment) [
11,
15,
20].
Although consistency is a vital characteristic of any concrete mix, the addition of extra water to the concrete mix to improve it complicates both the design process of any SAP-related experiments and the understanding of the SAP influence on the properties of cementitious composites. With an introduction of the additional volume of water into the mix, the proportions between all ingredients are subjected to changes (aggregate:cement:water). As SAP absorbs a portion of mixing water during the mixing stage, its volume influences the parameters of the pore network.
By increasing the overall volume of water in 1 m
3 of concrete mix, an increase in its porosity is expected, which leads to the deterioration of its mechanical properties and an overall deterioration in its durability [
15]. In this and previous research, the authors proposed a different approach to designing SAP-modified concretes—modification by SAP in a hydrogel form, with no additional water added to the system [
9,
11].
1.2. Modification of Concrete with Hydrogels—A New Approach
Different parameters characterize SAP hydrogels and non-saturated SAP. As the water absorption process in SAP is electrochemical in nature, in a cementitious environment, SAP in a non-saturated form tends to agglomerate, resulting in the formation of macropores in the cement matrix, which negatively impacts the mechanical performance of hardened concrete [
9,
11]. This SAP characteristic is one of the reasons why the amount of SAP added to the concrete mix usually is less than 1% m.c. [
20].
As the water absorption capacity of polyacrylic SAPs is much lower than in a tap water environment, the increase in the volume of SAP particles while added in a non-saturated form is a fraction of the volume of SAP that is fully saturated in tap water [
22,
23]. Of course, such an increase in the volume of SAP hydrogel saturated in tap water before mixing with other concrete mix components would negatively impact the homogeneity and overall pore network parameters.
However, due to the amount of water required to reach the maximal water absorption capacity, the SAP hydrogel is stretched to the maximum allowed by its polymer structure [
9]. This fact makes SAP hydrogel susceptible to fragmentation during mixing. In a study performed by the authors, forced change in granulation due to friction with other concrete mix components was investigated. It was found that, due to the mixing conditions, the granulation of tested SAP was changed—the median particle size fell down from approximately 330 µm (non-saturated SAP) to approximately 34 µm (SAP after fragmentation and desorption of water) [
9].
While modifying concrete with SAP, several phases of SAP influencing water transport can be distinguished. In the case of dosing non-saturated SAP, a water absorption phase lasts for few minutes after the addition of SAP to the mix, followed by a slow desorption phase caused by internal stresses within the cement matrix (due to hydration autogenous shrinkage, etc.) [
9]. In the case of adding SAP in a hydrogel form, the water absorption process takes place outside of the concrete mix, followed by fragmentation of the hydrogel during the mixing stage and intense water desorption from the SAP structure during the first 24 h after forming.
As SAP is used in concrete technology to desorb water in order to keep the internal humidity at a constant and high level for as long as possible, methods incorporating its use in cementitious composites should consider both the water absorption and desorption characteristics [
24]. The parameters of the capillary network of cementitious materials need to be taken into account, as those impact the parameters of water transport within the pore network [
24].
The method of modifying concrete with SAP in a hydrogel form is based on the assumption that SAPs water desorption parameters over time are important for achieving an optimal internal curing effect. As the primary goal of internal curing is to evenly distribute and control the water present within the cement matrix in order to mitigate self-desiccation of the cement matrix, one can argue that only the course of water desorption from SAP into cement matrix has a lasting impact on the hydration process of the binder. The SAP absorption capacity in any given environment is limited by the properties of the environment as stated previously.
In the case of dosing non-saturated SAP to the concrete mix, this deteriorates SAP water absorption approximately 10 times if compared to the absorption capacity in a tap water environment [
23]. By changing the environment of SAP water absorption to tap water and then placing the obtained hydrogel in the concrete mix, the water transport between SAP and the cement matrix is reversed [
9]. From the moment of introducing hydrogel to the mix, water from SAP is released to the cement mix until SAP reaches its water absorption capacity in a cementitious environment.
However, as the desorption process occurs in a cementitious environment, due to capillary pressures in the cement matrix and fine graded cement particles, the water release process from SAP is disturbed and, therefore, prolonged [
9]. In a cementitious environment, depending on the water-to-binder ratio of the cement paste and type of cement used for its preparation, SAP reaches its maximal water absorption capacity within minutes [
23].
Thereafter, due to cement paste being an electrochemically active environment, it desorbs water due to internal stresses within the hardening cement matrix. Due to the lowered amount of water initially absorbed, its water desorption potential is limited both in volume and time. This problem disappears altogether with the change in the dosing method. While initially, by saturating polyacrylic SAP in tap water, in an environment allowing to reach approximately a 10-times higher amount of absorbed water over water absorption in cement paste, water desorption from SAP particles due to the rheology of cement paste is limited.
Water is desorbed from SAP as an effect of both the electrochemical parameters of the water desorption environment and its physical properties, namely the impact of forming a capillary network in the cement paste. As cement particles in cement paste disturb the flow of water from the SAP to cement matrix, the water desorption rate from SAP slows down, prolonging the hydration time of the matrix and increasing its density over time.
The same mass amount of SAP can absorb different volumes of water (depending on the parameters of the water absorption environment) [
25]. In order to include this variable in the concrete design process, a new way of describing the SAP content in a concrete mix, except for the mass criterion, was proposed: the percentage of mixing water that is absorbed by SAP [
11]. By dosing non-saturated SAP to the concrete mix, SAP reaches its maximal absorption capacity in a cementitious environment within minutes after introducing it to the mix [
11]. On the other hand, in the case of introducing hydrogel, SAP reaches its maximal water absorption capacity in tap water prior to the mixing process [
9]. As those two water absorption capacities vary, for the same mass amount of SAP, the percentage of absorbed water during the mixing phase is different.
With changes to the pore network of cementitious composites caused by SAP addition, it is expected that the composite properties will change as well. Most notably, the mechanical performance of such a composite is going to be affected [
15,
26]. With a change in the pore size distribution and density of the cement matrix, the permeability of a SAP-modified material will also vary from the non-modified variant. SAP is sometimes compared to air-entraining agents, as its effect on the pore network increases the freeze–thaw resistance of cementitious composites [
27]. This property of SAP-modified concrete is usually linked with a presence of macropores that limits the continuity of the capillary network of the cementitious material [
20].
Any change to the pore network in concrete results in changes in the durability of the material. This can either facilitate the movement of corrosive agents into the matrix of the material or slow it down. As concrete is a construction material meant to last for decades, investigating the influence of admixtures and additions to the concrete mix is essential to study the durability. The main issue with changes to the permeability of SAP-modified concrete is caused by additional water replacing the amount of water stored in the SAP structure. By having an additional volume of water in the composite during the mixing stage, the porosity of the cement matrix is increased, thus, resulting in a deterioration in concrete’s mechanical performance, which usually has a negative effect on its durability [
28].
When SAP particles desorb water in a cement matrix, due to changes in pressures within the pore network, its structure is altered—the capillary network loses its continuity. As a result, the SAP-modified concrete is characterized by an increased freeze–thaw resistance [
28]. Due to this fact, one can assume that SAP addition will impact the transport properties of other substances through the pore network except for water. However, research attempts to study the diffusion of chloride ions to investigate the issue are sparse.
The durability of cementitious materials is linked with the susceptibility to aggression by chemical agents from the outer environment. The deterioration of hardened concrete can be caused by many corrosive agents that enter cementitious composites through its pore network. Therefore, altering the pore network distribution by any means of internal curing would change its durability. SAP influences several core properties of the cement matrix, including its density and pore distribution [
20]. With its impact on the properties of concrete, it is expected that chloride ion diffusion would also be impacted by SAP addition to the concrete mix.
Methodologically, internal curing by LWA and SAP is similar. Both methods usually include adding additional water to the concrete mix to increase the amount of free water in forming a cement matrix. However, while LWA addition is based on purely mechanical means of carrying water, in internal curing by SAP, this effect is strengthened by the impermanently bound water present in its structure and spread out within the entire volume of the cement matrix and not only to the close distance from LWA grains [
29,
30]. The main issue with concrete permeability is its dependence on the amount of water in the concrete mix. As this amount grows with the increase of the water-to-binder ratio, so does the permeability of hardened concrete, which would negatively affect the transport properties of corrosive agents through the pore network of the composite.
1.3. Research Significance and Aims
The conducted study was designed in such a way as to investigate the influence of the material characteristics of SAP (particle size), dosing method (non-saturated/saturated), and dosing methodology (the addition of extra water to the concrete mix to compensate for water initially stored in SAP) on the chloride ion diffusion in a hardened composite. SAP influences several properties of concrete.
To correctly assess its impact on ion transport properties through the pore network, the parameters of the aforementioned network had to be designed in a way that allowed comparison with a reference series. It is known that SAP mainly influences two of concrete’s properties—the pore network and cement matrix density. If the balance between those two altered parameters is met, the mechanical properties, namely the compressive strength of SAP-modified concrete, should be similar to the reference series.
If the amount of added SAP is too high due to either the methodology of dosing (addition of additional water) or the method of dosing (SAP in a non-saturated state tends to form conglomerates within the cement matrix [
26]). In that case, its influence over the pore network exceeds its impact on the cement matrix density and results in a deterioration in mechanical properties. By maintaining the balance and knowing that there are changes to the pore network due to SAP addition, this approach to the issue provides ground rules for investigating the influence of SAP on the concrete properties linked with the durability of the material.
4. Discussion
Concrete is a material whose properties are linked with the parameters of all phases that it constitutes: the aggregate, cement matrix, and pore network. SAP is one of the least known materials used in concrete technology [
20]. The design of SAP-modified concrete is a process that needs to take into account SAP’s influence on both the properties of concrete mix and hardened concrete.
Its influence can be modified by the type of SAP used, its water absorption and desorption parameters, granulation in the non-saturated state, morphology, method of dosing (non-saturated/hydrogel), or methodology of dosing (with or without extra water) [
12]. In the performed research, two polyacrylic superabsorbent polymers were used that differed both in water absorption capacities and particle size distribution in a non-saturated state in order to investigate chloride ion diffusion of SAP-modified concrete, as SAPs influence on ion transport parameters has yet to be fully established [
20,
36,
37].
Internal curing in concrete technology was introduced to positively influence the durability of high-performance concretes [
37,
38,
39]. With an increase in the amount of the binder within the composite, a reduction in the water amount, and an increase in the mass of fine aggregate, the influence of autogenous shrinkage on composite’s properties and durability had to be, among the influence of others, mitigated [
20]. As external curing methods proved inadequate to mitigate those phenomena, the issue of modifying the transport of water during and after the hardening of the cement matrix was raised [
40].
Currently, SAP represents one of the most curious materials used to modify water kinetics in cementitious materials [
20] and the hydration process [
41]. As presented in the introduction, SAP can be characterized by several material characteristics, including its particle size distribution and water absorption capacity. Both of those parameters need to be taken into account while designing SAP-modified concrete. As non-saturated SAP exists in the form of physical polymer grains, its particle size influences the effect on the parameters of the pore network within the cement matrix.
In that sense, SAP particle size distribution characteristics need to be taken into account while designing SAP-modified concrete. The other vital parameter of SAP is its water absorption capacity in both tap water (close to the reference water absorption capacity) and in an environment to simulate a cementitious environment. Those two material characteristics are crucial to designing concrete with SAP. With an increase in particle size of SAP in non-saturated form, a negative effect on the mechanical properties of cementitious composites can be observed [
42].
Apart from the parameters of SAP, the method and methodology of its dosing into the concrete mix affect its performance in the composite. We propose that differentiation needs to be made concerning the use of SAP and simultaneous addition of extra water to the mix to maintain the modified mix’s consistency on the same level as a reference one. While the workability of any concrete mix is an important issue during the production process [
15], maintaining it while modifying concrete with different admixtures and additions usually does not involve additional water in the system. The reason behind such an approach is that any change to the water amount in the mix changes the mechanical performance of hardened concrete. This also influences the durability [
43].
Polyacrylic SAPs can absorb different amounts of water in different water absorption environments. In the conducted research, SAP S was characterized by the water absorption capacity in a tap water environment of approximately 135 g/g and in a cementitious environment of approximately 12 g/g. SAP B had a water absorption capacity in the tap water of approximately 70 g/g, while, in a cementitious environment, that value dropped to approximately 7 g/g. The difference in the water absorption capacities allows using SAP as an internal curing agent via different dosing methods.
Suppose SAP is to be added to a water-containing environment. In that case, SAP grains will absorb water from that medium until they reach their water absorption capacity in that environment. Afterward, that water due to external pressures can be released. To compare two different polyacrylic SAPs, their difference in water absorption capacities had to be taken into account. Therefore, while designing SAP-modified series added in a non-saturated form to the mix, a percentage of absorbed water both by SAP S and SAP B was the comparative parameter. By maintaining it at the same level, due to differences in water absorption capacities between SAP S and SAP B, a different mass amount of each polymer was added to adequate series (in the case of SAP S—0.15% m.c. and in the case of SAP B—0.29% m.c.).
Between the two tested dosing methods (by introducing SAP into the concrete mix either in a non-saturated state or in a hydrogel form), two main differences influence the water desorption process from the SAP structure. The external pressures forcing SAP to release water from its structure, depending on the dosing method, vary. Suppose SAP is introduced in a non-saturated state after reaching its absorption capacity in cementitious environments. In that case, it releases water due to pressures resulting from ongoing hydration and volumetric changes present during hydration.
Suppose SAP is introduced in a hydrogel form into the concrete mix. In that case, the water desorption process is intensified as SAP is to reduce its initial high water absorption capacity. The intensification in water desorption was found to last approximately 24 h after the introduction of SAP into the concrete mix [
9]. For SAP in a hydrogel form, after the absorption of water increases its particle volume—the polymer itself is insoluble. Therefore it maintains a three-dimensional structure. With an extra volume of absorbed water, its network stretches out, making it prone to fragmentation during mixing, resulting in final granulation of the fragmented hydrogel after the desorption of water finer than that in a non-saturated state [
9].
Not all SAP-based hydrogels are susceptible to such a reduction in particle size. One of the factors controlling the absorption capacity of SAP is the amount of crosslinker is used to hold its structure—the more the crosslinker, the less water SAP can absorb and, in consequence, the less it is susceptible to a reduction in particle size [
44]. The introduction of a new variable in the form of the dosing method affected the amount of mixing water being initially absorbed by SAP in hydrogel form.
As the water absorption capacity of tested polyacrylic SAPs was approximately 10-times higher in a tap water environment than in an appropriate cementitious environment, the amount of initially absorbed mixing water increased approximately 10 times as well. In order to keep the series with different dosing methods comparable between each other, the mass content of SAP in the mix had to be kept at the same level (for SAP S—0.15% m.c. for all SAP S-modified series and 0.29% m.c. for all SAP D-modified series).
In the conducted research, to calculate the influence of both SAP and additional water added to the mix on the chloride ion diffusion, a comparison between all tested series had to be established. As the durability of concrete is of the essence for hardened concrete, not concrete mix, a comparative parameter had to be found between all SAP-modified concretes and reference ones. It was decided by the authors that, by maintaining the compressive strength of all concrete series at the same level, the impact of all discussed variables previously on the chloride ion diffusion could be investigated.
The prepared reference series (
Table 1) and SAP-modified series (
Table 3), due to the design process, were characterized by compressive strength at a comparable level (average compressive strength values after 28 days between 49.81 MPa and 53.55 MPa). Different SAP material characteristics [
44], SAP dosing methods [
9], and methodologies [
11] have various effects on the mechanical performance of concrete. These can be positive in some cases (increasing the degree of hydration without compromising pore network parameters [
15]) or negative (compromising the pore network by SAP-induced macropores/defects [
45,
46,
47]). Both of these effects had to be mitigated to establish a level field for accurately assessing the impact of all examined SAP-related variables on chloride ion diffusion.
The methodology behind SAP addition to the concrete mix is also relevant when investigating its influence on concrete’s properties [
9,
11]. From the conducted permeability tests, SAP S reduced the chloride ion diffusion through the pore network for the SAP-modified series, which did not involve additional water being added to the system (
Figure 6).
The chloride diffusion lowered by approximately 30% compared to the REF 1 series of water-to-cement ratio of 0.4 for the SAP S-modified series that did not involve any extra water (from 6.78 × 10−12 m2/s for REF 1 to 4.44 × 10−12 m2/s for SAP SD5 and 4.47 × 10−12 m2/s for SAP SH50). The SAP SH50 series in which w/ctot was increased by 0.02 (the amount of water that was entrained in SAP) resulted in an increase of chloride ion diffusion coefficient by approximately 70% compared to the REF 1 series (an increase from 6.78 × 10−12 m2/s for the REF 1 series to 11.63 × 10−12 m2/s for the SAP SHD50 series).
Additional water in the system, even if showing no influence on the mechanical properties of the material (
Table 7), impacts the parameters of the cement matrix. The method of increasing w/c
tot by the amount of water entrained in SAP led to significant changes in the properties of the cement matrix. Although in the performed tests, the volume of additional water was set to a minimal amount—only to the calculated mass of water that would be absorbed by SAP in a cementitious environment of water-to-cement ratio of 0.4, so that its influence on the mechanical performance would be negligible, it had an impact on the permeability of cement matrix modified in such way.
The consistency of the tested concrete mixes was also measured (
Table 6). We confirmed that the addition of extra water to the concrete mix in order to compensate for the water absorbed by SAP increased the flowability of the concrete mix. By increasing the total water-to-cement ratio by 0.02, the flowability of series changed (from SAP SH50 = 6.5 cm to SAP SHD50 = 15.5 cm under the slump test procedure). However, even with the set additional water in the system, the consistency of reference series was not reached (for the REF 1 series, the consistency was 18.0 cm). It is highly probable that if the method of calculating the entrained amount of water in SAP involved maintaining the consistency of tested concretes on a similar level, its negative influence on the permeability of such concretes would be even greater.
The case of SAP S shows the issue with the popular methodological approach to the design of SAP addition to the concrete mix [
11]. Suppose the volume of water entrained in SAP is to be treated as an additional volume of water added to the concrete mix to maintain the effective water-to-cement ratio. In that case, it results in an increase of porosity of cement matrix and, therefore, in an increase of diffusion of corrosive agents. Although if compared to reference series with additional water (REF 2 of water-to-cement ratio of 0.42—
Table 8) series modified by SAP S with additional water show the reduction in chloride diffusion, taking a step back to the original reference series shows an increase in permeability of such a concrete by approximately 70%.
Due to its particle size in non-saturated form and absorption capacity, SAP B indicated a smaller susceptibility to the mechanical reduction of its particles during the mixing period of concrete mix (
Figure 7). Due to this fact, its particles had a significant impact on the pore network of tested concretes. The combined effect of large SAP particle size (2–2.5 mm) in a non-saturated state and low water absorption capacity (approximately 70 g/g in tap water environment) contributed to the significant changes in the pore network.
Based on its properties, it caused an increase in the diffusion of chloride ions compared to the reference series regardless of the dosing method. For the SAP B modified series with no additional water in the system, the diffusion coefficients increased significantly, from 6.78 × 10−12 m2/s for reference series REF 1 to 9.16 × 10−12 m2/s for SAP BD5 series and to 9.43 × 10−12 m2/s for SAP BH50 series. An increase in the chloride ion diffusion was even more significant in the SAP BHD50 series (with additional water in the system), reaching 11.20 × 10−12 m2/s.
It was shown that, even with control over the mechanical properties of SAP-modified concretes established in the performed research, SAP influenced the chloride ion diffusion. However, to achieve such a setup, the total w/c ratio of the reference series had to be increased to the value of 0.4. In the case of the lower value of the aforementioned ratio, SAPs influence of mechanical performance was more difficult to be contained with the amount of variables that needs to be considered (the SAP material characteristics, the dosing method, and the presence of additional water in the system).
In the presented study, the authors proposed a method of standardization for testing chloride ion diffusion for both different SAP types and different methods of its introduction into the concrete mix. With SAP being introduced into high-performance concretes of lower w/c, SAPs influence on the chloride ion diffusion would depend on its material characteristics and the presence of additional water in the system.
Usually, if SAP is added to the concrete mix in a non-saturated state, the mass amount is higher than in the experiment plan proposed by the authors [
20,
46]. With an increase in its amount, the influence on the consistency of the concrete mix is even greater. If the concrete mix consistency is considered the parameter that is to be maintained at the level of the reference series, the amount of extra water required to compensate for the SAP-entrained one increases. One can speculate that it would further negatively impact the permeability of concrete.
SAP’s influence on the course of hydration of cementitious composites was investigated by measuring its impact on concrete’s different properties—its influence on the development of shrinkage deformations, mechanical strength, freeze–thaw resistance, permeability, hydration, etc. However, the exact phenomena that cause SAP to be an effective internal curing agent are still elusive. In
Figure 7, a small fragment of SAP-modified concrete is presented.
The micrograph was taken on a SAP BR5 sample after 30 days in a 3% NaCl solution. After that time, the sample was fractured, and its internal structure was photographed. It shows an intact SAP B particle, covered in a layer of crystallized NaCl. That level of salt crystallization was only observed on SAP particles, not within the cement matrix. That NaCl layer was partially peeled from the SAP B surface. Its inner volume showed no observable signs of salt presence.
The crystallization process during hydration of cementitious composites is a complex issue that is dependent on a number of variables. We believe that this observation provides indirect evidence of SAP water release mechanism within cement matrix and explains SAPs influence on multiple properties of cementitious materials. Intense crystallization on the surface of SAP indicates a presence of a thin layer of water that is not absorbed by SAP. It acts as an intermediate environment between that of saturated SAP and the outer environment.
It also serves as a crystallization environment, separate from crystallization not in the vicinity of SAP particles. Due to the low volume of water that it consists of, the convergence point for crystallization, in this case, the appropriate NaCl concentration, is achieved faster than in the rest of the composite, resulting in the crystallization of salt on the surface of polyacrylic SAP.
Due to the increase in Cl− ions concentration in that outer layer of free water on SAP structure, due to the electrochemical nature of SAP absorption capacity, its absorption capacity decreases—SAP desorbs water and self reduces its size, providing the additional water to its outer layer, which serves as a crystallization environment. One can imagine a similar mechanism taking place during the hydration process of binder in a cementitious environment. This approach to the issue of mechanisms behind SAP influence on the properties of cementitious composites and ultimately on the design of SAP-modified concrete is going to be investigated by the authors in future research.