The Examination of the Effect of Water-Soluble Hydrophobic Agents on Physical–Mechanical Parameters and Resistance to Aggressive Environment of Concrete

Abstract

The aim of this research was to examine the effect of water-based hydrophobic impregnations on concrete in order to improve its durability, chemical resistance, and physical–mechanical parameters. The purpose of this research was to prevent as much water evaporation as possible during concrete hydration, which resulted in the improvement in concrete properties including strength, durability, resistance of concrete in high-pressure water, water tightness, etc. Water-based hydrophobic impregnations based on silane and siloxane, epoxy resin, and oil were chosen to achieve improvements in concrete parameters. The comparison of water-based hydrophobic impregnations with solvent-based hydrophobic impregnations was also performed using the determination of absorbency, water pressure penetration depth, watertightness, concrete penetration depth, and resistance to aggressive environment. The concrete microstructure was observed using a digital microscope and a scanning electron microscope (SEM). Samples with hydrophobic agents exhibited a higher contact angle, lower absorbency, and higher resistance to aggressive environment, when compared to the untreated surfaces, confirming the water repellency of the water-soluble hydrophobic impregnations. The positive effect of hydrophobic agents in the compressive strength was also recorded. Furthermore, the benefit of the addition of 0.1 wt.% of nanocellulose into the hydrophobic impregnations was observed. Based on the obtained results, it was found that the addition of nanocellulose to water-soluble hydrophobic impregnations reduced the depth of pressurized water seepage and increased the compressive strength of the concrete after 28 days by 2 MPa.

1. Introduction

Concrete is a porous building material, and its ability to resist the effects of the surrounding environment depends to a large extent on the impermeability of its surface layers. The choice of an appropriate protection method can reduce the chances of premature damage or failure in service. Protection techniques for concrete vary, and include the following: controlling operating conditions or surrounding environment, improving the physical properties of concrete leading to increased resistance to the surrounding environment, creating a barrier between the concrete and its surroundings, or changing the electrochemical behavior in the case of corrosion of reinforcement. Methods that aim to protect concrete from damage serve to extend the period between planned repair cycles [1,2]. Concrete that is exposed to atmospheric conditions should be resistant to the penetration of water and able to slow down the penetration of harmful gasses mainly CO₂, NO_X, SO₂, or possibly other oxides in the atmosphere while allowing the concrete to dry. Concrete protection solutions should be durable in an alkaline environment and resistant to climatic conditions and UV radiation [3].

In recent years, emphasis has been placed on an increased service life of structures to support resource efficiency and energy conservation. The usual strategy for protecting concrete against the action of water and chlorides is the application of surface treatments, primarily coatings or hydrophobic impregnations. Polymers such epoxy and polyurethane resins are particularly effective [4]. Polymeric coatings form a continuous film on the surface that acts as a physical barrier against the surrounding environment, while impregnations penetrate the open pores on the surface of the material and react with it. These measures can be very effective, but their service life depends on different factors including the chemical or physical resistance of the material, the depth of penetration (below 20 mm for impregnation), and the interaction with the substrate [5,6]. Hydrophobic impregnations, e.g., siloxanes, alkylsilanes, alkoxysilanes, and related compounds, increase the contact angle of the concrete surface, prevent capillary water absorption, enhance surface resistance, and decrease water absorption and the transport of aggressive species [7]. In this way, they can inhibit the penetration of water or ions occurring in the surrounding environment and, at the same time, allow the entry and exit of water vapor [8]. Hydrophobic impregnations, e.g., those based on alkyl alkoxysilane and siloxanes, reduce the absorption and transport of liquids such as water, including salts dissolved in it [9]. Water-repellent substances must penetrate as deeply as possible into the concrete substrate to ensure guaranteed long-term durability. For this reason, the maximum depth of penetration is established, which provides the necessary assumption of the effective protection against chloride penetration and the subsequent corrosion of the reinforcement [10]. Knowledge of the transport mechanisms that take place in concrete with surface hydrophobic impregnation is necessary to determine their effectiveness. Silanes, siloxanes, oligomers, and mixtures of these components are currently the most commonly used agents. Silanes are colorless and are chemically known as alkyltrialkoxysilanes. They have a small-molecular structure (1.0·10⁻⁶–1.5·10⁻⁶ mm), so they can effectively penetrate even a dense structure. They are most often sold at a concentration of 20%. Silanes react chemically with silica or alumina-based materials and are very volatile due to their molecular structure [11]. Silanes, siloxanes, and hydroxy-silicone oil-modified epoxy resins penetrate the concrete and form a hydrophobic layer that prevents liquid water from passing through, while vapor is allowed to enter and exit [12,13]. The ability of concrete to absorb liquid surface treatment is affected by the viscosity of the liquid, the contact angle between the liquid and the material, and the pore radius [14]. Hydrophobic surface modifications showed substantial improvements in mechanical properties and corrosion resistance of concrete. The increase of 15% in flexural strength and the reduction of up to 50% in chloride ion permeability subsequent to the implementation of hydrophobic surface treatments were recorded [15]. The deterioration of the hydrophobic silane layer and the abrasion of the concrete surface can also lead to a decrease in the durability of the concrete [16]. The use of hydrophobic concrete, containing silane admixture as a surface treatment, is highly interesting in reducing the corrosion risk of steel reinforcement as it could cancel the detrimental effect of concrete cover cracking [17,18]. The silane surface hydrophobization significantly slows the creation of damage and microcracks and protects the concrete against deterioration during freeze–thaw cycles [19,20].

Hydrophobic impregnations are mainly applied in two manners: either a surface treatment that is a subsequent treatment on the material or an internal hydrophobic impregnation. Internal impregnation works on the principle of adding hydrophobic substances to the matrix of the composite in its fresh state [21]. Surface hydrophobic impregnation is compared to the concept of special surfaces, which include, for example, butterfly wings or lotus leaves [22]. The “lotus effect” shows that the microscopic structure of leaf surfaces can cause a self-cleaning effect, which is created due to the micron papillae on the rough surface and the presence of wax crystals [23,24]. The addition of nanoparticles (CaCO₃, SiO₂, and TiO₂) to a silicone emulsion was found to increase the chloride ion resistance rate by 76.13% [25]. The addition of nanoparticles did not block the capillary pores, and a biomimetic surface known as the lotus effect was created [26]. The principle of the lotus effect is that the contact area and adhesion between particles and surfaces due to epicuticular wax crystals is reduced, but both the contact area and the adhesion between particles and water droplets are greater. For this reason, particles stick to the surface of water droplets and are carried away [27].

Many natural materials have been used to reduce capillary absorption for thousands of years. In the past, various types of oils, waxes, fats, etc. were mainly used. Research shows that some substances contained in hydrophobic impregnations are not acceptable from an ecological point of view. These are mainly materials where the main component is solvent-based. Their use negatively affects the environment, increases energy consumption during production, and is associated with toxicity or subsequent lengthy processing [28,29]. The development of hydrophobic impregnations has therefore started to move towards alternative materials that are extracted from natural sources or environmentally friendly chemicals. Currently, water-soluble hydrophobic impregnations are receiving a lot of attention, as they are environmentally friendly and financially affordable, since they are produced on a water basis and can be diluted. This allows for their viscosity to be adjusted, and they are acceptable to users for their application on concrete structures. However, the effectiveness of water-based hydrophobic treatments must be thoroughly proven in terms of resistance to aggressive environments and long-term durability [30]. For this reason, the latest water-based hydrophobic treatments available on the market were thoroughly tested experimentally as a part of the current research, and their effect on various concrete substrates was monitored. In order to evaluate their positive effect on the properties of concrete structures, the results were compared with hydrophobic treatments containing solvents. Hydrophobic impregnations can also contain nanoparticles [31], which contribute to reducing water absorption and increasing the contact angle, as the contact area is reduced due to their addition. The dosage of nanoparticles is chosen with a view to maintaining the optimal price ratio, minimal agglomeration of nanoparticles, and overall improvement in the effectiveness of hydrophobic impregnation. Therefore, the research also examined the possible improvement in the properties of ecological hydrophobic treatments through the addition of small quantities of suitable nanoparticles.

2. Materials and Methods

2.1. Hydrophobic Impregnations

In total, five hydrophobic agents were tested, i.e., HY1—water-soluble oil-based hydrophobic agent (STACHEMA CZ s.r.o., Kolín, Czech Republic); HY2—silane–siloxane water-soluble hydrophobic impregnation (Redrock Constructon s.r.o., Nymburk, Czech Republic); HY3—two-component epoxy water-soluble hydrophobic agent (IN-CHEMIE Technology, s.r.o., Olomouc, Czech Republic); HY4—two-component epoxy water-soluble hydrophobic agent with a higher water content (HY3 diluted by 40% of water); and HY 5—hydrophobic impregnation based on acrylate copolymer containing xylene (IN-CHEMIE Technology, s.r.o., Olomouc, Czech Republic), which was used for comparison. The basic parameters of the used hydrophobization are stated in

Table 1. The basic parameters of the hydrophobic impregnations.

Type of Hydrophobization	Chemical Base	Consumption (kg/m2)	Application Method	Color
HY1	Oil	0.23	Spraying	White
HY2	Silane–siloxane	0.48	Spraying	Colorless
HY3	Epoxy resin	0.23	Painting	Faint white
HY4	Epoxy resin	0.23	Spraying	Faint white
HY5	Acrylic copolymers	0.15	Spraying	Colorless

.

HY1 is a product used for treatment of fresh concrete surfaces. After spraying, it creates a vapor-impermeable film that adheres well to the surface of moist concrete and prevents the premature drying of the concrete during the first days of setting and hardening. The dry film is not visible on the surface and does not affect the appearance of the concrete. The basis of HY1 is a water emulsion of paraffin waxes, which, after spraying on the surface of concrete, creates a film that does not affect the appearance of the concrete and is gradually degraded by operational and weathering influences, especially UV radiation. The chemical composition is paraffin wax (petroleum), hydrogenated, ethoxylated fatty acid alcohol C12-14, 2-octyl-2H-isothiazol-3-one, and octhilinone.

Hydrophobic silane–siloxane impregnation emulsion HY2 is a water-based hydrophobic impregnating agent for the protection of concrete, reinforced concrete, stone, mineral plasters, and masonry against the effects of weathering, chloride ions, and water penetration. It is suitable for both new and older structures. It is a one-component water-based polysiloxane penetration system that penetrates porous substrates, where it reacts and creates a bonded non-stick hydrophobic coating. It reduces the absorption of water and salts contained in it but, at the same time, allows water vapor to escape from the structure.

HY3 is a two-component epoxy-based water-soluble system used as a protective spray of machine-smoothed industrial floors with fillers, cement screeds, and other concrete structures. HY3 exhibits fast drying and high protection against water vapor and eliminates water absorption. The specific weight of HY3 is 1.1 kg/L, and the weight mixing ratio (epoxy/hardener) is 1:1.08. HY3 can be diluted up to 50% by water, so HY4 was prepared by adding water and slowly mixing to prepare an agent with lower viscosity.

Nanocellulose in the amount of 0.1 wt.% was added to the hydrophobic agents HY2 and HY4, showing the best improvements after application on the substrate.

2.2. Concrete Substrates

Three aggregate fractions were used for both concrete substrates. These were quarried fine aggregates of fractions (0–4 mm), coarse aggregates (4–8 mm) (PÍSEK ŽABČICE Co., Ltd., Žabčice, Czech Republic), and coarse crushed aggregates (8–16 mm) (Českomoravský šterk, Plc., Olbramovice, Czech Republic). Furthermore, the superplasticizing additive Stachement 2489 (STACHEMA CZ Co., Ltd., Kolín, Czech Republic) based on polycarboxylates with a high plasticizing effect (in the amount of 1.0% by weight for CS1, and 1.1% by weight for CS2) and water were added. These constituent raw materials are commonly available on the market and often used in concrete. The concrete substrate CS1 differed from CS2, as the former was based on Portland composite cement with limestone and blast furnace granulated slag CEM II/B-M (S-LL) 32.5 R, and high-temperature fly ash. CS1 concrete was designed with Portland composite cement and byproducts in order to support the drive for reduction in carbon emissions. The high-temperature fly ash had a specific gravity of 2330 kg/m³, specific surface area 394 m²/kg, SiO₂ content 49.8%, and CaO content 4.9%. The composition of the concrete substrates, presented in

Table 2. The composition of the concrete substrates per 1 m³ of concrete.

Component	Unit	CS1	CS2
Portland composite cement CEM II/B-M (S-LL) 32.5 R	kg	300	-
White Portland cement CEM I 52.5 R SR5	kg	-	380
High-temperature fly ash	kg	60	-
Water	kg	140	152
Fine quarried aggregates (0–4 mm)	Kg	950	908
Fine quarried aggregates (4–8 mm)	Kg	380	363
Coarse crushed aggregates (8–16 mm)	Kg	570	545
Superplasticizer	l	3.0	4.18
Aerating additive Micropon 0.3% of mc	l	-	1.14

, was designed based on the water/cement ratio and air content, and then recalculated through the equation of absolute volumes. The particle size distribution curve was fitted according to Fuller.

2.3. Nanocellulose

The effect of nanoparticles, specifically nanocellulose, on the properties of water-soluble hydrophobic agents was also investigated. Nanocellulose was added to the hydrophobic agents that appeared to be the most effective according to the basic results (HY2 and HY4). When compared to nanocrystalline cellulose, nanofibrillated cellulose has a longer length with a high aspect ratio (length to diameter), high surface area, and high adhesion to hydroxyl groups, easily accessible for surface modification [32]. Hydrophilic nanoparticles become hydrophobic when added to a coupling agent (e.g., silane). The surface of the coating then becomes rough, and water droplets are unable to pass through the nanoparticles. As a result, air is still present at the interface between the droplet and the coating. This is also why nanocellulose was chosen [33]. The selected cellulose nanofibers had a thickness of 10–20 nm, a length of 2–3 µm, a crystallinity of 92% and were obtained from wood fibrils during cellulose synthesis. Since they were hydrophilic fibers, they had to be dispersed in water using a high-pressure homogenizer. Compared to other nanoparticles such as nano SiO₂, nano ZnO₂, etc., nanocellulose should most significantly improve the effect of water-soluble hydrophobic agents. Homogenization of the nanofibers was carried out using the SONOPULS HD 4000 ultrasonic homogenizer (BANDELIN electronic GmbH & Co. KG, Berlin, Germany). The one-component hydrophobic impregnation HY2 allowed the addition of nanofibers directly into it, and then, mixing was carried out for 6 min at power 600 W. The two-component hydrophobic impregnation HY4 required a different preparation procedure. First, the necessary amount of water was weighed; then, the nanofibers were homogenized in it for 2 min at an amplitude of 44%; then, the mixture prepared was added to the pre-prepared and mixed components A and B of the epoxy hydrophobic impregnation.

2.4. Depth of Penetration of Hydrophobic Impregnation

The depth of penetration of the hydrophobic impregnation is considered as the penetration of the hydrophobic agents into the open pores of the concrete substrate and its structure. It depends on the viscosity of hydrophobic agents and the structure of the concrete surface. Ideally, hydrophobic impregnation should have the ability to flow as deeply as possible into the open pores and structure of the concrete substrate because there will be the greatest protection against water absorption. The penetration depth was measured at five spots for each hydrophobic impregnation on CS1 or CS2 concrete substrates using digital microscope types VHX-7000 and VHX 750F (Keyence Ltd., Osaka, Japan), and then, the average value was determined. The depth of penetration of the hydrophobic impregnation into the concrete substrate was determined after 28 days of application of the hydrophobic agents on the samples cut from the concrete cubes with an edge of 100 mm, which were impregnated. Firstly, a digital microscope VHX-7000 ((Keyence Ltd., Osaka, Japan) with the support of LIBS (Laser-Induced Breakdown Spectroscopy) was used. LIBS is an elemental analysis method that uses a pulse laser to identify elemental components technology [34]. All the hydrophobic agents used were organic, i.e., they contained carbon (C). Using LIBS, it was possible to determine at what depth in the surface layer of the concrete the detectable carbon was, which corresponded to the depth of penetration of the hydrophobic agents into the concrete. A total of 15 pulses were performed at each spot, and then, the 3D imaging of the resulting pit was performed after the laser beam with a VHX 750F digital microscope. The depth of the detected organic component in the concrete was determined, which corresponded to the depth of penetration of the hydrophobic agents. The principle of determination of depth of penetration of the hydrophobic agents into the concrete, including the hydrophobized sample, is depicted in Figure 1, where numbers represent the pulses of the laser beam.

2.5. Chemical Resistance

Chemical resistance was determined with CS1, on which hydrophobic impregnations were applied. First, plastic funnels were glued to the surface of hydrophobized concrete slabs with dimensions of 300 × 300 × 60 mm using silicone. After the silicone had dried, the funnels were filled with the selected chemical agents, and the opening of the funnel was hermetically sealed to prevent the evaporation of the solution. The surface was exposed to a chemically aggressive environment for 28 days. The samples exposed to aggressive media were stored at a temperature of 23 °C and a relative humidity of 50%.

The evaluation of chemical resistance was carried out visually and using a digital microscope type VHX 750F (Keyence Ltd., Osaka, Japan), when the microstructure of hydrophobized concrete was observed after chemical stress. For an adequate assessment of chemical resistance, inorganic and organic acids and bases, with which hydrophobic impregnation can come in contact in practice, were chosen. However, the concentration of chemical solutions was chosen to be more aggressive than is normally encountered in practice. Aggressive chemical environments in the form of 30% NaOH, 10% HCOOH, and 10% H₂SO₄ were chosen. The digital microscope was used to observe the surface of the hydrophobized concrete substrates exposed to aggressive media and was compared with the reference samples.

2.6. Compressive Strength

The compressive strength of concrete was determined according to the EN 12390-3 [35] standard after 28 days for the hydrophobized CS1 and CS2 and reference samples. The samples were kept in an air-conditioned chamber for 28 days at a relative humidity of 95% and a temperature of 15 °C until the time of testing.

2.7. Water Tightness Test

The determination of the waterproofness of hydrophobized concrete samples was carried out according to the ČSN 73 2578 standard [36]. The amount of water that was soaked into the surface of the tested sample within 30 min was determined as the loss of water in the burette, and then, the extent to which the concrete surface is waterproof was calculated in L/m². The test was performed at a temperature of 23 °C. The wetted area was equal to the area of a circle with a diameter of 6 cm, and therefore, the total wetted area was 0.002827 m². The water tightness test of hydrophobized samples CS1 can be seen in Figure 2.

2.8. Concrete Absorption

The absorbency was determined according to the standard ČSN 73 1316 [37] as the difference in the weight of the soaked-and-dried concrete sample, which was then divided by the weight of the sample in the dried state. The result was then multiplied by 100, while the absorbency (A) was given in %—see Equation (1). The absorbency of the hydrophobized samples was compared with the reference concrete samples without any surface treatment. The samples were stored in water at 20 °C.

$$A={\displaystyle \frac{m_w-m_d}{m_d}}·100 [\%]$$

(1)

where m_w is weight of the sample soaked in water, m_d is weight of dried sample, and A is absorbency of concrete.

2.9. Depth of Penetration of Water Under Pressure

The principle of determining the depth of water penetration under pressure was based on the action of water pressure of 500 ± 50 kPa for a period of 72 ± 2 h on the hardened concrete surface at a temperature of 23 °C. The test was performed according to the EN 12390-8 standard [38]. After the end of the action of the water pressure, the samples were split in half—perpendicular to the surface on which the water pressure was acting. The penetration boundary was then marked, and the penetration depth was measured to the nearest millimeter.

2.10. Adhesion of Hydrophobic Agents to Concrete

The adhesion of hydrophobic agents to the concrete substrate was performed according to the EN ISO 4624 standard [39]. Hydrophobic impregnations were applied by spraying on the concrete base and after they had dried, and the test targets were attached with glue, by waiting for a minimum time of 24 h for the epoxy two-component adhesive to dry. A pull-off adhesion test was then performed using an Elcometer 506 Pull-Off Adhesion Tester. After the test, the tensile strength of the surface layers and the point of failure were recorded.

2.11. Resistance of Concrete Against the Action of Water and Chemical Deicers

The determination of the resistance of the concrete surface against the action of water and chemical deicing substances was carried out according to the standard ČSN 73 1326 [40]. The test was performed on cubic samples with an edge of 100 mm with reference to method A according to the standard, with 100 cycles. The test specimens were placed in the bowls made of corrosion-resistant metal, into which a 3% NaCl solution was poured so that the test specimen was immersed in 5 ± 1 mm of the solution. The bowls with the test specimens were placed in the automatic freezer that allowed controlled temperature changes. The minimum temperature was −15 °C, and the maximum temperature was +20 °C, with cooling to the minimum temperature and heating to the maximum temperature taking place within 45 min. Extreme temperatures were maintained for 15 min, which means that the total time of one cycle was 120 min. Both the effectiveness of individual hydrophobic agents and the differences between the hydrophobized samples and the reference concrete were analyzed.

2.12. Frost Resistance

The determination of frost resistance of hydrophobized and reference concrete samples was carried out according to the standard ČSN 73 1322 [41]. A total of 100 freeze–thaw cycles were set up for the test. The evaluation of frost resistance took place by determining the weight loss and the coefficient of frost resistance from the compressive strength of the test specimens before and after the test. This is considered to be a sufficient parameter, since in this case, it is mainly an assessment of the effect of hydrophobic impregnations on increasing frost resistance.

2.13. Resistance to Aggressive CO₂ and SO₂ Gasses

The resistance to the action of carbon dioxide was determined on cubes with an edge of 100 mm, which were stored in an air-conditioned chamber for 90 days. The concentration of CO₂ in the corrosion chamber was set to 10% at a temperature of 23 °C and a relative humidity of 50%. Subsequently, the compressive strength and depth of carbonation were determined using the phenolphthalein test. This was determined by breaking the test specimens, spraying the broken surface with a 2% phenolphthalein solution, and finally measuring the width of the part that was not colored purple. The uncolored part determined the area where carbonation of the concrete had already begun.

Exposure of concrete to sulfur dioxide was assessed based on the EN ISO 3231 [42] standard. The test was performed on the cubes with an edge of 100 mm, which were left for 90 days in the corrosion gas chamber HK 800. It is a device for simulating the effect of harmful gasses, such as oxides of sulfur, nitrogen, carbon, etc., with the possibility of temperature and relative humidity regulation. The device for the supply and dosing of harmful gasses is built into a compact cabinet and is located in the immediate vicinity of the test area where the gas or gas mixture is dosed. The size of the test space is 800 dm³. A total of 74 cycles were performed as a part of the test. At the beginning of the test, the tub was filled with demineralized water, and then, the chamber was closed, and 2.7 l of sulfur dioxide was dispensed, which corresponded to a SO₂ concentration of 3.375‰. Once the SO₂ was supplied, the chamber was heated reaching a temperature of 40 ± 3 °C within 1.5 h, and this temperature was maintained for another 6.5 h. This phase lasted a total of 8 h. At the end of this period, the heating was turned off, and the door of the chamber was left open for 16 h. The procedure was repeated in the same way until the prescribed number of cycles was reached. After the end of exposure of the samples to the aggressive SO₂ environment, the compressive strength of the samples was determined.

2.14. Microstructure

The microstructure of CS1 samples exposed to SO₂ atmosphere was studied. This was performed using a TESCAN MIRA3 XMU scanning electron microscope (SEM) with 3D imaging capability (TESCAN, Brno, Czech Republic). The aim was to monitor changes caused by sulfation and new crystal products. In order to determine the microstructure using SEM, samples with an area of approximately 2 cm² and a thickness of 3 mm were taken from the surface of the hydrophobized substrate, which was exposed to the aggressive environment (SO₂). An energy dispersive X-ray analysis (EDX) was used to determine the elemental composition and chemical characterization of new crystals developed by the action of SO₂ atmosphere.

2.15. Rapid Chloride Penetration Test (RCPT)

The RCPT followed the test procedure of ASTM C1202 [43], which provides a method for measuring the resistance of concrete to chloride ion penetration. In the test, a DC voltage of 60 V was applied for 6 h to the 60-day samples that are 100 mm in diameter and 50 mm in thickness. One side of the specimen was exposed to a solution of NaCl (3%) and the other to NaOH (0.3 N), and the electric current and temperature were recorded throughout the test period. The transferred charge, expressed in coulombs, is an indicator of the chloride ion penetrability of the concrete, according to the classification in

Table 3. Standard chloride ion penetrability based on charge passed.

Charge Passed [C]	Chloride Ion Penetrability (CIP)
>4000	High
2000–4000	Moderate
1000–2000	Low
100–1000	Very low
<100	Negligible

.

HY1-, HY2-, and HY4-treated specimens on a CS1 substrate of concrete and an untreated reference specimen were tested to assess the effectiveness of hydrophobic agents to reduce the permeability of concrete to chloride ions. At the beginning of the measurement, the temperature of the solutions was 21 °C, and at the end of the measurement, during the test period of 6 h, the NaCl and NaOH solutions’ temperature increased gradually.

2.16. Vacuum Saturation Porosity

The vacuum saturation porosity of the concrete was determined on cylindrical 50 mm thick samples having a 100 mm diameter at 60 days. The samples were place under vacuum at −90 kPa for 3 h and then covered with de-aired water and kept under vacuum for a period of 1 h. The specimens were left in the desiccator under water for an additional period of 20 h. At the end of this period, the saturated surface dry mass and the buoyant mass of the specimen were determined. The samples were then placed in an oven for a period of 48 h at a temperature of 105 ± 5 °C, such that the oven dry weight did not vary by more than 0.1% between successive readings taken 24 h apart. The porosity (P) was determined using the following equation:

$$P=\left({\displaystyle \frac{W_s-W_d}{W_s-W_b}}\right)·100 [\%]$$

(2)

where W_b is the buoyant mass of the saturated specimen in water, g; W_d is the oven-dried mass of the specimen in air, g; and W_s is the saturated surface-dried mass of the specimen in air, g.

2.17. Concrete Sorptivity

Sorptivity tests were performed on 60-day CS1 cylindrical specimens of 100 mm diameter and 50 mm thickness in accordance with the EN 13057 standard [44]. The specimens were first oven-dried at 40 °C until they had a constant mass and then kept in the lab (21 °C and 50% RH) for 24 h to reach an equilibrium with the test environment before performing the sorption test. The lateral surface of the samples was sealed up to 20 mm depth with a silicon layer, and the specimens were put in a water basin resting on supports, in such a way that a 5 mm head over the bottom-exposed surface of the prism was always maintained. The water uptake was then measured, weighing the specimens at intervals over a 48-h period and plotting the water gain vs. the square root of time. The sorption coefficient was calculated as the slope of the stable part of the curve, expressed in kg/(m² h^0.5).

2.18. Effect of Nanocellulose Addition on Hydrophobic Agents

The positive effect of the addition of nanocellulose in the amount of 0.1 wt.% was investigated on two most suitable hydrophobic agents (HY2 + nano, HY4 + nano). The effect of nanocellulose on the compressive strength after 28 days, and depth of penetration of water under pressure were examined. The results were compared with samples treated with hydrophobic agents without nanofibers and with reference samples that were not surface treated in any way.

3. Results and Discussion

3.1. Dynamic Viscosity

Hydrophobic impregnation HY3 has the highest dynamic viscosity of 1073 mPa·s, which makes it impossible to apply by spraying, and therefore, it is applied by a brush. Other hydrophobic impregnating agents have a dynamic viscosity of less than 200 mPa·s, which allows their direct spray application without additional dilution. The differences in dynamic viscosity of the different hydrophobic impregnating agents are shown in the graph in Figure 3. For the comparison, the dynamic viscosity of nano-silica sol, non-toxic colloid formed using SiO₂ nanoparticles in water that is used for impregnation is 1870 mPa·s [45]. The curing agents of epoxy resins show low viscosities in the range of 19 to 1549 mPa·s [46].

3.2. Depth of Penetration of Hydrophobic Impregnation

The depth of penetration depends on the viscosity of the hydrophobic impregnations and the structure of the concrete surface. Ideally, the hydrophobic impregnation should have the ability to flow as deeply as possible into the open pores and structure of the concrete substrate, because then there will be the highest protection against water absorption. In the case of epoxy hydrophobization HY3, there was minimal penetration of the impregnation into the concrete structure, but it created the thickest protective layer on the concrete surface, mainly due to its high viscosity. Because of this fact, in the case of HY3, the depth of penetration was a sum of thickness and penetration of the hydrophobic agent. Tatar et al. [47] stated that epoxy resins were found to be chemically bonded to concrete by weak hydrogen bonds. The depth of penetration of HY2 was also influenced by the small-molecular structures of silanes and siloxanes, which allow a deeper penetration into the pores of concrete surface layers. For the CS2 concrete substrate, the penetration depth was generally lower, as its structure is denser and contained fewer open pores. The average penetration depth of the hydrophobic agents and a comparison of the penetration depths of the hydrophobic agents are presented in Figure 4. Based on these results, it can be seen that the penetration depth of hydrophobic impregnations is mainly influenced by their viscosity. The penetration depth of silane-based hydrophobic agents increases with the W/C ratio [48]. Generally, it is clear that a high penetration depth means a good performance, but it is not certain that a relatively low penetration depth means that the performance is poor. Yet this presents a risk [49]. The low penetration depth of the waterborne hydrophobic agent (0.43 mm) studied by Xue et al. [50] stems primarily from the large micelle size, which plays an important role in determining the penetration depth of a waterborne hydrophobic agent. The penetration depth of surface-treated silane into concrete mainly depends on four factors: the type of hydrophobic agent applied, the porosity of the concrete, the initial moisture content, and the surface treatment of the concrete substrate [51].

3.3. Chemical Resistance

Chemical resistance was determined on concrete substrate CS1, on a reference surface, and on surfaces on which hydrophobic impregnations were applied. In Figure 5, Figure 6 and Figure 7, the interface between the surface of concrete exposed to a chemically aggressive environment and the unstressed surface is clearly visible, with this boundary is marked by a red line. The effect of 10% sulfuric acid caused the degradation of the cementitious layer for samples HY1, HY2 and HY5 as in Figure 5b, Figure 5c and Figure 5f, respectively. After the removal of the cementitious surface layer, the exposed aggregate grains are visible on the concrete substrate [52,53]. On the other hand, for HY3 and HY4, a continuous red layer was formed by the action of 10% sulfuric acid as can be seen in Figure 5d,e.

The action of 10% formic acid caused a reaction with HY3 and HY4 to produce dark red products, as can be seen in Figure 6d,e. Conversely, surfaces treated with HY1, HY2, and HY5 recorded only a slight disruption of the surface and turned brown, as can be seen in Figure 6b,c,f. This implies that the long-term chemical resistance of the tested hydrophobic agents is quite low in some cases, especially in the case of exposure to strong acids. The main reasons for the poor resistance to acids are the unreacted amine curing agents and/or water trapped inside the surface treatment and, most importantly, an inhomogeneous film structure due to insufficient coalescence during the curing process [54]. As can be seen from Figure 7, after exposing the hydrophobized samples to NaOH, there was no significant surface damage, and only the precipitation of NaOH crystals on the surface was visible, while the surface was also smoothed.

3.4. Compressive Strength

Figure 8 gives a summary of the compressive strength evaluations for both concrete substrates (CS1 and CS2). Based on these data, it can be concluded that the hydrophobic impregnations had a positive effect on the resulting compressive strengths, probably due to the limitation of excessive water evaporation during concrete hydration. This effect is particularly pronounced for the CS1 specimens, where the improvement in compressive strength is the most evident. Yuan et al. [55] also proved the effectiveness of surface coating in improving mechanical properties and carbonation resistance. From the obtained results (Figure 8), this positive effect of HY is clearly evident, showing the difference between samples with and without hydrophobic agents, especially for the CS1 substrate.

3.5. Water Tightness Test

The results of the watertightness test (Figure 9) were noted to be the best for HY3, followed by HY4, which are epoxy-based. These two hydrophobic impregnations have the highest viscosity, allowing them to create a denser surface protection against water penetration. Epoxy resins performed better in resisting water absorption, chloride permeability, and diffusion as well as chemical attack when compared to other coating agents [56]. Water can contain corrosive substances (such as chlorides, sulfates, etc.) that penetrate inside the concrete through cracks, leading to the corrosion of steel bars or other durability problems. Therefore, the effective prevention of water penetration is essential to improve the durability and to prolong the service life of concrete structures [57,58].

3.6. Concrete Absorption

The average measured values of the water absorption are shown in Figure 10. The results of concrete absorption are similar for samples treated with hydrophobic agents with no significant differences It can be seen that hydrophobized samples showed an approximately 20% lower absorption than the reference sample. The positive effect of the hydrophobic agents was verified. However, concrete substrate CS1 showed a more porous structure than CS2 and therefore showed a higher absorption.

3.7. Depth of Penetration of Water Under Pressure

The results show that all hydrophobic agents had a positive effect on the resistance to pressurized water penetration into the concrete structure, with HY5 having the least effect. The CS2 substrate treated with epoxy hydrophobic agents showed zero water penetration into the concrete surface, while the diluted epoxy hydrophobic agent HY4 did not have such a significant effect on resistance to pressurized water. The application of hydrophobic agents did not show significant difference between HY1 and HY2 on CS2, which may be due to its denser structure. On the contrary, it is evident in CS1 samples that this substrate is more porous, as it showed more significant variations in the depth of pressure water penetration. The penetration depth of the seepage on reference samples can be seen in Figure 11. The measured values of penetration depth of the seepage on samples treated with hydrophobic agents are shown in Figure 12. The graphical comparison of the results is shown in Figure 13. Husni et al. [59] found out that the penetration of water into the concrete impregnated by rice husk ash dispersed in an ethanolic solution containing fluoroalkyl silane under a water pressure of 500 kPa was successfully reduced but not fully prevented.

3.8. Adhesion of Hydrophobic Agents to Concrete

Based on the adhesion testing of different hydrophobic agents on the concrete substrate, it was found that most of the failures occurred within the concrete substrate. The lowest adhesion was exhibited by HY1, where the failure occurred predominantly in the hydrophobic impregnation layer itself, as can be seen in Figure 14. Figure 15 graphically shows the tensile strength values of the surface layers and a comparison of these values. The adhesion between hydrophobic impregnated concrete and repairing materials should be considered in the case where repairing or strengthening is necessary after concrete structures repaired by hydrophobic impregnation agents are damaged [60]. Protective materials must tightly bound to the concrete substrate to provide long-term protection against corrosive environments [61].

3.9. Frost Resistance

Frost resistance index was highest for HY1, while similar values were observed for REF, HY2, and HY4—see Figure 16. The effectiveness of the hydrophobic agents was therefore evident only for HY1 and, to a lesser extent, for HY3. Additionally, as shown in the graphs, mass losses were lower for all treated samples compared to the untreated ones.

3.10. Resistance to Aggressive CO₂ and SO₂ Gasses

As a result of SO₂ exposure, surface degradation of the samples occurred, which was also manifested by a change in color. HY3 and HY4 showed a significant color change to brown, while the other samples showed only a yellowish change. Compressive strength was also determined on samples after exposure to SO₂, and none of the samples, including the reference concrete samples, showed a decrease in strength of more than 5%. The visual changes in the samples can be seen in Figure 17, and the changes in microstructure were also observed by SEM. Temperature changes can have an influence on the sulfate attack mechanism as well [62].

No significant visual changes were observed on the surface of the samples when CO₂ was applied. Based on the FF test results, it was found that HY3 and HY4 gave the best protection to the concrete against CO₂ as the depth of carbonation was less than 10 mm—see

Table 4. The results of carbonation depth using the FF test of samples exposed to 10% CO₂ atmosphere.

Type of Substrate	Type of Hydrophobization	Depth of Carbonation [mm]
CS1	REF	22
	HY1	12
	HY2	22
	HY3	9
	HY4	10
	HY5	18

. On the contrary, HY2 and HY5 showed the lowest protective effect. This confirms that the resistance of coatings to carbonation is closely related to their impermeability and the formation of a dense structure, as reducing gas penetration and diffusion inevitably leads to better carbonation resistance. The carbonation rate of concrete correlates with its short-term water absorption, when the samples treated with HY3 and HY4 showed the lowest water absorption. It was found out by Bu et al. [63] that the carbonation level of recycled aggregate concrete was three times that of natural aggregate concrete. Zhu et al. [64] reported that the carbonation depth of the silane surface-treated recycled aggregate concrete was lower than the concrete prepared with integral silane treatment.

3.11. Microstructure

The microstructure of samples shown in Figure 18 indicates the reaction of calcium compounds with SO₂. This process led to the decalcification of the cementitious matrix, which subsequently revealed the siliceous phase of the concrete. Such changes are indicative of chemical degradation caused by an aggressive environment, which has a significant effect on the structural properties of the surface.

The presence of the elements Ca, O, and C on the microstructure of the specimen shown in Figure 18 indicates that the hydrophobic impregnation of HY1 provided some protection to the concrete from the aggressive action of SO₂. The retention of calcium compounds such as Ca(OH)₂ or CaCO₃ indicates that HY reduced gas or water penetration, thereby minimizing the chemical degradation of the cementitious sealant. The EDX results indicate the absence of calcium hydroxide, while the particles of non-reactant lime are still present in numbers comparable to those in the control specimen [65].

The presence of the elements Ca, O, S, and Al in the EDX analyses shown in Figure 18 indicates that the aggressive action of SO₂ led to chemical changes in the concrete structure. Figure 18a shows CSH gels, which represent the intact cement matrix, and it can be seen that, even after the exposure of concrete to SO₂, its amount in the surface layers of concrete is high. The polymerization of the hydrophobic agent inside the pore structure could produce amorphous silica gel by auto-condensation and C-S-H by reaction with the cementitious matrix, leading to an increase in surface resistance [66]. A similar microstructure was observed in samples treated with HY1 hydrophobic agent, as shown in Figure 18b. Considering the form of the crystals and the results of EDX, it can be concluded that the observed neoplasms in Figure 18c, d probably represent gypsum. The presence of sulfur (S) confirmed the formation of sulfate compounds such as calcium sulfate (CaSO₄) and gypsum, which are the result of the reaction of SO₂ with calcium oxide compounds in the concrete. Once penetrated into the concrete, sulfates react with the lime formed during the hydration process of the cement, and one of the reaction products is calcium sulfate or secondary gypsum [67]. Gypsum leads to reduction in strength, expansion, and cracking and eventually to transformation of the material into a non-cohesive mass without a defined structure [68]. It can be seen that these products were formed mainly in the pores (Figure 18d) in the surface layers of concrete, where gypsum had sufficient space for its formation. The used hydrophobic treatments did not prevent the formation of these crystals under the influence of aggressive SO₂. The presence of aluminum (Al) may indicate a reaction with the aluminate constituents of the cement, which may have contributed to the formation of secondary products such as ettringite. These changes are clear evidence of the influence of SO₂ on the concrete surface.

3.12. Vacuum Saturation Porosity

The vacuum saturation porosity results obtained for the reference concrete are 1.85% for CS2 concrete substrate and 3.07% for the CS1 concrete substrate. This CS2 concrete had a lower permeable porosity, which also indicates that the same trend is obtained for the water absorption test and the reported depth of penetration of the hydrophobic agents, as concrete substrate CS1 showed a more porous structure than CS2.

3.13. Rapid Chloride Permeability Test (RCPT)

The charge values of the reference specimens and specimens treated with HY1, HY2, and HY4 on CS1 concrete substrate are shown in Figure 19, which also illustrate the effectiveness of each treatment on reducing the permeability of concrete to chloride ions according to the ASTM C1202 classification.

Based on the RCPT results, the samples refer to the same permeability category according to ASTM C1202, which has a low chloride ion penetrability. However, the reference sample with a value of 1866 coulombs and sample HY1 with a value of 1981 coulombs demonstrated higher chloride penetrability, when compared to samples HY2 (1692 C) and HY4 (1756 C). Therefore, the reference sample and HY1 had lower resistance to chloride ion penetration. Conversely, samples with the hydrophobic treatments, HY2 and HY4, indicate a somehow higher resistance of the concrete and better protection of the reinforcement against corrosion. The results were strongly influenced by the porous structure of the concrete samples. It was proved by Basheer et al. [69] that silanes and their mixtures with siloxanes used as waterproofing agents for concrete should create hydrophobic pore protection, which reduces the movement of moisture and chloride ions.

3.14. Concrete Sorptivity

Figure 20 shows that the sorption of the tested concrete over time was the lowest in the sample treated with the HY4 hydrophobic agent but higher for other hydrophobic treatments. HY1, in particular, demonstrated a higher sorption. The sorption coefficient “s” obtained for HY4 at 24 h was 0.0043 kg/m²/min^0.5 and 0.0033 kg/m²/min^0.5 at 48 h, an improvement and lower than the reference concrete with a sorption coefficient of 0.0053 kg/m²/min^0.5 and 0.0045 kg/m²/min^0.5 at 24 and 48 h, respectively.

3.15. Effect of Nanocellulose Addition on Hydrophobization

The addition of 0.1 wt.% nanocellulose into the water-soluble hydrophobic agents HY2 and HY4 contributed to a reduction in the depth of pressurized water seepage. Originally, the depth of pressurized water seepage for HY2 was 46 mm, and it decreased to 25 mm, while for HY4, it decreased from 39 mm to 0 mm—see

Table 5. The depth of penetration of water under pressure to samples treated with hydrophobization containing nanocellulose.

Type of Substrate	Type of Hydrophobization	Depth of Penetration of Water [mm]
CS1	HY2 + nano	25
	HY4 + nano	0

. Wu et al. [70] tested the concrete coating by incorporating nano-silica and polydimethylsiloxane in a bio-based epoxy resin, and resulting in a water contact angle of 160°.

It can be seen from

Table 6. The compressive strength of the samples after a 28-day hydrophobization treatment containing nanocellulose.

Type of Substrate	Type of Hydrophobization	Compressive Strength [N/mm2]
CS1	HY2 + nano	47.5
	HY4 + nano	46.1

that the compressive strength of CS1 hydrophobized by the hydrophobic agents with nanocellulose (HY2 + nano, HY4 + nano) was increased by approximately 2 MPa in comparison with concrete treated with hydrophobic agents without nanocellulose. This improvement in mechanical properties is related to the improvement in the hydrophobic properties of the concrete surface and the increased prevention of water evaporation from the concrete during hydration reactions and concrete maturation. The hydration of the concrete proceeded more intensively as the initial strength of the concrete developed, which can also be seen in the results shown in Table 6.

4. Conclusions

Based on the conducted research, the following key findings have been identified:

• The hydrophobized samples exhibited low resistance to acid attack, limiting their use in acidic environments. However, in an alkaline environment with NaOH, they showed better stability than reference samples.
• The application of hydrophobic impregnations had a positive effect on the mechanical parameters of the concrete—with an increase after 28 days in the compressive strength of CS1 by up to 30% to 45.2 MPa or of the hydrophobically treated concrete HY2. This positive effect was caused mainly by the reduction in water evaporation during concrete hydration. The same effect was recorded with CS2, when the concrete hydrophobized by HY1 showed an increase in compressive strength of 4 MPa with respect to the reference concrete.
• The depth of penetration of water under pressure, absorption, and watertightness were significantly decreased when water-soluble hydrophobic agents were used. CS2 impregnated by HY3 showed zero absorbency and watertightness of only 0.28 L/m².
• The effectiveness of HY was also confirmed with respect to the frost resistance of the concrete, when the weight loss of the CS1 hydrophobized using HY1 was only 1.68% in comparison to untreated CS1, which showed a weight loss of 2.85%.
• Samples treated with HY3 and HY4 showed higher resistance to the gaseous aggressive environment, whilst higher resistance to carbonation was confirmed through the FF test, when the depth of carbonation of the concrete impregnated by HY3 was 9 mm, which is 13 mm lesser than the untreated concrete.
• The best results for resistance to chloride penetration were achieved with samples treated with hydrophobic agents HY2 and HY4, which showed the lowest penetration of chloride ions.
• The presence of nanocellulose in HY2 and HY4 reduced the depth of penetration of water under pressure in CS1 (with no water penetration with concrete hydrophobized using HY4 + nano). There was even an increase in the compressive strength after 28 days in the case of CS1 to 47.5 MPa for HY4 + nano.

The experimental research provided a comprehensive overview of the properties of water-soluble hydrophobic agents and their effect on the concrete, providing a basis for the optimized selection of the appropriate HY according to the preferred application conditions and desired material properties. Water-soluble hydrophobic agents represent an exclusive option for the future, as water dilution reduces financial costs. There are many products on the market with different binder bases to choose from, and they are developed in accordance with sensitivity to environmental performance, unlike hydrophobic agents containing xylene or other solvents.

Future research can further focus on a more detailed examination of the effect of water-based hydrophobic agents not only on concrete but also on other cement composites. The testing of more types of water-based hydrophobic agents and, above all, their effect on the long-term durability of concrete in an aggressive environment are worth considering. It would be very appropriate to test these environmentally acceptable hydrophobic agents in practice, e.g., in the context of application on highways, where their effectiveness should be subsequently monitored over time when exposed to different actions in the real environment. It is also important to choose a suitable technology for the application of hydrophobic agents, both based on their viscosity and also from the perspective of the required secondary protection of concrete structures. Furthermore, it can be mentioned that it is also necessary to search for other suitable nanoparticles that would further improve the effect of water-based hydrophobic agents on cement composites.