Hydrophobic Protection for Building Materials

Abstract

The chapter “Hydrophobic Protection for Building Materials” highlights the significance of modifying wetting properties to enhance the durability and performance of construction materials. It addresses the theme of wetting in building materials, wetting assessment techniques, and factors influencing wetting, such as chemical composition, pore distribution, and surface properties. Traditional building materials are compared with innovative materials like geopolymers. Various methods of wetting modification are discussed in this chapter, including altering material composition through bulk additives and coatings. Research is presented to acquaint the reader with current trends in modifying the wetting of construction materials. The chapter underscores the importance of nanomaterials and bulk additives in altering surface properties and outlines investigations into coatings designed for surface protection. Practical applications of hydrophobic coatings are also demonstrated through examples of different commercial products.

1. Introduction

The realm of building materials is one of continuous evolution, driven by the pursuit of enhancing their durability, performance, and functionality in the face of diverse environmental conditions. Among the various challenges that materials encounter, the phenomenon of wetting stands out as a critical determinant of their behavior. The concept of wetting, encompassing the ability of materials to absorb fluids and propagate them along their surfaces, holds profound implications for material integrity and longevity. In the context of construction, where materials are subjected to multifaceted conditions, understanding and modifying wetting behavior emerge as crucial pursuits.

Section 3 of this chapter is dedicated to exploring the intricacies of wetting, shedding light on the factors influencing a material’s interaction with fluids and elucidating techniques for assessing and modifying wetting characteristics. This foundational understanding serves as a stepping stone toward enhancing material performance in construction applications. As the behavior and durability of building materials are deeply intertwined with wetting phenomena, comprehensive investigation and manipulation of wetting behaviors become pivotal. Figure 1 shows the application areas for hydrophobically modified cement-based materials.

Figure 1.

Applications of hydrophobically modified cement-based materials [1].

The assessment of wetting properties is a pivotal step in deciphering the interaction between building materials and fluids. Contact angle measurement, a key parameter in quantifying wetting behavior, is scrutinized through various techniques. The spreading method, laser beam method, and drop method each offer unique insights into the material’s interaction with fluids, unveiling the dynamic interplay between surface and liquid.

The wetting behavior of building materials is governed by a complex interplay of factors. The arrangement of pores within a material’s structure holds sway over its ability to absorb fluids, with parameters such as size and distribution significantly affecting the degree of wetting. As revealed by research, the manipulation of pore characteristics can lead to material optimization, ensuring efficient fluid interaction and enhanced durability.

Chemical composition and surface properties further influence wetting behavior. The presence of molecules and functional groups on a material’s surface can profoundly alter its interaction with fluids. Through meticulous study and manipulation of these surface characteristics, researchers can tailor material wetting behaviors to suit specific needs, thus fortifying material performance.

In this dynamic landscape, the protection of building materials assumes paramount importance. Techniques such as surface coatings, hydrophobic and oleophobic treatments, and anti-corrosion agents act as guardians, shielding materials from the deleterious effects of external elements. These interventions not only enhance material longevity but also provide avenues for creativity and esthetics in construction.

In summary, the exploration of wetting behaviors in building materials offers a gateway to unraveling the intricate interplay between materials and fluids. This understanding forms the bedrock for fortifying material performance, improving durability, and expanding the horizons of construction applications. Through the careful manipulation of wetting behaviors and the application of protective measures, building materials can transcend their inherent limitations and pave the way for resilient, functional, and visually appealing structures.

2. Comparison of geopolymers and Portland cement

Portland cement is the most commonly used type of cement and a fundamental component in the production of concrete and mortar. It is a finely ground, rocky mixture consisting mainly of calcium oxide (quicklime - 65%) and silica (silica dust - 21%), as well as, for example, alumina (5.6%) or iron oxide (3.4%). These substances occur primarily in the following forms [2]:

• Tricalcium silicate - (CaO)₃. SiO₂, commonly referred to as alite, often denoted as C₃S.

• Dicalcium silicate - (CaO)₂. SiO₂, commonly referred to as belite, often denoted as C₂S.

• Tetra-calcium aluminoferrite - (CaO)₄. Al₂O₃. Fe₂O₃, commonly referred to as celite, often denoted as C₄AF.

• Tricalcium aluminate - (CaO)₃. Al₂O₃, commonly denoted as C₃A.

It is produced by grinding and burning a mixture of calcium carbonate (in the form of limestone, chalk, or other rocks), silica (in the form of sand or clay, but old glass can also be used), bauxite (as a source of alumina, but recycled aluminum can also be used), iron ore (as a source of iron oxide, but recycled iron or fly ash from power plants can also be used), gypsum, and other additives. This mixture is hydrated after dissolution in water and solidifies by forming calcium hydroxide (allowing the cement to set even underwater). Therefore, the main difference between geopolymers and Portland cement lies in the reaction that occurs during solidification. Geopolymers undergo polycondensation, while cement undergoes hydration. Cement can simply be mixed with water, whereas geopolymers require activating solutions to achieve a high pH. These activating solutions increase the cost of geopolymers [3, 4].

Portland cement is the primary ingredient in concrete (specifically known as cement concrete - alternatively, asphalt, gypsum, or other substances can be used instead of cement). Concrete is the most widely used composite material in construction worldwide. Hydrated cement serves as the binder and matrix. Additionally, concrete contains aggregates (often called coarse aggregates, used to create a durable compressive skeleton formed by interlocking individual grains, thereby increasing the overall compressive strength). Commonly used aggregates include sand, gravel, or crushed stone with various particle sizes. Other additives such as metal or polymer fibers (used to increase tensile and flexural strength), other mineral or liquid admixtures, and accelerators or superplasticizers (to improve the workability and consistency of the concrete mix) are also added. The combination of concrete and iron or steel reinforcement allows for reinforced concrete, which can better withstand tensile loads than plain concrete [5, 6].

Geopolymers have significantly better mechanical properties compared to concrete made from Portland cement, particularly much higher compressive strength (around 100 MPa for geopolymers, 30 MPa for cement, and up to 60 MPa for cement with special modifications). However, tensile and flexural strength are lower in geopolymers. This can be compensated for by using reinforcements commonly used in conventional concrete, such as carbon steel bars or corrosion-resistant materials like carbon fiber. However, it is not possible to use any lightweight metals or alloys in the production of geopolymer composites as these materials quickly lose their strength due to the strong alkalinity of geopolymers. To produce geopolymer composites with glass fibers, it is necessary to use alkali-resistant glass fibers, for example, made of organic polymers [4, 7].

One of the most significant advantages of geopolymers is their higher thermal resistance. While concrete rapidly degrades when heated to temperatures above 300°C due to thermal decomposition, which also releases toxic substances, geopolymers remain stable up to a melting temperature of around 1265°C. However, even in this case, some changes occur in the structure or composition. In the temperature range of 100–200°C, there is an increase in porosity and a decrease in mass due to the release of bound water. As the temperature increases, there is further mass loss. At temperatures above 600°C, siloxane bonds begin to break down, releasing hydroxyl ions. With further temperature increase (above 800°C), porosity decreases due to partial melting or sintering of geopolymers into a more compact structure. However, these changes are much less destructive than the changes in concrete exposed to too high temperatures [4, 7].

Geopolymers also have low thermal conductivity, which can be further reduced by foaming. This can be achieved by adding powdered aluminum, which reacts with alkaline substances, such as hydroxides in the liquid geopolymer mixture, releasing hydrogen gas. All these properties, therefore, make it feasible to use geopolymers for passive fire protection. [4, 7].

Geopolymers are also much more resistant to chemical attack, making them suitable for use in harsh conditions, such as acidic rain, sewage treatment plants, or chimneys. Moreover, they exhibit significant adsorptive capabilities, making them potentially useful, for example, in wastewater treatment, where they can serve as adsorbents or filtering media. Additionally, they can seal various particles incorporated into the structure during geopolymerization, thus avoiding leakage into the surrounding environment. Consequently, they can be used for storing hazardous waste or other waste materials, such as exhaust gases from industrial plants or power plants [4, 7].

However, the main advantage of geopolymers lies in their much lower energy consumption during production and lower emissions. The production of geopolymer cement requires approximately 1230–1310 MJ/t, with most of the energy consumed during the calcination of clay materials. In contrast, the production of Portland cement consumes nearly three times more energy, about 3500 MJ/t, especially during the calcination of limestone. Furthermore, Portland cement production generates a large amount of carbon dioxide (approximately one ton per ton of cement), which cannot be significantly reduced since carbon dioxide is an unavoidable byproduct of limestone calcination. In 2005, cement production was responsible for 1.8 billion tons of carbon dioxide emissions, accounting for about 8% of the total emissions that year. In contrast, geopolymer production emits 50–80% less carbon dioxide, mainly due to lower temperature requirements (which reduces energy demand). Temperatures up to 1500°C are required for cement production, while 600–700°C are sufficient for thermal processing of clay components in geopolymers [8].

The main drawback of geopolymers is the need for their alkaline activation, which poses a logistical problem. This is due to the necessity of purchasing or producing, distributing, and more complex on-site use of activating solutions because of their strong alkalinity (corrosiveness). Materials based on ordinary Portland cement simply need to be mixed with water. Another disadvantage is the risk of variability in the composition and structure of materials used in geopolymer production, especially fly ash or slag. The variable composition of activators and binders leads to inconsistent properties of the obtained material, which is problematic in standardizing these materials, whereas the composition and properties of Portland cement are clearly defined. However, this problem can be addressed.

3. Wettability of building materials

The concept of wettability constitutes a fundamental aspect that profoundly influences the behavior and durability of construction materials, especially when exposed to various environmental conditions. This dedicated section of the chapter aims to delve into the nuances of wettability, examining numerous factors that affect a material’s ability to absorb fluids. Furthermore, it elucidates available techniques for assessing and modifying wettability characteristics. In essence, wettability pertains to the inherent tendency of a surface to absorb and facilitate the spread of fluids along its structure. Unlike droplet formation, this phenomenon involves fluids like water adhering to the material’s surface [1, 9].

The fundamental tool for expressing the level of wettability is the contact angle. This angle measures the inclination of a liquid droplet on the material’s surface. A smaller contact angle indicates strong wettability, where the fluid spreads widely. This implies a high surface energy. On the other hand, a larger contact angle signifies weak wettability, where the fluid tends to form droplets and roll off the surface due to lower surface energy. Through this measure, researchers can discern a material’s propensity for fluid absorption, its capillary action potential, or repellency, and how these properties directly impact its durability and overall performance [9, 10].

Hao Yao and colleagues [9], in their publication, focused on comprehending the mechanisms of wettability transition on microstructural surfaces, revealing the intricate interdependence between surface geometry and wettability behavior. By combining theoretical analysis with experimental methods, researchers illustrate how microstructures can be designed to induce specific wettability effects, potentially revolutionizing material design. Figure 2 presents an illustration of hydrophobic cementitious materials from three aspects: preparation, characterization, and properties.

Figure 2.

Illustration of hydrophobic cementitious materials from three aspects, covering preparation, characterization, and properties [9].

The composition and microstructural organization of the material have an impact on wettability behavior. Jihui Zhao et al. [1] investigate the effects of hydrophobic modifications on concrete and their influence on surface free-energy. François-Xavier Coudert and colleagues [11] analyze changes in wettability properties and surface free energy due to concrete hydrophobization. Meanwhile, Chenzhi Li and colleagues [12] focus on the mechanisms of wettability transition on microstructural surfaces and their significance for the durability of construction materials.

The influence of pore structure on wettability has been described by Jihui Zhao and his team [13], emphasizing the importance of surface analysis techniques such as X-ray photoelectron spectroscopy (XPS) and surface energy analysis (SEA) in understanding and manipulating wettability. Similarly, Wang et al. [14] concentrate on testing and modeling wettability transition on microstructural surfaces.

In summary, wettability serves as a foundation for understanding the behavior and durability of building materials. The mentioned publications highlight the multidisciplinary nature of wettability’s influence, encompassing microstructural considerations, surface modifications, and broader material applications. Together, these studies contribute to a more precise analysis of the impact of wettability on material behavior and their diverse applications in the field of construction.

3.1 Methods for assessing wettability

The assessment of wetting in building materials is a crucial step in understanding how fluids interact with their surfaces. This assessment is based on the measurement of the contact angle – the angle at which a liquid droplet rests on the material’s surface. This angle provides insights into the wetting behavior of the material, offering information about the spreading or droplet formation of fluids on the surface. Various techniques for measuring the contact angle are utilized, each revealing different aspects of the material’s interaction with fluids. These techniques include: [15, 16].

3.1.1 Spreading method

In this method, the change in contact angle over time is carefully monitored by introducing a liquid droplet onto the material’s surface. This dynamic analysis reveals how the fluid interacts with the surface over time, shedding light on the material’s ability to absorb or repel fluids. The spreading method provides subtle insights into the evolution of wetting in response to fluid exposure.

3.1.2 Laser beam method

The laser beam method employs a laser beam to measure the angle of inclination of a liquid droplet resting on the material’s surface. This technique offers precise measurement of the contact angle, allowing for an accurate assessment of wetting characteristics. The precision of the laser beam ensures reliable quantification of contact angles, contributing to a thorough understanding of the material’s response to fluids.

3.1.3 Drop method

The drop method takes a different approach by capturing micrographs of droplets on the material’s surface. The shape and dimensions of these droplets provide significant information about the material’s wetting tendencies. Analyzing the geometry of droplets – whether they spread or maintain their shape – offers a qualitative and visual representation of how fluids interact with the surface.

Shirong Zhu et al. [16] conducted extensive research on advanced methods for assessing surface-free energy and wetting. Their work delves into the intricacies of these measurement techniques, offering insights into their applications, advantages, and limitations. This comprehensive review serves as a valuable resource for researchers and practitioners seeking to understand the complexity of wetting measurement.

Fan Ding and Manglai Gao [17] examined wetting measurement methods to enhance oil recovery. Although focused on a different context, their review provides valuable insights into various techniques used across different fields for wetting assessment. This broad perspective highlights the versatility of wetting measurement and its applications beyond the realm of building materials.

Significant research was conducted by Grzegorczyk-Frańczak et al. [18]. Their work focused on the changes in wetting properties and surface energy during the hydrophobization of concretes with the addition of boiler slag and coal dust. These studies provided valuable insights into the influence of these additives on the hydrophobic properties of concrete, which is crucial for the durability and performance of building materials under various environmental conditions.

On the other hand, the work of Adhikary et al. [19] provided a review on lightweight self-compacting concrete. The authors concentrated on analyzing various aspects of this type of concrete, including its components, properties, and potential applications in construction. Their research emphasizes the importance of lightweight self-compacting concrete in the context of sustainable construction and the need for further studies to optimize its properties and applications.

In conclusion, wetting measurement through contact angle assessment forms the basis for uncovering the intricate dynamics between materials and fluids. The presented methods showcase a multifaceted approach to understanding wetting behavior. These measurement techniques, supported by in-depth research, serve as essential tools for delving into the interaction of building materials with fluids, ultimately influencing their performance and durability in construction applications.

3.2 Factors influencing wettability

The wetting of building materials is a multifaceted phenomenon shaped by a range of factors related to their structural, chemical, and physical properties. Within this complex structure, several key determinants play vital roles in shaping the wetting characteristics of the material, each offering a unique perspective on its behavior during interactions with fluids. An essential aspect of these properties is wettability, which refers to the material’s capacity to absorb liquids, such as water. The influence of the material’s structure on its ability to absorb fluids has been the subject of numerous studies and analyses, contributing to a better understanding of this phenomenon.

3.2.1 Pores - size and distribution

The significance of pore distribution and size in building materials regarding their interaction with fluids, particularly water, is well-documented in scientific literature. Figure 3 shows the complexity of the correlation study of concrete pore structure based on WOS (2018–2023). The distribution of pores within the material has a significant impact on various aspects of its interaction with fluids. It is often observed that materials with a higher percentage of larger pores exhibit the ability to absorb a greater amount of water and other fluids. However, the quantity of pores is not the only crucial factor; their size and distribution also play a key role. These parameters determine to what extent fluids can penetrate the material. For example, materials characterized by an organized network of small pores often demonstrate much better wettability compared to materials with a more chaotic pore structure.

Figure 3.

Correlation of concrete pore structure study based on WOS (2018–2023) [20].

In a study conducted by Song Jin et al. [21], the impact of pore structure on the ability of concrete with varying strengths to absorb water and the permeability of chloride was analyzed. Another example comes from the research of Jianzhuang Xiao et al. [20]. Their work focused on investigating the influence of pore structure on concrete’s ability to absorb water. These studies revealed that the distribution and distribution of pores have a complex impact on the material’s response to varying humidity levels. The results provide valuable insights into optimizing concrete properties related to water interaction. In the studies presented by Dinghua Zou et al. [22], the authors demonstrate that the pore structure of the aggregate affects the equilibrium of residual water, but has a negligible impact on the rate of water release.

Furthermore, it is worth noting that the influence of pore structure on the ability to absorb water is not limited to traditional materials such as concrete. Shikun Chen et al. [23] conducted research on the pore structure and water absorption capability of geopolymers. In the context of studying the influence of distribution, size, and pore structure on the hydrophobic properties of alkali-activated materials, it is essential to consider the work of Bang et al. [24]. Another study by Ruan et al. [25] focused on the analysis of the microstructure and mechanical properties of sustainable cementitious materials with an ultra-high substitution level of limestone and calcined clay powder.

The research presented by Lawrence M. et al. [26] offers tangible insights into how pore distribution influences a material’s water absorption behavior. This study underscores not only the pivotal role of pore structure in interaction processes but also accentuates the imperative to comprehend and regulate these characteristics to enhance the efficacy of building materials. Grasping these facets is paramount for engineers and scientists striving to develop materials with optimal characteristics concerning water resistance and longevity in diverse environmental scenarios.

The size and distribution of pores in building materials significantly influence their interaction with fluids, especially water. Figure 3 illustrates the complex relationships between the pore structure in concrete and its interaction with fluids, summarizing publications from WoS between 2018 and 2023. Materials with a higher proportion of larger pores tend to absorb more water. However, the sheer number of pores is not the only determinant; their size and distribution are equally important. Materials with a well-organized network of small pores often exhibit better wettability than those with a random pore structure. Numerous studies have focused on the impact of pore structure on concrete’s ability to absorb water and other properties. Notably, the influence of pore structure is not limited to traditional materials like concrete but also extends to newer materials such as geopolymers. Understanding the role of pore distribution and size is crucial for optimizing water resistance and durability of building materials in various environmental conditions.

3.2.2 Chemical composition and surface properties

The wettability of materials is shaped not only by their physical structure but also by their chemical composition (Figure 4) and surface properties (Figure 5). Molecules and functional groups present on the material’s surface can interact with fluids in various ways, directly affecting the material’s absorption capacity. This complex interplay between surface chemistry and wetting behavior has been thoroughly investigated and documented in various scientific studies. The aforementioned issue will be discussed in detail in Section 4 of the above chapter.

Figure 4.

Effect of chemical composition modification on wetting properties [27].

Figure 5.

Effect of surface modification on wetting properties [28].

4. Methods of protecting building materials

In the field of construction, modifying the wettability of materials plays a crucial role in improving their durability, performance, and functionality. The literature presents various approaches that contribute to understanding ways to shape wettability to enhance material performance. One approach involves adding additives to modify the material’s structure and improve wettability, while another direction involves applying appropriate hydrophobic coatings.

The following information presents methods of modifying material wettability through both additives and applied coatings. By understanding and utilizing these approaches, researchers and engineers can create materials with tailored surface properties that positively impact their durability and performance across various applications in construction and beyond.

4.1 Bulk additives in modifying wettability of building materials

The way building materials interact with water and their ability to withstand it are of paramount importance to their strength and durability. This section focuses on altering material behavior by modifying its structure through additives. As people seek materials with improved properties, researchers investigate how to modify the surface properties of these materials.

Research of Shidong Li et al. [29] delves deep into the influence of silica nanofluids on wettability alteration. By employing advanced visualization techniques, Li et al. were able to provide a comprehensive understanding of how surface properties, when interacted with specific chemical compositions, can significantly alter a material’s ability to absorb water. Their findings underscore the importance of nano-level interactions and their profound impact on macro-level properties.

In the multifaceted exploration of water absorption in materials, the incorporation of recycled materials into building mixes offers a unique dimension. A pivotal study in this context was conducted by Alena Sicakova et al. [30]. They examined the characteristics of building mixes that incorporate fine particles from selected recycled materials. Sicakova and her team’s findings suggest that the inclusion of these recycled particles can significantly influence the water absorption properties of the resulting composite. This highlights not only the potential for sustainable material development but also emphasizes the intricate interplay between chemical composition and surface properties when recycled components are introduced. Their research lays the groundwork for understanding the potential benefits and challenges of integrating recycled materials into building mixes, offering a fresh perspective on optimizing water absorption properties while championing sustainability.

A seminal study by Ernesto Mora et al. [27] provides invaluable insights into this endeavor. Their research delves into the control of water absorption in concrete materials through modification with hybrid hydrophobic silica particles. The introduction of these silica particles alters not only the inherent properties of the concrete but also significantly reduces its water absorption capacity. Mora and his team’s findings underscore the transformative potential of such modifications, suggesting a promising avenue for enhancing the durability and longevity of construction materials.

In summary, wettability, a key property of materials, is shaped by both physical structure and a complex balance of chemical composition and surface properties. The behavior of building materials in contact with liquids results from a complex interaction of factors, each contributing a unique aspect to the material’s reaction to interactions with liquids - pore structure, surface chemical composition, and type of surface. All these factors together form the wettability profile of the material. This subtle interdependence is fundamental to understanding how building materials interact with liquids, which has profound implications for their durability and performance in various construction applications. Molecules and functional groups on the material’s surface play a crucial role in determining how it interacts with liquids (Figure 4). Recent research sheds light on this intricate relationship. Shidong Li and his team [29] highlighted the role of silica nanofluids in altering wettability, emphasizing the importance of interactions at the nano level. Meanwhile, studies conducted by Sicakova et al. [30] revealed the potential of incorporating recycled materials into construction mixes, showing how such additives can affect water absorption properties and promote sustainable development. Additionally, the works of Mory, González, Romero, and Castellón [27] demonstrated the potential of modifying concrete with hydrophobic silica particles to reduce its water absorption. These findings underscore the importance of understanding and manipulating surface chemistry to design materials with tailored wettability properties, suitable for a wide range of applications in construction and beyond.

4.2 Surface protection of building materials

Surface protection of building materials is a crucial element that ensures the durability and longevity of construction. Building materials such as concrete, sandstone, bricks, and ceramics are exposed to various external factors, including water, moisture, salts, pollutants, UV radiation, temperature changes, and other harmful substances. To maintain their quality and properties, various methods and means of surface protection are employed, tailored to specific requirements and operational conditions.

One of the pioneering studies in this field is the work of Dario S. Facio and Maria J. Mosquera [28], in which the researchers introduced a simple strategy for producing superhydrophobic nanocomposite coatings in situ on a construction substrate. Their method, based on an innovative approach to nanocomposites, opens up new possibilities for creating durable and efficient hydrophobic coatings for various construction applications. Figure 6 illustrates the hydrophobic effect of the material without and with the coating.

Figure 6.

Hydrophobic coating effect [28].

In the context of advancements in superhydrophobic surfaces, the work of Yun-Yun Quan et al. [31] provides a review of the latest achievements in creating durable superhydrophobic surfaces. The authors focus on various aspects of structures and materials that contribute to achieving desired hydrophobic properties, emphasizing the evolution of techniques and materials used in this field.

Another significant contribution in the realm of hydrophobic coatings is the study conducted by Zhang et al. [32]. The researchers presented functional and versatile superhydrophobic coatings obtained through stoichiometric silanization. Their approach to creating hydrophobic coatings, which are both durable and versatile, holds potential for application across a broad spectrum of industries, from construction to manufacturing.

In summary, the development of hydrophobic coating technologies in recent years has brought about many innovative solutions that have the potential to revolutionize the construction sector and other fields. Thanks to studies like those presented by Facio, Mosquera, Quan, Chen, and Zhang, the future of hydrophobic coatings seems promising, offering materials with improved performance, durability, and functionality.

4.2.1 Painting and impregnation

One of the most popular and effective approaches to modifying the wetting of building materials is through surface coating techniques. The use of paints and impregnating agents allows the creation of a protective layer on the material’s surface, shielding it from water infiltration and other substances. The coating can also serve esthetic purposes, providing the desired color and appearance to the material’s Surface [33]. By applying a hydrophobic material layer to the surface, the material’s interaction with liquids can be significantly altered. The study conducted by Bing Yin et al. [34], for example, focused on applying superhydrophobic coatings to concrete surfaces. Their research indicates that such coatings effectively reduce water absorption by concrete, potentially enhancing its durability in moist conditions. This example underscores the role of hydrophobic coatings in improving the ability of building materials to repel water.

In the study conducted by Yao et al. [9], the authors thoroughly analyzed the advancements in hydrophobic cement-based materials. Their research focused on the preparation, characterization, and properties of these materials, emphasizing the importance of appropriate coating techniques to achieve desired hydrophobic properties. This study underscores that proper painting and impregnation techniques can significantly influence the ability of building materials to repel water, which is crucial for maintaining their integrity and extending their lifespan in various environmental conditions.

In a comprehensive review by Pan et al. [35], the authors delved into various methods of concrete surface treatment. Their research, encompassing a wide range of techniques, highlighted the significance of painting and impregnation in the realm of building materials. The study meticulously categorizes different types of treatments and elucidates their underlying mechanisms. This work underscores the pivotal role that surface treatments play in enhancing the durability, performance, and esthetic appeal of concrete structures. By understanding the intricacies of these methods, professionals can make informed decisions to ensure the longevity and resilience of their constructions in the face of environmental challenges.

Surface coating techniques serve as versatile tools for modifying the wetting properties of building materials. The research examples illustrate that the application of protective coatings can significantly impact the durability, performance, and longevity of materials in humid and adverse conditions.

4.2.2 Hydrophobic and oleophobic coatings

Hydrophobic and oleophobic coatings have gained widespread attention due to their exceptional ability to provide protection against water and oil. These coatings work by altering the surface properties of materials, making them less prone to wetting and absorption while promoting fluid runoff instead of absorption. This inherent feature makes these coatings particularly effective in safeguarding materials constantly exposed to moisture, including building facades, roofs, and architectural elements (Figure 7) [36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48].

Figure 7.

Silicone-based products penetrate deeply, forming a repellent layer within the substrate [36].

Coating the surface of cement-based materials with substances of low surface free energy, such as silanes and fluorosilicates [37]. The primary mechanism of this process is the reaction of the molecular functional groups of these substances with the hydroxyl groups on the surface of cementitious materials. As a result, a layer of hydrophobic molecules forms on the material’s surface, converting it into a hydrophilic one [38, 39].

The differences between silane-based hydrophobic coatings and traditional waterproof coatings mainly arise from the molecular characteristics of silanes. Due to their small molecular structure, silanes can easily penetrate open cracks or capillary pores in the material, creating an effective hydrophobic barrier [40].

Contemporary research in the field of hydrophobic modification increasingly combines various concepts and techniques. For instance, the studies by Genga et al. focused on combining surface roughness with graphene oxide and reducing surface energy using a silane solution [41]. Other research, such as that conducted by Wong et al., centered on utilizing waste in the process of creating hydrophobic coatings [42].

An important aspect in the context of hydrophobic coatings is also the use of nanoparticles. Their presence can significantly increase surface roughness, leading to enhanced hydrophobic properties [43, 44, 45]. Nanoparticles can also influence the structure and compactness of coatings, further increasing their resistance to water penetration [46, 47].

In the context of preserving cultural heritage, protecting building materials from degradation is a pivotal issue. In response to this challenge, Mosquera, Carrascosa, and Badreldin introduced an innovative approach to creating superhydrophobic and oleophobic coatings on historic building materials [48]. Their research focuses on techniques and methods that effectively enhance not only the resistance of materials to water and oils but are also suitable for delicate and historical surfaces (Figure 8). Through this approach, it is possible to maintain the integrity and authenticity of historic structures while providing them protection against damaging factors. This work represents a significant step toward integrating modern technologies with the conservation of cultural heritage.

Figure 8.

The effect of hydro- and oleophobic coatings [48].

The effectiveness of hydrophobic and oleophobic coatings in safeguarding building materials, surfaces, and architectural elements is highlighted by the presented research. By altering surface properties to deter water and oil interactions, these coatings play a crucial role in enhancing the resistance and esthetic preservation of structures in various environmental conditions.

4.2.3 Anti-graffiti agents

In urban settings where graffiti poses a frequent issue, the application of anti-graffiti agents has become a common solution aimed at protecting surfaces from permanent graffiti painting. In Figure 9, a schematic of the anti-graffiti coating is presented. These agents play a crucial role in creating a protective layer, which in turn facilitates the removal of unwanted paints and marks while minimizing damage to the substrate.

Figure 9.

Anti-graffiti coating [49].

Modern cities around the world grapple with the issue of graffiti, which, although often considered a form of street art, can lead to the degradation and damage of building materials. In response to this challenge, Amrutkar et al. [49] conducted an in-depth analysis of the latest solutions in the field of anti-graffiti coatings.

In their study, the authors focused on various technologies and materials used in anti-graffiti coatings. They highlighted the fact that an ideal anti-graffiti coating should not only effectively protect the surface from spray paint but also be durable, weather-resistant, and easy to maintain.

One of the key findings of the study was that anti-graffiti coatings can be divided into two main categories: permanent and removable. Permanent coatings create a durable barrier on the surface of the building material, preventing paint adhesion. On the other hand, removable coatings allow for easy removal of graffiti without damaging the surface.

The authors [49] also emphasized the importance of nanotechnology in creating modern anti-graffiti coatings. Thanks to the use of nanoparticles and nanocomposites, it is possible to create coatings with exceptional resistance to various types of paints and markers, while ensuring excellent adhesion to various building materials.

In the context of sustainable development and ecology, attention should be paid to the need to create environmentally friendly anti-graffiti coatings. The publication [49] mentioned research on biodegradable and ecological coatings, which effectively protect not only against graffiti but also minimize the negative impact on the environment.

In conclusion, the review publication [49] provides a comprehensive overview of the field of anti-graffiti coatings, highlighting the latest achievements and directions of development. Thanks to such studies, it is possible to create effective and durable solutions that help maintain the esthetics and durability of urban structures in the face of challenges associated with graffiti.

The application of anti-graffiti agents presents a valuable strategy for protecting urban surfaces from permanent graffiti. Research underscores the efficacy of these coatings in deterring vandalism, facilitating maintenance, and preserving cultural heritage. In cities grappling with graffiti-related challenges, the use of anti-graffiti agents remains a crucial component of conservation and urban environment maintenance.

4.2.4 Self-cleaning coatings

The urban environment is constantly exposed to various pollutants, which can degrade the appearance and functionality of building materials. In recent years, the development of self-cleaning coatings has emerged as a promising solution to address this challenge. These coatings maintain not only the esthetics of structures but also contribute to their longevity by reducing the need for frequent cleaning and maintenance.

In a study by Cedillo-González et al. [50], the authors delved into the self-cleaning properties of TiO₂ coatings for building materials. Their research highlighted the crucial role of morphology and humidity in the stain removal performance of these coatings. Specifically, they found that the efficiency of TiO₂ coatings in removing stains is significantly influenced by their morphological characteristics and the ambient humidity levels. This insight underscores the importance of tailoring the properties of self-cleaning coatings to specific environmental conditions to achieve optimal performance.

Rabajczyk et al. [51] took a broader perspective, examining self-cleaning coatings and surfaces of modern building materials designed to remove various air pollutants. Their research emphasized the potential of these coatings to keep surfaces not only clean but also contribute to improving air quality. By effectively removing pollutants from their surfaces, these coatings can play a pivotal role in creating healthier urban environments.

One of the intriguing aspects of self-cleaning surfaces is their ability to interact with different types of pollutants. Quan et al. [52] explored the role of surface wettability and dust types in the self-cleaning process. Their findings revealed that the efficiency of self-cleaning is significantly influenced by the type of dust and the wettability of the surface. For instance, certain dust types can be more easily removed from hydrophobic surfaces, while others may require hydrophilic properties for effective cleaning.

The mechanism behind many self-cleaning coatings is the photocatalytic activity of materials like TiO₂. When exposed to sunlight, these materials can break down organic pollutants, making it easier for them to be washed away by rain or wind. This not only ensures cleaner surfaces but also reduces the organic load on building materials, potentially extending their lifespan [50].

Another significant advantage of self-cleaning coatings is their potential economic benefits. By reducing the frequency and intensity of cleaning required, these coatings can lead to significant savings in maintenance costs over the lifespan of a building or structure [51].

However, it is essential to note that while self-cleaning coatings offer numerous benefits, they are not a one-size-fits-all solution. As highlighted by the studies, the effectiveness of these coatings can vary based on environmental conditions, the specific pollutants in question, and the properties of the coating itself [52].

In conclusion, the development and application of self-cleaning coatings represent a significant advancement in the field of building materials. As urban environments continue to grapple with pollution and its associated challenges, these coatings offer a promising avenue to maintain the esthetics, functionality, and longevity of structures. Self-cleaning coatings represent an innovative solution in the field of building materials and are crucial for maintaining the esthetics, durability, and functionality of surfaces. Research examples on the applications of self-cleaning coatings for building materials highlight the diversity of applications and the potential of such solutions. As the construction industry emphasizes durability, performance, and esthetics, self-cleaning coatings remain a key tool in achieving these goals.

5. Application of hydrophobic coatings in practice

To test the surface protection capabilities of concrete, geopolymers, and sandstone, 19 commercially available products were tested. These products were selected based on their specific applications, particularly those intended for surface treatment of concrete or other mineral materials, as well as in accordance with previous research. Furthermore, the selected products were primarily designed for surface hydrophobization, such as primer coatings (mainly used as an adhesive bridge between the surface and other coatings, but often applied independently for surface hydrophobization).

Below are the coatings that exhibited high hydrophobicity:

Stoprim micro - A primer, transparent coating for mineral substrates based on a concentrated aqueous solution of siloxanes. Depending on the dilution ratio with water, it can be used as a primer or for hydrophobization. For the following tests, a dilution ratio of 1:4 was used.

Lukofob classic - Hydrophobic coating based on polysiloxane resin in organic solvents. Intended for treatment of mineral surfaces (natural stones, concrete, etc.). To be used in an undiluted form.

Repesil Aqua - Hydrophobic coating based on a water emulsion of siloxanes. Designed for absorbent mineral substrates (discolorations may occur on weakly absorbent substrates). To be used in an undiluted form.

Hydrofobizant Fortesil - Final hydrophobic coating for natural and artificial stone, based on a water emulsion of siloxanes. To be used in an undiluted form.

To determine the coating’s ability to hydrophobize surfaces, a series of tests were performed, including the drop test described in the earlier part of the chapter, and the results are presented below.

5.1 Stoprim micro

This product achieved very good results in hydrophobizing the surfaces of the tested materials. The initial contact angles were 125.1° for concrete, 130.5° for geopolymers, and 119.5° for sandstone. At the same time, the droplet size changed only slightly during measurements after several minutes, indicating more of an evaporation process than infiltration. Furthermore, the appearance of the samples remained unchanged (Table 1).

t(s)	Concrete	Geopolymer	Sandstone
	Contact angle (°)
0	125.1	130.5	119.5
1	120.1	128.7	118.6
2	119.8	128.2	117.0
3	118.5	125.1	116.5
4	117.0	125.0	114.3
5	115.8	123.6	111.9

Table 1.

Contact angles of samples subjected to the action of Stoprim micro.

5.2 Lukofob classic

This product showed excellent results in hydrophobization. The initial contact angle was 137° for concrete, 133.8° for geopolymers, and 129.8° for sandstone. At the same time, the droplet size changed only slightly during measurements after several minutes, indicating more of an evaporation process than infiltration. The droplet size also changed only slightly. The appearance of the samples remained unchanged after application (Table 2).

t(s)	Concrete	Geopolymer	Sandstone
	Contact angle (°)
0	137.0	133.8	129.8
1	134.4	131.7	128.0
2	133.4	130.0	127.9
3	133.0	129.5	127.5
4	132.7	127.8	126.6
5	132.3	127.0	126.0

Table 2.

Contact angles of samples subjected to the action of Lukofob classic.

5.3 Repesil aqua

This product showed excellent results in hydrophobization of the tested materials’ surfaces. The initial contact angle was 131.6° for concrete, 124.2° for geopolymers, and 121.9° for sandstone. The droplet size varied only slightly, and the appearance of the samples remained unchanged after application. The main advantage of this product is that it is water-based (water emulsion of siloxanes), which reduces the risk of contamination or damage to treated objects by potentially aggressive solvents (Table 3).

t(s)	Concrete	Geopolymer	Sandstone
	Contact angle (°)
0	131.6	124.2	121.9
1	125.0	120.8	119.9
2	124.7	117.3	118.3
3	123.2	114.7	116.2
4	121.0	114.3	115.9
5	120.2	112.4	113.9

Table 3.

Contact angles of samples subjected to the action of Repesil aqua.

5.4 Hydrofobizator fortesil

Although this product did not alter the appearance of the samples and effectively hydrophobized sandstone and concrete, achieving high contact angles and minimal droplet changes, it did not perform as well on geopolymers. While the initial contact angle was very high for geopolymers, the droplet quickly infiltrated, and the infiltration rate was highly inconsistent. This suggests that although the agent is strongly hydrophobic, it is unable to fill the highly porous surface of geopolymers (Table 4).

t(s)	Concrete	Geopolymer	Sandstone
	Contact angle (°)
0	129.0	114.9	129.0
1	126.5	103.4	127.3
2	125.0	84.2	125.9
3	123.8	45.1	125.1
4	122.0	—	124.0
5	120.9	—	119.6

Table 4.

Contact angles of samples subjected to the action of hydrophobic agent Fortesil.

6. Conclusions

The chapter “Hydrophobic Coatings for Building Materials” explores the significance of modifying wetting properties to enhance the durability and performance of construction materials. It presents research conducted on commonly used building materials such as Portland cement and sandstone, as well as new materials like geopolymers. To introduce readers to the realm of construction materials, the first part of the chapter compares alternative materials, such as geopolymers, with the widely used Portland cement, highlighting their distinct reactions during the solidification process, as well as their physicochemical, mechanical properties, and environmental impact.

The concept of wetting is comprehensively discussed in the context of building materials, with a particular focus on factors influencing wetting behavior, such as pore size and distribution, chemical composition, and surface properties. Various methods for assessing wetting are presented, including the spreading method, laser beam method, and drop method.

The chapter underscores the importance of surface protection for building materials. In its subsequent sections, it elaborates on the significance of modifying wetting behavior through bulk additives and coatings, providing examples of nanomaterials and polymers employed to alter surface properties. Strategies such as hydrophobic and oleophobic coatings and self-cleaning coatings are discussed, aiming to enhance the durability and performance of materials in diverse environments.

The final part of the chapter presents research on selected hydrophobic coatings aimed at extending the lifespan of building constructions. During tests of hydrophobic protection, a range of commercially available products dedicated to protecting concrete and other mineral materials were examined, evaluating their impact on water absorption on the surfaces of concrete, geopolymers, and sandstone (and additionally sandstone) using the drop test method. The testing of hydrophobization on these materials revealed that many of the products already available on the market can also be applied to protect the surfaces of geopolymers and sandstone. The most effective were found to be hydrophobic coatings based on siloxanes and protective coatings based on epoxy resins.

This chapter provides fundamental knowledge about hydrophobic protection for building materials and serves as a basis for further research, demonstrating their efficacy in modifying surface wetting properties and enhancing material protection.

Acknowledgments

This chapter was supported by the project titled “Development of Geopolymer Composites as a Material for the Protection of Hazardous Wrecks and Other Critical Underwater Structures Against Corrosion,” with project number TH80020007. The support was provided by the Technology Agency of the Czech Republic (TACR) under the Epsilon Program during the 2021 M-ERA.Net3 Call. Additionally, the chapter was carried out at the Technical University of Liberec, Faculty of Mechanical Engineering, with the Institutional Endowment for the Long-Term Conceptual Development of Research Institutes, as provided by the Ministry of Education, Youth, and Sports of the Czech Republic in 2023.