Geopolymer Cement-Based Composite as Self-Cleaning Coating: Comparison
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Geopolymer is the combination of the pozzolanic compound, which is also known as aluminosilicate source materials with an alkaline activator solution. The concept of green blending materials has led to the development of a new binding material for green materials, which is geopolymer with an addition of photocatalyst. Moreover, concepts of self-cleaning have received great attention in construction building materials. Self-cleaning with the presence of photocatalyst has been applied in building materials to overcome the problem of building surfaces becoming dirty after exposure for a long time in highly polluted areas.

  • self-cleaning
  • geopolymer
  • nanoparticles
  • photocatalyst
  • coating

1. Introduction

Concrete is most widely used as a material in the construction industries [1]. Among others, Portland cement is the most common type of concrete utilized as construction material [2]. The demand of this type of concrete increases exponentially with city developments. Besides being used as concrete, cement is also used as a paste for surface finishing for buildings. However, the high utilization of cement or concrete causes air pollution, resulting in the negative impacts on the environment [3]. In the Ordinary Portland Cement (OPC) production processes, high amounts of carbon dioxide (CO2) are released into the atmosphere, causing global warming [4]. Due to the high emission of greenhouse gases during manufacturing of OPC, geopolymer is an alternative material to replace the cements-based binder with industrial by-product, e.g., fly ash. In order to be developed for OPC substitution, geopolymer is an innovative, green alternative for construction material. Geopolymer was invented by Davidovits [1] and belongs to the family of inorganic polymers.
Geopolymer was applied as paste, mortar, and concrete for construction materials. In addition, it is also commonly used in ourthe surroundings, i.e., exterior tiles, paving blocks, etc [6][5]. 
Due to its physical and morphological properties, fly ash (FA), which is an industrial (coal power plant) by-product, is the most commonly used material in the production of geopolymers [7][6]. Fly ash is also cheap and available worldwide [8,9,10][7][8][9]. Thus, it becomes a potential source for greener cement production [10][9]. Instead of using Portland cement, fly ash-based geopolymer paste is becoming a viable alternative because it has the potential to reduce Portland cement usage while maintaining the physical and mechanical properties of Portland cement concrete [2]. Geopolymer was also reported to exhibit better compressive strength than OPC concrete [9][8].
In order to maintain the appearance of the building, the construction industry has been focusing on producing cementitious materials that not only have good physical and mechanical performance but also have an additional property to clean the surface of buildings alone. The self-cleaning concept has been applied in conventional cement and has proven better performance in terms of physical, mechanical, and self-cleaning properties. Meanwhile, in certain cases, the strength was decreased with the increase in nanoparticles photocatalyst [11][10]. On the other hand, few years ago, the application of nanoparticles in cement-based materials gained attention due to the unique properties of nanomaterials owing to the high reactivity, fine particle size. and specific functional properties. With the implementation of nanotechnology in construction and building materials, the performance of concrete-based material was improved and enhanced in terms of increased strength and also new materials development for other functional properties (i.e., self-cleaning properties, discoloration resistance, etc.) [12,13,14][11][12][13]. The nanoparticles of the photocatalyst, which are zinc oxide (ZnO) and titanium oxide (TiO2), were added in the cementitious material to develop self-cleaning properties. Photocatalyst will decompose organic and inorganic pollutants to the lesser toxic form under exposure to sunlight and ultraviolet (UV) light. The combination of photocatalyst with cementitious materials has significant advantages. For example, it has the ability to decompose unwanted organic compounds on the surface of concrete buildings, which can be easily removed by rain [15,16,17][14][15][16]. As a consequence, buildings’ aesthetic appearance is maintained by keeping the surface of the buildings free from dirt and preserving the colour, even in industrial areas and polluted conditions [18][17].

2. Self-Cleaning

2.1. Self-Cleaning Mechanism

Self-cleaning is an automatic process of cleaning. The botanist, Wilhelm Barthlott discovered the principle of self-cleaning in 1973 [16][15]. This is due to the photocatalytic action, which removes the pollutants when the photocatalyst has been added [19][18]. The properties of self-cleaning include that it is a “bio-inspired” material, mimicking biological systems that exhibit the “lotus effect.” The surface exhibiting the lotus effect was created due to functionalized nanoparticles, such as TiO2, ZnO, silicon oxide (SiO2), calcium carbonate (CaCO3), or hydroxyapatite (HA) [20][19].
Self-cleaning refers to the ability of a mechanism to clean surfaces by itself. Self-cleaning is used widely in cementitious materials. This is because of improved mechanical properties and performance owing to the self-cleaning properties [21][20]. Photocatalytic reaction consumes the energy from ultraviolet (UV) rays in order to oxidize organic compounds and to degrade the other pollutants to a lesser toxic form under exposure to sunlight. With the presence of UV radiation from the sun, photocatalytic reaction occurs. When heat and light strike the concrete surface, which are covered by a layer of photocatalytic materials, photocatalyst uses the energy to disintegrate the organic pollutant into the oxygen, water, nitrate, sulphate molecule, CO2, etc. [19,22,23,24][18][21][22][23].
Due to the superhydrophilic surface of the photocatalyst, the reaction products from the self-cleaning process are easily removed by simple rinsing or rain [16][15]. The photocatalytic components consume the energy from sunlight to oxidize organic and inorganic compounds. The air pollutants, which cause discoloration to the exposed surfaces, are removed, and their residues are washed away by rain. Thus, this innovation minimizes the costs of maintenance while ensuring cleaner environment [25][24].

2.2. Self-Cleaning Technology

Self-cleaning technology is commercialized to be applied in indoor and outdoor applications. In the last few decades, indoor and outdoor pollutions are becoming significant concerns due to the exponential growth in the industrialization and urbanization over the world [30][25]. Nowadays, abundant technologies have evolved from nature, one of which is self-cleaning technology. There are numerous practical applications of self-cleaning technology, for example, car mirror, solar panels, window glasses, textiles, tiles, ceramics, paints, cementitious materials, etc. Self-cleaning materials have attained substantial attention due to their unique properties and have the potential to reduce maintenance costs of cleaning the surface of civil infrastructure. Self-cleaning technology has been used in a variety of products, but the most popular application is for cementitious materials that have self-cleaning properties [31,32,33,34][26][27][28][29].

2.3. Conventional Self-Cleaning

The self-cleaning paste is a promising new invention in cementitious materials and has a potential for new approach to maintain cleaner city by reducing the air pollutants in the urban areas. The photocatalytic reaction is capable of accelerating the natural oxidation process and speeding up pollutant decomposition [19,23,24][18][22][23]. The self-cleaning property of building materials displays multiple photocatalytic functions by preserving the aesthetic appearance of the building even in harsh urban environments, which reduces the costs of routine maintenance effectively and depollution that contributes to mitigating heat and urban microclimates in ensuring a cleaner environment [23,31,38,39,40][22][26][30][31][32]. In another aspect, the photocatalytic action also contributed to antimicrobial, air purifying, and self-cleaning properties in construction materials [39][31].

2.4. Method of Self-Cleaning Cement Preparation

There are a lot of methods and techniques for introducing photocatalyst into building materials besides directly mixing photocatalyst with cementitious materials, for example, sol-gel dip coating method, precipitation method, ball milling, hydrothermal method, microwave assisted synthesis, pulse combustion-spray pyrolysis method, sputtering, electrophoretic deposition, thermal oxidation, chemical vapor deposition, and wet coating [33,47][28][33]. However, this traditional method has significant drawbacks, particularly coating techniques, which typically have weak adhesion substrates, resulting in low durability in harsh outdoor environments [21,48][20][34]. Consequently, the end products of cement-based photocatalytic materials included losing the ability to decompose, leading to hydration of cement product [49][35].

2.5. Photocatalytic Materials

Nanomaterials have attracted scientists’ interest extensively and have become a hot topic in the field of research and development due to the new potential particle usage within less than 100 nanometres (<100 nm), which is nanoscale sized. Therefore, in the future, nanomaterials are considered as the most promising materials [59,60][36][37]. Nobel laureate Feynman first introduced nanotechnology in 1960. Since then and until now, nanotechnology has become a main topic in research scope and is applied in various engineering fields, for instance, electronics, mechanics, medicals, biomechanics, and coating in building materials. However, since the middle of the 1990s, the application of nanotechnology focused on the construction industry and building materials, which continue to grow from research areas worldwide, which led towards the development of introducing new materials into existing materials [12,33][11][28].  Nanomaterials are new materials that have emerged and been utilized for enhancing mechanical strength and modifying the properties of cementitious materials. In these recent years, in order to apply nanotechnology in construction and building materials, research has been extensively conducted on the effects of the addition of nanoparticles towards properties of cementitious materials [58][38]. With the implementation of nanomaterials in construction and building materials, other types of features and additional new functionalities properties have been developed, such as self-cleaning, anti-bactericidal, anti-microbial, anti-fogging, self-sensing capabilities, depollution, air quality improvement, air decontamination, and discoloration resistance [12,31,62,63][11][26][39][40]. Nanoparticles of photocatalysts are applied in construction materials, especially in paste, mortar, concrete, cement, paints, pavement, glass, etc., to enhance the properties of self-cleaning and improve the cement properties performance. Nanoparticles have been chosen due owing to their properties on cementitious materials, which are high reactivity, ultrafine size of particle, and unique physical and chemical properties and specific functional properties. In addition, nanomaterials are very reactive because they have a high specific surface, which shows a great potential in improving the mechanical and durability properties of the reaction product [41]. The nanotechnology implementation in cement-based materials has attracted much attention due to the unique properties of nanomaterials and its beneficial effects, which can enhance the performance of concrete properties in terms of gaining strength and functional properties [14,59][13][36].

3. Geopolymer

Geopolymer Paste

Geopolymers have been manufactured as paste, mortar, and concrete [74][42]. For finishing materials, geopolymer paste has been chosen to be applied in the outer wall of the building. Geopolymer paste is a hardened cementitious paste that combines waste products into useful product. The reaction between source material that has higher content of silica and alumina with alkaline liquid results to form a geopolymer paste [74,75][42][43]. For testing purpose, the prepared geopolymer paste was cast into a mould and cured at ambient temperature without elevated heat for hardening and for the geopolymerization process to occur. The development mixtures of geopolymer are suitable for curing at room temperature, which was the extent of its application [76][44].
However, based on published literature, fly ash is most commonly chosen as a base for geopolymer paste. There is a study on fly ash-based geopolymer containing low calcium, which was cured in ambient temperature (23 °C) without additional heat, and the results show that geopolymer mixtures are suitable for ambient curing, as the condition of the moisturized specimens after hardening was improved and under control [74][42]. However, in certain cases, geopolymer paste was cured at the required temperature for a specific time in order to be applied in construction materials, especially for precast concrete applications [76,77,78,79][44][45][46][47].
Geopolymer paste demonstrates various advantages in terms of properties and characteristics, including sustainable high mechanical strength, which is gained in a short time; excellent durability; low shrinkage; and high temperature resistance and acid resistance, which is in contrast with OPC [74,80,81][42][48][49]. Geopolymers are ideal for applications in building, repairing infrastructures, and also pre-casting due to high early strength properties and remaining intact under exposure for a long time, and setting time could be controlled [80][48].The mechanical strength of geopolymer paste was believed to increase by considering the reaction amounts of combination between SiO2 and Al2O3 in the fly ash. An experimental result shows that the Si/Al ratio has a significant impact on the compressive strength of geopolymer paste [79][47]. 

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