Keywords

8.1 Introduction

Global climate change due to industrialization and urbanization has badly affected our environment. Although industries are essential for our economic growth and development, their effects on ecosystem are adverse. Byproducts of the industries have significantly affected our environment including agriculture and water resources. It also has negative impact on air quality consequently human health. Further, rapidly developing concrete industry to fulfill human needs also impacted our environment (Nath and Zain 2016; Ziming and Shen 2020). Extensive use of cement and sand has hampered the sustainable development and has degraded the air and water quality (Ziming and shen 2020). Portland cement which is an important ingredient of concrete is the major contributor of pollution. Production of cement generates large amount of CO2 along with other hazardous gases such as NOx and SO3 that leads to enhanced greenhouse effect and acid rain (Shen and Lyu 2020). However, by selecting eco-friendly cement material in construction environment, pollution can be reduced to some extent (Zhong and Haghinghat 2015). Recently, a new construction material with self-cleaning ability has been developed (Zhong and Haghinghat 2015; Sikkema and Alleman 2014; Ohama and Gemert 2011). Self-cleaning mixture can retain its color for longer period of time than existing construction matrix, so need not be substituted frequently and can also minimize the risk of air pollution (Sikkema and Alleman 2014; Ohama and Gemert 2011).

8.1.1 How Self-Cleaning Material was Invented and It’s History

Self-cleaning property of cement was described by Italian chemist Luigi Cassar in his pioneer work. He incorporated TiO2 with cement, which gets oxidized with sunlight and breaks down pollutants (Husken and Hunger 2009; Cassar 2004). A building material with long-term stability and ability to retain a bright color in toxic environment was prepared by a method called “photocatalysis”, which keeps away dirt with the help of sun's energy. Surprisingly, the air around the tested concrete depicted 80% reduction in nitrous oxide. Reduction in other toxic compounds such as lead, carbon monoxide, and sulfur dioxide were also observed. Some of the important historical events about the research and development of photocatalytic compound are shown in Fig. 8.1 (Sikkema and Alleman 2014; Hamidi and Aslani 2019). In early 2000, in a church Dives in Misericordia, in Italy, photocatalytic material was analyzed using Rhodamine B discoloration test. Lately, first generation of photocatalytic material was prepared with start of PICADA project (Fig. 8.1). Thus, a thin layer of cement coating with photocatalytic compound enabled the cement to clean concrete and also clean the air in the city (Husken and Hunger 2009; Cassar 2004).

Fig. 8.1
figure 1

History of photocatalysis cementitious materials (Bondioli et al. 2009; Cassar 2004)

Full size image

8.1.1.1 Self-Cleaning Concrete

Concrete and cementitious building material are generally used in the construction of offices, residential buildings, public places and area of aesthetic values, etc. (Nath and Zain 2016). Dust produced by construction and demolition of buildings causes the pollution and makes building surfaces looks dull and dirty (Nath and Zain 2016; Shen and Lyu 2020). These situations have forced the researchers and industries to design an eco-friendly alternative material known as self-cleaning concrete for the maintenance of healthy environment. Self-cleaning concrete is also known as “Green concrete” as it is capable of retaining the visual properties of buildings for longer duration even in highly polluted environment (Shen and Lyu 2020). Self-cleaning concrete keeps itself dirt free and maintains its brightness in polluted urban areas (Shen and Lyu 2020). Some of the important aspects of self-cleaning materials are shown in (Fig. 8.2).

Fig. 8.2
figure 2

Smart functions added to the structural materials and their capabilities (Fujishima and Rao 2000)

Full size image

Geo-polymer concrete, supplementary cementitious materials, and alkali-activated materials are some of the environment friendly concrete with detoxification abilities. (Nath and Zain 2016; Fujishima and Rao 2000; Paola and Marci 2012). Geo-polymer cement is able to tolerate high temperature, high salt concentration, and acidic environment. This material not only decreases the CO2 emissions but is also economical and durable (Nath and Zain 2016; Shen and Lyu 2020). Other alternative cementitious materials such as fly ash, furnace slag, silica fume, limestone dust, rice husk ash, palm oil fuel ash, cement kiln dust, and metakaolin instead of Portland cement have also been used to reduce CO2 emission (Hipolito and Martinez 2019; Morsy and Alsayed 2012).

Furthermore, addition of “photocatalyst” also provides self-cleaning effect to the building material (Nath and Zain 2016; Fujishima and Rao 2000; Paola and Marci 2012). Photocatalytic compound is applied to the surface of concrete as a thin layer which adds the property of air purification to the concrete (Nath and Zain 2016; Shen and Lyu 2020). Photocatalyst removes pollutants like hydrocarbons, sulfur dioxide, carbon monoxide, carbon dioxide, and nitrogen oxides. It converts organic pollutants into carbon dioxide and water by photodegradation reaction (Nath and Zain 2016; Shen and Lyu 2020). TiO2 is widely used photocatalyst material, it can be used on wide range of surface like mortars, paints, and tiles (Diamanti and Ormellese 2008; Bondioli and Ferrari 2009). But, TiO2 has certain limitations as its activation due to solar radiation is limited therefore it is modified with other transition metals or non-metallic anionic species (Pinho and Rojas 2015; Luna and Juan 2018). Further, Portland cement is also modified by adding Titanium Oxynitride (TiO2-xNy) to enhance its activation under visible light spectrum (Pinho and Rojas 2015; Luna and Juan 2018). Recently, an attempt has been made to replace traditional TiO2 with bismuth due to its high photocatalytic activity and cost efficiency (Pinho and Rojas 2015; Luna and Juan 2018). The tendency of the photocatalytic self-cleaning coating activities is: Bi2O2CO3 (49%) > BiOI (30%) > BiVO4 (15%) > BiPO4 (14%) > Bi2O3 (5%) (Hipolito and Martinez 2019). Bi2O2CO3 exhibited the lowest crystallite size (27 nm) among the studied compound (Hipolito and Martinez 2019). Importantly, all the titanium and bismuth-based photocatalyst exhibit self-cleaning ability of photodegradation of organic compounds. Moreover, photocatalytic compounds are low cost and they won’t affect the mechanical property of the concrete.

In this view, in future, nanotechnology can play a crucial role in the construction of functional buildings. Nanosized materials addition to existing material will not only improve basic properties but also can add specific functionalities to them like antimicrobial, self-cleaning, and pollution minimizing properties (Zaho and Zhou 2020). Some of the commonly used nanomaterials are carbon nanotubes, nano silica, and graphene oxide. Typical carbon nanotubes have strength that of normal steel tubes. All the nanomaterials improve the physical (strength, elasticity, ductility, etc.) as well as chemical property of the cement and concrete and can have the potential to be used as better self-cleaning material (Zaho and Zhou 2020,Chuh and Pan 2014; Kumar and Kolay 2012).

8.1.1.2 Importance of Self-Cleaning

Concrete and cementitious materials are one of the major sources of air pollution in the city. Commonly produced pollutants by these materials include nitrogen oxide (NO2), Sulfur dioxide (SO2), volatile organic compounds (VOCs), etc. (Nath and Zain 2016; Shen and Lyu 2020). These pollutants cause deposition of organic matter and contaminants which result in external damage to the buildings. Self-cleaning concrete can be potential approach for keeping the city pollution free. Self-cleaning concrete helps to minimize the air pollutants, reduce maintenance cost, and extend the life of buildings (Nath and Zain 2016; Zhong and Haghinghat 2015; Sikkema and Alleman 2014). In addition, self-cleaning materials reflect light and reduce the heat builds up on buildings, and keep the city cool.

Furthermore, various photocatalytic materials improve air quality and keep city clean and beautiful. Self-cleaning is greatly beneficial for cementitious materials as it maintains its mechanical strength and functions. It has been reported that photocatalyst-TiO2 nanoparticles contribute to increased tensile and flexibility of cement (Hamidi and Aslani 2019; Paola and Lopez 2012; Witkowski and Hubert 2019). TiO2 in concrete gets activated in the presence of light radiation and mostly remains unutilized. Studies have investigated the exploitation of TiO2 as surface coating of concrete as a protective coating for cement hydration products. TiO2 is used as photocatalyst due to its low cost, resistance to corrosion, low toxicity, and it is activated by solar radiation (Quiroga and Viles 2018; Folli and Pade 2012). Recently bismuth has been proposed as alternative to TiO2 with much higher photocatalytic activity than TiO2(Hipolito and Martinez 2019). The hydroxyl radicals and superoxide anions produced in photocatalytic reactions can react with pollutant molecules (SO2, NO2, VOCs, etc.) and remove them. Photocatalytic materials use atmospheric O2 as an oxidant agent. Apart from this, outer coating with TiO2 maintains the visual aspects and brightness of buildings due to photocatalytic action. Use of natural wollastonite powder as the binder also reduces the CO2 emission and global warming (He et al. 2019). Results of some of the important studies on self-cleaning material are presented in Table 8.1. Thus, self-cleaning coatings reduce aesthetic damage and associated deterioration of building materials.

Table 8.1 Literature review on different existing research techniques of self-cleaning
Full size table

8.2 Photocatalytic Cementitious Materials

Photocatalytic structure materials have been considered as a good alternative to existing environmental polluting construction materials. Long-term maintenance of aesthetic properties of white cement is the additional benefit of photocatalysis cement-based structures (Nath and Zain 2016, Shen and Lyu 2020). Mixing of Photocatalytic material such as TiO2 in construction material does not alter the final characteristics of cementitious products. The development of construction materials mainly depends on two factors:

  1. (1)

    Surface quality and visual appearance.

  2. (2)

    Structural stability.

Thus, materials should be selected carefully to keep a balance between mixture constituents and rheological properties of resulting mixture (Paola and Marci 2012). Materials selection and processing are the most critical parameters which affect properties and functionality of cementitious matrix. (Quiroga and Viles 2018; Awadalla and Arafa 2011; Teoh and Scott 2012). Factors influencing the performance of TiO2-based photocatalytic construction material and major steps used for dispersion of TiO2 in construction material are shown in Fig. 8.3.

Fig. 8.3
figure 3

Factors affecting the performance of photocatalytic cement-based materials (Tsang and Cheng 1997; Vittoriadiamanti and Pedeferri 2013)

Full size image

8.3 Process Parameters

Processing mainly includes mixing/dispersion, molding, and curing. Amalgamation procedure of TiO2 nanoparticles in cement material is one of the vital steps in production of construction mix. Mixing method affects consistency and the properties of the final products (Addamo and Augugliaro 2008). Nano-size TiO2 particles owing to high surface energy can easily agglomerate and it’s hard to break these aggregates (Addamo and Augugliaro 2008; Palmisano and Augugliaro 2011). Homogeneous distribution of nano-TiO2 in the construction matrix is complicated step. Therefore, this is a challenging task for researchers to develop an optimum method for dispersion of TiO2 particles in construction material (Addamo and Augugliaro 2008, Tsai and Cheng 2007).

8.3.1 Environmental Parameters

The important features of photocatalytic construction material are their self-cleaning property and long-term photocatalytic ability for the degradation of air pollutants. Various studies have indicated the same properties of photocatalytic construction material with a reduction in the photocatalytic efficacy of the TiO2-based cement composites with time. The photocatalytic ability of TiO2-based materials may decrease significantly for both surface coatings and TiO2 added in the main mix after a 4-month duration. Aging of photocatalytic concrete also affects the air pollutants removal due to carbonation of the matrix and partial inactivation of the catalytic sites on the Titanium oxide surface (Guerrini and Plassais 2007).

  1. a.

    Cement Parameters: Various studies have reported different factors like cement matrix pore organization, type of binder, and cement surface coarseness for the performance of photocatalytic construction materials.

    1. (i)

      Type of Binder: The chemical characteristics of the binder also affect the photocatalytic activity of composite material (Cassar 2004; Cucitore and Cassar 2011a; Murata and Obata 1999; Vittoriadimanti and Pedeferri 2013). The ordinary Portland cement shows lower photocatalytic activity them white cement because of metallic compounds (Hamidi and Aslani 2019).

    2. (ii)

      Roughness: Few researchers have investigated the effect of surface roughness on the performance of photocatalytic construction materials. The surface areas for medium and rough samples were, larger than sample with fine area. However, NOx removal was directly proportional to surface topography.

    3. (iii)

      Cement Pore Structure: Highly porous structure does not lead to higher photo-activity (Hamidi and Aslani 2019). However, pores > 1 µm (pores of air) and < 0.05 µm exhibited decreased degradation of NOx and organic dyes.

8.3.1.1 Techniques to Evaluate Photocatalytic Efficiency in Cementitious Materials

Qualitative and Quantitative parameters of Photocatalysis technology need to be assessed for better performance and functionality of cementitious materials. Various assessment tests have been designed to measure the detoxification effect of different photocatalytic materials, however no standard testing method has been approved. Similarly, there is no optimum method available for the evaluation of self-cleaning ability.

  1. a.

    Efficiency on the basis of environment toxicants: Different test methods are used for photocatalytic cement on the basis of category of environment toxicants (e.g., NOx, organics, etc.), formulation of construction matrix and physio-chemical properties, etc. (Quiroga and Viles 2018; Folli and Pade 2012).

    1. 1.

      NOx Tests: NOx test methods include four main types as given below:

      1. (i)

        NOx flow-through test: In this, air purification efficiency of the photocatalytic material is assessed on the basis of relative concentration of NOx in the water and sample.

      2. (ii)

        Dynamic method: It is used for inorganic materials to reduce the NOx concentration.

      3. (iii)

        Static method: Like dynamic method, it is also used for inorganic materials to reduce the NOx concentration.

      4. (iv)

        Photocatalytic Innovative Coating: The photo-conversion of NOx is assessed with time on the basis of chamber coated with photocatalytic materials on wall. The efficiency of the photocatalytic method for NOx removal is proportional to contact time), high temperature, and less relative humidity ().

    2. 2.

      (BTEX) Tests: BTEX (Benzene, Toluene, Ethylbenzene, and Xylene) test series is a quantitative test method to assess the photocatalytic ability for degradation of hydrocarbon-based molecules. This test measures the degradation of hydrocarbon molecule in air and at the topmost layer of the cement-based materials by using stirred flow reactor. Stirred flow reactor maintains the homogeneous concentration of reactant deposited on the surface of the material (Vittoriadimanti and Pedeferri 2013).

    3. 3.

      Colorimetric Tests: Colorimetric tests are used to measure the dye degradation and self-cleaning abilities of photocatalytic construction materials. Degradation of rhodamine B, on the TiO2 (photocatalyst) in the cement matrix confirmed the photocatalytic activity (TiO2-sensitized photoreaction). However, this test is invalid for spongy, coarse, and colored materials because in the case of spongy material, homogeneous distribution of the dye is not possible and even in red-color materials, the red color of the dye cannot be transformed. One study has advised the use of a terephthalic acid-based fluorescence probe to quantify the rate of hydroxyl radical production, and ultimately photocatalytic activity. The main advantages of this test are that it is rapid, highly sensitive, and can be used for colored materials. Incorrect hypothesis of existing test methods related to the removal of air pollutants may result in incorrect measurement of photocatalytic efficiency of a construction material. Most of these existing standard test methods require costly equipment and time (Guerrini and Plassais 2007; Cassar 2004; Zhong and Haghighat 2015).

8.4 Various Techniques for Self-Repairing

Broadly five parameters are used to measure the efficiency of self-repairing. These parameters are shelf life, perverseness, quality, reliability, and versatility which have been discussed with different techniques in Table 8.2. However, reliability data is not available for any of the discussed methods.

Table 8.2 Various parameters to test efficiency of self-repairing
Full size table

8.4.1 Taxonomy of Self-Cleaning Techniques

In this section different techniques of self-cleaning have been discussed. These are as follows:

  1. a.

    Autogenous/natural cleaning: In traditional concrete material, approximately 20–30% cement is deficient in water. After formation of cracks in cement, ingress water interacts with cement particles and hydrations start again to repair the cracks. Self-repairing of cracks is termed as autogenous cleaning.

  2. b.

    Autonomic cleaning: In this process repairing of cracks is performed with help of repairing agent to concrete at normal temperature (Cucitore and Cangiano 2011b). This can be either biological or chemical based.

    1. i

      .Chemical self-cleaning: In this process chemical self-repairing of concrete, chemical molecules like glue are added to concrete. Mainly hollow pipette and encapsulation are used for preparation of this mix. Active and passive modes are used in self-repairing concrete. In active, mode chemical agent is added externally (Virginie and Junker 2011).

    2. ii

      Biological self-cleaning: Biologically based self-repaired concrete is an eco-friendly process which utilizes micro-organisms in development of self-repairing concrete (Papanikolaou and Arena 2019; Jiaqi and Zhang 2019). Micro-organisms consist of bacteria, virus, and fungi. Mainly bacteria are used for this process. The main advantage of biological-based method is ease of growth of micro-organisms. Micro-organisms can be added in concrete in form of broth, as spores, in immobilized or encapsulated form. Mainly spore form and encapsulation method are preferred because of the harsh environment in concrete. However, encapsulation method is quite complicated and costly.

    3. iii

      Engineered self-cleaning: Although different self-repairing methods of cracks in concrete have been investigated, still a convenient method is not designed. Every process has its own pros and cons. Bacteria-based method is efficient but it’s hard to grow bacteria in harsh environment (Wang and Dewanckele 2014). Vascular method may have pre-maturation of repairing agents even before the appearance of crack. Out of available approaches, concrete technology has shown promising result. Different areas and environmental conditions also affect self-repairing techniques (Table 8.3). Bacteria-based and autogenous methods require water for repairing process to occur, which makes this optimum for water exposed structures. However, the glue-based agents are not optimum for under water structures, because water presence can interfere with release of glue-based agents. In case of underground structures, any repairing method can be used but if water level is high then use of adhesive agent should be avoided.

      Table 8.3 Self-cleaning techniques versus structural environment where symbol √ represents highly preferred and symbol × represents rarely preferred
      Full size table

8.5 Cost Analysis

Yang and colleagues (Alfani 2013) investigated the photocatalytic efficiency of TiO2 maintained on mortar surfaces vs TiO2 scattered as in mortar in 2019. Considering photonic efficiency as just an indicator, the impacts of environmental factors like NO concentration as well as fluid velocity, UV light intensity, with relative humidity upon photocatalytic performance also were studied in this work. In photocatalytic mortars, the research resulted in much greater utilization efficiency (approximately 150 times higher) over TiO2. TiO2's advantages in photocatalytic concrete technology were further proven by its efficiency and affordable cost. Chen et al. (Yang and Hakki 2019) created compound photocatalysts by coating recycled clay with brick sands, as well as recycled glass with nano- TiO2. They also conducted research on the photocatalytic mortar created using such photocatalysts’ rheological behavior, the mechanical performance, including NOx elimination. The usage of nano- TiO2 in combination with recycled clay with brick sand, as well as recycled glass improves rheological behavior. Photocatalysis was already discovered to be improved by the use of composite photocatalysts. Furthermore, due to mixing, NOx elimination was claimed to be boosted by 18.8%, whereas cost is lowered by 80%.

8.6 Challenges and Future Prospects

Formulation of photocatalytic construction matrix with improved function is vital for environment safety and socio-economic growth. Efficient exploitation of sunlight with the help of a suitable light stimulating photocatalyst could help in the achievement of desired goals related to photocatalysis-based construction materials. Photocatalytic material degrades pollutants and cleans the surface as well as air. Mixing of TiO2 in construction material may decrease its band gap and can affect the efficacy of photocatalytic activity (Alfani 2013). TiO2 has been used as photocatalytic material due to its non-toxic nature, cheap availability, less corrosive property, etc. It absorbs ultraviolet rays and oxidizes most organic and some inorganic pollutants.

Additionally, to expand the potential use of photocatalysis structure materials, their efficiency must be increased. Increased surface area may prove to be a significant approach to enhance efficiency of structure material. However, for stable and long-term increase in efficacy of material, different parameters should be considered and optimized like (1) electron-hole recombination, (2) number of active sites on the surface, (3) dispersion control of TiO2 for optimum pore size and to maximize photocatalytic activity for both organic dyes and gases, (4) use of efficient photocatalysts, and (5) fine pore arrangement.

Furthermore, byproducts released in photocatalytic reaction must be assessed carefully and their effect on health and ecosystem must be investigated. A deep study about the effect of photocatalyst addition on cement mix structure and stability is also required. Energy consumption is an important factor that will decide the applicability of photocatalytic material (Yang and Hakki 2019). Effectiveness of TiO2 addition to other eco-friendly material like magnesium phosphate cement can be investigated as an alternative in the construction sector (Chen and Kou 2020). TiO2 included cement has many characteristics like sustainability, self-cleaning photo-induced hydrophilicity and elimination of the urban heat from urban building, etc.

8.7 Conclusions

In the present review, main emphasis was on self-cleaning process based on photocatalytic method. Photocatalyst keeps the environmental air clean. This approach had been followed by various researchers to develop various photocatalytic material with different photodegradation abilities to keep the indoor and outdoor air pollution free. Photocatalyst acts as an oxidizing agent and decomposes various organic and inorganic pollutants by photodegradation. Thus, photocatalyst does not allow the pollutants to accumulate on the surface of buildings. In this review an attempt has been made to analyze various characteristics, effect of different parameters, and various tests to assess the performance of construction material. Based on this review, following research gaps are identified:

  • TiO2 is used as photocatalyst in most of the composite mixture. It should be replaced with more effective photocatalyst such as bismuth, graphene, nano platelets g-C3N4 nano sheets having better self-cleaning properties.

  • Alternative cementitious material should be made with fly ash and other alkaline activators. Nevertheless, to better apply cementitious material containing highly efficient photocatalyst, research should be done in-depth in future.

  • At present, research on self-cleaning construction material is mostly based on laboratory tests and theoretical analysis. It should be available at field level and at field level these materials should be available at low cost and with better efficiency