*2.3. Nano-Titanium Oxide (Nano-TiO2)*

The addition of nano-TiO2 in concrete specimens can provide self-cleaning properties to the concrete. The concrete containing these nanoparticles can allow a photocatalytic degradation of pollutants (e.g., VOCs, CO, NOx, aldehydes and chlorophenols) from industrial and automobile emissions. However, this effect is less efficient with aging due to carbonatation [94,95].

**Figure 9.** *Cont.*

**Figure 9.** Examples of photocatalytic cement-based coatings that contain TiO2 thin film: (**a**) parking lot view (Phoenix, AZ, USA) and (**b**) bike lane (Brooklyn, NY, USA). Reprinted with permission from [23]. Copyright©2016, Higher Education Press and Springer-Verlag Berlin/Heidelberg.

Figure 9 depicts two applications of photocatalytic cement-based coatings, which allows a self-cleaning effect in function the decomposition of gases and organic pollutions [23]. This is due to a TiO2 thin film on the concrete surface that can provide active oxygen under UV light present in sunlight. Thus, it catalyzes the degradation of organic matters located at the nano-TiO2 coated concrete surface [27]. The concrete surface is cleaned with the rainwater, which can prevent the buildup of dirt. Another important characteristic of nano-TiO2 is the chemical stability and low price in comparison with other materials. Moreover, nano-TiO2 can enhance the resistance to water permeability of cement-based structures [25]. Figure 10 depicts the SEM image of fracture surfaces of cementitious composites considering two sizes of nano-TiO2 [96]. Wang et al. [97] investigated the mechanical and physical properties of cement mortar specimens considering different contents of nano-TiO2 under curing temperatures of 0, 5, 10 and 20 ◦C. They used natural river sand, Portland cement (type I ordinary), and TiO2 nanoparticles with size of 15 nm. In the experimental tests were used nano-TiO2 dosages of 1%, 2%, 3%, 4% and 5% by cement weight, respectively. In the fabrication of the specimens, the nano-TiO2 was dispersed in water through ultrasonication. After, cement and sand are mixed during 1 minute. Then, the well-dispersed nano-TiO2 was added and mixed during 60 seconds and after water was incorporated. In the following stage, the mortars are collocated into molds and cured using different temperatures. For the specimens were used a water to binder ratio of 0.5. Figure 11 depicts the SEM images of cement pastes including 2 wt.% nano-TiO2, at curing age of 28 days under different temperatures. The compressive strength characterization is determined according to ASTM C109 [98] employing a hydraulic testing machine under a controlled load of 1350 N/s. The flexural strength test was evaluated regarding the ASTM C293 [99]. This characterization is determined at curing ages of 3, 7, 28 and 56 days. Figure 11 depicts the results of the hydration degree of the mortar specimens. First, hydration degree of the mortar specimens enhanced through the increment of the nano-TiO2 dosage. TiO2 nanoparticles can supply an extra space for the precipitation of hydration products. Figures 12 and 13 depict the response of the compressive and flexural strength of the cement mortar samples. Both compressive and flexural strength registered downward trend at low curing temperature. On contrary, flexural and compressive strength of mortar specimens containing nano-TiO2 had fast increment with respect to ordinary mortar until that the nano-TiO2 content achieved up to 2 wt.%. This increase slowed down for TiO2 nanoparticles dosages higher than 2 wt.%. The enhanced strength of mortar samples is caused by TiO2 nanoparticles that facilitated the cement hydration and filled the pores in C–S–H gels [97]. These nanoparticles present large surface area to volume ratio, allowing an extra surface area to precipitate hydration products. In addition, TiO2 nanoparticles form a bond between them self and C–S–H gel that improves their strength [97].

**Figure 10.** SEM image of the fracture surfaces considering: (**a**) control cementitious composites (1200×); (**b**) nano-TiO2 (50 nm size) modified cementitious composites (1200×); (**c**) nano-TiO2 (500 nm size) modified cementitious composites (1200×); (**d**) aggregations of nano-TiO2 (50 nm size) in cementitious composites (1200×). Reprinted with permission from [96]. Copyright©2019, Elsevier B.V.

Feng et al. [100] examined the microstructures of concrete matrices incorporating nano-TiO2 as well as the mechanical properties of the cement pastes. Figure 14 is a SEM image of TiO2 nanoparticles and their selected area electron diffraction. The incorporation of nano-TiO2 (0.1%, 0.5%, 1.0% and 1.5% by cement weight) in cement paste using a water-cement ratio of 0.4 improved the flexural strength (4.52%, 8.00%, 8.26% and 6.71%) at 28 days age.

Jalal et al. [101] studied the characteristics of high resistance self-compacting concrete containing fly ash and nano-TiO2. They used Portland cement that was replaced up to 15% weight of waste ash and up to 5% weight of nano-TiO2. The addition of nano-TiO2 in the concrete improved the consistency and reduced the segregation probability. Considering the water absorption and capillarity, a significant decrease was obtained due to the nano-TiO2.

**Figure 11.** SEM images of cement samples with addition of 2 wt.% nano-TiO2 cured under temperatures of (**a**) 0 ◦C, (**b**) 5 ◦C, (**c**) 10 ◦C, and (**d**) 20 ◦C at 28 days. Reprinted with permission from [97]. Copyright©2018, Hindawi.

The weight losses in concrete samples were caused by the rapid formation of hydrated products. The self-compacting concrete with nano-TiO2 registered a microstructure more refined, which enhanced the resistance to mechanic failures. Other researchers, Yu et al. [102] reported the improvement of concrete microstructure incorporating nano-TiO2, which increased its mechanical strength. The TiO2 nanoparticles catalyze the decomposition of harmful gases in the air. In addition, the concrete with nano-TiO2 achieved a maximum compressive strength that was 7% higher in comparison with the non-added nanoparticle concrete. In addition, Yu et al. [102] investigated the changes of temperatures that can induce cracks and accelerate the hydration reaction.

Chunping et al. [103] investigated the durability of ultra-high performance concrete due to the incorporation of nano-TiO2. This concrete added with 1% nano-TiO2 improved its mechanical properties. They investigated the effects on the dry shrinkage, carbonation resistance, freeze-thaw resistance and resistance to chloride ingress. The addition of nano-TiO2 in concrete could allow it a self-cleaning and photocatalytic behavior. In addition, the normal concrete containing nano-TiO2 could decrease the capillary porosity.

**Figure 12.** Compressive strength of cement mortar samples incorporating different nano-TiO2 dosages. Reprinted with permission from [97]. Copyright©2018, Hindawi.

**Figure 13.** Flexural strength of cement mortar samples containing different nano-TiO2 dosages. Reprinted with permission from [97]. Copyright©2018, Hindawi.

**Figure 14.** TEM image of the morphology of the TiO2 nanoparticles and their selected area electron diffraction (SAED). Reprinted with permission from [100]. Copyright©2013, American Chemical Society.
