*2.4. Nano-Alumina (Al2O3)*

The use of nano-Al2O3 can accelerate the formation process of C–S–H gel, especially at early-ages, which enhances the strength of composites [104]. For instance, Muzenski et al. [50] fabricated ultra-high strength cement-based materials using Al2O3 nanofibers with a content of 0.25% by weight of cementitious materials, which improved the compressive strength up to 200 MPa. This represents an increment of 30% in comparison to material strength with only 1% of silica fume. This high compressive strength was achieved with low amount of silica fume. This improved performance is caused by the nanofibers that act as a seed to generate hydration products and contribute the reinforcement for the C–S–H formations, which decrease the number of micro-cracks. In addition, to reach the maximum mechanical performance of the cement-based materials is necessary a suitable dispersion of the Al2O3 nanofibers. A longer dispersion time could reduce the fibers agglomeration, allowing the enhance of mechanical performance. For instance, the compressive strength at 28 days age achieved higher values for specimens with Al2O3 nanofibers dispersed for 3 h. Nevertheless, higher quantities of Al2O3 nanofibers and supplementary cementitious materials did not increase the mechanical behavior of the cement-based materials. Figure 15a,b depicts SEM images of the Al2O3 nanofibers diluted in cement pastes.

**Figure 15.** SEM image of the Al2O3 nanofibers diluted in cement pastes at (**a**) 9000× magnification and (**b**) 20,000× magnification. Reprinted with permission from [50]. Copyright© 2019, Elsevier B.V.

Yang et al. [105] investigated the effect of nano-Al2O3 on the chloride-binding capacity of cement paste samples. These samples were prepared with nano-Al2O3 dosages of 0.5%, 1.0%, 3.0% and 5%. The chloride-binding capacity was examined using conventional equilibrium tests, in which the samples were exposed with a NaCl solution at 0.05 mol/L, 0.1 mol/L, 0.3 mol/L, 0.5 mol/L and 1.0 mol/L, respectively. Based on the experimental results, the bound chloride content had an increase of 37.2% at NaCl solution (0.05 mol/L) by adding 5.0% of nano-Al2O3. Thus, an appropriate adding of nano-Al2O3 improved the chloride-binding of cement paste samples.

Mohseni at al. [106] studied the effects of nano-alumina and rice husk ash (RHA) in polypropylene fiber (PPF)-reinforced cement mortars. The RHA is an agricultural waste material, which can be recycled to obtain economic and environmental benefits. Figure 16 shows the SEM images of nano-alumina and RHA. The compressive strength of the mortar samples is increased up to 18% and 20% due to the addition of 3% nano-Al2O3 with 20% RHA at 28 and 90 days. The flexural strength of the mortar samples increased up to 34% and 41% by adding 3% nano-Al2O3 with 10% RHA. This addition of nano-Al2O3 generated a denser microstructure in the mortar samples.

**Figure 16.** SEM image of the (**a**) nano-Al2O3 and (**b**) rice husk ash. Reprinted with permission from [106]. Copyright© 2016, Elsevier B.V.

Barbhuiya et al. [107] examined the influence of the incorporation of nano-Al2O3 on the microstructural properties of the cement paste hydrated at 7 days age. Ordinary Portland cement is substituted with nano-Al2O3 powder with 2% and 4% by cement weight and the water-cement ratio is fixed to 0.4. In this early-age, they did not note changes at the compressive strength of the cement specimen at early age.

Based on the XRD analysis, Barbhuiya et al. [107] did not find a new crystalline phase developed by adding nano-Al2O3 within 7 days of curing. They reported the generation of dense microstructure with larger crystal of portlandite within the cement matrix due to the nano-Al2O3 addition, as shown in Figures 17 and 18. Gowda et al. [108] reported the influence of nano-Al2O3 in the water absorption and electrical resistivity of cement mortars. They used 1%, 3% and 5% of nano-Al2O3 by cement weight. The water absorption had a small reduction with the addition of 1% and 3% nano-Al2O3. However, the water absorption registered a small increment with the addition of 5% nano-Al2O3. The highest electrical resistivity of the cement mortar is achieved with 5% nano-Al2O3.

**Figure 17.** SEM image of 2% nano-Al2O3 by weight of cement paste hydrated up to 7 days. Reprinted with permission from [107]. Copyright© 2014, Elsevier Ltd.

**Figure 18.** SEM image of 4% nano-Al2O3 by weight of cement paste hydrated up to 7 days. Reprinted with permission from [107]. Copyright© 2014, Elsevier B.V.

#### *2.5. Carbon Nanotubes (CNTs)*

Recently, several researchers [109–123] have reported the effects of CNTs on the electrical and mechanical properties of concrete samples. For instance, CNTs can decrease the formation and growth of micro-cracks in concrete. The CNTs have important mechanical and electrical properties, including their high strength and high conductivity. For instance, CNTs have high mechanical performance with high aspect ratios (length to diameter ratio) that may generate stronger cement composites [27]. The CNTs cement-based composites have strain-sensing behavior that can measure their electrical parameters under applied loads [124]. This behavior can allow the development of strain-sensing systems of concrete structures for potential applications of damage detection and structural health monitoring [75,125].

García-Macías et al. [124] developed a micromechanics model to determine the piezoresistive behavior of cement-based nanocomposites incorporating CNTs and considering the waviness and non-uniform distributions of nanoinclusions. In order to validate the theoretical model, they tested cement-based samples that were doped with multi-walled CNTs (MWCNTS) and exposed to uniaxial compression. These samples were fabricated of concrete, mortar and composite cement paste. Figure 19 depicts the SEM pictures of the MWCNTs dispersion in water solution after sonication and in a cement mortar sample. For the compression loads on the MWCNTs reinforced cement-based composites, they used an equipment of servo-controlled pneumatic universal testing with load capacity of 14 kN, as shown in Figure 20a,b. For the cement paste, mortar and concrete samples are incorporated

MWCNTs with electrical conductivity between 101 and 10<sup>4</sup> S/m. Figure 21a–c illustrates the response of the electrical conductivity of different cement-based composites using the theoretical model and experimental setup. The cement paste, mortar and concrete specimens used filler concentrations of 1%, 0.75% and 0.75% by cement weight. The proposed analytical model may predict the electrical resistance performance of MWCNTs reinforced cement-based materials under compression loads. Ruan et al. [118] reported the influence of different types and dosages of MWCNTs on the mechanical properties of RPC under water or heat curing. They fabricated RPC including four types of MWCNTs with dosages of 0%, 0.25% and 0.50% with water/heat curing, respectively. The mechanical performance of the MWCNTs filled RFC specimens were examined. This mechanical performance considered the flexural strength, fracture energy, compressive/ toughness and flexural strength to compressive strength ratio. The fabrication of the RPC specimens included MWCNTs, water, water reducer, fly ash, quartz sand, cement and silica fume. The cement, silica fume and quartz sand ratio was 1:0.25:1.1. In addition, 20% of cement was substituted by fly ash to enhance the mobility of the mixtures and decrease the cement amount. The four types of MWCNTs used were classified as T1 (functionalized MWCNTs with carboxyl groups), T2 (functionalized MWCNTs with hydroxyl groups), T3 (helical MWCNTs through catalytic cracking) and T4 (nickel-coated MWCNTs).

**Figure 19.** SEM images of the MWCNTs dispersion in (**a**) water suspensions after sonication and (**b**) in a mortar specimen after curing. Reprinted with permission from [124]. Copyright© 2017, Elsevier B.V.

**Figure 20.** (**a**) Compression load versus time and (**b**) uniaxial testing machine used in the MWCNTs reinforced cement-based specimens. Reprinted with permission from [124]. Copyright© 2017, Elsevier B.V.

**Figure 21.** Results of the theoretical and experimental electrical resistance versus applied mechanical strain for (**a**) cement paste, (**b**) mortar and (**c**) concrete samples. Reprinted with permission from [124]. Copyright© 2017, Elsevier B.V.

Figure 22 illustrates the flexural strengths of the MWCNTs filled RPC under water curing. With exception of the specimen filled with 0.5% MWCNTs dosage of T3, all the others specimens filled with dosages of 0.25% and 0.50% MWCNTs showed enhanced flexural strength. The specimen T2 with 0.25% MWCNTs content had the maximum increase (27.2%) of the flexural strength. However, the specimen with 0.50% MWCNTs content registered a decrease (3.8%) of the flexural strength. For the specimens T1, T2 and T3, the flexural strength had better results for low dosage of MWCNTs than that by high dosage of MWCNTs. In addition, the incorporation of the four types of MWCNTs with dosages of 0.25% and 0.50% improved the compressive strength of the RPC specimens under water curing. The compressive strength increased 18.1% with the incorporation of 0.50% MWCNTs content, compared with the RPC without MWCNTs. The dosage of MWCNTs improved the compressive toughness of all the RPC specimens with water curing, as shown Figure 23. The highest compressive toughness was measured in the specimen T2 with 0.25% MWCNTs content, which represented an increase of 39.2% in comparison to the RPC without MWCNTs. Figure 24a,b depicts SEM images of the wide distribution network of MWCNTs in RPC, which can improve the mechanical properties of RPC.

**Figure 22.** Flexural strengths of the MWCNTs reinforced RPC specimens under water curing. Reprinted with permission from [118]. Copyright© 2018, Elsevier B.V.

**Figure 23.** Compressive strengths of the MWCNTs reinforced RPC specimens under water curing. Reprinted with permission from [118]. Copyright© 2018, Elsevier B.V.

**Figure 24.** (**a**,**b**) SEM images of the extensive distribution of MWCNTs in RPC. Reprinted with permission from [118]. Copyright© 2018, Elsevier B.V.

Lushnikova and Zaoui [120] studied the effect of different types of CNTs incorporated into cement specimens. They used molecular dynamics simulations to determine the influence of CNTs on the mechanical properties of C–S–H such as shear modulus, bulk modulus, elastic constants and Poisson ratio. The results of these simulations registered an improvement of all the studied mechanical properties. Thus, the CNTs are nanomaterials that could enhance the mechanical properties of concrete. Moreover, Sedaghatdoost and Behfarnia [121] examined the influence on the mechanical properties of the Portland cement caused by addition of MWCNTs at the ratios between 0 and 0.15% by weight of cement specimens. These specimens were heated using high temperatures (200–800 ◦C). The incorporation of 0.1% MWCNT by cement weight improved the compressive, tensile and flexural strength by 35%, 8%, and 11.2%, respectively. In addition, the cement paste was more stable and denser due to the addition of MWCNT. Also, Hawreen and Bogas [122] studied the effects on the long-term creep and shrinkage of concrete due to the incorporation of different types of CNTs. They used concretes with 0.05%–0.5% of unfunctionalized and functionalized CNTs and water to cement ratios of 0.35–0.55. The compressive strength of the concrete with CNTs was increased up to 21%. The addition of CNTs caused a reduction in the early and long-term shrinkage of concrete up to 54% and 15%,

respectively. The concrete with addition of CNTs had 17%–18% lower long-term creep in comparison to the concrete without CNTs. Carbon nanotubes are innovative materials for the construction industry that can decrease the formation of nano-cracks. Moreover, the inclusion of CNTs in concrete may increase the compressive and flexural strength of the concrete.

#### *2.6. Graphene-Based Nanomaterials*

Recent studies [126–147] have examined the performance of cement-based materials incorporating graphene family nanomaterials (GFN) such as graphene, graphene oxide (GO), reduced graphene oxide (rGO), and graphene nanosheets (GNS). These nanomaterials have extraordinary electrical, mechanical, chemical and thermal properties. Thus, GFN reinforced cement-based materials can improve their structural strength and durability, as well as allow self-cleaning surfaces and self-sensing abilities [148–158].

Hu et al. [158] fabricated cement composite including nano-silica coated GO, which enhanced its mechanical properties. The compressive strengths of cement composites containing GO and nano-silica coated GO were studied at curing ages of 1, 3, 7 and 28 days. The nano-silica coated GO reinforced cement composites increased their compressive strength up to 120.6%, 124.1%, 126.7% and 133% compared to plain cement with curing ages of 1, 3, 7 and 28 days, respectively. For the GO reinforced cement composites without nano-silica, their compressive strengths improved up to 106.0%, 106.7%, 112.2% and 113.6% with respect to plain cement at curing time of 1 day, 3 days, 7 days and 28 days. The coated nano-silica on GO allowed a finer surface structure and better dispersion, which helped to eliminate the agglomeration of GO in pore solution. The nano-silica coated GO promoted the deposition and growth of C–S–H, refining the cement composite microstructure and improving their macro-mechanical properties. Moreover, Hu et al. [159] functionalized GO via triethanolamine (TEA), which was added into oil well cement (OWC) to enhance its mechanical behavior. The incorporation of TEA-GO and GO was at 0.3 wt.%, keeping a fixed water-to-cement ratio of 0.44. First, they mixed GO/TEA-GO power with water and after added cement within 15 s. In the second stage, the cement was collocated in the cup of Waring blender at 4000 rpm and after was mixing during 15 s at 120,000 rpm. Finally, the cement composites are cured at 60 ◦C for 1, 3, 7 and 28 days. Both GO and TEA-GO had good dispersion in water; although, TEA-GO with smaller size reported better uniformity. The TEA-GO incorporated to cement allows nucleation sites and acts as seeds to provide the cement hydration. In order to evalue the influence of GO/TEA-GO on the mechanical properties of cement composite, compressive and flexural strength were obtained through mechanical hydraulic pressure testing machine. For the compressive strength tests, the TEA-GO reinforced samples (50.8 <sup>×</sup> 50.8 <sup>×</sup> 50.8 mm3) were characterized a loading rate of 1.2 KN/s. On the other hand, the flexural strength tests used TEA-GO reinforced samples (160 <sup>×</sup> <sup>40</sup> <sup>×</sup> 40 mm3) under a loading rate of 0.2 KN/s. Figure 25 illustrates the average compressive and flexural strengths of GO and TEA-GO reinforced cement samples. The TEA-GO reinforced cement samples presented higher increments (9.4%–31%) of the compressive strength than that of GO reinforced cement (4.1%–17.2% ). This is due to that TEA-GO modified cement provides crystals more mature and fewer pores and cracks compared to blank cement and GO reinforced cement samples (see Figure 26). Therefore, the TEA-GO significantly refines the microstructure of cement specimens. On the contrary, the TEA-GO reinforced cement samples achieved the higher increments (8.1%–36.7%) with respect to those of cement incorporating GO (7.8%–20%). The TEA-GO enhanced the mechanical performance of cement due to the increment of hydration degree and limitation to crack propagation. Figure 27 depicts SEM images of the cracks types in three different OWC composites. In the blank cement sample, the cracks penetrate in a straight-through form. For GO reinforced cement sample, the cracks are thinner with random branches. Finally, the cement containing TEA-GO registers fewer cracks with thinner dimensions.

**Figure 25.** Response of the (**a**) compressive and (**b**) flexural strength of OWC composites containing GO and TEA-GO. Reprinted with permission from [159]. Copyright© 2019, Elsevier B.V.

**Figure 26.** SEM image of the (**a**) blank OWC sample, (**b**) GO reinforced OWC specimen and (**c**) TEA-GO reinforced OWC specimen after compressive strength test at age 28 days. Reprinted with permission from [159]. Copyright© 2019, Elsevier B.V.

**Figure 27.** SEM images of the (**a**) plain OWC matrix including a straight-through crack, (**b**) GO/OWC specimen containing fine cracks and (**c**) TEA-GO/OWC specimen. Reprinted with permission from [159]. Copyright© 2019, Elsevier B.V.

Tao et al. [160] investigated the influence of graphene nanoplatelets (GNPs) on the microstructure, pore structure, piezoresistive and mechanical behavior of cement mortar. They quantitatively examined the piezoresistive performance of GNP-reinforced cement mortars exposed to cyclic compressive loads. A PI 42.5 cement is used as binder and natural quartz sands are employed as aggregates. Different dosages (M0 = 0%, M1 = 0.05%, M2 = 0.1%, M3 = 0.5% and M4 = 1%) of GNPs by cement weight were incorporated in cement matrix. The compressive and flexural strength of the GNP-reinforced cement mortars were characterized through a 25 KN high-performance fatigue testing machine. For the compressive and flexural tests were employed loading rates of 144 kN/min and 3 kN/min, respectively. The four-probe method is employed to measure the piezoresistive properties of the cement mortars including GNPs. The cement mortars are dried at 80 ◦C during 24 h to eliminate the capillary water, which affects the piezoresistive response [161]. Figure 28 illustrates the fracture surfaces of cement

mortars containing different GNPs dosages. Later, the cement mortars specimens, regarding the probes and cables, are examined using the mechanical testing machine (see Figure 29). First, the initial resistance of the specimens is determined at stable voltage. Then, the external loads are applied to specimens using constant loading rate of 0.5 kN/s and the piezoresistive properties are obtained through an Instron actuator. Figure 30 depicts the compressive and flexural strengths of the cement mortars. Both strengths firstly increment their values and after decrease when the GNP dosage increases.

**Figure 28.** SEM images of the cement mortars added with different GNP content: (**a**) M0 (0%), (**b**) M1 (0.05%), (**c**) M2 (0.1%), (**d**) M3 (0.5) and (**e**) M4 (1%). Reprinted with permission from [160]. Copyright© 2019, Elsevier B.V.

**Figure 29.** Measurement setup of piezoresistive test of the GNP-added cement mortars under cyclic compressions. Reprinted with permission from [160]. Copyright© 2019, Elsevier B.V.

**Figure 30.** Experimental results of the (**a**) compressive and (**b**) flexural strengths of the GNP reinforced cement mortars. Reprinted with permission from [160]. Copyright© 2019, Elsevier B.V.

Higher magnitudes of compressive and flexural strengths (53.6 MPa and 8.9 MPa, respectively) are reached in the cement mortar with GNP dosage of 0.25% (M1) by cement weight. These values represent increments of 8.3% and 15.6% compared with the compressive and flexural strengths of the cement mortar without GNP. However, the values of both strengths decrease when the GNP dosages exceed 0.05%. Thus, cement hydrates with homogeneous spatial distribution could be obtained with appropriate values of GNPs dispersed in cement matrix.

Qureshi and Panesar [162] characterized the influence of GO and rGO on the performance of cement-based composite. They investigated the microstructural properties of GO and rGO using X-ray diffraction (XRD), optical microscope, Fourier-transform infrared spectroscopy (FTIR), SEM, Energy dispersive X-ray (EDS) and Raman spectroscopy techniques. The average C:O ratio of 54.46 and 82.18 in GO and rGO, respectively, were employed in the cement-based materials. To enhance the dispersibility of rGO in water, rGO was processed with superplasticizer. The dosages of both GO and rGO were of 0.02%, 0.04% and 0.06% of cement weight. To reach uniform mixture and efficient dispersion of both GO and rGO in the cement specimens, a water to cement ratio of 0.45 is implemented. In comparison with the control cement sample without GO and rGO, the final setting time and workability of GO reinforced cement specimens gradually decreased using higher GO content up to 0.06% of cement weight. This is caused by the dominant oxygen functional groups and hydrophilic behavior of GO. On the other hand, final setting time and workability of rGO-added cement specimens increased with respect to the control cement sample. It is due to the superplasticizer content and the almost hydrophobic behavior of rGO. The GO composites had greater dosage of C–S–H and Ca(OH)2 than the rGO composites at ages of 1, 7 and 28 days. In addition, the GO composites showed micropores filled with crystalline compounds and C–S–H gel. For the rGO composites was found random pore filling nature with ettringite elements. Figure 31 depicts the SEM-EDX results of the microstructure of GO composite pores. Based on the EDX and SEM results of GO and rGO, these nanomaterials present suitable compatibility with cement hydration products, reinforcing the microstructure of the cement composites. Figure 32 depicts the response of the flexural strength and compressive of cement specimens added with GO and rGO at curing age of 28 days. In comparison to control cement sample, cement composites incorporated with 0.02%, 0.04% and 0.06% GO and rGO dosages had an increase of 10.2%, 7.8% and 10.6 %, and 9.6%, 13.3% and 14.9%, respectively. This is due to the high number of functional groups of GO in chemical bonding with cement hydration products and the high mechanical strength of rGO.

**Figure 31.** SEM-EDX results of the microstructure of GO composite pores in: (**a**) 0.06% GO composite, (**b**) amplification of (a) showing a surface growth nature of ettringite and flakes structures, (**c**) a small pore of 0.02% GO and (**d**) a small pore of 0.04% GO. Reprinted with permission from [162]. Copyright© 2019, Elsevier B.V.

**Figure 32.** Experimental response of the (**a**) compressive and (**b**) flexural strength of GO and rGO reinforced cement specimens at curing age of 28 days. Reprinted with permission from [162]. Copyright© 2019, Elsevier B.V.

Krystet et al. [163] studied the mechanical properties and microstructure of cementitious materials with addition of electrochemically exfoliated graphene (EEG). EEG enhanced the mechanical properties, microstructure and workability of cementitious materials. EEG did not provide aggregate in alkaline environment and the cement mortars incorporating EEG did not decrease its workability and fluidity. The mixture of 0.05 wt.% of graphene with ordinary Portland cement improved the compressive and tensile strength of the cement material up to 79% and 8%, respectively. EEG contributes to

hydration reactions of calcium silicates, allowing an intense generation of C–S–H phase and a compact microstructure.

Kaur and Kothiyal [164] compared the effect of polycarboxylate superplasticizer (PCE-SP) added GO and functionalized CNT (SP@GO and SP@FCNT) on the mechanical properties of cement nanocomposites (CNCs). They used two types of SPs to alter the GO and FCNTs structural features, and to enhance the dispersion of these nanomaterials in aqueous solution and cement matrix. The stabilized GO and FCNT allowed to enhance the mechanical strength of the CNC specimens. After, they fabricated three cubes of CNC specimens (70.6 mm <sup>×</sup> 70.6 mm <sup>×</sup> 70.6 mm3) containing SP@GO and SP@FCNT with different dosages (i.e., 0.02%, 0.04%, 0.08% and 0.16% by cement weight). These specimens were water curing at ages of 7, 14 and 28 days to characterize their mechanical strength. The mechanical tests were done using universal testing machine, applying load at the rate of 3.8 kN/s and 0.5 kN/s, respectively. Figures 33 and 34 show the measurements of average compressive and flexural strength of the SP@GO and SP@FCNT modified CNC specimens. With respect to cement specimen, the maximum values of compressive and tensile strengths of CNC specimens were improved up to 23.2% and 38.5% due to addition of 0.02% and 0.08% SP@GO by cement weight, respectively. On the other hand, addition of 0.08% and 0.04% of SP@FCNT by cement weight enhanced the compressive and tensile strengths of the CNC specimens by 16.5% and 35.8%, respectively. Figure 35 depicts FE-SEM images of CNC specimens containing different SP@GO dosages.

**Figure 33.** Experimental results of the (**a**) compressive and (**b**) flexural strength of SP@GO modified CNC specimens at curing ages of 7, 14 and 28 days. Reprinted with permission from [164]. Copyright© 2019, Elsevier B.V.

**Figure 34.** Experimental results of the (**a**) compressive and (**b**) flexural strength of SP@FCNT modified CNC specimens at curing ages of 7, 14 and 28 days. Reprinted with permission from [164]. Copyright© 2019, Elsevier B.V.

**Figure 35.** FE-SEM images of CNC specimens incorporating (**a**) 0.02% SP@GO, (**b**) 0.04% SP@GO, (**c**) 0.08% SP@GO and (**d**) 0.16% SP@GO at curing age of 28 days. Reprinted with permission from [164]. Copyright© 2019, Elsevier B.V.

The addition of graphene family nanomaterials in cement composites can enhance their mechanical strength. This will allow the construction of lighter concrete components with extended durability, thus, the consumption of concrete components could be decreased. This will help with the reduction of the gas pollutants resulting from concrete production.

#### **3. Challenges**

The nanotechnology has allowed the fabrication of nanomaterials that can be incorporated in cement-based materials to generate higher mechanical properties of the concrete structures. The effect of the nanomaterials on the performance of cement-based materials includes the enhance of their compressive, tensile and flexural strength, reduction of the total porosity (i.e., refinement of the microstructure), acceleration of C–S–H gel generation and increment of Young modulus. Furthermore, incorporation of nanomaterials such as nano-TiO2 and CNTs can provide self-cleaning and self-sensing properties, respectively, of the products obtained with cement-based materials. To achieve the optimal mechanical properties of the cement-based materials it is very important to mix a suitable dosage of nanomaterial with the cement-based materials. For instance, excessive quantities of nanomaterials added to cement can result in lower compressive, tensile and flexural strength of the cement-based structures. This is caused by nonhomogeneous dispersion of nanomaterials in the cement paste. Thus, the mechanical properties of the nanomaterial reinforced cement-based materials depend of several factors such as the dosage and type of nanomaterial, dispersion method, curing days and curing method. Between these factors, the dispersion method can have a significantly effect on the performance of the nanomaterial reinforced concrete.

An important challenge for the application of nanomaterials in the construction industry is the development of efficient methods for the dispersion of nanomaterials in cement samples. An alternative solution to incorporate the nanomaterial into cement-based materials consists in the dispersion of the nanomaterial in water before of incorporating it to the dry components of the cement-based materials [165]. For this case, ultrasonic dispersion can be employed as an effective method for the dispersion of the nanomaterials, although, this method requires electrical energy that increases its cost. A bad dispersion of the nanomaterials into cement specimens and the formation of great amount of agglomerates may alter the kinetics of the hydration process, modifying the properties of the cement specimens. For instance, Singh et al. [166] reported that the method used to incorporate nano-silica into cement composites can affect the porosity and mechanical properties of the composites. A bad nano-silica dispersion in cement-based materials may generate voids and weak zones, altering the mechanical properties of the materials. Surfactants such plasticizers and superplasticizers can be used to improve the dispersion of nanomaterials in cement-based materials [51,167,168]. Thus, surface active agents can enhance the homogeneity of dispersion due to the generation of aggregates around nanoparticles [169]. This good dispersion is achieved because of both hydrophobic and hydrophilic groups. The nanomaterials interact with the hydrophobic groups and the hydrophilic groups decrease the water-surface tension, increasing the dispersion of the nanomaterials [167]. Nevertheless, several surfactants (e.g., polymeric matrices) employed for the dispersion of nanoparticles can affect the cement hydration kinetics. Figure 36 depicts a nanomaterial dispersion process employed to obtain a cement-based composite.

**Figure 36.** Schematic view of a common nanomaterial dispersion process employed to fabricate a cement-based composite. Reprinted with permission from [167]. Copyright©2018, MDPI AG.

To develop the large scale production of these modified cement-based materials it will be necessary to develop an efficient nanomaterial dispersion method that allows a stable and satisfactory dispersion in cement-based materials [167]. However, the re-agglomeration of the nanoparticles may change their size, which could affect their behavior of nanomaterials in cement-based materials [170].

The application nanomaterials in cement-based composites is attractive due to that enhances their mechanical properties only using small dosages of nanomaterials. However, the high cost of these nanomaterials is a limitation to achieve their commercial application in cementitious materials. In the case of graphene and graphene oxide, their fabrication scale is small and relatively expensive. In the future, an important challenge is the reduction of fabrication costs of the nanomaterials. For instance, SiO2 nanoparticles may be generated with low cost from hydrothermal solutions generated due the magmatic ore intrusion [77]. Thus, nano-silica and other minerals can be recovered when the steam is condensed in water during the operation of a geothermal power plant [77].

Moreover, another challenge is the optimal dosage determination of the nanomaterial added in the cement paste to obtain the higher mechanical strength and greater durability. To meet this goal, more studies about the effect of different types of nanomaterials on the mechanical properties of cement-based materials are required. These studies must include the effect of the combination of nanomaterials with other cementitious materials. Thus, the optimal quantities of the combination of these materials must be examined to find the best mechanical behavior of the cement-based materials. The main challenge of the use of carbon nanotubes (CNTs)/nanofibers in cement paste is the dispersion due to their strong self-attraction and high hydrophobicity [171,172]. This poor dispersion may cause defects zones in cement/CNTs composites, which constrain the use of CNTs in cementitious matrices [27]. Thus, more investigations must be made with respect to developing methods to improve the dispersion of CNTs in cement-based composites. For example, some researchers [173,174] have studied chemical and surface modification methods for carbon nanotubes to improve their dispersion and bonding between carbon nanotubes and the matrix. Other studies [175–177] had examined the mechanical properties of cement samples, which were prepared with different combinations of carbon nanotubes and nano-silica. In addition, future researches must consider prediction models of the relationship between external mechanical deformations and electrical resistivity of cement-based composites incorporating CNTs.

In addition, the future applications of nanomaterials in the construction industry will require one to consider the local environmental conditions (e.g., elements of the local environmental dust). These conditions could damage the performance of the concrete, reducing its durability and increasing the cost of maintenance. For instance, The Jubilee Church in Rome (2003) was one of the first buildings that used self-cleaning and reinforced concrete [178–180]. This construction had three iconic shells constructed from 2001 and 2002, which employed precast panels with photocatalytic nano-TiO2 particles. Thus, the nano-TiO2 particles could absorb energy from light and employ it to achieve a photocatalytic degradation of pollutions. In 2019, Cardellicchio [179,180] reported a study about premature evidence of decay of the three shells that showed failure of their self-cleaning performance. This study considered the material pathologies and their possible damage sources. The surfaces of the shells still contain nano-TiO2 particles in the form of anatine, which was detected by this study through chemical analysis. Nonetheless, the self-cleaning properties of the shells with nano-TiO2 are only activated when both the sunlight allows the redox of pollutant and the photo-induced hydrophilicity permits the cleaning of the shells [180]. For this case, the hydrophilic characteristic is limited by two main conditions. One condition is linked with common composition of the pozzolanic powder in Rome that cannot be oxidized by titanium dioxide. Another condition is the abrasive effect of the rainwater on the surface of the shells that improves the superficial roughness, increases the bond between powder particles and concrete [180]. The erosive action occurs on the convex surface, which was registered by a colorimetric analysis showing a tendency towards whitish-grey hue caused by the scattering of the sunlight. These two conditions generate a patina which decreases the photocatalytic effect of the surface of the concrete shells (see Figure 37). The efficiency of the nano-TiO2 particles incorporated in concrete is affected by the porosity and roughness of the concrete surface. The porosity of the concrete surface may allow the water retention and its roughness may help the adhesion of powder on the surface. The chemical and abrasive characteristics of pozzolanic powder may decrease the efficiency of the self-cleaning of the nano-TiO2 added concrete. Therefore, future buildings that use concrete with self-cleaning properties may be affected by powder of volcanic origins or precipitations incorporating desert dust [180].

**Figure 37.** View of the damages on the surface of the self-cleaning concrete shells of Jubilee Church in Rome, which were caused by the local environmental dust. Reprinted with permission from [180]. Copyright©2019, Taylor & Francis Group.

More investigations about new nanomaterials to improve the mechanical properties of cement-based composites are required. In addition, to achieve the commercial application of these nanomaterials reinforced cement-based composites is necessary to know the effect of the nanomaterials on the mechanical properties of these composites. Therefore, future researches must include the development of novel theoretical models that can predict the mechanical properties of the cement-based materials as function of dosage level of the nanomaterials. Also, more researches about the impact of the nanomaterials in the public health and environment must be developed. There are few studies about the effects of the nanomaterials used in cement-based materials on the public health and environment. For instance, Lee et al. [181] reported some effects of nanoparticles used in the construction industry on environment health and safety. Lam et al. [182] presented a review about the carbon nanotube toxicity, which could produce pulmonary inflammation and cardiac toxicity. Moreover, nano-TiO2 could generate inflammation in mammalian cells [183]. Thus, many researches must be done to reduce the negative impacts of the nanomaterials on the health and environment. In addition, more studies about the bioavailability and environment mobility of nanomaterials required to be done [182].

#### **4. Conclusions**

The incorporation of nanomaterials in concrete can improve their compressive, tensile and flexural strength. Recent investigations had considered nanomaterials such as nano-silica, nano-titania, nano-ferric oxide, nano-alumina, CNT, graphene and GO. The addition of these nanomaterials in concrete can achieved denser microstructures, decreasing the water absorption. The workability of the concrete could be improved by adding these nanomaterials. The nano-TiO2 modified concrete can provide it self-cleaning properties and other benefits to help the environment clean. In addition, nano-TiO2 added in concrete can allow the photocatalytic degradation of pollutants (e.g.; NOx, VOCs, CO, chlorophenols, and aldehydes) from automobile and industrial emissions. The CNTs reinforced cement-based composites can have self-sensing abilities for applications of structural health monitoring or damage detection. In addition, graphene and GO added in cement-based materials can increase their mechanical strength and durability, as well as develop self-cleaning surfaces and self-sensing abilities.

In the construction industry, the fabrication of cement-based composites can generate high levels of CO2 gas. To address this problem, one solution is the addition of nanomaterials to cement-based composites, which can provide structural components with high mechanical strength and great durability. Thus, the maintenance requirements and replacement frequency of the cement-based structural components can be decreased. These advantages can allow the reduction of the percentage of cement used in the construction industry. This in turn will decrease the CO2 emissions caused by the cement fabrication process.

The application of nanotechnology in cement-based materials is still in a research stage. The results of experimental tests of nanomaterials-reinforced cement specimens have demonstrated that they can enhance the mechanical strength and durability of the resultng concretes. Moreover, these nanomaterials can allow a novel generation of smart cement-based composites with strain-sensing abilities for damage inspection and structural health monitoring.

**Author Contributions:** K.P.B.-G. and A.H.-M. contributed with the sub-sections of nano-silica, nano-Fe2O3 and nano-Al2O3; J.M.S.-L. supervised all the sections of the manuscript; S.A.Z.-C. reviewed the sections of introduction and challenges, and A.L.H.-M. wrote the sections of nano-TiO2, carbon nanotubes and graphene-based nanomaterials.

**Funding:** This research was supported by the project "PFCE 2019 DES Técnica Veracruz 30MSU0940B-21".

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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