*2.1. Nano-Silica (Nano-SiO2)*

Nano-silica is a nanomaterial employed for civil engineering applications that can replace micro-silica and silica fume. Nano-silica reacts with lime during the cement hydration process and it generates a C–S–H gel that may improve the mechanical strength and durability of concrete. A good dispersion of nano-silica into cement-based materials can accelerate the hydration process of cement paste, allowing a denser microstructure. On the other hand, an excessive number of nanoparticles can cause agglomeration due to their high surface energy, which will provide a non-uniform dispersion. Figure 1 shows the scale ranges of several materials used in concrete fabrication [77].

**Figure 1.** Scale range of several materials used in the concrete fabrication. Reprinted with permission from [77]. Copyright©2017, Higher Education Press and Springer-Verlag, Berlin/Heidelberg.

Flores-Vivian et al. [77] used Portland cement containing nano-silica to modify the rheological performance and improve the durability and strength. They used a nano-silica content of 0.25% by weight of cement-based materials. Other researchers such as Braz de Abreu et al. [22] reported the use of stabilized nano-silica particles (between 3 and 200 nm in size) in Brazilian-type CP V ARI PLUS Portland cement. They fabricated three types of concrete mixes: a reference concrete, a concrete added with stabilized nano-silica and a concrete including stabilized nano-silica with silica fume. After, they studied the results of concrete compressive strength tests at curing ages of 3, 7 and 28 days. The concrete compressive strength with only stabilized nano-silica increased up to 27%, 20% and 11% at 28 days compared with the reference concrete. On the contrary, the concrete with stabilized nano-silica and silica fume registered even higher compressive strength values (i.e., 28%, 37% and 24% at 28 days) compared to the control concrete. Thus, a mixture of nano-silica and silica fume with Portland cement generated a concrete with higher compressive strength.

Heidari and Tavakoli [78] fabricated a mixture using nano-silica and ceramic powder. They investigated the properties of ceramic power based on the ASTM C 618 standard, using 92% as material in the mixture. In this mixture, the cement is replaced with ceramic powder (phase A). In the second phase (phase B), the ceramic powder percentage is reduced, and the nano-silica is added. They employed the binder content as a constant (320 kg/m3) and a water-cement ratio of 0.5. During phase A, mixtures were made with a ceramic powder percentage of 0%, 10%, 15%, 20%, 25%, 30% and 40% of the cement weight using the same proportion of aggregates and water. During phase B, mixtures are made with 0.5% and 1% of nanosilica and different ceramic powder content of 10%, 15%, 20% and 25% of the cement weight. All concrete mixtures are fabricated considering the ASTM C192 standard. The results of the compressive strength tests of the concrete (phase A) were obtained with different curing ages (7, 28, 56 and 91 days). These results show that the concrete strength proportionally decreases with the amount of ceramic powder added to the concrete. The concrete specimen containing 1% of nano-silica and 10% of ceramic powder improved its compressive strength. The impact on the pozzolanic reaction of nano-SiO2 is more effective at an early age.

Supit and Shaikh [79] determined the durability properties of high-volume fly ash concrete with addition of nano-silica. They used type I Portland cement and different series of mixtures with a water-cement ratio of 0.40. Compressive strength tests for all concrete mixtures were measured at ages of 3, 7, 28, 56 and 90 days. The incorporation of nano-silica into ordinary concrete increased the compressive strength reaching up to 150% more at early ages. For ages of 28, 56 and 90 days, the compressive strength showed an increment between 45% and 75%. The nano-silica accelerated the hydration process and allowed a cementitious matrix with denser microstructure. The 4% nano-silicamodified concrete decreased its water absorption (between two and three times lower) in comparison to concrete without nano-silica. The resistance of chloride penetration was studied at ages of 28 and 90 days, in which the mixture with 2% nano-silica registered the lowest penetration value. Based on microstructure analysis, nano-silica-modified concrete mixtures presented denser microstructures. Thus, nano-silica modified concretes could be classified as low permeability concretes.

In order to reinforce reactive powder concrete (RPC), Han et al. [80] added nano-SiO2-coated TiO2 (NSCT) to RPC. These nanomaterials were studied by scanning electron microscopy (SEM), thermogravimetry (TG) analysis and powder X-ray diffraction (XRD). The acceleration of cement hydration due to the effect of the nucleus played a dominant role in the first days. The CH crystals particles size registered a reduction when the content of NSCT was increased (see Figure 2). The flexural and compressive strength of NSCT reinforced RPC (NSCTRRPC) specimens were investigated at curing ages of 3 and 28 days, considering different contents of NSCT (i.e., 1%, 3% and 5% by cement weight). The NSCTRRPC specimens enhanced their flexural and compressive strength in comparison to RPC specimens without NSCT. Figures 3 and 4 depict the flexural and compressive strength of NSCTRRPC specimens at curing ages of 3 and 28 days. For 3% NSCT dosage at a curing age of 3 days, a maximum flexural strength value (9.77 MPa) of the NSCTRRPC specimen was achieved. It represents an increment of 83.3% compared with the RPC without NSCT. For curing age of 28 days and 5% NSCT content, the flexural strength (14.38 MPa) of the NSCTRRPC specimen was increased up to 87% with respect to RPC without NSCT. Thus, NSCT increases the flexural strength of RCP specimens at both early age (3 days) and later age (28 days). The composites with NSCT registered small increments in their compressive strength at curing age of 3 days. On the contrary, maximum levels of the compressive strength of NSCT modified composites were measured at a curing age of 28 days. Thus, the highest compressive strength (111.75 MPa) of NSCTRRPC specimens is obtained with 3% NSCT dosage. This strength value registered an increase of 12.26% in comparison with RPC without NCST. However, the flexural strength of NSCTRRPC specimens had higher increment levels than that of compressive strength for the same test composites. This is caused by the NSCT that significantly enhances the toughness of the RPC [80].

Li et al. [81] examined the properties of ultra-high-performance concrete, which is obtained with particles of nano-limestone (nano-CaCO3) and nano-silica. They used type I Portland cement and fly ash, and silica fume as binding agents. The percentages of nano-silica and nano-limestone by cement weight were of 0.5%, 1.0%, 1.5% and 2.0% and 2.0%, 3.0% and 4.0%, respectively. The mixture workability was reduced with respect the control specimen and it was maintained when the amount of nano-limestone is increased. This is due to the small size of nanoparticles that are found on the surface, leaving less water to contribute towards fluidity. The compressive and tensile strength of concretes including nano-limestone and nano-silica were improved with respect to concretes without any additions. The microstructure with highest values of density and mechanical strength was obtained with content levels of 1% nano-silica and 3% nano-limestone, respectively. The mechanical strength of concrete containing nano-silica and nano-limestone is increased with the reduction of the water-cement ratio.

Sadeghi et al. [82] reported non-destructive compressive strength tests of self-compacting concretes added with steel fibers, polypropylene and nano-silica. They employed the ultrasonic pulse velocity technique in concrete to register mechanical strength of concrete specimens. These concrete specimens (100 <sup>×</sup> 100 <sup>×</sup> 100 mm3) were fabricated based on II Portland cement at ages of 7, 28 and 90 days. In addition, they used 40 different types of mixtures considering 2%, 4% and 6% of replacement with nano-silica and superplasticizer. In the specimens were measured the wave transmission velocity

and compressive strength using the exponential relationship between both parameters. An increment of steel fiber volume above 3% increased the wave pulse transmission velocity in the specimens. The compressive strength and wave pulse transmission velocity increased when the percentage of nano-silica achieved above 4% of cement weight; however, both decreased afterwards.

**Figure 2.** SEM micrographs of CH crystals in concrete with nano-SiO2-coated TiO2 at curing age of 28 days (20,000×): (**a**) sample T0; (**b**) sample T3; (**c**) sample T5. Reprinted with permission from [80]. Copyright©2017, Elsevier B.V.

**Figure 3.** Flexural strength of NSCTRRPC test specimens with different values of NSCT content at curing age of 3 and 28 days. Reprinted with permission from [80]. Copyright©2017, Elsevier B.V.

**Figure 4.** Compressive strength of NSCTRRPC test specimens with different values of NSCT content at curing age of 3 and 28 days. Reprinted with permission from [80]. Copyright©2017, Elsevier B.V.

Najigivi et al. [83] implemented tests using ordinary Portland cement and different nano-silica particles types according to average size. They named each one with the letters N and M, which both particle types reached an amorphous structure with a high pozzolanic reaction. These researchers used a water-cement ratio of 0.40, including nano-silica particles with proportions of 0.5%, 1.0%, 1.5% and 2.0% within the N particles and 2% in the M particles. In all the combinations of these tests, both nano-silica particles types decreased the concrete fluidity. The lime-cure concrete with maximum compressive strength was achieved using 2% nano-silica particles of M type with quicklime solution. This concrete reached the maximum values of compressive strength (40.2 MPa, 53.5 MPa and 57.1 MPa) at curing ages of 7, 28 and 90 days. This increment is due to the calcium hydroxide compounds reacted with nano-silica at a superficial level, generating additional C–S–H gel.

Zhang et al. [84] investigated the durability of concrete specimens containing nano-silica and steel fiber. They used five different contents of nano-silica (1%, 3%, 5%, 7% and 9%) and five-volume levels of steel fiber (0.5%, 1%, 1.5%, 2% and 2.5%). The durability tests of concrete specimens included the carbonation and cracking resistance, and permeability and freezing-thawing resistance. The durability tests are examined considering the carbonation depth of the specimens, total cracking area per unit area of the concrete specimen, cracks number, relative dynamic elastic modulus of the samples obtained after of the freezing-thawing cycles, and permeation depth of the water. For instance, a reduction in both the generated cracks number and water permeation depth of the concrete specimens can improve the concrete durability. Figures 5 and 6 show the total cracking area per unit area and cracks number of concrete samples containing 15% fly ash and five different nano-silica dosages. The cracks number in the concrete specimens decreased when the nano-silica dosages increased from 1% to 7%. The minimum number of cracks is achieved with 7% nano-silica dosage, but this number is increased when the nano-silica content is 9%. In addition, the total cracking area significantly decreased for nano-silica contents between 3% and 5%. Although, a nano-silica dosage of 9% caused an increment of 71.8% of the total cracking area compared with 5% nano-silica content. On the contrary, water permeation depth of the concrete specimens is showed in Figure 7. When the nano-silica content increments between 1% and 5%, the water permeation depth of the concrete specimens is significantly reduced. This improvement level decreases for nano-silica dosages of 7% and 9%, respectively. Based on these results, the nano-silica added concrete specimens enhanced their durability when the nano-silica content is within a certain limit. However, a high content of nano-silica could affect the durability of the concrete.

**Figure 5.** Influence of nano-silica dosage on the total cracking area per area unit of concrete specimens. Reprinted with permission from [84]. Copyright©2019, MDPI AG.

**Figure 6.** Effect of nano-silica dosage on the cracks number of concrete samples. Reprinted with permission from [84]. Copyright©2019, MDPI AG.

**Figure 7.** Effect of nano-silica dosage on the water permeation depth of concrete samples. Reprinted with permission from [84]. Copyright©2019, MDPI AG.

Tavakoli et al. [85] reported the effect on the compressive strength caused by addition of silica fume and nano-silica in concrete samples at curing age of 7, 28 and 56 days. They used type II Portland cement with different percentages of nano-silica (0.5% and 1%) and silica fume (5% and 10%) by cement weight. For each case, concrete samples containing nano-silica and silica fume increased their compressive strength compared to control specimen without these materials. The concrete samples achieved the highest compressive strength (52.9 MPa) using 10% of silica fume and 1% of nano-silica particles at curing age of 56 days. This strength value is 42.2% higher than that of the concrete sample without nano-silica and silica fume. More investigations about of nano-silica modified cement were

reported by Nazerigivi and Najigivi [86]. They studied the influence on the mechanical strength of concrete specimens caused by incorporation of two different nano-silica sizes (15 nm and 80 nm) with percentages of 0.5%, 1.0%, 1.5% and 2.0% by cement weight. They employed ordinary Portland cement and 16 different concrete samples and one control concrete sample for each mechanical test type, as indicated Table 1. A water-to-binder ratio of 0.40 was used into all the concrete samples. With lime solution, these samples are cured at ages of 7, 28 and 90 days. Tables 1–3 indicate the measurements of the compressive, split tensile and flexural strength of all the concrete samples. For the three curing ages, the nano-silica added concrete samples improved their compressive strength with respect to that of the control specimen. The compressive strength had a gradual increment when the nano-silica dosage was increased up to 2% of 15 nm plus 1.5% of 80 nm; after, it had a small decrease. The generation of C–S–H gel may be accelerated due to ultra-high specific surface and ultra-fine particle size of nano-silica incorporated in concrete samples [82]. The split tensile and flexural strength of all the nano-silica modified concrete specimens were improved with respect to the control sample. Both split tensile and flexural strength registered gradual increments with the incorporation of nano-silica up to 2% of 15 nm plus 1.5% of 80 nm; after, these mechanical strengths had a small reduction. It could be caused because the total quantity of nano-silica is higher than that to obtain the lime-silica hydration reaction [82].

Mastali and Dalvand [87] reported a theoretical and experimental study of the effects on the mechanical properties of concrete samples due to the presence of 1.0% nano-silica and 7% silica fume, respectively. They realized 270 tests with different designs of self-compacting concrete, in which the impact resistance and mechanical properties of concrete samples were enhanced. The incorporation of nano-silica and silica fume in the cement of silica fume and self-compacting concrete increased 70% its impact resistance for the first crack. Fiber reinforced specimens with water-cement ratio of 0.34 and 0.27 registered the highest average of tensile and flexural strength, respectively.


**Table 1.** Compressive strength of concrete specimens containing several nano-silica contents. Reprinted with permission from [86]. Copyright©2019, Elsevier B.V.


**Table 2.** Split tensile strength of concrete specimens containing several nano-silica contents. Reprinted with permission from [86]. Copyright©2019, Elsevier B.V.

**Table 3.** Flexural strength of concrete specimens containing several nano-silica contents. Reprinted with permission from [86]. Copyright©2019, Elsevier B.V.


Mohammed et al. [88] evaluated the influence on the properties of concrete due to the nano-silica inclusion. This nano-silica incorporation caused a reduction of 13% in the pore amount of the cementitious paste. They studied the relationships that improved the compression strength of the concrete. The workability was modified negatively, which was not affected with the incorporation of superplasticizer to the concrete paste. When the nano-silica inclusion was increased in the experimentation, the permeability and infiltration rate were reduced based on the SEM results.

The incorporation of nano-silica optimal dosage in concrete samples may improve their compressive, tensile and flexural strength. The nano-silica added in cement with other materials such as polypropylene, glass and steel fibers with fixed proportions can increase the mechanical properties of the concrete. Concretes with nano-silica absorbed Ca(OH)2 crystals, filling the voids of the C–S–H structure, leading to a denser microstructure.
