2.1.1. Nanoscience

Science studies and deals with materials at atomic and molecular levels at a scale not exceeding 100 nm. Nanoscience is considered twenty-first-century science because it reveals the secrets of materials, their interactions, and why they have different properties and specifications [18].

2.1.2. Nanotechnology

Nanotechnology enables us to exert direct control over materials, harnessing the unique properties of particles smaller than 100 nm. We can bestow matter with novel properties and behaviors by manipulating these tiny particles, smaller than characteristic lengths in various phenomena. The field delves into atomic assemblies, comprising five to a thousand atoms. Notably, nanotechnology operates on scales significantly smaller than bacteria and living cells, offering multiple dimensions of exploration.

Nanotechnology or miniature technology (microscopic) deals with atoms and single molecules that make up materials by using precise tools to build and operate a smaller group of materials with repetition of the process of building and operating for smaller groups of materials more than once to reach the required size or is the technology that is concerned about designing and manufacturing materials and machines at the nanometer scale, and in general, this technology aims to apply nanoscience to create and produce means, techniques, inventions, and useful products that are characterized by their extremely small size, as well as their economic cost that does not exceed the raw material and energy used in the manufacturing process of these means and products [18].

2.4.1. Nanomaterials for Concrete

Using nanotechnology in the building and construction field leads to the creation of improved concrete with technical characteristics that differ from ordinary concrete and are environmentally friendly. This trend comes after studying the negative effects of the toxic gases that pollute the environment and pose a great danger to natural resources because they play a vital role in developing infrastructure. Nanomaterials are considered one of the best solutions when mixed with cement because studies have proven mechanical and chemical activation processes for concrete and its materials, as they improve tensile and shear resistance, durability, and bending strength. In addition, it was noted that these materials have found the solution to many complex structural designs, heralding a promising future for producing concrete with new and desirable specifications [23].

2.4.2. Use of Nanomaterials to Improve the Mechanical Properties of Concrete

Nanotechnology has played an important role in enhancing the mechanical properties of concrete, thanks to which it has enhanced strength, durability, resistance to bending and tensile strength, resistance to corrosion and spalling, and reduced carbon dioxide emissions, which cause global warming. From this point of view, nanomaterials have been adopted to solve many problems, especially those related to complex structural designs, and used to produce environmentally friendly concrete. The reason for this improvement in the properties of concrete is due to the direct effect of nanomaterials on the particles of concrete materials, as they rearrange them, strengthen the interconnections between their particles, and increase the permeability resistance [6].

2.4.3. Improvement of Compressive Strength Concrete

Nanomaterial is one of the most important and latest means to increase the compressive strength of concrete. Many studies have shown that replacing cement with different nanomaterials enhances compressive strength, Figure 3.

Narasimhan et al. [24] studied the effect of CNTs and NS on the compressive strength of lightweight concrete. The researchers noted that the concrete containing 1% NS and 2% CNT achieved the highest compressive strength, recording an improvement rate of 39% over the reference mixture. The study showed that increasing the content of nano-CNTs to 3%, while the remaining 1% NS reduces the compressive strength by 16%. The researchers showed that the increase in compressive strength was due to good packing, which enhanced the microstructure and density and thus improved the compressive strength.

Medah et al. [25] studied the replacement of nanoalumina with Al2O₃ and its effect on compressive strength. Replacing 3% nanoalumina led to an improvement in compressive strength by 35%. This improvement occurred in the presence of rice straw ash, which was 10% of the weight of the cement. The decrease in calcium hydroxide crystals was due to the pozzolanic reaction, which produced calcium hydrate in abundance, so the strength increased. Adhikary, Rudzionis, and Ghosh [26] published a study on the use of CNTs and graphene nanosheets and the extent of their effect, together and individually, on the compressive strength of concrete. The data showed that using 2% CNTs gave the highest compressive strength, recording a 10% improvement over the control mixture, indicating that the rest of the percentages (4%, 6%) gave a good improvement. While only 2% of graphene improved the compressive strength, the rest of the percentages (4.6)% reduced the compressive strength. The process of combining the two nanomaterials (1% + 1%, 2% + 2%, and 3% + 3%) improved the compressive strength by (4%, 6.5%, and 7.5%), respectively.

Sugumaran et al. [27] studied the effect of NMK and NGO on the compressive strength of lightweight self-compacting concrete. Models were evaluated at 180 days of age. The results showed that using 2% NMK + 2% MgO recorded the highest compressive strength, which increased by 11%. The researchers attributed this improvement to the quality of packing, the consistency of the mixture, and the increase in density, which improved the microstructure and thus the resistance to erosion.

Albaijan et al. [28] used nanoattapulgite (NAT) to demonstrate its effect on compressive strength. The study showed that NAT enhanced the strength of concrete by 16% when the replacement of coarse aggregate was 50% and NAT was 6%. The researchers attributed this improvement to the nanomaterial filling the cracks and pores, which strengthened the interfacial transition zone (ITZ), and led to improved compressive strength.

Yoo, Oh, and Banthia [29] studied the effect of nanomaterials on the compressive strength of HPC. The researcher showed the addition of 0.04 graphene oxide (GO) increased compressive strength by 14.5% at 90 days, but a further increase in GO content decreased compressive strength. Also, incorporating 0.15 nanocellulose filaments increased compressive strength by approximately 10%, but at a higher dosage of 0.3, the strength was reduced.

Prasanna Kumar and Rajasekhar [30] studied the effect of colloidal NS on the compressive strength of concrete. The researchers found that replacing NS from 0% to 2.5% enhanced the compressive strength. Increasing the dose of the additive led to a decrease in the compressive strength. The optimum dosage of colloidal silica was determined to be 2.5% by weight of cement, as it showed improved strengths in the compression of concrete by reducing voids.

Feng et al. [31] published the effect of nanomaterials on the compressive strength of HPC containing recycled sand. The results showed that adding 1% NS increased the compressive strength by 14% at the age of 28 days. The researchers justified this improvement to the improvement of the microstructure and density and the production of C-S-H in large quantities, which increased the compressive strength.

Luo et al. [32] studied the effect of nanomaterials on the early compressive strength of HPC. The researchers found that the 1-day compressive strength of UHPC samples with 0.3 wt% C-S-H seeds and NS increased by 457% and 25%, respectively, compared to the control sample. C-S-H seeds were more effective in accelerating the early hydration of UHPC compared to NS, producing hydrates and refining the pore structure.

Nanomaterials such as NS, nanoalumina, NMK, CNTs, and nano–titanium oxide have been shown to effectively improve the compressive strength of concrete [29–31]. These nanomaterials enhance the mechanical characteristics of concrete by increasing its strength and durability [33]. The addition of nanopowders, such as nano-SiO₂, nano-Fe2O₃, and nano-TiO₂, to concrete, has been found to significantly enhance its compressive strength [31]. Graphical representations of the effects of oxide nanoparticles on concrete have facilitated the comparison of their impact on compressive strength. Additionally, the incorporation of nanomaterials in recycled sand ultra-HPC (RSUHPC) has been shown to counteract the negative impact of increased recycled sand content on compressive performance. Overall, these studies demonstrate that various nanomaterials can effectively improve the compressive strength of concrete, making them promising additives for enhancing the quality and performance of concrete structures, as shown in Table 3.

2.4.4. Improvement of Flexural Strength for Concrete

Flexural strength is one of the important properties of concrete, and the concrete mix must be designed in an amount commensurate with the requirements of ASTM No. C78 and C293. Many researchers have been using a lot of natural and industrial materials in the concrete mixture to enhance the bending properties of the concrete; some of them got good results, and others failed to achieve their goal of enhancing this property. After the advent of nanomaterials, some researchers resorted to using them to enhance the mechanical properties of concrete, including bending resistance, Figures 4, 5, 6, 7, and 8 and Table 4.

Mohseni et al. [50] studied the effect of nanoaluminum and rice husk ash on flexural strength. Nanoaluminum and rice husk ash were added in proportions of (1%, 2%, 3%) and (19%, 20%, 30%), respectively. The maximum bending resistance recorded in this study was when (3% NS + 10% rice husk ash) was added, where the bending strength recorded an improvement value of 41%. The researchers attributed this improvement to the improvement of the microstructure, density, and homogeneity of the mixture.

Elrahman et al. [39] studied the effect of NS on the flexural strength of light concrete. Two doses (2% and 4%) were used. The 4% NS ratio was ideal for giving the best flexural strength, with an improvement rate of up to 25% when compared with the reference mixture. The study also showed that resistance remained stable at this point when evaluated at 90 days.

Ghanbari, Kohnehpooshi, and Tohidi [51] studied the effect of NS on the flexural strength of concrete. Three mixtures were designed, each containing (2%, 4%, and 6%) NS. The researchers noted that the highest bending resistance recorded at 28 days of age was 35% when 4% NS was added. Improvement in microstructure, density, and bonding of components was the main reason for this condition. In addition, improving the contact area between the aggregate and the cement paste increased the strength of their adhesion, thus increasing the bending resistance.

Hamad and Sarhan [52] published a study on the effect of NC on the flexural strength of concrete. The models were evaluated at 7, 28, and 56 days of age. The researchers used doses of (2%–19%) NC. The researchers found that 10% NC gave the best results. The highest improvement in bending resistance (16%) was recorded at 56 days.

Oh et al. [15] explored the effect of CNTs at different percentages (0.05%, 0.15%, and 0.45%) on flexural strength when mixing them with NS at percentages (0.2%, 0.6%, and 1%). The improvement rate ranged between 10% and 50% when using both nanomaterials. The highest bending resistance was recorded when 0.15% CNTs + 0.2% NS were added, where the improvement amounted to 90%. The researchers noted that increasing the nanoparticle content leads to a decrease in flexural strength.

Othman Mydin et al. [53] used nanoscale calcium carbonate to demonstrate its effect on flexural strength. The researchers added (1%–6%) of CCNP. The study showed an improvement in mixtures from 7.2%–58.75%. 4% was ideal and gave an improvement in bending resistance of 64%. The researchers explained that the reason for this improvement is due to the strength of the calcium carbonate network, which led to improved contact between the particles and thus increased flexural strength.

2.4.5. Improvement of Tensile Strength for Concrete

Nanomaterials, such as NS, polypropylene fibers, and various nanoparticles, have been found to improve the tensile strength of concrete, Table 5. Studies have shown that the addition of NS to concrete mixes can significantly increase the initial compressive strength and splitting tensile strength of the concrete, leading to improved mechanical properties [55]. Similarly, the use of NS material and polypropylene fibers in concrete mixes has been found to enhance the split tensile strength and flexural tensile strength of the concrete [56]. Additionally, the addition of nanomaterials like nanocement, NS fume, nano-fly ash, and NMK as partial replacements for cement has been shown to improve the impact strength of concrete at elevated temperatures [56]. Overall, the incorporation of nanomaterials in concrete has demonstrated the potential to enhance its tensile strength and durability, making it a promising approach for improving the quality of traditional concrete [57].

The main reason behind the improvement of the mechanical properties of concrete when adding nanomaterials is the improvement of the porous structure of the cement matrix and ITZ through the formation of C-S-H. The use of nanomaterials such as NS increased the compressive strength of concrete. Studies showed that using nanomaterials reduced the workability of concrete and its flowability.

Saad et al. [58] showed the effect of NS on tensile strength. Different doses of NS were used (0.75%, 1.5%, and 2%) by weight of cement. All doses recorded an improvement in tensile strength, but the highest improvement in strength was recorded when adding 2% NS, where the improvement rate reached 45% at the age of 28 days, and as the age increased, the tensile strength increased.

Mansi et al. [59] studied the effect of CNTs on the tensile strength of concrete containing geopolymer and basalt fibers. The researchers noted that the tensile strength increased by 32% over its counterpart without CNTs and that the highest tensile strength chosen was when adding 0.15% CNs + 0.6% basalt fibers. The researchers found that CNTs reduced cracks and formed a dense structure, which enhanced tensile strength.

Othman Mydin et al. [60] published a study on the effect of nano-iron oxide (Fe₂O₃) on tensile strength. The researchers used different doses ranging from (0.1%–0.%3). The study recorded the highest increase in tensile strength (51%) when adding 0.25% nano-iron oxide at 56 days of age. The improvement in tensile strength came due to the production of large quantities of calcium hydrate (S-C-H), which is responsible for the strength of concrete.

Hong et al. [14] studied the effect of doses of NGO on tensile strength. Five doses were used (0.02%, 0.04%, 0.05%, 0.06%, and 0.08%). A dose of 0.05 achieved an improvement of 24%, which is the highest tensile strength. Consumption of calcium hydroxide by NGO and production of calcium hydrate played the main role by increasing the tensile strength.

Ali and Awad [61] studied the effect of NS on the tensile strength of lightweight concrete. Doses less than 1% were used in this study. Forms are evaluated for up to 90 days. The study showed that adding NS at various doses enhanced tensile strength for all ages, but the highest improvement rate (11%) was recorded when 1% was added at 90 days. Ali justified this improvement to the improvement of the transitional zone between the aggregate and the cement paste, which improved the microstructure and thus increased the tensile strength.

Nanomaterials such as nano-SiO₂, nano-Fe2O₃, nano-TiO₂, NC, and nano-Al₂O₃ have been found to be effective in improving the tensile strength of concrete [62]. Nanoparticles have a high potential for enhancing concrete properties due to their small fragment size and increased surface area [33]. In particular, nano–titanium dioxide and nanoalumina have been shown to significantly improve the tensile properties of concrete members, with ultimate tensile strengths reaching 11.54 and 9.56 MPa, respectively [17]. Additionally, nano–titanium dioxide and nanoalumina can be used to replace cement in concrete, reducing its carbon footprint and improving its mechanical response [63]. Overall, the addition of nanomaterials such as nano-SiO₂, nano-Fe₂O₃, nano-TiO₂, NC, nano-Al₂O₃, nano–titanium dioxide, and nanoalumina can effectively enhance the tensile strength of concrete.

Finally, the key findings from the mentioned studies provide some statistical components, Table 6.

2.4.6. Improvement of the Microstructure of Concrete

The microstructure of concrete can be improved through the incorporation of nanomaterials, such as NS and NC, which enhance the hydration reaction and promote the formation of more hydration products [30]. These nanomaterials act as fillers, making the concrete matrix more compact and uniform, resulting in a decrease in porosity and an increase in packing density [32]. Additionally, nanomaterials provide extra nucleation sites, leading to the generation of more C-S-H gel clusters and a finer pore structure [29]. The improved microstructure of the concrete matrix, achieved through the refinement of microcracks and pore filling, enhances the homogeneity and compactness of the concrete [31]. This, in turn, improves the mechanical properties and durability of the concrete, making it more resistant to deterioration and extending its service life [4].

Wang et al. [64] used three types of nanomaterials (nanoaluminum (Al₂O₃), NMK, and NS) to investigate their effect on the ITZ between aggregate and cement paste. The researchers found that the best additive was NS among the three materials. The researchers found that NS increased the bond strength of concrete by 109% when compared with the reference mix. The researchers indicated that the rest of the materials, such as NMK, improved the bond strength, but by 31%, and Al₂O₃, by 20%.

Onizaz et al. [4] studied the effect of calcined NAT. The researchers found that the nanomaterial filled the cracks and pores inside the concrete mortar and the ITZ, and thus improved the harmful pores and cracks as well as the density, which reflected positively on the strength of the concrete.

Goel et al. [33] published a review to demonstrate the effect of nanomaterials on concrete. The study showed that many nanomaterials have been found to enhance the pore structure and density of concrete, acting as a filler and reducing porosity.

Huo et al. [65] studied the effect of nano-SiO₂ and nano-Al₂O₃ on concrete microstructure. The researchers observed that the addition of nanomaterials increased the gelatinous products containing calcium in the geopolymer concrete matrix, resulting in a denser and more uniform microstructure. The researchers found that this improvement was due to reducing the number of pores and their fractal dimension. The researchers found that increasing the dose of NS leads to agglomeration of the material, which weakens the microstructure and creates heterogeneity in it.

Wang et al. [66] found that adding nanomaterials has a positive effect on concrete that contains recycled bricks. The researchers found that concrete devoid of nanomaterials contains small cracks and defects in the concrete, especially the interstitial area between the aggregate and the cement paste. The SEM examination showed that the nanoconcrete was denser and had fewer cracks due to nanomaterials filling these cracks and that the transition zone was more compact and durable.

Du et al. [67] studied the effect of NS and nano–iron oxide, together and individually, on the microstructure of concrete. The researchers found that the ideal percentage (3.45% NS, 4.24% nano–iron oxide) that achieved the highest strength of concrete was the same and showed the best improvement in microstructure. The researchers also found that the two nanomaterials improved the microstructure when added individually. The researchers justified this improvement by saying that the nanomaterials filled the small pores, making them more cohesive.

Kishori et al. [68] studied the dispersion and agglomeration of nanomaterials in geopolymer concrete and its effect on the microstructure and thus its effect on the strength of the concrete. Through the researchers’ use of the transmission electron microscope (TEM) technique, the researchers found that the dispersion of nanomaterials is of great importance for concrete, as it helps speed up the interaction with concrete components and the formation of C-S-H, which is responsible for strength, and that their agglomeration forms weak areas and hinders the performance of the compounds.

Chen et al. [69] studied the effect of adding nano–calcium carbonate (CaCO₃) (CN) to concrete reinforced with polypropylene fibers. The results showed that NC improved the concrete hydration rate and heat of hydration in the early stage, and also participated in the chemical reactions of hydration in the late stages, which enhanced the hydrate production and microstructure, Figure 8, 9, and 10 [70, 71].

2.4.7. Effect of Nanomaterials on Water Absorption of Concrete

Al-Jabri and Shoukry [72] published a study on the effect of NMK on water absorption. The results showed that the use of the additive reduced water absorption. This is attributed to the improvement of the microstructure of the concrete mixture. It is known that the microstructure improves due to the pozzolanic interactions of the cement dough.

Diab et al. [73] studied the effect of NMK and NS on the water absorption capacity of HPC. The researchers added nanomaterials in different doses. The researchers found that adding NS reduced water absorption by (7%, 11.4%, 16.8%, and 21.1%) for mixtures containing (0.5%, 1%, 1.5%, and 2%), respectively, compared to the reference mixture. The researchers also showed that using NMK had the same effect on water absorption when using the same doses. The researchers attributed the reason for this decrease to the increase in solid volume and improvement of the microstructure, which in turn reduces the movement of water within the concrete.

Vanitha et al. [74] published a study on the effect of nano-TiO₃, Al₂O₃, and CaCO₃ on water absorption capacity. The researchers found that NT reduces water absorption by (4.5%, 7.5%, and 13.6%) when used by (2%, 4%, and 6%), respectively, when compared with the reference mixture. The study also showed that the effect of NC was clear, as the water absorption rate decreased by 16%, 23%, and 31% when doses of 2%, 4%, and 6% were used. In addition, the study showed that nanoaluminum was the most effective additive in reducing water absorption. The percentage of decrease in water absorption reached (31%, 35%, and 38%) at levels of (2%, 4%, and 6%). The researchers justified that the reason for this decrease was that nanomaterials filled the tiny gaps, which impeded the transfer of water.

Abdullah et al. [75] studied the effect of several nanomaterials on the ability of concrete to absorb water. The researchers found that incorporating 2% NS reduced water absorption by 85%, while 0.08% CNTs reduced water absorption by 36%. Also, NGO reduced water absorption by 46% when a dose of 0.8% was added to it. 3% nano-calcium carbonate reduced water absorption by 65–70%. Adding 4% of nano–iron oxide achieved a reduction in water absorption of 74%, while nanoalumina had a negative effect on water absorption.

2.4.8. Improvement of Sulfate and Chloride Penetration of Nanoconcrete

Diab et al. [73] studied the effect of NMK and NS on the sulfate resistance of HPC. The researchers added nanomaterials in different doses. The researchers found that adding NS reduced the loss of compressive strength by 4.6%, 11%, 15%, and 18.6% for mixtures containing 0.5%, 1%, 1.5%, and 2% NS, respectively, compared to the reference mixture. Adding doses of NMK also showed the same effect, as the amount of loss of compressive strength was reduced by 11.4%, 20.7%, 32.5%, and 41.4% for an NMK content of 1%, 3%, 6%, and 9%. This positive improvement was linked to suppressing the growth of gypsum and ettringite when exposed to sulfates.

Maiti et al. [76] presented a study on the effect of NT permeable geopolymer concrete against chlorides and harmful compounds. The study showed that adding NT increased the ability of geopolymer concrete to resist the penetration of chlorides, which contributes to the possibility of treating the environment. It also increases the possibility of using geopolymer concrete modified with NT in combating water pollution.

Abdullah et al. [75] studied the effect of several nanomaterials on the ability of concrete to resist chloride permeation. The researchers found that incorporating 0.3% NS increased concrete’s resistance to permeability by 29%, while incorporating 0.03% GO reduced chloride penetration by 52%. The researchers noted that CNTs (0.05% to 0.1%) reduced the penetration of chlorides by 12%. Nano-iron oxide (2%) reduced the permeation of chlorides into concrete by 44%, while NT reduced the permeation of chlorides by 88%.

Wu et al. [77] developed concrete containing capillary crystalline cement insulation agents and studied the effect of five compounds including NS (EDTA, aspartic acid, sodium sulfate, sodium silicate, and NS) on the permeability of concrete. The researchers determined the ideal amounts for each compound (0.5 EDTA, 2% aspartic acid, 3% sodium sulfate, and 2% sodium silicate). The resistance of concrete against penetration was enhanced by EDTA 225%, L-aspartic acid 200%, sodium sulfate 150%, sodium sulfate 130%, and NS 125%. It was found that NS enhanced the penetration resistance by 125%. This is a positive development that indicates the effectiveness of nanomaterials in enhancing the properties of concrete against the penetration of harmful substances.

Qu et al. [78] studied the effect of NS and NT on enhancing concrete resistance against chlorides and carbonation. Nanomaterials have been used to enhance concrete coatings. The results indicated a significant improvement in resistance to carbonation and chlorides, as penetration decreased by 38% and carbonation by 12.8%. The researchers also noted that nanomaterials reduced the damage of ultraviolet rays. Moreover, the study showed that the effect of NT on resistance to carbonation and chlorides is higher than the effect of NS.

Han et al. [79] studied the effect of CNTs on enhancing the resistance of concrete against chloride penetration. The researchers found that the diffusion coefficient and diffusion depth of chloride ions decreased by 19% and 26%, respectively, when incorporating a 0.15% CNT into concrete. The study showed that the additive improved the microstructure and thus reduced self-shrinkage and improved porosity and pore size distribution, which had a positive impact on resistance to chloride penetration.

2.4.9. Improvement of Thermal Insulation of Concrete

The building and construction sector poses the greatest danger to the environment and to energy and natural resources. The reason is due to several factors, the most important of which is the consumption of large quantities of natural resources such as wood, stone, soil, rubber, and many other materials to comfort humans. The second main reason is the consumption of energy sources such as oil and its derivatives and electrical energy closely related to oil and its derivatives. As a result of providing the necessary electrical energy, large quantities of fuel must be consumed, which results in burning huge quantities of natural and industrial fuels, which causes pollution of the environment with carbon dioxide and the rest of the gases that pose the threat of the environment and increases the global warming. Therefore, it has become necessary to find radical solutions to reduce the emission of heat and toxic gases into the atmosphere and reduce the consumption of electrical energy and natural resources. The researchers sought to find a material with properties that enhance the ability of thermal insulation and resistance to high temperatures and are environmentally friendly. The researchers found that nanomaterials significantly improve the mechanical ability of concrete. When studying them in a focused manner, they found that they significantly improved the resistance of cement compounds to high temperatures. The nanosized materials contributed to the modulation of cement hydration, the degree of compaction of its structure, and the improvement of cement compounds’ mechanical and fracture properties, thus improving their strength and mechanical performance under high-temperature conditions, Figure 11.

Sikora, Elrahman, and Stephan [80] used nanomaterials in vacuum insulation panels or polystyrene foam, as they showed a significant decrease in the thermal conductivity coefficient, and the electrical energy consumption decreased to 23%, compared to ordinary insulated walls. On the other hand, nanoglass was applied instead of the two-layer glass used for insulation. This type of nanoglass saved approximately 11% of the value of the average energy consumption. If two layers were used, the insulation and conductivity would be reduced by less than half. In addition, when using insulating panels and insulating nanoglass, the study showed a decrease in energy consumption to 47.5%, with a decrease in thermal conductivity to more than half. This process was described as an efficient way to isolate buildings and consume less energy.

Bhutto, Alsaadi, and Erklig [81] studied the effect of NC and NS on the thermal properties of concrete. The study showed that adding NS and NC to the mixture reduced the thermal conductivity coefficient to 0.5%, indicating that using nanomaterials contributed to reducing heat transfer. The contrast in mitigating heat transfer between the materials above is noteworthy. The research demonstrates that incorporating NC results in a more pronounced reduction in the thermal conductivity coefficient compared to NS. This disparity arises from an augmented bonding between the matrix and the epoxy resin due to NS, alongside the influence of weight content.

Saleh et al. [40] published a study on the effect of NS on the thermal properties of concrete (thermal conductivity, specific heat capacity, and thermal diffusivity). The researchers used 3% (1%, 2%, and 3%). The results showed a clear decrease in thermal conductivity values from (0.9–0.5) W/m. Kelvin. The decline rates were 42%, 53%, and 66%, and the replacement rates were 1, 2, and 3, respectively. The highest reduction was achieved by a dose of 3%. In addition to thermal conductivity, heat capacity was measured, as it increased with higher replacement ratios. The increase in heat capacity recorded values of 21.5%, 24.25%, and 32.45% for the three ratios.

Babalu and Pimplikar [82] studied the effect of NS on the thermal properties of concrete. Three mixtures were designed containing NS (1%, 2%, 3%). The researchers noted that increasing the percentage of added nanomaterial reduces the conductivity coefficient. The percentage of decrease in the conductivity coefficient was 9%, 24%, and 39% when the dose was increased from 1%–3%. Another study showed that the thermal conductivity coefficient decreased from 15% to 59% when the dose of NS was increased. The researchers attributed this improvement in thermal insulation to the presence of air voids that prevent heat transfer. In addition, the air voids increase the ability of concrete to store heat without transferring it, which enhances thermal insulation.

2.5.1. Background of Lightweight Concrete

Over two thousand years have passed since the invention of lightweight concrete. Its technical development is still going on. Although it is well known that it has the potential to perform well in these areas, there are still many uncertainties and hesitations in its implementation. One of the main challenges the developers face is adapting lightweight concrete’s design and production rules to accommodate their varying requirements [84].

Lightweight concrete is not considered one of the modern achievements, but it has been known since ancient times, as it was invented two thousand years ago. The first references to light concrete were invented in the early Roman Empire, and there are pillars of these buildings until now, such as the Pantheon in Rome, one of the most famous buildings in this style, as it showed wonderfully the regular use of lightweight natural aggregates. In the 19th and 20th centuries, lightweight concrete became a historical turning point that kept pace with materials and technology [84].

The successful use of lightweight concrete in shipbuilding greatly aided its introduction into the construction industry. The first factory to produce expanded aggregate was established in 1920 in Kansas, USA, where experiences have proven that the quality and uniform composition of industrially produced aggregate is more beneficial than natural aggregate. Expanded slabs have been used as lightweight concrete on many bridges. More than 200 lightweight concrete bridges have been built in the United States and Canada. In the middle of the twentieth century, the use of lightweight concrete increased in structural engineering, as the high-rise Marina City Towers building in Chicago is considered one of the distinctive buildings built with light concrete, and the Prudential Plaza building is also one of the giant towers in light concrete. Its concept has remained largely confined to individual cases despite using light concrete for structural reasons. Technical regulations for lightweight concrete were modified in the nineties in Germany, which witnessed a qualitative leap due to the issuance of these new regulations and the continued development of science and technology. Despite all these improvements, the number of buildings and facilities remained relatively low in terms of the use of light concrete. Recent developments have focused on reducing concrete density while retaining as high and sufficient strength as possible. Monolithic structures made of lightweight concrete with a glazed face have recently gained great popularity in Europe. Low density, high resistance to loads and fires, and excellent durability are the main characteristics of the promising material. Recent developments aim to find appropriate solutions for thermal insulation using lightweight concrete, as this technology has helped many engineers and researchers to find appropriate solutions without the need to add insulating layers to the origin. However, obtaining this concrete requires technical approvals related to the project [84].

2.5.2. Properties of Lightweight Concrete

These properties of lightweight concrete can be exploited in several ways as a structural material primarily or used to enhance thermal insulation when incorporated into the structure. In this research, lightweight concrete with lightweight aggregate will be one of the most important materials used in thermal insulation and reducing the conductivity coefficient because of its great insulating properties, as shown in Figure 12 [85].

2.5.3. Types of Lightweight Concrete

Three types of lightweight concrete will be mentioned.

2.5.4. No-Fine Aggregate Concrete

Concrete free of fine aggregates has a relative density that varies according to the different mixing ratios, as its proportion decreases by increasing the cement, coarse aggregate, and water content. The relative density of concrete free of fine aggregate ranges from 1770 to 2015 kg/m3, and this density is much lower than the density of ordinary concrete, which is 2400 kg/m3, due to its high nails. As for the porosity, it is affected by the difference in the size of the coarse aggregate in the concrete mixture, where studies have shown that the porosity of concrete free of fine aggregate ranges between 27% and 36%. Figure 13 [86] shows a sketch of no-fine concrete. As for its mechanical properties, most studies focused on compressive strength depending on the type and size of coarse aggregate, as well as the weight of the concrete unit free of fine aggregate, the ratio of water to cement, and aggregate to cement. Studies have shown that the greater the volume of the coarse aggregate, the greater the compressive strength. Most of the mixing ratios that have been studied in which the compressive strength is lower than the compressive strength of ordinary concrete due to its low density. The best compressive strength for this type of concrete is 11.25 MPa. From the above, we conclude that light concrete free of fine aggregate cannot be used in structural parts that need high compressive strength, but it can be used in some nonstructural parts and for thermal insulation purposes [86].

2.5.5. Aerated Concrete

Many factors affect the strength of structural concrete. The sample size, shape, water content, properties of concrete components, method of concrete formation, loading direction, and curing method all affect the compressive strength of aerated concrete [70].

Aerated lightweight concrete does not contain coarse aggregate, and the difference in mixing properties is one of the most important features that distinguish it from ordinary concrete, its low density with high strength, and thermal and sound insulation. The lightness of the concrete weight reduces the dead load on the other concrete parts, which enhances this by reducing the structural elements, which reduces the loads on the foundations and increases its endurance [87].

2.5.6. Lightweight Aggregate Concrete

This concrete consists of cement, water, fine aggregate, and lightweight aggregate, completely or partially replaced with the usual heavy aggregate. The main purpose of using lightweight aggregate is to reduce the overall density of concrete, which in turn reduces the dead weight on the structure and, at the same time, increases the thermal insulation of concrete. Lightweight aggregates can be natural, such as scoria or pumice, as the origin of this aggregate is volcanic stones, or they can be made, such as kiln slag, vermiculite aggregate, and clinker. This aggregate is characterized by high porosity, which is used to reduce the total weight of concrete [88].

2.6.1. Improving Compressive Strength for Lightweight Concrete

Narasimhan et al. [24] studied the effect of CNTs and NS on compressive strength. Lightweight concrete is designed using expanded clay as a lightweight coarse aggregate. The researchers found that the LWC samples added to 1% NS and 2% CNTs gave the maximum compressive strength, as the improvement rate was 39% higher than the reference mixture. The results showed that increasing the additives more than the ideal percentage reduced the compressive strength to 16% higher than the reference mixture.

The impact of nano- and microsilica on the compressive strength of light concrete was examined by Afzaleh et al., 2029. Three different combinations were created, with 1%, 3%, and 5% NS in each; 10% of the microsilica was also included. The two additives were utilized both singly and in combination by the researchers. The researchers discovered that while mixing the two materials increased the compressive strength, doing so alone did not. When 3% NS and 10% microsilica were combined to create the optimal mixture, the improvement rate over the reference mixture was 31% [1].

Sugumaran et al. [27] published a study on the effect of NMK as an additive on the compressive strength of lightweight concrete. The researchers found that using this material by 15% improved the compressive strength by 15% when compared with the reference mixture. The researchers justified this improvement to the strengthening of the microstructure as well as the pozzolanic interactions that produced abundant calcium hydrate responsible for strength. In addition, the ITZ between the aggregate and the cement paste is improved.

Hong et al. [90] on the effect of using NGO in different proportions on compressive, tensile, and bending resistance. The results showed that using NGO, material increased the compressive strength in varying proportions according to the amount of NGO replacement from the cement weight. The results showed that the best compressive strength was obtained when replacing the NGO by 8% at 56 days, as the compressive strength increased by 24% compared to the reference mixture, as shown in Figure 14. This increase is because the nanomaterials regulated the formation and growth of flower-like crystals and filled the pores.

Zhang et al. [37] published an article about using NS in very low doses and investigating the improvement in concrete’s mechanical and physical properties. The results showed that the use of 0.1% of NS gave the best results for compressive strength when compared with the reference mixture, as well as when compared with other percentages of NS, where the compressive strength increased by up to 41%, as shown in Figure 14.

Najaf et al. [56] published about the addition of NS in lightweight concrete and its effect on compressive strength, and it was found that replacing 3% of NS increased the compressive strength to more than 47% at the age of 28 days when compared with the reference mixture, and then, the resistance begins to decrease when the dose of NS is increased.

Nour El-Din et al. [91] published an article about using NS material and studied its effect on lightweight concrete. They found that the optimal ratio that gave the best results for compressive strength is 3%, and then, higher percentages give a decrease in compressive strength. Nour El-Din et al. showed that doses of less than 3% are very useful for increasing compressive strength and that doses higher than 3% are useless, as shown in Table 7, Figure 15.

2.6.2. Improving Flexural Strength for Lightweight Concrete

Nazari et al. [47] concluded that replacing 1% of nano-ferric material with lightweight concrete enhanced the flexural strength by 27% at the age of 90 days. The results also showed that increasing the replacement dose of nano-ferric material reduces the bending resistance, so 1% of nano-ferric material is considered the best replacement ratio.

Salman et al. [94] studied the effect of adding NT to lightweight concrete and investigated whether there was an improvement in the bending property of lightweight concrete. The results showed that the percentages of NT (0.25% and 0.75%) increased the bending resistance, and the best percentage added was 0.75%, as it improved the bending resistance by 15.1%. An increase of more than 0.75% of NT reduced the flexural strength, but there was an improvement when compared with the reference mixture, as shown in Table 8.

Zhang et al. [37] used NS in very low doses and studied the effect of the nanomaterial on flexural strength. The study showed that adding NS in doses (0.1%–0.5%) increased the bending resistance, especially by 0.2%, which gave the best results compared to the rest of the ratios. Increasing the dose by more than 2% has reduced the bending resistance, which indicates that the optimum ratio is 2%.

Alomayri [95] conducted a study on combining nanoalumina with geopolymer pastes in doses of 1% and 2% and showed that the bending strength increased by 5.6% when adding 1% nanoalumina and 44.5% when adding 2% nanoalumina. In the same study, Alomayri indicated that adding calcium carbonate nanomaterials in doses of 1%, 2%, and flexural strength increased by 88% and 151%, respectively. The study also showed that using calcium carbonate with glass fibers gave better flexural strength when compared with mixing nanoalumina only in concrete, as shown in Figure 16.

Hong Lee and Qian [90] published an article on the use of NGO in lightweight concrete and showed the effect of the nanomaterial on flexural strength. The results showed that the addition of 8% of NGO gave an increase in bending resistance of up to 20% when compared with the reference mixture, which is the highest result obtained when adding different percentages of nanomaterials (2,4,6%), respectively, as shown in Figure 17.

Alomayri and Adesina [96] stated that the addition of calcium carbonate nanoparticles to geopolymer pastes at a rate of 2% gave the best results in resisting bending, but when the dose was increased to more than 2%, the resistance to bending began to decrease. Alomayri explained in his article that the reason for the increase in the bending resistance was due to the improvement of the microstructure of the geopolymer upon the addition of nano-calcium carbonate, as well as the acceleration of the polymerization reaction, which works to densify the matrix. Al-Omairi indicated that the decrease in flexural strength when increasing the dose to 3% is due to the agglomeration of the nanoparticles, as shown in Table 9 and Figure 18.

To confirm their impact on the flexural strength of lightweight concrete, Du and Korjakins [97] investigated CNTs at a dose of (0.05%, 0.15%, and 0.45%) and NS at a content of (0.2%, 0.6%, and 1%) by weight of cement. The reference combination was used to compare the results of the tests. The study found that all combinations comprising the two nanomaterials increased flexural strength by 10% to 50%. With an improvement rate of 90% over the reference mixture, the dose of 0.15% CNT with 0.2% NS was found to have the maximum flexural resistance, while the sample with the 10% NS and 0.45% CNT demonstrated the least improvement in flexural strength, with an improvement of only 7.53%.

The impact of nano-calcium carbonate on flexural strength was investigated by Othman et al. in 2023 [53]. The investigators employed six nano-calcium carbonate dosages. A dose of (1%, 2%, 3%, 4%, 5%, 6%) nano-CaCO₃ was added, and the results showed that the flexural strength rose by 7.46%, 22.39%, 30.26%, 58.75%, 23.5%, and 7.2%, correspondingly. When 4% nano-CaCO₃ was added, the greatest improvement in flexural strength (64%) was observed. According to the study, the robust networked nature of nano–calcium carbonate, which is typified by van der Waals forces that enhance molecular communication is the cause of this enhancement. Additionally, the compact arrangement of wetting products produced by nano–calcium carbonate results in smaller pores and larger particles, which in turn shortens the interfacial distance and boosts strength. Lastly, raising the dose by more than 4% reduced the flexural strength due to the agglomeration of particles or due to the entry of calcium carbonate nanoparticles between the cement paste particles, which weakened its microstructure, Figure 19.

2.6.3. Improving Splitting Tensile Strength for Lightweight Concrete

Different amounts of nanoaluminum were substituted in Nazri et al.’s 2010 study to examine its impact on lightweight concrete. Four nanoaluminum dosages (0.5%, 1%, 1.5%, and 2%) were employed. Five combinations were created in order to contrast the effects of different concentrations of nanoaluminum with a reference mixture that contained no nanomaterial. The models were assessed when they were 7, 28, and 90 days old. The investigation revealed that all mixes’ tensile strength increased more than that of the control mixture. When 1% nanoaluminum is introduced, the tensile strength rises; however, as the dose is raised, the tensile strength starts to fall. The rise in nanoaluminum in the concrete mix, which resulted in nanoaluminum particles that were not mixed, was the reason given by the researchers for this drop [98].

The combined impact of fibers (0.25%, 0.5%, 0.751%, and 1.5%) and NS (2%, 4%, and 6%) on the tensile strength of lightweight concrete was investigated by [51]. A total of twenty-four concrete mixtures with varying percentages of the two elements mentioned above were created and compared. When compared to the reference mixture, the investigation revealed that the tensile strength of the 24 combinations increased from 3%–55% at the age of 28 days. This was attributed to the combined influence of both fibers and NS. The results of the study indicated that while employing each material alone increased tensile strength, the improvement was not as great as that which was achieved by combining the two materials. The study found that adding 1.5% fiber and 4% NS produced the highest tensile strength. The findings demonstrated that the pozzolanic activity of NS, which interacts with the constituents of the concrete to strengthen interconnections between the aggregate and cement paste, is primarily responsible for the improvement in the tensile strength of lightweight concrete. The restricted hardness of the light aggregate is increased when the paste’s strength is increased because it prevents the lightweight aggregate from breaking. According to the study, adding NS to lightweight concrete produced superior outcomes than including fibers [51].

Saad et al. [58] published a study on the use of NS in lightweight concrete to verify its effect on tensile strength. Three doses were used (0.75%, 1.5%, and 2%). All models were evaluated at 28 days of age. The researchers found that all added proportions improved tensile strength, but the best dose that improved tensile strength was 2% NS, which reached 45% higher than the reference mixture.

Hong et al. [90] showed that adding NGO to the lightweight concrete mixture improves tensile strength. The addition of 5% nanographene gave the best tensile strength at 28 days, where the percentage of improvement was 17% when compared with the reference mixture. The study also showed that this increase was due to the ratio of water to cement.

The influence of nanosized Fe₃O₄ on tensile strength was investigated by Othman et al. [60]. Six combinations with varying nano-Fe₃O₄ (0.10%, 0.15%, 0.20%, 0.25%, and 0.30%) by weight of cement were created by the researchers. The models were assessed when they were 7, 28, and 56 days old. The combinations adding micro Fe₃O₄ all improved, but to varying degrees, the researchers found. At 56 days of age, adding 0.25% nano-Fe₃O₄ and 51% yielded the maximum tensile strength. The results of the investigation demonstrated that the nanomaterial’s ability to speed up the production of calcium hydrate, which gives concrete its strength, was the cause of the gain in tensile strength [60].

The use of NS material to enhance the tensile strength of lightweight concrete is one of the nanomaterials that the researcher Najaf used with his colleagues in 2022. The results showed that using this substance by 3% enhanced the tensile strength by 25% at the age of 28 [56].

Najaf et al. [56] studied the effect of NS on splitting tensile strength for lightweight concrete containing polypropylene fiber. The best tensile strength was achieved by using 1.5% fibers, 3% NS, and 25% waste glass powder, with a 28-day tensile strength that is 1.6 times that of the control sample.

Aseel et al. [59] studied the effect of CNTs on the tensile strength of lightweight geopolymer concrete with basalt fibers. Furnace slag and CNTs were used as binders. The researchers found that using these additives increased the tensile strength by 32%. The researchers attributed this improvement to the fact that basalt fibers and CNTs improve the bond strength between the cement paste and the aggregate and prevent cracks. In addition, filling the voids with nanomaterials enhanced their density. The researchers noted that the maximum tensile strength was at 0.15% CNTs and 0.6% basalt fibers.

Different doses of NGO were employed by Hong et al. [14] to show how it affected the tensile strength of lightweight concrete. Five mixes with NGO were created; the percentage of cement in each combination was 0.02, 0.04, 0.05, 0.06, and 0.08 by weight. The researchers combined NGO-containing mixes with rice husk ash. Tensile strength was increased by all five mixes, according to the study. When more than 0.05% NGO was added, the researchers saw a drop in tensile strength. By adding 0.05% NGO, the tensile strength was the highest (24%) as compared to the reference combination. When NGO was used in the right proportion, it was completely consumed and a substantial amount of calcium hydrate was produced, which improved the tensile strength. However, by exceeding the recommended dosage, more NGO particles were added, and they interfered with the concrete particles without interacting, weakening the binding and reducing joint strength [17].

Ali et al. [61] published a study on the effect of NS on the tensile strength of lightweight concrete. Three models were selected and evaluated, each containing 0.5%, 0.75%, and 1%. The results showed that the tensile strength improved by 3%, 10%, and 16% for the three doses at the age of 28 days. The highest tensile strength was 1% NS. The researchers justified the improvement due to the pozzolanic reactions that produced more calcium hydrate, which is responsible for strength. In addition, the ITZ between the aggregate and the cement paste is improved, as shown in Table 10 and Figures 20 and 21.

In comparison with various research articles exploring the improvement of concrete properties using nanomaterials, our study contributes by addressing a wide range of nanomaterials, including CNTs, NS, NGO, NT, nanoalumina, and nano-calcium carbonate. We extend the understanding of their impact on concrete’s compressive, flexural, and splitting tensile strengths in lightweight concrete.

Narasimhan et al. [24] focused on CNTs and NS, showing a 39% improvement in compressive strength with specific proportions.

Sugumaran et al. [27] explored NMK’s impact, resulting in a 15% increase in compressive strength. In our study, we go beyond by encompassing various nanomaterials, including NT and NGO, showcasing a broader spectrum of potential enhancements.

The work of Hong et al. [90] on NGO replacement exhibited a 24% increase in compressive strength, aligning with our findings. Zhang et al. [37] also investigated NS, showing a notable 41% improvement in compressive strength with a specific dosage.

Additionally, studies by Najaf et al. [56] and Nour El-Din et al. [91] highlighted the role of NS in lightweight concrete, providing insights into the optimal dosage for enhanced compressive strength. Our study complements these findings by encompassing a broader array of nanomaterials and their varied effects.

For flexural strength, our study aligns with the findings of Nazari et al. [47], Salman et al. [94], and Zhang et al. [37], emphasizing the positive impact of nanomaterials on flexural strength. Furthermore, the work of Alomayri et al. [95] on combining nanoalumina with geopolymer pastes echoes our exploration of diverse nanomaterials, showcasing their influence on flexural strength.

In the realm of splitting tensile strength, our study builds upon the insights provided by Ghanbari et al. [51], Saad et al. [58], and Najaf et al. [56], offering a comparative analysis of various nanomaterials’ effects on this crucial property.

Finally, our study extends beyond individual nanomaterials, encompassing a diverse range and comprehensively exploring their impact on different concrete strengths. This approach adds depth to the current body of knowledge, providing a more nuanced understanding of the potential improvements achievable through nanomaterial incorporation in lightweight concrete.