1.1          General Background

Concrete is the backbone of structural engineering as it is the main material used in construction processes. Therefore, many engineers, researchers and students always seek to learn the secret of this material and know its physical, mechanical and chemical properties and its strong relationship with the reinforcing steel used. The constant effort is to work on increasing the efficiency of this concrete either by improving one of its basic components in aggregate, sand, water or cement or by adding different chemical additives that help to obtain a material with superior properties that help to withstand more loads and different stresses on the structure. There is also a constant desire to obtain a concrete material that has high hardness, great fire resistance and great ability to resist the compression that concrete is exposed to. Concrete must also have a great ability to resist corrosion and obtain great durability that can withstand loads over time. Or resist all lateral loads placed on it, whether wind loads or seismic loads. Therefore, we must be in front of a super-capable material that can meet all these needs and at the same time must be stable, safe and economical in order to achieve the successful engineering triangle.

1.2          Nano Silica

Nano silica or nano silica (NS) is defined as silicon dioxide which may be in the form of particles having size less than one hundred nanometre. This material comes with fascinating characteristics arising from its porous nature and high reactivity and its high surface area; hence it can be finding application in different sectors of the industries. Nano silica is used in construction particularly in concrete as a pozzolanic material that improves on its strength, and durability and low permeability. Also, it constitutes a vital component in manufacturing new-age materials like composites and coatings used where the material’s mechanical characteristics and thermal stability resistance are of paramount importance. Nano silica is also being used in pharmaceuticals and cosmetics since it would act as effective carrier for active pharmaceutical ingredients and improve the texture of the final product. In any case, due to the multifunctionality of nano silica, it is of high importance in the modern material science and engineering.

1.3          Problem Statement

Concrete is one of the most popular constructional materials around the world which is well-known for its strength. However, there is always some weakness associated with ordinary concrete including low tensile strength, tendencies to crack and inadequate ability to withstand rough climates. These limitations can greatly impact the life of concrete structures as well as the effectiveness and efficiency of concrete structures construction. The use of nano silica is therefore a viable solution to these problems. Nano silica is a fine powder which when added to the concrete improves its characteristics due to the modification of the concrete microstructure. Nano silica when incorporated in concrete, contributes to the reduction of void space within the cement paste matrix hence improves its density. This leads to increased compressive strength and reduced permeability to water. Furthermore, utilisation of nano silica in concrete can also go along with sustainability standards due to its ability to enable the creation of concrete with less cement content. As cement production is one of the primary sources of carbon emissions improving the efficiency of cement as a material in concrete through additives such as nano silica can help lessen the ecological effect. The statement problem that comes with the use of traditional concrete is the acknowledgment of its shortcomings and the search for other concrete forms including nano silica. With the help of improved properties of concrete through special materials, it is possible to achieve better results in strength and durability of structures for today’s construction.

1.4          Research Objective

The research objective for the utilization of nano silica in concrete involves improving on the concrete material properties. Nano silica is obtained from silicon manufacturing waste; it has been discovered to make concrete more durable and sustainable. One of the main objectives is to know at how nano silica can be incorporated in order to enhance compressive strength. Several researchers have observed an improvement in the properties of fresh concrete when nano silica particles are incorporated in optimum proportions into the mix; the particles flow into the micro-cracks present in the matrix and formulate a denser structure of the concrete. It also improves mechanical properties of the material as well as increases its durability attributing to the influence of adversative factors like moisture or chemical reaction. Another major aim is to find out the possibility of enhancing workability of concrete mixtures by inclusion of nano silica. In other words, by controlling the particles of the cement paste during its production or after its production as a building material the behaviour is changed, and this enables construction engineers to handle the material and insert it into the construction process and materials while not providing any compromise on the final durable structure. Moreover, studies find ways to determine the durability changes in concrete structures with moreover additives such as Nano silica. This research aims to create a basis for developing cementitious systems with better and more sustainable energy efficiency with the use of nanotechnology.

1.5          Research Content

This research is divided to five chapters and references as shown below:

Chapter one: Introduction

Chapter two: Literature Review

Chapter three: Methodology

Chapter four: Results and Discussion

Chapter five: Summary, Conclusion and Recommendation

Although nano-silica is new material in the family of nanotechnology materials, it has shown remarkable impacts on the aspect of hydration, strength, durability as well as microstructural properties of concrete. Nano-silica which has been incorporated into concrete blends acts as nucleation sites to enhance the hydration reactions hence the enhancement of mechanical characteristics. This improvement causes improved particle packing and densification in the concrete structure and therefore a consistent and better enhancement of the resultant compressive strength and a reduction in permeability. In addition, the incorporation of nano-silica enhances the durability of concrete through improving the concrete resistance to chemical attacks and carbonation processes. Maximum mechanical and physical properties are usually found when the nano-silica content is comprised between 2% and 4%. Therefore, factors such as the surface area of the granules, the size of the particles and the water cement ratio in the concrete mixture may affect the effectiveness of the nano-silica as presented below. Based on some of the experiments it can be seen that nano-silica enhances the flow of hydration by which the work of battering was increased, and this is evidenced by the compressive strength result. In the same way, the use of nano-silica leads to the reduction of porosity and therefore increases durability. In an interfacial transition zone, the nano-silica particles penetration was also established to enhance the properties of the cement paste through microscopy findings. [1]

Figure 1.  Effect of consolidation material on filling of pores (a) Pure Portland cement, (b) cement with Nanomaterial, and (c) cement with gels of calcium-silicate-hydrate

It is important understanding how nano-SiO2 affects concrete and hence the need to evaluate the mechanical properties of concrete containing nano-SiO2. The parameters that were investigated in the study included durability, shrinkage, slump and the setting time. For examining the micro structural changes, scanning electron micrographs were made of the concrete samples. Getting the results disclosed that nano-SiO2 conferred increased shrinkage together with better durability, shorter setting time and least slump value compared to samples without the additive. This improvement can be attributed to the small size of the nano-SiO2 particles which have high surface area to increase water absorption within the mixture thereby giving less slump. Besides, nano SiO2 can also play a positive role for the formation of C-S-H gel which help to fill the voids and increases the degree of hydration in concrete matrix. [2]

This review paper focuses on the works done on the use of nano-silica, popularly referred to as silica nanoparticles in concrete. Other researchers have documented that the incorporation of nano-silica can improve the concrete’s microstructure and this makes it denser. It improves the cement-aggregate interfacial zone, since micro voids are filled, crystalline calcium hydroxide (CH) particles are consumed, while the calcium silicate hydrate (CSH) gel formed is increased. The study reviews the enhancement of mechanical properties; compressive, flexural, and tensile strengths, permeability to water, resistance to sulphate, and chloride permeability where nano-silica was added. [3]

Research done in this area reveals that these materials improve on the early strength of concrete thereby shortening construction time. More explicit, concrete mixtures with 30% and 70% GGBS were produced, and 1% and 2% nano-silica were incorporated thereon. Thus when 30% GGBS replaced cement, and 1% nano-silica added into this mixture the gain in compressive strength was 13%. 5%, 7. 8%, 8. 1%, and 2. 2% at one day, three days, seven days and twenty-eight days of starting the medication, respectively. In this regard, nano-silica addition of 2% produced a relatively smaller increase in the value of compressive strength in the 30% GGBS mixture by 4. 3%, 7. 6%, and 4. 9% at three days, seven days and twenty-eight days, respectively as compared to the 1% addition. On the other hand, the 1% and 2% nano silica led to reduction in effective compressive strength of the 70% GGBS mixes. Moreover, it was found that the addition of nano-silica has reduced the deformability of the mixtures, thus enhancing the Young’s modulus. As observed, the flexural strength of 30% GGBS mixtures had almost same trend with that of the 28 days compressive strength and the flexural strength of 70% GGBS mixtures increased with the increase in the percentage of nano-silica content. The results of RCPT test analysis and the SEM images of the mixtures also supported the findings regarding the improvement of microstructure and interfacial structure due to high pozzolanic activity and nano-fillers property of the nano-silica. [4]

On the same note, studies have been made aiming at establishing the effect of nano silica on the mechanical properties and the durability of mortar and concrete based on the micro structures of the samples. The results suggest that the use of nano silica related closely with the water content present in the mixture and, therefore, requires a higher volume of water to obtain appropriate workability. In addition, the findings showed that nano silica increases the value of the compressive strength and acts as a water-solids accelerating agent. A lot of research has been conducted in the area of enhancing the durability and mechanical properties of mortar and concrete and also the formulation of mathematical model for analysis. Recent studies focused on the application of nano silica in different fields; as a result, the number of methods used in the investigation of the material has multiplied rapidly.

[5]

Studies have shown that nano-silica has drawn a lot of attention in the area of concrete technology for more reasons, mainly owing to the fact that it has greatly influenced the strength of bonding between the cement and aggregate particles. Research shows that the addition of nano-silica in concrete at 2- 4 percent improves mechanical and physical properties that lead to concrete’s durability. This enhancement is even more noticeable when examination of the microstructural properties of concrete is considered. [6]

Figure 2. SEM microstructure for mortar mix with nano SiO2

Because the topic of discussion is so important hence, different studies have been conducted with an aim of analysing the impact of micro silica and nano silica in concrete. These studies’ intention was to perform trials that would evaluate properties of concrete, incorporating these materials, with th e goal to identify the best content of these materials for the improvement of concrete functions. The results showed that it is possible to enhance the properties of the concrete through the addition of nano-silica within the concentration of 1-2 wt.% Thus, the beneficial role of the discussed additives was proved. The research methodology mainly included a number of well-planned experiments where concrete samples were prepared with different proportions of nano silica and were put through a number of tests. Some of the factors include workability, setting time, and curing conditions were carefully controlled for the purpose of obtaining very good result. Compressive strength results of the concrete cubes tested for a given curing age as illustrated above showed that the concrete strength in the mixes containing silica additives were significantly higher than those containing plain concrete mix. This can be attributed to the smaller particle size of nano-silica which helps in filling the pores existing in the concrete matrix which in overall enhance the concrete density. Also, the pozzolanic activity which arises from the reaction between the silica and the calcium hydroxide found in the concrete also play an important part in contributing to the formation of extra C-S-H needed to bolster strength. [7]

Figure 3. The value of strength at 28 days with W/CM ratio with different MS and NS.

Another reason for using the nano-silica is that it can effectively absorb into the pores of the matrix of the concrete thus creating a more compact structure. This behaviour narrows the porosity and permeability, which are key determinant for increasing the resistance of concrete to the environmental conditions such a moisture and chemical attacks [8,9] The alkali silica reaction between nano silica and calcium hydroxide generated from cement hydration produces more C-S-H gel which is strength contributing phase in concrete [10,11] Research has pointed out that even with concentrations as low as 2% on the weight of cement, it is possible to achieve a remarkable improvement in the compressive strength, in fact, some studies have pointed out gains of about 70% when using 4% nano-silica. In addition, the impact and fatigue resistance of concrete is enhanced while tensile and flexural strength is also increased by the incorporation of nano-silica. The studies show the addition of nano-silica generally falls between 1 % to 3 % by weight of cement which enhances the gain in different mechanical properties [12,13].Another significant characteristic that improves the overall performance of the concrete which incorporates nano silica modified concrete is the bond strength at the interfacial transition zone (ITZ) between the coarse aggregate and cement paste [14] Aside from the aforementioned mechanical improvements, nano-silica is involved with chemical durability of concrete. The factual nature of the constructed microstructure that exists by the addition of nano-silica helps to minimize the leaching of calcium ions and hence increases its durability in an acidic environment [15].This characteristic is well pronounced in the cases where concrete is subjected to harsh chemicals or varying weather conditions. In addition, concrete containing nano-silica more effective on the sustainable development of concrete. Nano-silica enables a cut down in the amount of cement required in emulsions to a level that does not affect the quality thus giving a reduction in the carbon emissions from cement production [16].This is due to the increasing concern of sustainable construction and development and the search for sustainable materials in construction practices.

The behaviour of Nano Silica

Nano silica in concrete has become a forefront discovery in construction materials especially on improving the microstructure of concrete. The key features of nano silica the particle size which varies from 1 to 100 nanometres; is responsible for the improvement of the microstructure of concrete. The nano silica when incorporated into the concrete mix reacts with the cement particles through a pozzolanic reaction that not only increases the packing density of the concrete matrix but also increases the mechanical properties of the concrete. This interaction led to the formation of a denser microstructure which is crucial in enhancing the durability and service life of concrete structures. Specific steps that define the approach of incorporating nano silica in concrete include the following: The first step is to choose the right nano silica particles that are suitable for use in concrete, incorporation of the nano silica particles in the concrete mix design. The concentration of nano silica is decisive because its incorporation should enhance the performance of the concrete in a way that would not impugn the workability of the material. Usually, the nano silica is added to the cement mixture, or it can be incorporated in the mixture in a separate form during the mixing. It is imperative to achieve the proper distribution of the nano silica in the concrete matrix as the agglomeration of particles would overshadow its advantage. 

In addition, curing conditions and duration the nature of nano silica also depends on duration of curing which is considered critical in effecting the process of hydration and microstructure formation. It is generally accepted that the properties and characteristics of concrete can be assessed in terms of its microstructure and morphology where techniques such as scanning electron microscopy (SEM) and X-ray diffraction (XRD) need to be employed in order to determine the impact of nano silica. These methods enable the investigators to make observations of variations in the morphological and crystalline structure of the concrete at the nano-level. Incorporation of nano silica has been established to decrease the pore size and volume, thereby improving on the microstructure of the material. These changes in the nanoscale improve the mechanical properties of concrete including the compressive strength, porosity and permeability, which are essential in concrete structures depending on the exposure environment. In general, the application of nano silica into concrete can be regarded as an effective strategy to augment the development of construction material science with potential for concrete’s durability and sustainability.

Mechanical properties of Nano Silica

1. A few more definitions of Nano-Silica (NS):

   1. Filler and Activator: NS functions as an activator while using it as a filler in order to enhance the microstructure as well as make pozzolanic compounds within the concrete matrix.

   2. Nucleation Site: NS is useful in solving the issue of C-S-H (calcium silicate hydrate) crystals growing and serves as a nucleation site for the C-S-H crystals resulting in faster cement setting.

   3. Consumption of C3S: Following the application of the NS, the rate of consumption of C3S (tricalcium silicate) is enhanced, which helps in the development of smaller-sized portlandite (CH) crystals, as well as homogeneous structures of CSH (calcium silicate hydrate) composition.

   4. Interfacial Transition Zone Enhancement: NS improves the microstructure of the interfacial transition zone between aggregates and cement pastes.

2. Effects on Mechanical Properties:

   1. NS incorporation enhances compressive, split tensile, and flexural strengths of concrete.

   2. By reducing porosity, NS contributes to improved mechanical performance.

3. Durability Enhancement:

   1. Microstructural Characterization: NS modifies the microstructure, leading to reduced micropores and improved durability.

   2. Eradication of Structural Micropores: NS has been shown to decrease structural micropores, enhancing concrete’s resistance to environmental factors.

   3. Bond Strength, Creep, and Shrinkage: Rigorous evaluation of fresh and hardened properties is essential to understand NS’s thermal and acoustical characteristics.

4. Prospective Research:

Researchers continue to explore NS application in cement concrete, seeking further optimization and understanding of its behaviour.

Physical properties of Nano Silica [17]

• Diameter: Approximately 20 ± 5 nm 

• Surface to Volume Ratio: Approximately 220 ± 20 m²/g

• Density: Less than 0.20 g/cm³

Chemical properties of Nano Silica [17]

• SiO₂: 98.31%

• CaO: 0.392%

• Al₂O₃: 0.076%

• Na₂O: 0.328%

• Fe₂O₃: 0.293%

• SO₃: 0.185%

• Cl: 0.044%

• TiO₂: 0.064%

• K₂O: 0.08%

• MgO: 0.05%

• P₂O₅: 0.129%

• ZnO: 0.021% 

• CuO: 0.02%

• Total: 99.99%

Several studies have been conducted to establish the effect of several parameters such as surface area and amount of NS on the concrete. The results suggest that higher performance of NS particles is attributed to the large surface area of the tested particles. Notably, the sample having a surface area of 250 m²/g was found to be most preferable out of all the samples. Moreover, the optimum amount of NS is dependent on its surface area which is subjected to vary between 2 to 5% by weight of cement. It is found that the amount of NS being recommended increases as the particle size of NS and the w/b ratio increases. NS particles promote the formation of the gel due to the high surface area that they possess and exposes more water molecules. Impact of NS on the compressive strength is not only conspicuous at the age of 7 days but it is slightly higher as compared to the strength at an age of 28 days. SEM studies included found modifications in the qualities of the concrete microstructure due to the addition of NS. General when incorporating silica admixtures to concrete it is important to ensure that w/c ratio is greater than 0. 35. If too much water is used, it can weaken the strength of the concrete though if little water is used in the mixture it may lead to things like cracking and shrinkage. These results have shown that it is crucial to pay attention on proportionality of the components present in actual concrete compositions for the optimal result.

The nana silica content that should be included in concrete mix is another aspect that necessarily determines the performance and durability of the compound. The studies suggest that the addition of nano silica can help improve the mechanical characters of concrete and their resistance to the stresses of the environment, including compressive strength, tensile strength and the like. However, a more accurate proportion of the nano silica should be used since lesser or more may result in a poor outcome. Concentration generally has an optimal value that can be expected to fall within a narrow band, and this is determined by the concrete mix and its use. The use of nano silica in the concrete does not only enhance the concrete mechanical properties but also enhances the microstructure of the concrete. This enhancement is as a result of pozzolanic reaction which takes place between Nano silica and calcium hydroxide and leads to the formation of more C-S-H gel which are benefactors for strength as well as durability of concrete. Hence, the prime consideration is the careful measurement of the proportions of the nano silica since it influences the concrete’s hydration and the microstructure. The research has indicated that the most appropriate combination with the proper content of contents can reduce porosity while at the same time increasing the density of the concrete matrix. Moreover, the extent of incorporation of nano silica with concrete depends on the type of cement utilized, WCR and other forms of SCM. As a result, various experimental studies must be carried out to determine an optimum concentration of nano silica for particular concrete services. Hence, by managing this ratio engineers and materials scientists can design a concrete that must only satisfy the structural performance standards but should also possess better durability as well as resistance towards various environmental forces used in constructions which will help in increasing the sustainability of constructions.

Figure 4. Microstructure of Nano-Silica concrete at different scale. [17]

It commences with the constraint of normal concrete that include low tensile strength, vulnerability to cracks, ineffective ordeal towards severe weather conditions thus degrading the durability of concrete projects. Nano silica is marketed as a viable answer to enhance the properties of concrete wherein the particle size and greater surface area facilitate better water absorption and beneficial microstructure alteration. Performance is assessed based on the principles such as durability, shrinkage, slump and setting time of concrete through experimental approaches such as scanning electron micrography to reveal the microstructure of concrete samples. Consequently, it was established that the concrete with nano-SiO2 has enhanced durability, reduced working time, and lesser slump compared with the normal concrete. The study focuses on the influences of particle characteristics of cement paste on the concrete improvement without affecting the structural performance. The search shows the possibility of using nano silica for the further development of the concrete material and eliminating drawbacks of classical concrete materials, as well as suggesting the need for further studies and case studies of nano silica in cementitious systems to create new sustainable and energy-effective constructions.

The conclusion of this research emphasizes the significant benefits of incorporating nano silica into concrete mixtures. The research demonstrates that nano silica enhances the mechanical properties of concrete, leading to improved durability, reduced permeability, and increased compressive strength. These enhancements are attributed to the unique characteristics of nano silica, such as its small particle size and high surface area, which facilitate better particle packing and densification within the concrete matrix. The findings suggest that optimal incorporation levels of nano silica, typically between 1% and 4%, can yield the best results in terms of performance. Furthermore, the use of nano silica aligns with sustainability goals by potentially reducing the overall cement content required in concrete, thereby lowering carbon emissions associated with cement production. The research advocates for continued research and practical applications of nano silica in concrete technology, highlighting its potential to address the limitations of traditional concrete and contribute to the development of more resilient and environmentally friendly construction materials.

The recommendation in the study is aimed at the extension of the study of the impacts of nano silica on the concrete technology. Therefore, the further research is recommended to examine the impact of nano silica on the performance of concrete at long term both in different environmental condition and loading pattern. Also, it can used for other comparative study of other nano material on the properties of concrete. The research also advocates for the outfield guidelines for the use of nano silica in concrete mixtures to enhance a standard and credibility of results in the particular applications. In addition, it points to the need of developing a synergy between researchers, engineers and industries by integrating nano silica for construction practices. Such partnership could result to more effective solutions to the problems associated with conventional concrete thereby enhancing the formation of more resilient structures. Finally, it is recommendation that awareness and education of the benefits of using nano silica in concrete be created to the stakeholders in the construction industry, hence improving its usage.

The authors declare that they have no conflict of interest

No funding sources

The study was approved by the University of Thi-Qar, Civil Engineering Department, Thi-Qar, Iraq.

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