The Impact of Nano Clay on Normal and High-Performance Concrete Characteristics: A Review

The use of nano clay to improve the qualities of construction materials and engineering applications has attracted a lot of discussion in recent years. This review article summarizes the influence of nano clay as a cement substitute and supplement on the performance of conventional and high-performance concrete. The addition of nano clay to high performance concrete revealed an increase in compressive and flexural strength, as well as durability attributes such as resistance to elevated temperatures and sulfate attack, while simultaneously decreasing porosity, permeability, and water absorption. This enhancement is a result of nano clay’s roles as nano reinforcements, nanofillers, nucleation sites, and reactive pozzolans, which promote hydration and increase material characteristics.


Introduction
Nanotechnology has emerged as a viable alternative for significantly increasing the strength and durability of construction materials over the last few decades. The phrase "nanotechnology" refers to the process of designing, fabricating, and utilizing functional structures with at least one characteristic dimension measured in nanometers. Due to the small size of its constituent particles or molecules, these materials and systems can be created to display innovative and physical, chemical, and biological characteristics have improved significantly. The explanation for this unique and extremely valuable behavior is that when structural features are between 10-9 m and 10-7 m (1 to 100 nm) in size, objects can exhibit physical properties that are markedly different from atoms or bulk materials. This can lead to new technical challenges and opportunities [1]. Clay minerals, which typically have a one-dimensional layer structure with a thickness of 0.7nm for 1:1 and 1nm for 2:1 ratio, exhibit a variety of surface shapes, including planar and interlayer. They are easily connected to internal and external surfaces and can be improved via ion exchange, adsorption, and grafting procedures. They

Production Methods of Nanomaterial
Methods for manufacturing nano materials can usually be classified into top-down and bottom-up methods, as presented in Fig. 1. A top-down method is an easy method to fabricate sculpture from large marble block. Whereas, the bottom-up techniques function in the opposite direction: starting from the atomic or molecular precursors, the nano material, such as a nano-coating, is obtained and gradually assembled until the desired structure is formed [22]. The classification of top-down methods is shown in Table 1.   [24].

Mechanical Energy Methods
Mechanical energy methods are utilizing for ease, less expensive and less time-consuming fabrication of functional structural in bulk quantity. For such category, ball milling method is the simplest production method. Further, ball milling method uses kinetic energy of medium grinding to produces nanomaterials. The other mechanical energy methods are compaction and consolidation in which nano materials are "put back together", and improved their properties. It is possible to make metallic alloys this way. Industry uses many top-down mechanical methods [25].

Ball Milling
Ball milling is a technique for producing ultrafine powders and nanostructured powders down to the submicron range, and it's widely employed across a wide range of industries. J. Benjamin invented the ball mill in 1966. The ability to create nano-crystalline powders and obtain huge quantities of

Comments Top-down fabrication methods No
Mechanical methods employ a physical process that does not involve chemical change (a reaction).

Mechanical Energy 1
The thermal method employs a physical process (heating) that does not initiate chemical change (a reaction).

Thermal Fabrication Methods 2
The top-down production of nanomaterials is frequently accomplished using high-energy sources such as arcs, solar flux, and plasmas. These are not thermal procedures due to the fact that the source of heat is not traditional.

High Energy Fabrication Methods 3
The methods that use chemical transformation during a production process.

Chemical Fabrication Methods 4
For numerous decades, lithography has been used to create printed circuit boards and computer boards.

Lithographic Fabrication Methods 5
Most of these natural processes are quite familiar, and there is no need to allocate any more time or space.  [26]. Ball milling is a critical mechanical top-down process, a technique that uses mechanical attrition to manufacture nanoscale materials. Ball milling is a process in which the kinetic energy of a grinding medium (e.g., stainless steel ball bearings, zirconia, or porcelain) is immediately transmitted to coarse-grained metal, ceramic, or polymeric sample materials with the goal of reducing their size.

Types of Nano clays
Nano clay is nanoparticles, composed of layered mineral silicates, and according to their chemical compositions and morphologies, they are categorized into numerous categories such as bentonite, montmorillonite, kaolinite, halloysite, and hectorite. Table 2 and Figure 2 show the classification types of Nano clays and a schematic diagram of the structures. In this study, Nano clay is a kaolinite type. Kaolin or china clay is the name given to kaolinite-rich rocks [27].
It is a clay mineral that belongs to the industrial minerals group in which a layered silicate mineral along with one tetrahedral sheet is connected to octahedral sheet through oxygen atoms [29,30]. Table 2: Classification types of Nano clays [31].

Family Group Formula
Phyllosilicates TO There are six-membered silicate rings in the kaolinite structure, which are linked to an octahedron of edge-sharing AlO6 octahedral molecules that form four-member aluminate rings by shared oxygen atoms at the C-axis. The apical oxygens of the silicate and aluminate layers generate strong ioncovalent contacts that hold the other layers together, although the hydrogen bonds of the other layers are weaker [32]. It is found in the white mineral kaolinite, which is also known as dioctahedral phyllosilicate clay. It is composed of clay that is created as a result of the chemical weathering of aluminum silicate minerals such as feldspar [29,33]. In mineralogy, the chemical formula for kaolinite is Al 2 Si 2 O 5 (OH) 4 .

.Properties of Nano clay 4.1. Physical Properties
Nano clay (NC) is derivative of kaolin (Al 2 Si 2 O 5 (OH) 4 ), which changes from crystal to amorphous after treatment or endothermic dehydration [34]. Subsequent to dihydroxylation at temperatures between 600°C and 800°C, the large portion of octahedral alumina is changed into penta and more active tetra coordinated units [35]. Additionally, the high amorphous metakaolin (MK) and disordered phase variation are formed when the bonds between kaolinite layers are damaged, or the crystal structure is fully and/or partially damaged [36]. Finally, mechanical grinding is used to obtain NC. The equation (1) can be used to represent the exothermic dihydroxylation reaction at 500 ℃~925 ℃ [37]: Metakaolin is a highly reactive pozzolan that has a similar reaction to silica fume. Compared to that silica fume, metakaolin has a refined microstructure which caused a better substantial strength and durability, permitting better water penetration resistance, and being cost-effective [38]. Table 3 illustrates that the surface area increases from 2 to 8 times when there is a change in clay size from micron to nanometer.

Chemical Properties
The chemical compositions of NC are presented in Table 4. NC has the highest content of SiO 2 (45.5 %to 89.6%), followed by Al 2 O 3 (18.9 % to 42.3 %), both of which are important in improving cement hydration rate and setting time, while NC has a low content of CaO.

Hydration
According to some studies, NC enhanced the cement hydration reaction [47]. In the cement hydration process, Shoukry et al. [48] observed that as the NC replacement rate increased, CH content and X-ray diffraction peak intensity decreased marginally, while the C-S-H peak value increased. Moreover, about 6.7% reduction in CH content of NC cement paste has been observed as compared to CH content of ordinary cement paste [49]. However, beyond 10% replacement, there was no considerable variation in CH content of NC cement paste [50]. The reduction of 30.23J/g to 14.17J/g for CH enthalpy was observed when the NC replacement increased from 0% to 8%, and led the phase transition from good crystal to non-crystal. Moreover, the enthalpies of C-S-H, C 3 ASH 6 and C 2 ASH 8 were raised from 45.81 J/g, 0.85 J/g and 0.289 J/g to 46.91 J/g, 0.881 J/g, and 1.68 J/g respectively [41]. It was also noted a considerable enhancement in the crystalline structural forms [51]. The following points are the main aspects of the NC mechanism combination for accelerated cement hydration [52]: 1-The fine particles of NC induce nucleation, consume free CH, and generate needle-like secondary hydration products, significantly reducing the induction period's hydration. 2-When NC is added to cement particles, H2SiO4 is generated and combines with the available Ca2+ to make more C-S-H. These C-S-H molecules spread in the water between the cement particles and serve as seeds for the formation of denser C-S-H phases. 3-Unlike pure C3S, the production of the C-S-H phase does not occur exclusively on the particle surface, but also occurs in the pore region.

Workability
Aiswarya et al. [53] discovered that inducing more NC production Cement paste consistency could be improved by adding more water to the mixture. Cement paste's initial and final setting times have also been extended when the NC content was increased from 2% to 20%, and then dropped, reaching their maximum values at 6% and 4%, respectively. Similarly, in the Ultra-High-Performance Concrete (UHPC) test, a comparable result was observed [18]. In terms of fluidity, cementitious materials demonstrated a decreasing tendency when the nonlinear increase in NC content occurred. When 9% of NC was added to concrete, slump decreased by roughly 15.7 percent and workability of UHPC decreased from 178mm to 150mm [40], which was consistent with the results found by Senff et al [54]. Additionally, Alani et al. [19] and Alobaidi et al. [20] concluded that replacing cement with nano clay decreased the workability qualities of SCC. This is largely due to the fact that the surface area of NC is so large, is easily formed into a flocculent network structure, has a low dispensability [55], and absorbs more free water during wetting [56]. This effect becomes more pronounced as the amount of NC in the solution increases. As a result, the range of NC content should be regulated to ensure adequate workability.

Compressive Strength
It has been observed through numerous studies that the presence of NC caused an improvement in the compressive strength of the cementitious materials [55,56]. Table 5 displays the impact of the addition of NC on compressive strengths of cementitious materials at 28 days. , and its dispersion defect causes to produce fewer points of contact with the matrix and produce a negative effect on bonding with cement particles [72,73]. Although the other mixing information such as chemical and mineral admixtures, curing conditions, and w/b ratio are almost similar, but the best contents of NC has a significant effect on compressive strengths. As a result, more studies should be conducted and discussed better to apprehend the various effects of NC on compressive strengths [74]. Table 6 illustrate that the addition of NC can significantly increase the flexural and splitting tensile strengths of cementitious materials.

The impact of filling.
Nanometer-sized of NC has a substantial impact on the filling capacity of cement ingredients. Although the NC particles are 1000 times smaller than the size of cement particles (micrometer), but still NC could fill the microstructure gaps, voids and could diminish the ITZ defects [75,76]. The SEM image as shown in Figure 3 shows that the incorporation of NC fills the voids and pores and resulting a more compact structure, also opposed to the weak microstructure of control mortar [48], which corresponds with the other studies [46,66].

The effect of nucleation.
Figure 4 depicts that the NC helps clinker hydration by the available surface area to sedimentation hydrates from the interstitial solution [77]. The high surface energy of NC provides the thermodynamic drive for this process. Rather than critical nucleation, NC behaves as a source to initiate precipitation [41], and the hydrate phases start as the ionic concentration reaches saturation. Meanwhile, NC enables continuous growth of the C-S-H phase, prefabricate nucleus for the hydration process, aggregation of C-S-H in the network structure. It could also promote hydration reaction of cement [72], and develop uniform and dense microstructure [78]. The nucleus of a crystal is beneficial for increasing the early strength of cement, but the nucleation effect is lost when the cement reaction completes [47].

The influence of bridging.
The inclusion of NC particles works as a fiber between cement products, creates bridge between matrix and reinforcement, and provides resistance to cracks and crack propagation as shown in SEM images Figure 5 Table 7 summarises the effect of NC on the coefficient of capillary water absorption of the cementitious materials. In general, as the NC content increases, the water absorption coefficient decreases [79]. Also, the geometry and size of the pore system should be directly linked to the mechanism of capillary water absorption [80]. The improved pore structure can be attributed mostly to the additional C-S-H gel produced by NC's pozzolanic reaction and to its own microaggregate impact, which fills the micropores between cement particles [48,49,81] and reduces the water absorption of cement paste [45].  [82]. The results illustrated that when the temperature was below 300 ℃, NC concrete increased strength; when the temperature was between 440 and 580 ℃, strength was significantly reduced; and when the temperature exceeded1000 ℃, the strength was less than 10% of its original strength.

Water Absorption
The excess hydration of unhydrated cement particles results steam effect due to internal autoclaving, and the pozzolanic reaction of NC resulted extra C-S-H gel with smaller Ca/Si ratio. The additional hydration products help to bridge the pore system, reduce the thermal stresses around the pores. Moreover, the calcium aluminates hydrate (C-A-H) phases can increase the compressive strength of blended cement mortars up to 200℃~250℃ [83]. Furthermore, the incorporation of NC in cementitious products would be made the cement matrix had greater inclusions of gel-like hydration products which could make it more fire resistant. However, the excess hydration can cause a loss in strength of the cementitious products. It is mainly due to that CH decomposes into CaO and H 2 O at 500℃, and CaCO 3 decomposes with the effusion above 600℃,, and caused a reduction in free water, and decreased strength and elasticity modulus [84]. Furthermore, the temperature gradient causes high thermal tension, which leads to an increase in micro cracks and a weakening of structure's compactness [85].

Resistance to Sulfate Attack
The porosity and pore size distribution of concrete significantly impacts its durability. Moreover, such factors influenced the transport properties of the main harmful agents such as gases, water, and ions. These transport properties can be deteriorated concrete by physical and/or chemical attack [86]. The presence of sulfate ions in seawater, groundwater, and soil can decline reinforced concrete structures by means of expansion, spalling, cracking, swelling of concrete elements. These characteristics can be caused by a variety of factors such as the type of cement, type of sulfate cation, sulfate concentration, and exposure period. Many sulfate-damaged structures need rehabilitation or, in the most violent cases, reconstruction [87]. Sulfate ions can react with sources of alumina (hydration products such as mono sulfa aluminate (AFm) and unhydrated tricalcium aluminate (C 3 A)) to produce ettringite. Also, the produces gypsum with the help of calcium (calcium hydroxide (CH)) or C-S-H gel). However, such production resulting in loss of substantial strength and crack initiation and propagation. It is mainly because of the driving force of crystallisation pressure impacts on the pore walls, and such driving force results crystal growth of gypsum and ettringite [88]. Tian  formation of ettringite in tiny pores (10-50 nm) resulted tensile stress of 8 MPa, and caused crack expansion which led to failure of the cementitious structure. If the attack solution is magnesium sulfate (MgSO4), all cement minerals, including C-S-H, are targeted. The stability of the C-S-H gel is decreased as a result of the development of the relatively insoluble and low-alkaline brucite (Mg(OH)2) in interaction with dissolved CH. Additionally, MgSO4 reacts immediately with calcium silicate hydrate, transforming it into gypsum and a weak magnesium silicate hydrate (M-S-H). Due to the presence of a thick brucite coating on the surface, surface damage is unnoticeable until late in the attack, whereas strength loss begins early [91]. Equations (2) and (3) can illustrate the mechanics of the magnesium sulfate attack . Many researchers have shown in recent years that using nanomaterials may greatly increase the sulfate resistance of cement-based materials, with this effect becoming more noticeable at earlier ages [92,93]. The great pozzolanic reactivity that caused the reduction of CH, as well as hardened cement-based materials were studied for its influence on the microstructure [94]. Cement mortar and concrete's susceptibility to sulfate attack has only been studied by a handful of scientists. The microstructure of cement mortar is strengthened by the addition of nano clay as a filler. It also serves as a mortar activator, encouraging the pozzolanic reaction. Hakamy et al. [95] investigated the effects of Nano clay on cement composite properties. They reported a decrease in porosity and water absorption in cement nanocomposites containing 1 w.t% Nano clay. They also discovered that cement nanocomposites had higher in density, strength, fracture toughness, and hardness than the control mix. Hosseini et al. [96] studied the effects of adding 0.25, 0.50, 0.75, and 1% Nano clay to SCC mix. The water penetration depth was lowered by 17, 27, 39, and 43 percent, respectively. Water penetration into the SCC mix is reduced, making it more resistant to sulfate attacks.

Conclusions
Previous research has demonstrated that using nano clay as a cement substitute and addition in concrete has the potential to increase the strength, microstructure, hydration and durability performance of conventional and high performance concrete. nano clay is a hydrophilic material that swells and absorbs water when used in cement, this behavior can be minimized by altering the nano clay structure. All the qualities of nano clay, such as catalyst, nano fillers, highly reactive pozzolanas and nano reinforcements, counterbalance and nucleation sites influence the performance of high performance concrete. Concrete's mechanical characteristics and microstructure performance can be enhanced by nano clay, however the concrete's water permeability can be decreased as a result of this enhancement. Accordingly, the use of nano clay in the high performance concrete has a potential in order to develop the strength property which is also enhance the durability performance such as permeability, chemical resistance and water absorption of concrete. On the other hand, nano clay improve the cohesion and homogeneous of microstructure of concrete. However, the segregation and bleeding characteristics of concrete reduced by nano clay incorporation in the concrete mixtures.