Microstructural characteristics and nano-modification of interfacial transition zone in concrete: A review

Zhong Xu; ZhiJie Bai; JiaNing Wu; HongYuan Long; Hui Deng; ZanZhu Chen; Yuan Yuan; Xiaoqing Fan

doi:10.1515/ntrev-2022-0125

Open Access Published by De Gruyter June 1, 2022

Microstructural characteristics and nano-modification of interfacial transition zone in concrete: A review

Zhong Xu , ZhiJie Bai , JiaNing Wu , HongYuan Long , Hui Deng , ZanZhu Chen , Yuan Yuan and Xiaoqing Fan

From the journal Nanotechnology Reviews

https://doi.org/10.1515/ntrev-2022-0125

Abstract

The interfacial transition zone (ITZ) has long been considered as a zone of weakness in concrete. Many scholars have carried out relatively scattered tentative explorations to improve the performance of ITZ. The application of nanomaterials to enhance ITZ in concrete is a promising research. In order to further study the basic principles and practical applications of this field, it is urgent to systematically sort out the existing results. In this study, the nano-modification of ITZ in concrete is systematically reviewed and summarized. First, this study reviews the microscopic characterization of ITZ, including hydration products, porosity, and microhardness. Then, the influence of nanomaterials on ITZ is reviewed from the view of the above three aspects. Finally, the method and mechanism of the ITZ modified by nanomaterials were preliminarily clarified, which provided theoretical and empirical data support for the team’s next experimental work. A large number of research results show that nanomaterials improve the mechanical properties and microstructure of the ITZ, thus enhancing the mechanical properties and durability of concrete. The results of this article will provide source materials for the development of this field, a broader research basis for peer research, and a basis for further development of green engineering materials.

Keywords: interfacial transition zone; nanomaterials; porosity; microhardness

1 Introduction

Since the industrial revolution, the earth, the common home of mankind, has gradually been facing progressively serious environmental crises, such as global warming, extreme climate, and sea level rising [1,2]. In recent years, human beings’ awareness and attention to environmental issues have been unprecedentedly high and people from different regions, industries, and positions are striving to participate in the defense of our earth. Gradually realizing the wider use of green and environmental engineering materials is one of the important contributions to environmental protection in the field of engineering technology. Many researchers take this as a hot spot and continue to carry out theoretical and application exploration [3,4,5].

Concrete is one of the most frequently used engineering materials, which has the advantages of high strength, low price, strong plasticity, and convenient constructions [6]. However, concrete has the disadvantages of occupying non-renewable resources and producing environmental pollutants during production and use. How to improve or replace it has become a research hotspot in this field in recent years. Existing research shows that researchers hope to fully turn waste into treasure, to make full use of cheap mineral materials and industrial waste materials. Based on this concept, there are two main ways to realize the green and sustainable development of concrete materials: one is to modify or replace the cementitious materials, such as geopolymers, and the other is to replace the aggregates, such as using building waste blocks as recycled aggregates. The research work on cementitious materials and aggregates is an academic hotspot. Many scholars have carried out tests and analyses of macroscopic engineering properties to explore the effects of green cementitious materials [7,8,9,10] or aggregates [11,12,13] on concrete engineering performance. The bond strength between cement matrix and aggregate and the mechanism of ITZ are considered to have a great impact on engineering performance, which is the key to concrete research and development [14,15].

ITZ is defined as an area between coarse aggregate and cement mortars in concrete while between fine aggregate and cement matrix in mortar. The ITZ of natural aggregate concrete (NAC) is relatively simple. If considering recycled aggregate concrete (RAC), ITZ can be divided into the following three types: ITZ₁ between virgin aggregate and old attached mortar, ITZ₂ between attached old mortar and new mortar, ITZ₃ between virgin aggregate and new mortar, as shown in Figure 1 [16]. Over the past few years, many scholars hope to explore the mechanism of ITZ affecting the mechanical properties of concrete through engineering performance testing and microscopic performance analysis and have obtained many tentative explorations. Scrivener and Gartner found that the average porosity of ITZ is 30% higher than that of the cement matrix. Although the average thickness of ITZ is only tens of microns, its volume ratio in concrete can reach 20–30%, which is very important for concrete strength and durability [17]. Mitsui et al. compared the mechanical properties of mortars with different thicknesses of ITZ and found that the compressive and tensile strengths of mortars with thinner ITZ were higher [18]. Through model experiments and numerical simulations, Xiao et al. found that ITZ in recycled concrete had a significant effect on the strength of concrete. When the relative strength of ITZ and cement matrix decreased from 1.0 to 0.5, the compressive strength of concrete decreased by 9.8 MPa/24.0%, and the tensile strength decreased by 0.5 MPa/35.0% [19]. Sicat et al. found that the deformation of ITZ under freeze–thaw cycles was much larger than that of the matrix and aggregate, which indicated that ITZ had a heavy impact on the frost resistance of concrete [20]. Huang et al. found that ITZ has an important influence on thermal fatigue properties of concrete [21]. The research of Erdem et al. showed that the mechanical properties of ITZ have a significant impact on the dynamic response of concrete stronger and denser ITZ provides stronger impact toughness [22]. It can be seen from above that ITZ has an important impact on the performance of concrete.

Figure 1

ITZ in concrete (a) ITZ s of RAC, (b) ITZ of NAC [16].

Nanomaterials have been proved to play an obvious role in improving the performance of concrete, which is the key to the development of high-performance and green concrete. In terms of improving the mechanical properties of concrete, Han et al. found that adding 3% nano-ZrO₂ (NZ) can increase the compressive and flexural strength of concrete by 14.4 MPa/14.5% and 4.2 MPa/36.6%, respectively [23]. Kumar et al. found that adding 3% nano-SiO₂ (NS) can significantly improve the mechanical properties and durability of concrete because the pozzolanic effect and filling effect of NS make the cement matrix denser [24]. Wu et al. found that 0.03% graphene oxide (GO) can significantly improve the mechanical properties of concrete; the maximum compressive, flexural strength, and splitting resistance can be increased by 34.08, 15.06, and 24.81% [25]. On the other hand, nanomaterials can also improve the durability of concrete. For example, Said et al. found that NS optimized the pore structure of concrete, reduced the penetration depth of chloride ions, and improved the durability [26]. Liu et al. found that the close spacing and high specific surface area of GO effectively prevent the penetration of water and aggressive chemicals into concrete and control the growth of microcracks under aggressive exposure [27,28,29]. There are many studies on the engineering performance of nano-modified concrete, and many achievements have been made, but scholars mainly focus on studies with the cement matrix, with the ITZ being less. Considering that ITZ often has a controlling effect on the engineering performance of concrete, this research needs to be further strengthened [30].

Environmental protection and sustainable development are highly valued in China. By optimizing the industrial structure and energy structure, China’s State Council formulated a plan to achieve peak carbon emissions by 2030 and carbon neutrality by 2060 [31]. The above national policies had become a new driving force for the development of green engineering materials, prompting the author to try to consider the research on the ITZ of concrete modified by nanomaterials. In recent years, the research in this field is relatively scattered and lacks systematic understanding and review. In order to better understand the existing research basis and working methods, it is necessary to effectively sort out and review the existing research.

In order to strengthen the research on the mechanism of nano-modified ITZ, so that researchers can have a more systematic and in-depth understanding of the structural characteristics, performance characteristics, test methods, and modification techniques of ITZ, this article focuses on the nano-modification of ITZ, which reviews the basic characteristics microstructural of ITZ, the influencing factors of ITZ, and the modification of ITZ by nanomaterials. It tries to sort out the origin, development, status quo, and hotspots of further research in various aspects, form an organic analysis and systematic understanding, and then provide a theoretical and literature basis for researchers in the same field. It not only provides a new perspective for nanomaterials modified concrete but also contributes to the scientific research and exploration of green building materials.

2 Microscopic characteristics of ITZ structures

The formation of ITZ is due to the wall effect [32] caused by the addition of aggregate and the micro-bleeding effect [33] caused by gravity, as shown in Figure 2. The addition of aggregate breaks the accumulation form of cement particles so that the local water-to-cement (w/c) ratio around the aggregate is greater than that of the cement matrix. Because of gravity, water molecules accumulate at the bottom of the aggregate. Thus, the microstructure of ITZ is different from that of the cement matrix.

Figure 2

Illustration of the “wall effect” and micro-bleeding effect.

2.1 Composition and distribution of hydration products of ITZ

Hydration products of ITZ consist mainly of Ca(OH)₂ (CH), ettringite, calcium silicate hydrate (C–S–H) gel, and unhydrated cement particles, as shown in Figure 3 [34]. It was formed by the diffusion and mutual reaction of the ions dissolved from the cement slurry to the liquid film on the aggregate surface [35]. The ions (Ca²⁺, OH^–, Al³⁺, and SO₄ ^2–) are easier to migrate than silicate ions, resulting in the enrichment of CH and ettringite. Due to the small size of the ettringite crystal, it is not easy to observe, so there are few studies on it. The enrichment of CH is an important feature of ITZ. Moreover, the low ion concentration in ITZ makes the CH crystal particles larger than the matrix. Kong et al. found that a large number of CH crystals were enriched in ITZ using scanning electron microscopy (SEM) [36], which can also be confirmed by measuring the average atomic ratio of hydration products using energy-dispersive spectroscopy (EDS). The molar ratio of Ca/Si of the main hydration products of cement is C–S–H (0.8 ≤ Ca/Si ≤ 2.5), CH: (Ca/Si ≥ 10); Wu et al. confirmed that the Ca/Si of ITZ is greater than that of the cement matrix and gradually decreases with the increase in distance [37]. CH crystals grow in orientation in ITZ. Some scholars analyzed the orientation of CH using X-ray diffraction (XRD) [38], and the others observed that CH crystals grow perpendicular to the aggregate surface using SEM [39].

Figure 3

SEM image of ITZ in concrete. (a) 2000×; (b) 8000×; (c) 30000 × [34].

According to the literature [40], the distribution of hydration products of ITZ can be statistically analyzed by a backscattered electron microscope (BSEM). Existing studies show that the content of CH gradually decreases with the increase in the aggregate distance. Mukharjee and Barai found that with the distance increase from the aggregate, the CH content in ITZ gradually decreases and the average value is greater than that of the cement matrix [41]. The cement particles in ITZ exhibit an ascending gradient distribution, so the content of C–S–H and unhydrated cement particles increase with the distance to aggregate increase.

2.2 Porosity of ITZ

Porosity is an important indicator to characterize the performance of ITZ. Existing research shows that the porosity of ITZ is higher than that of the cement matrix, and the porosity gradually decreases with the distance of aggregate. The porosity of ITZ can be determined by the following three methods. The first is the quantitative analysis by sectional plane of concrete, as shown in Figure 4. After grayscale conversion by the BSEM image analysis method, black represents pores, and then, the porosity is calculated by those strips [39]. The second is to obtain the pore size distribution of the whole concrete through mercury intrusion porosimetry (MIP). It is assumed that the pores of large size and not observed in the slurry sample are generated by the appearance of ITZ, which can be used as an indirect method to study the porosity of ITZ [42]. The third is the qualitative method to judge the micro morphology of ITZ through SEM images. A more compact microstructure means the reduction of porosity [43]. Most researchers used the first method; for example, Scrivener et al. found that the closer to the aggregate surface, the greater the porosity, and the maximum porosity is three times that of the matrix [44]. Sun et al. found a sixfold higher porosity in inner ITZ (5 mm from the aggregate surface) than that in outer ITZ (50 mm from the aggregate surface) [45].

Figure 4

Example of the BSE–SEM image analysis procedure: (a) the BSE-SEM image, (b) pore segmentation, and (c) strip delineation [39].

In recent years, researchers had also developed other methods to determine the porosity of ITZ, such as computer tomography (CT) technology. The scanning process itself will not introduce any change or damage, to maintain the integrity of the interface. Cui et al. used dual CT scanning to study ITZ for the first time; the results showed that the average porosity of ITZ was 1.36 times that of cement matrix [46]. Kim et al. and Chung et al. used three-dimensional micro-CT to study the actual microstructure of ITZ and obtained the three-dimensional microstructure of ITZ pore distribution. As shown in Figure 5, the gray part is aggregate, and the porosity of ITZ is greater than that of the matrix [47,48].

Figure 5

3D microstructure of ITZ (quartz and limestone as aggregates, w/c ratios of 0.4 and 0.6, respectively) [48].

The porosity of ITZ is affected by many factors, such as the w/c ratio, hydration age, and fine mineral mixtures. The local w/c ratio near the aggregate change with the change of the overall w/c ratio, thereby changing the porosity of ITZ. Jia et al. found that the decrease in w/c ratio will reduce the porosity of ITZ and increase the unhydrated cement particles of ITZ [49]. Xie et al. found that when the w/c ratio increased from 0.4 to 0.5, the maximum porosity of ITZ increased from 27 to 43% [50]. Hydration age will also affect the porosity of ITZ. Gao et al. found that with the increase in curing age, the porosity of ITZ decreases, and the compactness of hydration products increases [51]. In the early stage of the hydration reaction, due to the larger w/c ratio near the aggregates, the hydration products near the aggregates consist of relatively large crystal accumulations, so the porosity is high. As the hydration reaction progresses, more mobile ions, such as Ca²⁺, move from the cement matrix to crystallize in the pores of the ITZ, and the porosity of the ITZ gradually decreases. Superfine mineral admixture can reduce the boundary effect by pozzolanic effect and filling effect [42]. Gao et al. and Dobiszewska et al. found through BSEM that slag [52] and basalt powder [53] significantly reduce the porosity of ITZ.

In addition, the size, type, and saturation of aggregate will also affect the porosity of ITZ. According to the micro-bleeding effect, the local w/c ratio near the surface of larger aggregate is high, so the porosity of ITZ is relatively high. Elsharief et al. found that the porosity of ITZ decreased after the size of the aggregate decreased from 4.75 to 0.3 mm [54]. The micro-bleeding effect will increase the w/c ratio under the aggregate, especially the coarse aggregate, so the porosity of ITZ around the same aggregate is uneven. The chemical properties and surface structure of different types of aggregates are different, which will lead to the change in ITZ porosity. Kong and Du believed that the aggregates of quartzite, granite, limestone, and basalt react with the cement slurry to affect the hydration of ITZ cement particles, which may increase or reduce the porosity of ITZ [55]. Lyu et al. found that the porosity of ITZ of smooth aggregate is less than that of rough aggregate on the surface because the stacking of cement particles on the surface of rough aggregate is not as good as that on a smooth surface [56]. The high porosity under the aggregate was also found by Lyu. Wu et al. studied the porosity of ITZ of smooth river sand, rough steel slag, and recycled fine aggregate under the same mix proportion. The results show that the porosity of ITZ in turn is recycled aggregate ITZ > steel slag ITZ > river sand ITZ [57]. Aggregate water absorption will affect the local w/c ratio near its surface, thus affecting the porosity of ITZ. For example, lightweight aggregate absorbs water from the surrounding paste, leading to a reduction in the porosity of the ITZ [58]. Le et al. found that the porosity of ITZ in RAC is affected by the saturation degree of aggregate [59]. The dry aggregate will absorb water on the aggregate surface, resulting in low ITZ porosity. In order to avoid the reduction of a concrete slump, recycled aggregate is usually used under saturated surface-dried. The redundant absorbed water will be released to disrupt the w/c ratio of ITZ, resulting in higher porosity of ITZ than natural aggregate [60,61]. Moreover, Tam et al., Li et al. and Zhang et al. found that for recycled aggregate, the advanced mixing method will also make the ITZ microstructure compact and reduce its porosity [62,63,64].

To sum up, the porosity of ITZ is mainly affected by w/c ratio, curing age, and aggregate. The influence of aggregate on the porosity of ITZ is more complex and changeable, while the influence of w/c ratio and age on the porosity of ITZ is simple.

2.3 Microhardness of ITZ

The microhardness of ITZ can be tested using Vickers hardness tester and nanoindentation, while the indentation modulus and elastic modulus can also be obtained by nanoindentation [65]. The microhardness of ITZ is related to hydration products and porosity, which reflects the elastic–plastic deformation characteristics of materials. It is an important mechanical property index and one of the important characterization parameters of ITZ properties. The existing research results show that the microhardness and elastic modulus of ITZ are lower than those of aggregate and cement matrices. Mondal et al. studied the micromechanical properties of ITZ in concrete through nanoindentation experiments, and the results showed that the average elastic modulus of ITZ was 70–85% of the matrix [66]. Xiao et al. plotted the indentation modulus of ITZ based on nano-indentation test (Figure 6). It was found the average indentation modulus of new ITZ was about 80–90% of the modulus of new cement mortar, while the indentation modulus of old ITZ was about 70–80% of that of old cement mortar [67]. In addition, Otsuki et al. and Du et al. found that the Vickers hardness of ITZ gradually increased with increasing distance from the aggregate and remained unchanged after 100 µm [68,69]. Lee and Choi tested the Vickers hardness of ITZ in RAC. The results showed that the average hardness of old ITZ was 3.35 (average hardness of old mortar, 20), and the average hardness of new ITZ was 5.56 (average hardness of new mortar, 30) [70]. Wang et al. found that the Vickers hardness of substances in RAC from low to high is old ITZ < new ITZ < old cement matrix < new cement matrix < aggregate [71]. The reason for the lowest hardness of old ITZ is the deterioration of old ITZ caused by the recycled aggregate manufacturing process.

Figure 6

Elastic modulus distribution of old and new ITZ in RAC [67]. (a) Old ITZ modulus contour map (GPa), (b) new ITZ modulus contour map (GPa), (c) properties and thickness of old ITZ, and (d) properties and thickness of old ITZ.

The microhardness of ITZ is also affected by w/c ratio, aggregate, and so on. Hussin and Pooleb studied the effect of different types of aggregates on the microhardness of ITZ [72]. It was found that the microhardness of granite ITZ is higher than that of limestone ITZ. Sidorovato et al. found by nanoindentation test that the elastic modulus of ITZ gradually decreased when the w/c ratio increased [73]. Nežerka et al. found that a chemical reaction occurred between the broken bricks and the lime mortar, resulting in a higher elastic modulus of ITZ than that of the matrix [74]. Zhao et al. believed that the surface pre-wetting treatment of recycled aggregates would significantly reduce the strength of ITZ₃ and increase the strength of ITZ₂, while the effect on ITZ₁ was small [75]. Bosque et al. studied the ITZ mechanical properties of recycled concrete aggregates (including impurities such as glass, asphalt, plastic, and wood) using nanoindentation technology. It was found that the ITZ of impurities showed a lower elastic modulus, and the ITZ elastic modulus of organic aggregates and smooth surface aggregates was relatively low [76]. In addition, Li et al. found through nanoindentation experiments that the indentation modulus of ITZ in RAC was related to the mixing procedure, and the two-stage mixing method could significantly improve the indentation modulus of the new ITZ [77].

2.4 Thickness of ITZ

The thickness of ITZ is not strictly defined in the existing research, because ITZ has high instability and complexity [78]. Leemann et al. defined the scale of the area whose porosity is higher than 15% of the matrix from the aggregate surface as the thickness of ITZ [79]. While most researchers only gave a general concept: when the parameter (like, porosity, microhardness, CH content, and Ca/Si molar ratio along the aggregate surface) characteristic curve tends to be flat, it means that it has been over to the matrix. For example, based on the variation of porosity, Zhang et al. found that the thickness of ITZ in lightweight aggregates (LWA) concrete was obtained at about 50, 40, and 30 µm, respectively, at different hydration ages, as shown in Figure 7 [39]. It was previously found that the ITZ thickness would decrease with a higher absorption capacity of lightweight aggregate since less water can accumulate in the vicinity of the aggregate particle. However, in Figure 7, the porosity (10 µm from aggregate) is up to 60%, which does not seem to conform to the distribution characteristics of the ITZ porosity of LWA. This is because the 24 h water absorption of lightweight aggregate used by the author is 1.93%, which is basically consistent with the water absorption of natural aggregate, so the porosity of ITZ is high. Based on the distribution of average porosity and CH content, Scrivener et al. determined that the thickness of ITZ was about 15–20 µm [80]. By measuring the change of the Ca/Si molar ratio, Rossignolo et al. determined that the thickness of ITZ was about 55 µm [81].

Figure 7

The porosity profile starting from aggregate surface at different hydration ages [39].

The thickness of ITZ is also an important parameter to characterize its performance, which is mainly affected by w/c ratio, aggregate size, and type. In previous studies, ITZ thickness was generally considered to be between 10 and 50 µm when measured in a mortar [38], and ITZ thickness hasthe potential to exceed 50 µm when measured in concrete around coarse aggregates [56], which indicates that the size of the aggregate affects the ITZ thickness. Through BSEM image analysis, Lyu et al. found that the ITZ thickness of mortar was 31.08, 26.64, 26.64, and 22.20 μm with sand particle sizes of 2.36–1.18 mm, 1.18–0.60 mm, 0.60–0.30 mm, and 0.30–0.15 mm, respectively [82]. In addition, Gao et al. found that the content of fine aggregates would affect the thickness of ITZ, the mortar with 10 and 50% sand content was, respectively, 10 and 15 µm, and this difference comes from different effective w/c ratio and air content [83]. Duan et al., Rossignolo et al., and Nežerka et al. found that silica fume, slag [84], sugarcane industrial ash [85], and fly ash [86] could effectively improve its strength and reduce the thickness of ITZ. As shown in Figure 8, the thickness determined by the indentation modulus is about 80 µm. After adding silica fume, the thickness decreased to about 20 µm.

Figure 8

Counter map of indentation modulus (the w/c ratios of a and b are 0.45 and 0.35, respectively, and the right side is modified by adding silica fume) [85].

The thickness of ITZ is usually obtained by porosity and microhardness; at the same time, it is difficult to define exactly the thickness of ITZ. The thickness of ITZ is not only related to the composition and mix proportion of raw materials that are discussed above, but also related to the sample preparation method and observation method [87]. When studying the thickness of ITZ, the experimental conditions and test methods are described in detail, which has a great reference value for comparing the thickness of ITZ.

3 Improvement of structures of ITZ by nanomaterials

A nanomaterial refers to a material that has at least one dimension in a three-dimensional space is a nanoscale (1–100 nm) [88]. The price of nanomaterials is reduced, and their applications are more extensive due to the rapid development of nanotechnology [89]. In recent years, the use of nanomaterials to modify the properties of concrete has become a research hotspot in the construction industry [90,91]. ITZ is considered to be the weakest part of the concrete and plays a controlling role in concrete performance. Therefore, the study of the influence of nanomaterials on ITZ has aroused people’s interest [92,93]. The research of nanomaterials modified ITZ mainly focuses on the characteristics of hydration products, porosity, and microhardness. The methods of modification mainly include direct mixing and coating aggregate by nanomaterials.

3.1 Influence of nanomaterials on hydration products of ITZ

Different from the cement matrix, the content of CH in ITZ is enriched and the crystal is larger. Existing studies show that nanomaterials can reduce the amount of CH in ITZ and reduce its size. Najigivi et al. found that NS can reduce the size and amount of CH [94]. Palla et al. and Durgun et al. proved by SEM that ITZ was extremely dense after incorporation of NS, and no large CH crystals were found [95,96]. Carriço et al. found that carbon nanotubes (CNTs) tightly connected the hydration products together, optimizing the microstructure of ITZ and cement matrix [97]. By measuring the change of Ca/Si molar ratio along with aggregate distance using EDS, Liu et al. and Xiao et al. found that NS can effectively reduce the thickness of ITZ. When the w/c ratio is 0.3, the thickness of ITZ is reduced from 50 to 20 µm [98,99]. Erdem et al. and Wang et al. found that with the addition of NS, the Ca/Si molar ratio of ITZ decreased from 3.09 to 1.5, and the flaky CH was scattered in SEM image [100,101]. The large-size CH crystal has a lower van der Waals force, thus reducing the cohesive force of ITZ. Wang et al. studied the effect of nanomaterials on the size of CH crystal in ITZ by SEM. As shown in Figure 9, the CH size of the control group is about 55 µm; after adding 3% NS, 2% nano-TiO₂ (NT), 0.3% nickel-coated carbon nanotubes (Ni@CNT), the CH size reduced to 34, 24, and 9 µm [102].

Figure 9

CH crystal size after nanomaterial addition [102]. (a) Cement paste matrix surface microstructure of control specimens (1,000×), (b) CH crystals in the cement paste matrix surface of control specimens (5,000×), (c) cement paste matrix surface microstructure of specimens 3 wt% of NS (1,000×), (d) CH crystals in the cement paste matrix surface of specimens with 3 wt% of NS (5,000×), (e) cement paste matrix surface microstructure of specimens 2 wt% of NT (1,000×), and (f) CH crystals in the cement paste matrix surface of specimens with 2 wt% of NS (10,000×).

In addition, nanomaterials also have an impact on the formation of C–S–H. Xu et al. used nanoindentation and statistical nanoindentation techniques to study the effect of nanomaterials on ITZ. The results showed that after adding 1% NS, both low-density C–S–H (LD C–S–H) and high-density C–S–H (HD C–S–H) of ITZ increased by 24.2 and 10.2%, respectively [103]. Nanomaterials not only increase the quantity of C–S–H, but also enhance its quality, thereby improving the compactness of hydration products.

3.2 Influence of nanomaterials on porosity of ITZ

Many scholars draw conclusions that the addition of nanomaterials will reduce the porosity of ITZ using the quantitative and qualitative analysis. Quercia et al. and Alhawat et al. found that the addition of NS made more dense and small size C–S–H gel, resulting in more compact ITZ [104,105]. Mukharjee and Barai studied the modification of RAC by NS through BSEM images. The test results showed that NS reduced porosity and unhydrated cement quantity in ITZ [106,107]. Shaikh et al., Li et al., and Liu et al. found through MIP and SEM that the large capillary porosity of concrete decreased significantly and the microstructure of ITZ became denser with the addition of NS, but nano-CaCO₃ (NC) could not effectively improve the microstructure of ITZ due to agglomeration of particles [108,109,110]. Zhang et al. found by MIP and SEM-EDS that NS can not only consume a large amount of CH to form dense C–S–H, but also exert the grading filling effect, resulting in the decline of porosity [111]. Younis and Mustafa soaked the recycled aggregate with NS slurry to make the aggregate surface covered with a layer of nanoparticles. They found that the porosity of ITZ is greatly reduced by BSEM [112]. Abreu et al. and Du et al. believed that NS makes ITZ compact and denser, which means that its porosity is reduced [113,114,115]. Sun et al. studied the effect of GO on ITZ of mortar through BSEM. It was found that the pores in ITZ accounted for 45–68% of the whole pore volume in mortar, and the addition of GO can significantly reduce the porosity of both ITZ and the bulk paste, as shown in Figure 10. At 0.35, 0.45, and 0.55 of w/b, the porosity in ITZ of PC/GO specimens was reduced by 64.71, 46.54, and 28.37%, respectively; at the same time, the thickness of ITZ was reduced by 6.62, 10.40, and 11.64%, respectively [116].

Figure 10

Porosity in specimen control specimen (PC) and specimen with GO (PC/GO) at varied w/b: (a) porosity in ITZ and bulk paste; (b) porosity in the whole mortar [116].

In summary, due to the extremely high specific surface area of nanomaterials, the hydration of cement is manipulated, the composition and distribution of hydration products are changed, ITZ has a denser microstructure, and the porosity and thickness of ITZ are reduced.

3.3 Influence of nanomaterials on microhardness of ITZ

Many scholars have studied the microhardness of ITZ modified by nanomaterials using Vickers hardness tester and nanoindentation. The results show that nanomaterials significantly improved the microhardness and elastic modulus of ITZ, and the thickness of ITZ was also reduced. Zhu et al. found that carbon nanofiber (CNF) can greatly improve Young’s modulus of ITZ, as shown in Figure 11 [117]; the modulus of the histogram plot was shifted to the right for CNF-reinforced samples. Gao et al. found that 0.1% CNF improved the elastic modulus of cement stone, cement mortar, and concrete by 21, 34, and 29%, respectively, which indicated the enhancement of CNF to ITZ. It was also found that the average elastic modulus of ITZ increased by 36.7% after adding CNF using atomic force microscopy [118].

Figure 11

Elastic modulus of ITZ modified by CNFs [117].

Nanomaterials also improve the microhardness of ITZ in RAC which has a more complex ITZ compared with NAC [119,120]. Zhang et al. soaked recycled aggregate with NS slurry. The nanoindentation test results showed that NS enhanced the elastic modulus of the new ITZ in RAC, but the old ITZ was not strengthened [121]. In the followup study, Zhang et al. used NS + NC and NS + cement composite slurry to soak the recycled aggregate. The results showed that the thickness of the new ITZ decreased by about 10 and 20%, respectively, while the thickness and elastic modulus of the old ITZ almost did not change [122]. Li et al. treated recycled aggregate by pre-spraying NS slurry and found by digital Vickers microhardness that NS significantly improved the hardness of ITZ [123,124]. Yue et al. proposed an industrialized mixing method: covering the surface of recycled aggregate with NS and micro-CaCO₃. Through the nanoindentation test, it was found that the elastic modulus of the new ITZ ranges from 0.38 to 9.46 GPa. After adding 2% NS and 1% MC, the elastic modulus of the new ITZ ranges from 6.33 to 12.75 GPa. Due to the agglomeration of particles, NC often has a negative effect on the performance of ITZ [125]. The specific surface area of micron-CaCO₃ is greatly reduced compared with NS, so the compounding of NS and MC can play a better role in ITZ.

In addition, nanomaterials can also enhance the ITZ between fiber and cement-based ultrahigh performance concrete (UHPC) [126]. For example, He et al. coated CNF on the surface of polyethylene (PE) fibers, which successfully enhanced ITZ and improved the interfacial bond strength between PE fibers and cement-based matrix [127,128]. Yeke et al. found that GO can enhance the mechanical properties of the ITZ between the steel fiber and cement matrix [129]. After adding 0.04% GO, the thickness and average elastic modulus of ITZ changed from the original 45 µm, 19.63 GPa to 35 µm, 20.68 GPa.

The microhardness of ITZ can be represented indirectly by interfacial bonding strength, which can be obtained by stretching and shearing an artificial interface. Wang et al. studied the effect of different nanomaterials on the bonding strength of ITZ through a three-point bending test. The addition of 2 wt% nano-TiO₂, 0.3 wt% CNT, and 0.5 wt% graphene can increase the interfacial bonding strength by 2.32 MPa/50.0%, 2.76 MPa/59.5%, and 3.03 MPa/65.3% respectively, as shown in Figure 12 [102]. Wang et al. (2021) studied the effects of NS, nano-metakaolin (NM), and nano-Al₂O₃ (NA) on the ITZ through the push-out test. It was found that when the addition of NS, NM, and NA was 3%, the bond strength increased by 108.54, 43.41, and 20%, respectively. The nanoindentation test shows that the lower limit of indentation modulus was increased by 71.30, 54.39, and 10.78%, respectively [130]. Song et al. found that the addition of NS and CNTs can generally improve the interfacial tensile and shear strengths of RAC by 51 and 53% for dosage not exceeding 2.0 and 0.5 wt%, respectively [131]. From the above results, the bonding strength of ITZ is different when different nanomaterials with different content are used, among which graphene has the best effect on improving the bonding strength.

Figure 12

Bonding strength of concrete artificial interface [102].

4 Mechanism nanomaterials modifying ITZ and its effect on concrete

The microstructure of ITZ is defective compared to cement matrices, like high porosity and low microhardness. The excellent properties of nanomaterials have a positive impact on ITZ. This chapter reviews the modification mechanism of nanomaterials for ITZ, the relationship between ITZ and concrete, and the effect of nano-modified ITZ on the macroscopic properties of concrete.

4.1 Mechanism of nanomaterials on ITZ

From the above research, the wall effect leads to a high local w/c ratio of ITZ, so the porosity is high, resulting in stress concentration and microcrack development. A large number of CH crystals are filled in the ITZ pores and grow in orientation. At the same time, the specific surface area of larger CH crystals is smaller and the van del Waals force is weakened. The low compactness caused by the high porosity of ITZ makes its microhardness lower than that of the cement matrix. The modification of ITZ by nanomaterials is essentially the adjustment of the composition and structure of the hydration products.

Using different modified methods, the movement of nanomaterials in concrete is different. For example, when the concrete is modified by the direct addition of nanomaterials, the wall effect and the water absorption of the aggregate lead to the nanoparticle content in ITZ being 1.65–1.98 times that of the cement matrix; that is, the nanomaterials converge from the cement matrix to the ITZ [132]. When the concrete is modified by pre-treated the aggregate with nano-slurry, the nanomaterials are directly present in ITZ and radiate to the cement matrix. This method can also enhance the mechanical properties of the recycled aggregate. Which of these two methods is more advantageous in improving concrete performance is not discussed in this review.

Nanomaterials have an ultra-high specific surface area, so they have some excellent properties, which can adjust the hydration of ITZ at the nanoscale. The effect can be divided into chemical effect and physical effect. The chemical effects are shown in Figure 13; due to the nucleation effect of nanomaterials, it promotes cement hydration, reduces the proportion of unhydrated cement particles in ITZ, produces more hydration products, and fills pores, thus reducing porosity. Because the nanomaterials are negatively charged, the metal cations in the solution such as Ca²⁺ and Al³⁺ are adsorbed, which limits the formation of CH and ettringite [133]. Some nanomaterials with pozzolanic activity can also react with CH to form additional C–S–H [134]. At the same time, this mechanism will not limit the generation of C–S–H gel, but also improve the order of C–S–H and reduce the distance between C–S–H layers [135]. The physical effects of nanomaterials refer to the filling effect and bridging effect. Due to the small particle size of nanoparticles, nanomaterials can fill the voids of hydration products. One-dimensional and two-dimensional nanomaterials can also connect hydration products [136,137].

Figure 13

ITZ modification mechanism of nanomaterials [102].

4.2 Influence of ITZ on mechanical properties and durability of concrete

ITZ which has microstructure defects is a bridge of stress transfer between aggregate and cement matrix, so it is considered to be the weakest area in concrete [138,139]. Many studies have shown that ITZ performance is closely related to the mechanical properties of concrete. For example, Xiao et al. used white cement and black aggregate to make concrete and found that cracks were most likely to first appear at ITZ using digital image correlation technique [140,141]. Duan et al. found that the average Vickers hardness of ITZ was positively correlated with the compressive strength of concrete [84]. Zhao et al. defined the comprehensive elastic modulus of ITZ (E_ITZ) in RAC, that is, the sum of the product of ITZ₂, ITZ₃ modulus, and their length. It was found that the E_ITZ was positively correlated with the compressive strength of RAC [75]. Zhu et al. found that the elastic modulus and thickness of ITZ have a significant impact on the elastic modulus of concrete [117]. Wang et al. found that the Vickers hardness of the old ITZ was positively correlated with the compressive strength, as shown in Figure 14 [71]. It can also be seen that the correlation coefficient between the strength and compressive strength of the new ITZ is as low as 0.09 because the lowest strength of the new ITZ (7.8 MPa) is much higher than that of the old ITZ (6 MPa). Liu et al. used polymer emulsions to pretreat aggregates and found that the porosity of ITZ was doubled; the average hardness was reduced by 30%, with the compressive strength of concrete reduced by 20% [142]. In addition, Kepniak et al. found that the micro-cracks in ITZ were closely related to the compressive strength of concrete using SEM [143]. Many people used the reduction of micro-crack width in ITZ to explain the improvement of mechanical properties of concrete [144,145,146].

Figure 14

Relationship between strength of ITZ and compressive strength [71].

It can be seen from the above test work that improving or reducing the performance of ITZ will lead to a corresponding increase or decrease in the mechanical properties of concrete. The above research only gives the correlation between ITZ and mechanical properties of concrete. How to determine a reasonable and reliable interface parameter, and furthermore, to quantitatively investigate the influence of ITZ on the concrete is a challenge for future research [147,148,149].

Durability of concrete is the ability of concrete to resist the environmental media and maintain its good performance and appearance integrity for a long time. It is characterized by transport properties (impermeability, chloride penetration resistance, etc.) [150]. There are two contradictory statements about the relationship between ITZ and the transport properties of concrete. On the one hand, due to the high porosity of ITZ, some researchers believe that ITZ may become the dominant transport channel for harmful substances. For example, Yang et al. found that when the w/c ratio is 0.35, 0.45, and 0.55, the chloride ion migration coefficient of ITZ is 40.6, 35.5, and 37.8 times that of matrix mortar, respectively. After the w/c ratio increases, the porosity of the mortar matrix increases, so the multiple gradually decreased [151]. Sun et al. found through numerical simulation that after coating aggregate with slag and silica fume, the porosity of ITZ decreased significantly and the penetration speed of chloride ions decreased by around 40 and 60%, respectively [152]. On the other hand, some researchers obtained the opposite conclusion in experimental studies on real mortar and concrete. Hornain et al. [153] found that the tortuosity and dilution effect due to the presence of aggregate had more influence on transport properties than the ITZ effect. By adjusting the amount and diameter of aggregates to change the content of ITZ, Wu et al. found that the chloride ion permeability of concrete decreased first and then increased with the increase in ITZ content [154]. Rangaraju et al. changed the overlapping degree of ITZ by changing the sand particle size distribution, and the results showed that the increase in the overlapping degree of ITZ had no obvious effect on the chloride ion penetration rate of concrete [155].

To sum up, ITZ plays a leading role in the mechanical properties of concrete, but the durability of concrete is still under debate. The research on the quantitative statistical analysis in ITZ and concrete performance needs to be furthered, so that makes it possible to analyze the characteristics of concrete engineering based on ITZ performance indicators in the future.

4.3 Performance of concrete when ITZ modified by nanomaterials

Conventional view holds that the ITZ is the weakest zone in concrete. The microstructure of ITZ has been studied by many researchers using advanced characterization instruments, and the results show that this view is beyond doubt. After modification of ITZ with nanomaterials, the performance of ITZ is significantly improved. In order to prove the feasibility of using nanomaterials to modify ITZ so as to improve the performance of concrete, the author makes statistics on the performance of ITZ and its corresponding concrete after using nanomaterials, as shown in Table 1 (NAC) and Table 2 (RAC).

Table 1

Performance of ITZ and NAC

Ref.	Nanomaterials	Method	Performance of ITZ at microscale	Performance of concrete at macroscale
[96]	NS	Direct mixing	ITZ did not exist anymore and the bond was much stiffer and stronger	Elastic modulus of the concretes had been significantly improved but compressive strength does not
[94,95]	NS	Direct mixing	More compact and no large CH, Ca/Si was reduced from 1.51 to 1.1	Increasing compressive strength by 30% and flexural strength by 23% at 28 days
[103,105]	NS	Direct mixing	The microstructures were more homogenous and its thickness was reduced. Pore structure was refined. The ration of Young’s modulus of ITZ to that of bulk paste increased from around 50–80%	The compressive strength and flexural strength increased and water penetration depth, chloride migration coefficient, and diffusion coefficient were reduced significantly
[115]	NS	Direct mixing	The microstructure was more compact and homogeneous	Water penetration depth, moisture sorptivity, chloride migration, and diffusion coefficient were reduced
[130]	(NS, nano-metakaolin, nano-Al₂O₃	Direct mixing	The modulus of ITZ was improved by 54.39% and 71.03%, respectively, after the addition of NS and nano-metakaolin. The microscopic morphology of ITZ is improved	The bond strength of non-dispersible underwater concrete was increased by 134.12 and 87.70%, respectively, at 3days and by 108.54 and 43.41%, respectively, at 28 days
[157]	NS	Direct mixing	The microstructure was denser and uniform	The compressive strength increased by 22% and 18% at 3.7 days
[157]	NS	Direct mixing	The porosity and ratio of Ca/Si were reduced
[34]	NS	Coating aggregate	ITZ was much more denser and porosity was reduced	The compressive strength and the chloride penetration resistance were improved
[98]	NS	Direct mixing	Nano-silica can micrify the thickness of the ITZ and reduce the width of the abrasion cracks in the ITZ	The enhancement rate of the anti-permeability of concrete becomes significant
[97]	CNTs	Direct mixing	By filler and nucleation effects, refining the microstructure of ITZ; by bridging effect, contributing to reduction of the formation and propagation of macrocracks	Mechanical strength and durability properties could be improved as much as 25%
[117]	CNF	Direct mixing	The ITZ is enhanced mechanically (elastic modulus is increased) and chemically (Ca/Si ratio is reduced)	The modulus of elasticity is increased by 21, 34, and 29% for cement paste, mortar, and concrete, respectively
[129]	GO	Direct mixing	The number of ITZ microcracks and CH crystals between steel fiber and matrix decreased	The compressive strength at 28 and 90 days increased by 15.8 and 14.5%, respectively, and the flexural strength increased by 14 and 13.5%, respectively. Besides, the tensile strength ranges from 7.55 to 8.7 MPa
[129]	GO	Direct mixing	The thickness of ITZ decreases from 45 to 35 μm, and the elastic modulus of ITZ increases from 19.63 to 20.68 GPa

Table 2

Performance of ITZ and RAC

Ref.	Nanomaterials	Method	Performance of ITZ at microscale	Performance of concrete at macroscale
[100]	NS	Direct mixing	New ITZ became denser and more uniform. Vickers microhardness was enhanced. The porosity and Ca/Si ratio were reduced	Compressive strength, tensile strength, non-destructive parameters, and permeability of recycled concretes were enhanced. Dynamic modulus of elasticity values increased
[109]	NS, NC	The second mixing method	The NS- and NC-modified RAC had denser microstructures and less micro-cracks in ITZs	NS significantly enhanced the compressive strength of RAC
[109]	NS, NC	The second mixing method		NC even decreased the compressive strength at a late age due to the poor dispersion of NC
[121]	NS	Pre-soaking or pre-spraying aggregate	The thickness of the new ITZ was reduced. The elastic modulus obtained from nanoindentation tests was proved to be significantly improved	The compressive strength was increased by 34.22% at 28 days. The volume of permeable voids, water sorptivity, and chloride ion penetration measured after 28 days of curing was also reduced
[123]	NS	Pre-soaking or pre-spraying aggregate
[125]	NS and micro-CaCO₃	An industrially applicable mixing method	The average elastic modulus and hardness of new ITZs in RAC-2% NS-1% MC were 3.02 times and 2.92 times higher than those of new ITZs in reference concrete, respectively. Old ITZ did not change	The increase ratio of RAC incorporating 2% NS and 1% MC reached 19.46% at 90 days while the strength improvement of RAC containing 2% NS at 90 days was merely 4.55%

Tables 1 and 2 show that when the ITZ performance is improved, the compressive strength, flexural strength, elastic modulus, and other mechanical properties of concrete are improved accordingly. Using aggregate pretreatment to modify ITZ, the improvement of mechanical properties of concrete is directly related to ITZ, because there are almost no nanomaterials in the cement matrix. However, the direct addition method often does not distinguish its effect on ITZ and matrix; that is, the improvement of concrete mechanical properties is more due to the improvement of ITZ microstructure or the optimization of matrix properties. Some scholars have focused on this problem. Through the study of the microstructure of ITZ by SEM, XRD, and EDS, Nili et al. and Xu et al. confirmed that the improvement of mechanical properties of concrete should be attributed to the improvement of ITZ properties by nanomaterials [103,156]. Zhang and Islam found that with the increase in the content of NS, the strength of ITZ gradually increased. When the strength of ITZ increased to a certain extent, the failure mode of concrete changed from crack propagation along ITZ to through aggregate. When the content of NS was higher than 2%, the compressive strength of concrete was not increased due to the limitation of aggregate strength [157]. Ren et al. used the grey theory to make a quantitative analysis of the relationship between the porosity of ITZ and the macro-mechanical properties of concrete after nano-modification. They found that the porosity of the ITZ phase was the most important for improving the compressive strength [158].

Tables 1 and 2 show that many scholars report that the transportation performance of concrete decreases after the performance of ITZ is improved. There is no doubt that nanomaterials are positive to reduce the transmission performance of ITZ because nanomaterials significantly reduce the porosity of ITZ. For example, Gao et al. found that the addition CNTs micrified the ITZ thickness by 14.3–30%, and the corresponding impermeability properties of concrete present a linear correlation with ITZ thickness, which can be strengthened by approximately 10.9–35.8% [159]. But the porosity of the cement matrix was also reduced, it is impossible to determine whether the reduction in the transportation performance of concrete is the reason for the improvement of ITZ or cement matrix [160]. In a word, although nanomaterials have a positive impact on the durability of concrete, it is uncertain whether ITZ plays a controlling role.

To sum up, the improvement of concrete performance by nanomaterials should be attributed to the improvement of ITZ performance. The use of nanomaterials to modify ITZ to improve concrete performance provides a new perspective for the green development of the construction industry.

5 Summary and prospect

ITZ is a relatively weak area in concrete, which plays a controlling role in the performance of concrete. Improving the performance of ITZ is the main way to improve the performance of concrete. Nanomaterials have an ultra-high specific surface area, which is very effective for improving the performance of ITZ. In this article, the microstructure characteristics and influencing factors of ITZ and the modification of nanomaterials are reviewed. The following important conclusions are obtained:

The types of hydration products of ITZ are the same as those of matrix, but the quantity distribution is different. With the increase in the distance from the aggregate, the content of ettringite and CH in ITZ decreases, while the content of C–S–H gel and unhydrated cement particles increases, and the CH grains are larger than the matrix and grow in orientation.
ITZ have a higher porosity and lower microhardness than those of cement-based aggregate, and they change with the increase of the distance from the aggregate. Most scholars use these two parameters to determine the thickness of ITZ, which generally does not exceed 100 µm. These parameters are only relative. In the future, the absolute reference values of ITZ thickness and porosity of concrete with different strengths can be obtained according to a large number of data statistics.
The reduction of the w/c ratio, the increase in curing age and the addition of ultra-fine mineral additives will enhance the performance of ITZ. The influence of aggregate on ITZ is complex, including aggregate type, size, and lithology. The mechanism of the interrelatedness of these influencing factors remains to be studied. Only by understanding the complex factors affecting the performance of ITZ can the ITZ be improved theoretically.
Nanomaterials affect the morphology and distribution of hydration products in ITZ, including reducing the number and size of CH and increasing the proportion of C–S–H. Nanomaterials also reduce the porosity and increase the microhardness of ITZ, which can be attributed to the nucleation effect, pozzolanic effect, filling effect, etc. At present, there is no comparative analysis on the influence of nanomaterials dispersion method and dosage on ITZ performance, and research in this aspect needs to be strengthened.
ITZ plays a controlling role on the mechanical properties of concrete. Whether ITZ has a leading negative effect on durability remains to be investigated. The current research only points out the positive correlation between ITZ and concrete mechanical properties. Establishing the quantitative relationship between ITZ and concrete mechanical performance, developing the ITZ index becomes a parameter of concrete performance, and providing a scientific basis for concrete modification are the focus of future research.
A large number of studies show that the improvement of concrete performance by nanomaterials is mainly due to the improvement of ITZ performance. Therefore, improving the performance of concrete from the perspective of nanomaterial-modified ITZ is the main way to realize green concrete.
Although the price of nanomaterials gradually decreases, it will still make the high cost of concrete. If nanomaterials are only used for ITZ modification, the use of nanomaterials will be reduced so that improving the applicability of nanomaterials in concrete.

Acknowledgments

The writing of this article has been supported by many projects, which can be seen in funding information. At the same time, the project team members and all authors have supported this article, a note of thanks to them. The authors thank the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology) for providing the working environment and scientific research conditions for the their scientific research team.

Funding information: This study was supported by the Philosophy and Social Science Research Fund Project of Chengdu University of Technology (YJ2021-ZD002), Special Project of Marxist Theory Research of Chengdu University of Technology (20800-2021MLL005), Project of Western Ecological Civilization Research Center (XBST2021-YB002), Chengdu University of Technology Development Funding Program for Young and Middle-aged Key Teachers (10912-JXGG2021-01003), Sichuan Mingyang Construction Engineering Management Co., Ltd. Specialized Project (MY2021-001), and College Students’ Innovation and Entrepreneurship Training Program (202010616009, S202110616011, S2021106160114, S202110616013, S202110616099).
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2022-03-17

Revised: 2022-04-27

Accepted: 2022-05-11

Published Online: 2022-06-01

This work is licensed under the Creative Commons Attribution 4.0 International License.

Microstructural characteristics and nano-modification of interfacial transition zone in concrete: A review

Abstract

1 Introduction

2 Microscopic characteristics of ITZ structures

2.1 Composition and distribution of hydration products of ITZ

2.2 Porosity of ITZ

2.3 Microhardness of ITZ

2.4 Thickness of ITZ

3 Improvement of structures of ITZ by nanomaterials

3.1 Influence of nanomaterials on hydration products of ITZ

3.2 Influence of nanomaterials on porosity of ITZ

3.3 Influence of nanomaterials on microhardness of ITZ

4 Mechanism nanomaterials modifying ITZ and its effect on concrete

4.1 Mechanism of nanomaterials on ITZ

4.2 Influence of ITZ on mechanical properties and durability of concrete

4.3 Performance of concrete when ITZ modified by nanomaterials

5 Summary and prospect

Acknowledgments

References

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