## 1 Introduction  
Carbon has always been an important part of human lives. Over the past century, the use and function of carbon have taken many roles in the world of material science. During the 1960s, scientists created new types of carbon materials, which included carbon fibers made from various materials, pyrolytic carbon formed from vapor, hard crystalline carbons, dense carbons made by pressing, compounds that could conduct electricity well, and thin carbon sheets resembling diamonds [1, 2, 3]. During the 1990s, the potential of carbon became even more prominent as the attention shifted toward nanocarbons, like fullerenes, carbon nanotubes, and graphene [4, 5, 6]. In the realm of advanced materials science, the development of nanocomposites has marked a significant leap forward, offering unprecedented improvements in mechanical, thermal, and electrical properties.  
Among them, composites with nano-structured carbon materials are gaining popularity due to their exceptional strength, stiffness, and conductivity. When integrated into composite materials, CNTs can improve mechanical performance and push the boundaries of what is achievable with traditional composites [7, 8, 9, 10].  
Carbon nanotubes are composed entirely of carbon atoms arranged in a hexagonal lattice, and rolled into a tubular form, CNTs exhibit a unique combination of properties that is difficult to find in other materials. Their high aspect ratio, along with the inherent strength of the sp\({}^{2}\) carbon-carbon bonds, allow CNTs to exhibit extraordinary tensile strength and stiffness. Moreover, the delocalized \(\pi\)-electron cloud along the tube's surface contributes to their unique electrical properties, making them excellent conductors of electricity [5; 11; 12; 13; 14].  
The features that make CNTs exceptional pose challenges in the application of the product. The strong van der Waals forces between individual tubes lead to clusters, hindering their uniform dispersion within a matrix. Moreover, the static nature of the CNT surfaces complicates their interaction with other matrices, limiting mechanical load transfer [15; 16]. Their impurities and weak affinity toward metals are also some of the bigger challenges [17; 18]. To overcome these obstacles, surface modification of CNTs has been developed as a crucial strategy. By introducing functional groups or coatings onto the CNTs' surface, their compatibility with different matrices can be significantly improved. This modification not only enhances the dispersibility of CNTs but also facilitates stronger interfacial bonding between the CNTs and the matrix, which is essential for effectively transferring stress and enhancing the mechanical properties of the composite [19; 20; 21; 22].  
The exploration of surface-modified CNTs in reinforcing various composite matrices opens up new avenues for engineering materials with tailored properties. This paper focuses on the advancements in the mechanical performance of nanocomposites reinforced with surface-modified carbon nanotubes. The application of such nanocomposites has a wide range of applications in industrial sectors, including 3D printing [23; 24; 25], automotive [26; 27; 28], construction, and coatings [29; 30; 31; 32], highlighting their growing importance in modern engineering and materials science.  
This paper aims to provide a comprehensive review of the latest advancements in the field of surface-modified CNTs as additives in polymeric, metallic, and cementitious composites. Various production and synthesis processes are highlighted to provide necessary context, followed by an exploration of the resulting composites' innovative applications, existing challenges, and future potential. This review aims to offer a detailed overview of the current State of the Art in the field of CNT-reinforced nanocomposites of polymers, metals, and cement and provide an understanding of the mechanisms of enhancement in their mechanical and functional properties with the incorporation of CNTs.