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Figure 7g shows the structure of the untreated NS-CNTs composite in cementitious composite. A few ellipsoids are formed, and there are only a few pore fillings. Figure 7h shows the structures of the activated NS-CNTs composite in cementitious composite. As the size range of the composite is basically within the pore range of the cementitious composite and the surface of the composite structure contains NS, the Si-OH produced during the hydrolysis of trifluoropropyltrimethoxysilane could also undergo a dehydration–condensation reaction with (CaOH2). Therefore, the composite could fill into the pores of the cementitious composite hydration products, and it could combine well with the cementitious composite hydration product. 1. IntroductionAttributed to its excellent structural properties, cementitious composite is widely used as a building material, such as in the construction of bridges, tunnels, dams, railways, and wharfs. At the extraction of oil, Cl− can degrade the rebar performance and shorten the service life of the reinforced cementitious composite . Hence, to enhance the properties of cementitious composite, nanomaterials in the size range of 1–100 nm have been added in various commercial applications . Due to their large surface areas, nanomaterials can impart beneficial properties to the cementitious composite paste, such as a good early bond strength, reduced permeability, accelerated cementitious composite hydration, and controlled fluid loss. As such, more calcium silicate hydrate (CSH) and less calcium hydroxide are produced in the presence of the nanomaterials, which makes the cementitious composite denser .Nanomaterials possess small volume and adding nanomaterials such as nano-silica (NS), Sardon et al. used (3-aminopropyl) triethoxysilane (APTES)-modified SiO2 to reduce the agglomeration of NS significantly . The addition of nano-titanium dioxide (nano-TiO2) , nano-iron (nano-Fe2O3) , nano-alumina (nano-Al2O3) , carbon nanotube (CNTs) , and nano-clay particles into cementitious composite can improve its overall properties. Among these nanomaterials, NS and CNTs are particularly significant in cementitious composite . NS is composed of very fine SiO2 particles that are nearly 1000 times smaller than ordinary cementitious composite particles, with high pozzolanic activity. Moreover, the addition of NS curing paste can shorten the setting time. Therefore, NS is widely used to increase the impermeabilities of pastes and the mechanical properties of hardened materials. On the other hand, CNTs have garnered increasing attention due to their relatively low densities, which can improve the thermal properties of the cementitious composite. Their positive effects on the performance of cementitious composite-based composites are mainly affected by two phenomena: hydrate nucleation and filling effects. However, the greatest challenges in implementing these nanomaterials in cementitious composite are the compatibility and dispersibility of these nanomaterials in the cementitious composite, which warrants an urgent resolution.Physical methods, such as mechanical force and ultrasonic dispersion, and chemical methods are mostly used in the preparation of composite. For the chemical methods, modifiers are added to reduce the hydroxyl groups on the surface of NS through chemical reaction, and to reduce the nanoparticles’ agglomeration. For instance, Mahadik et al. studied the preparation of TEOS-based silica gels by a two-step sol–gel method followed by surface-treating the fumed SiO2, and modification of its surface with a silylating agent. Hydrophobic aerogel comprising trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) can be realized using an ambient pressure drying method . As such, these functionalized NS and CNTs can chemically interact with the cementitious composite particles, which in turn can positively influence the hydration process. However, chemical modification of CNTs may reduce their mechanical strengths, which could reduce the overall properties of the composites. This is because the chemicals used in the functionalization of CNTs can have a negative impact on the hydration kinetics. Studies by Yakovlev and Mendoza-Reales et al. have shown that the combination of CNTs and NS can accelerate the hydration kinetics, resulting in greater production of C-S-H . Due to the chemical compatibility of the NS and the C-S-H matrix, SiO2 located on the surfaces of the CNTs can enter the matrix and form aggregates. Therefore, due to the improved bonding between the CNTs and NS in the matrix, enhanced load transfer capabilities can be achieved. Monfared et al. have shown that the incorporation of nanoparticles enhanced the fiber–matrix interfacial strength, toughened the surrounding matrix, and improved resin adhesion to the fiber, which increased the tensile properties of incorporated composites with hybrid nano-fillers . Rui Wang and others proved that the incorporation of the as-prepared nanosilica/graphene oxide (m-SGO) hybrid into epoxy resin (EP) resin not only obviously increases the flame retardancy, mechanical, and thermal stability properties, but also endows EP resin with high thermal conductivity, low dielectric loss, and high dielectric constant. This study yielded similar experimental results . MR Ayatollahi also showed that the combination of carbon nanotubes and carbon nanotubes strengthens the fiber–matrix interface strength, and the stiffness of the multi-scale composite material . Miaomiao Hu discovered through research that the nanosilica particles on MWCNT-81 G/VMQ(GO) surface formed a physical barrier to maintain good dispersion of GO and NS. This is similar to the findings of this experiment . Bratati Pradhan also improved the performance of MWCNT-G/VMQ at the same time, because Nanocomposites can be attributed to the synergistic effect of hybrid fillers in pore solution, respectively . Majid R Ayatollahi also studied the effect of carbon nanotubes and nanosilica on the tribological properties of carbon fiber cloth composite materials and concluded that which indicates enhan cementitious composite in bonding strength between carbon fiber and epoxy matrix due to the interfacial reinforcing action of the nano-particles .In this study, the effect of NS and CNT composite structure on the properties of cement materials was systematically studied, including the mechanical properties, water resistance and corrosion resistance of the cement base. Nuclear Magnetic Resonance (NMR) (Bruker, Switzerland, Germany) and Fourier Transform In-fared (FTIR) (Nicolet 5700. Bruker, Switzerland, Germany) were used to study the dehydration condensation of nano-silica and trifluoropropyltrimethylsiloxane. Scanning electron microscopy (SEM)( FEI Verios 460, Hillsboro, OR, USA) and X-ray Diffraction (XRD)(Bruker D8-Discover instrument, Karlsruhe, Germany), X-ray Photoelectron Spectroscopy (XPS)(British AXIS SUPRA), TG equipment, and calorimetry have been used to study the influence of nano-silica and CNT composite structure to enhance the microstructure and hydration of the cement slurry. The freeze–thaw resistance experiment was used to study the influence of NS and CNT composite structure on the corrosion resistance of cement materials. The contact angle experiment was used to study the waterproofness of cement materials. This article also introduces the experimental procedures, results analysis, and discussion in detail.

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