## 2 Materials and Methods  
In the present study, ordinary Portland cement was used as a primary binder. MWNTs, a product of Beijing DK nano technology Co., Ltd. in Beijing, China, were used, and their dimensions and physical properties are given in Table 1. NS, a product of SAT nano Technology Material Co., Ltd. in Dongguan, China, was used in a dry and fine powder form. The NS exhibited hydrophilic characteristics and had an SiO\({}_{2}\) content of 99.8%. The sizes and other details are provided in Table 2. The water used in the fabrication of the mixture was supplied from a tap. A polycarboxylic-acid-based super plasticizer (SP) was utilized for all sample preparations in an effort to control the fluidity of the fresh mixtures. The mixture proportions and fabrication procedures of the cement composites were as follows. A total of 16 samples were prepared by adding different amounts of MWNT and NS. Four different NS contents, 0%, 1%, 3%, and 5% by weight of the cement were examined. In a similar fashion, four different MWNT contents, 0%, 0.3%, 0.6%, and 1% by weight of the cement were applied. The target flow of fresh mixture ranged from 127 to 166 mm, and all the mixtures were prepared in accordance with the target flow to enhance the distribution of nanomaterials [26; 32]. [PERSON] et al., 2015 and [PERSON] et al., 2014 demonstrated that a fresh mixture prepared with a low fluidity led to enhancements of the MWNT distribution in the resultant cement composites [25; 32]. Different amounts of the SP were used in the fresh mixtures, and the amount of SP varied based on the content of MWNTs used. The amounts of water and cement were fixed, and the SP content was varied in an effort to achieve the target flow. Details of the mixture proportions are given in Table 3. \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **MWNT** \\ \hline Exterior diameter & \textless{8 nm} \\ Length & 10–30 μm \\ Purity & 98\% \\ Ash & \textless{0.5 wt.\%} \\ Specific surface area & \textgreater{350 m}2/g \\ Electrical conductivity & \textgreater{100 S/cm} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions and physical properties of multi-walled carbon nanotubes (MWNTs). \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **NS** \\ \hline Particle size & 10–20 nm \\ pH & 4–7 \\ Specific surface area & 170–220 m2/g \\ SiO2 content & 99.8\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Dimensions and physical properties of nano-silica (NS). To ensure the integrity of the composite samples, the following fabrication procedures were carefully followed. Cement, NS, and MWNTs were weighed based on the specified proportions, and the mixture was poured into the bowl of an automatic mixer. Dry mixing was conducted for 3-4 min for preliminary mixing. After the pre-mixing, water and SP were added to the dry mixture and mixed again for another 2 min. A flow test was carried out to examine the fluidity, and the fresh mixture was decanted into molds with dimensions of 40 mm by 40 mm by 160 mm if the flow value was in the target flow range. The fresh mixtures in the molds were placed on a table-type vibrator and compacted for 60 s. The mixtures were subsequently cured in a storage cabinet under an ambient temperature of 18-20 \({}^{\circ}\)C for 28 days. Figure 1 shows images captured in each step of the fabrication procedure. The testing methods are briefly described as follows. A three-point flexural test was conducted in compliance with American Society for Testing and Materials (ASTM) C 78 after 28 days of sample curing. The fabricated prismatic composite samples were subjected to flexural loading applied with a displacement-control loading system. The  
Figure 1: Schematic illustrating the fabrication procedures of the multi-walled carbon nanotubes/nano-silica (MWNT/NS)-incorporated cement composites.