## 3 Results and Discussion  
### Tensile and Compressive Strength  
We investigated the influence of TEOS/PVP nanofibers, including those modified by carbon nanotubes, on the mechanical characteristics of hardened cement pastes using 28-day tensile and compressive strength tests. Figure 3 shows that the tensile strength of the nanofibers-blended cement pastes increased slightly by around 6% and 10% compared with that of the plain paste in comparisons between the tensile strength results of the T28S and T28 CS samples, respectively, and the T28 PL sample. These unimpressive results show the inefficiency of TEOS/PVP NFs and CNT-TEOS/PVP NFs in increasing the tensile strength of the cement pastes. Previous studies have clarified the increase in tensile strength of the pastes when blending nylon 66 nanofibers and their modified versions with cement using the same method [11; 12]. The comparison results are shown in Table 6. In contrast, there was a significant increase in compressive behaviors when modifying cement with the proposed fibers compared with those of plain paste (see Figures 4 and 5 and Table 7). The results show an increase of 28% and 38% in compressive strength and 54% and 66% in toughness when comparing the C28S and C28 CS samples, respectively, with C28 PL. These results highlight the important role played by TEOS/PVP NFs and CNT-TEOS/PVP NFs in improving the compressive strength and toughness of the cement pastes. Observations in previous studies about the nylon 66 nanofibers-modified cementitious materials did not show the efficiency of nanofibers from this perspective [11; 12] (see Table 6). Above all, our findings from the mechanical strength tests show a better performance in compressive strength than in tensile strength when introducing TEOS/PVP NFs and CNT-TEOS/PVP NFs into cementitious materials.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & \multicolumn{3}{c}{**Previous Works [11; 12]**} & \multicolumn{3}{c}{**Present Work**} \\ \hline  & **N66 NFs** & **N66 NFs** & **SWCNT-N66** & **MWCNT-N66** & **T28S/C28S** & **T28 CS/C28 CS** \\  & **MCP (1)** & **MCP (2)** & **NFs MCP** & **NFs MCP** & **T28S/C28S** & **T28 CS/C28 CS** \\ \hline \hline \
\begin{tabular}{c} CNTs content (wt\% in the \\ cement-based materials) \\ \end{tabular} & 0 & 0 & 0.015 & 0.015 & 0 & 0.02 \\ \hline Tensile strength (MPa) & 32 & 28 & 43 & 57 & 6 & 10 \\ \hline Compressive strength (MPa) & 6 & 8 & 10 & 14 & 28 & 38 \\ \hline \hline
\begin{tabular}{c} Toughness (J/m\({}^{3}\)) \\ \end{tabular} & 42 & 49 & 30 & 12 & 54 & 66 \\ \hline \hline \end{tabular} Note: N66 NFs MCP (1): nylon 66 nanofibers-blended cement paste (formic acid–dichloromethane solvent). N66 NFs MCP (2): nylon 66 nanofibers-blended cement paste (formic acid–chloroform solvent). SWCNT-N66 NFs MCP: hybrid nanofibers (single-walled carbon nanotube and nylon 66)-blended cement paste. MWCNT-N66 NFs MCP: hybrid nanofibers (multi-walled carbon nanotube and nylon 66)-blended cement paste.  
\end{table}
Table 6: Effectiveness of TEOS/PVP NFs, CNT-TEOS/PVP NFs, N66 NFs, and CNT-TEOS/PVP NFs on mechanical properties of cement pastes (increase in % compared with parameters of control samples).  
Figure 4: Results from compressive strength test.  
Figure 3: Results from tensile strength test.