# Effect of Electrospun Nanofiber Additive on Selected Mechanical Properties of Hardened Cement Paste
## 3 Results and Discussion
### SEM and TEM Results  
Figure 4 presents the cross-section morphologies of the modified hardened cement pastes including electrospun nanofibers. As is illustrated, some common cement hydration products can be observed, such as calcium hydroxide (CH) and calcium-silicate-hydrate (CSH), with the morphologies of large prismatic crystals and small fibers, respectively [32]. The electrospun nanofibers with a diameter of around 200-300 nm could interleave easily in the space from 10 nm to 1 \(\upmu\)m of the capillary cavities or voids, which were created when the initial spaces occupied by water could not become totally filled by the hydrated products such as CH or CSH [32]. Therefore, these electrospun nanofibers not only act as the bridging agents, linking the hydrated products physically, but also act as the filling agents, filling up the spaces among the hydrated products. As a result, the existence of electrospun nanofibers inside the cement matrix can enhance the mechanical properties of this material. Based on the model of the well-hydrated Portland cement paste from [32], the bridging effect as well as the filling effect of nanofibers in the matrix can be summarized schematically (Figure 5).  
From the tensile strength test results presented above, the tensile strength of the group containing nylon-66-based nanofibers was higher than that of the group containing TEOS/PVP-based nanofibers. This observation can be explained due to the structure of the electrospun nanofibers, as presented in TEM images (see Figure 6). As shown in Figure 6, the structure of nylon-66-based nanofibers presented a solid and dense state [33], in contrast with the porous and hollow structure of TEOS/PVP-based nanofibers [34]. As a result, the density of the structure of nylon 66 nanofibers resulted in the higher tensile strength among the nanofiber-modified cement pastes. In addition, the existence of CNTs inside  
Figure 4: The microstructure of hardened cement paste containing electrospun nanofibers: (**a**) the paste including N66 NFs; (**b**) the paste including CNTs-N66 NFs; (**c**) the paste including TEOS/PVP NFs; (**d**) the paste including CNTs-TEOS/PVP NFs.  
the electrospun nanofibers was observed from the TEM images, as shown in Figure 6. The nanofibers with a diameter of around 200 nm were strengthened by CNTs with a diameter of around 10 nm. As a result, the tensile strength of the hardened cement pastes containing the hybrid electrospun nanofibers was higher than that of the pastes containing the raw electrospun nanofibers. Besides the bridging effect, nanofibers also formed among the hydrates and interleaved among them as mentioned above. Therefore, the porous structure of the cement matrix can be filled up by these nanofibers. Consequently, more compacted structures of hardened cement pastes were formed, and greater compressive strength was observed.  
Figure 5: The bridging and filling mechanism of electrospun nanofibers in the hydrated cement paste.  
Figure 6: TEM images of electrospun nanofibers: (**a**) the hybrid nanofiber containing CNTs and nylon 66; (**b**) the hybrid nanofiber containing CNTs and TEOS/PVP.