## 3 Results
### Mechanical Strength  
In Figure 3a, the samples were grouped as MWNT0, MWNT0.3, MWNT0.6, and MWNT1.0, and the compressive strength results are presented as a function of NS content. In the MWNT0, MWNT0.3, and MWNT0.6 groups, the compressive strengths tended to increase with the addition of NS, and the largest increment reached 8.6%, 9.5%, and 29.2% compared to the strength of the composite without NS, respectively. However, the increasing tendency was not pronounced for the MWNT1.0 group. In the MWNT1.0 group, enhancements of the compressive strength were achieved by the incorporation of 1% and 3% NS, but the compressive strength dropped due to the incorporation of 5% NS, which is considered to be a relatively large NS content. Figure 3b shows that compressive strengths of the NS0, NS1, NS3, and NS5 groups varied as the content of MWNTs increased. In all the groups, the increase in the compressive strength was significant when the MWNT contents were 0.3% or 0.6% compared to the strength of the composite without MWNTs. The largest increases were 5.2%, 21.2%, 3.9%, and 8.0% in the NS0, NS1, NS3, and NS5 groups, respectively. In the view of the compressive strength results of all the composites, it is noteworthy that enhancement of the strength was the largest in the composite with 0.6% MWNTs and 5% NS (corresponding to a strength of 114.8 MPa), which was 17.2% greater than the strength of the control with no MWNTs and NS. The best enhancement rate and strength level were not achieved by the incorporation of a single nanomaterial, that is, either MWNT or NS alone. This signified a synergistic effect of the MWNTs and NS on the compressive strengths of the composites. However, in the composite groups having relatively large amounts of NS, such as the NS3 and NS5 groups, the compressive strength decreased due to the incorporation of 1% MWNTs, which is considered to be a relatively large MWNT content. [PERSON] et al., 2019 demonstrated that compressive strength increased when appropriate amount of MWNT was incorporated in the cement composites, and discussed that this outcome was accomplished due to that distributed MWNT played a role of additional nucleation site to form C-S-H [30]. Accordingly, it can be said that appropriate amount of MWNT can provide additional nucleation site for C-S-H formation, and addition of NS promoted C-S-H generation, which ultimately lead to synergistic effects in the compressive strength. Figure 4a shows the flexural strength results of the MWNT0, MWNT0.3, MWNT0.6, and MWNT1.0 groups as a function of the NS content. Similar to the compressive results shown in Figure 3a, enhancements of the flexural strength with the addition of NS were evident in the MWNT0, MWNT0.3, and MWNT0.6 groups, and the greatest increases  
Figure 3: Compressive strength results of the MWNT/NS-incorporated cement composites after 28 days of curing: (**a**) NS content varied and (**b**) MWNT content varied. reached 30.6%, 9.9%, and 14.3%, respectively. In contrast, a decrease in the flexural strength was evident in the MWNT1.0 group, of which the MWNT content was relatively large, and this outcome was similar to the compressive strength results of the MWNT1.0 group shown in Figure 3a. Figure 4b shows that the flexural strengths of the NS0, NS1, NS3, and NS5 groups changed as the MWNT content ratio varied. In all the groups, the compressive strength increased due to the incorporation of 0.3% or 1.0% MWNTs compared to the strength of the composite without MWNT. The largest increases in the NS0, NS1, NS3, and NS5 groups were 56.2%, 36.1%, 18.5%, and 16.7%, respectively. However, in the composite groups fabricated with relatively large amounts of NS, such as the NS3 and NS5 groups, it was found that the incorporation of 1% MWNTs led to a marginal enhancement or deterioration of the strength.