## 4 Conclusions  
In this study, the cement composites were prepared by adding various contents of MWNTs and NS. The microstructure and MWNT distribution were examined using SEM images and electrical conductivity tests. The compressive and flexural strengths of the composites were tested, and they were analyzed using thermal analysis and porosity evaluations. The experimental findings are summarized as follows. The exponential increase in the electrical conductivity with the increase in the MWNT content indicated that the MWNTs were satisfactorily distributed in the cement composites. The percolation threshold of all the composite types was shown in the range between MWNT 0% and 0.3%. The similar level of the percolation threshold with regardless of the composite type represents that contribution of NS addition to the electrical characteristics of the composites was insignificant. However, it is noteworthy that NS, which is an insulating material, did not have an adverse effect on the conductive MWNT networks. The composite type with 0.6% MWNTs and 5% NS and the composite type with 0.3% MWNTs and 5% NS exhibited enhancements of the compressive and flexural strengths of 17.2% and 52% compared to the control samples, respectively. In particular, the 17.2% enhancement rate of the compressive strength was only achieved by the simultaneous incorporation of MWNT and NS. This signified a synergistic effect of the MWNTs and NS on the compressive strengths. It was discussed that addition of NS promoted C-S-H generation and the distributed MWNT played a role of additional nucleation site to form C-S-H. However, composites with relatively large amounts of NS and MWNTs, e.g., the C1.0N5 composite, showed degradation in the mechanical strength. The porosity results agreed with the mechanical strength results. The porosity tended to decrease as the NS or MWNT content increased in the composites, but this was not applicable to the composite types that included relatively large amounts of NS and MWNTs, e.g., C1.0N5. This was likely because the densification of the microstructure developed by NS addition was minor. Figure 8: Mass losses computed at two different ranges during TGA. In the microstructural observations, it was apparent that MWNTs were distributed and embedded in the cement hydration products. However, agglomerated MWNTs were found in the C1.0N5 composite, and in the best endeavors of the authors, those were not found in other composite types. Unlike individual CNT strands, which led to a bridging and packing effects in the cement hydrate or interface, the agglomerated CNTs did not yield these positive effects, but rather they acted similarly to pores in the microstructure of the cement composites. The thermal analysis agreed with the porosity and mechanical strength results. In the composite including a relatively large amount of MWNTs and NS, C-S-H generation was reduced. This agrees with marginal changes of the porosity in the MWNT1.0 group and small changes or deterioration in the compressive and flexural strengths of the MWNT1.0 group. Additional C-S-H generation due to the reaction between NS and CH was limited due to the dense microstructure of the MWNT/NS-incorporated cement, although the incorporation ratio of NS increased. This finding endorses insignificant change of the porosity in the MWNT1.0 group and minor changes or decrease in the compressive/flexural strengths of the MWNT1.0 group which possesses relatively dense microstructure. The following are available online at [[https://www.mdpi.com/2076-341/11/152/s1](https://www.mdpi.com/2076-341/11/152/s1)]([https://www.mdpi.com/2076-341/11/152/s1](https://www.mdpi.com/2076-341/11/152/s1)), Figure S1: Additional SEM images obtained from the fractured surfaces of each composite type. Conceptualization, I-W.N. and H.-K.K. ; methodology, I-W.N. and H.-K.K. ; formal analysis, A.R., I.-W.N., and H.-K.K. ; investigation, A.R., M.B., and H.-M.S. ; data curation, A.R., M.B., and H.-M.S. ; writing--original draft preparation, A.R. and I.-W.N.