## 3 Results and Discussion  
### Compressive Strengths  
As shown in Table 3, the compressive strength at three days for MC00 (40.0 MPa) was higher than that for mixtures from MC01 (37.7 MPa) to MC05 (33.4 MPa). As time progressed (up to 91 days) MC00 developed a higher compressive strength than the other mixtures. MC00 and MC01 developed almost identical compressive strengths for 91 days, except for the result at 28 days, which indicates that the addition of 0.1 wt% CNT to the total binder weight does not significantly influence the mortar strength.  
The test results showed that an increase in the CNT dosages added to mixtures led to a decreasing compressive strength. At early ages (three and seven days), the compressive strengths ratio of MC03, MC04, and MC05 to MC00 were 80.4%, 90.9%, and 83.5% for three days and 81.2%, 86.8%, and 85.6% for seven days, respectively. However, compressive strengths for mixtures with 0.3 wt% of CNT solids developed as much as that of MC00 after 28 days.  
Thus, it can be considered that CNT hindered the strength development due to the Van der Waals forces at early ages. Low water-to-binder ratio and large amount of CNT contents in the mixtures caused greater non evaporated water-to-binder ratio and chemical shrinkage, and it was influenced on the low hydration reaction at early age due to agglomerated CNT [20]. This caused the reduced compressive strengths at early age.  
In addition, it can be considered that CNTs were not actively involved in cement hydration because they have the ability to absorb water. In theory, CNT was known for hydrophobic materials, but experimental results indicated that strength development for CNT mixtures was larger than reference  
Figure 3: Experiment setup for heating carbon nanotube-cement (CNT-cement) composite blocks.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Curing Time**} & \multicolumn{6}{c}{**Compressive Strength (MPa)**} \\ \cline{2-7}  & **MC00** & **MC01** & **MC02** & **MC03** & **MC04** & **MC05** \\ \hline
3 d & 40.0 & 37.7 & 37.1 & 32.1 & 36.3 & 33.4 \\
7 d & 49.6 & 50.4 & 46.7 & 44.2 & 43.0 & 42.5 \\
28 d & 61.9 & 58.1 & 58.6 & 56.2 & 56.0 & 55.9 \\
56 d & 63.9 & 63.9 & 61.0 & 58.2 & 59.4 & 58.0 \\
91 d & 66.1 & 65.9 & 64.7 & 63.1 & 62.1 & 62.6 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Compressive strengths of mortar mixtures.  
mixtures. It was revealed that the entropy of water molecules increases in limited nano-spaces such as nano channels with hydrogen bonds [21].  
No significant differences were observed in the flexural strengths as shown in Table 4. All mixtures exhibited the strength of less than 10 MPa at 28 days, and less than 12 MPa at 91 days. That is, CNT addition did not play an important role in flexural strength development.  
Thus, strength development was relatively small in the mixtures with a large amount of CNTs owing to internal curing effects. Figure 4 shows the compressive strength development, and the data indicated strength enhancement for mixtures with CNT after 28 days. Although several researchers asserted that CNT played a role in accelerating effect in the first 14 days [22], the experimental results as shown in Figure 4 indicated strength decrease at early age due to agglomerated CNT [20].