## 3 Results and Discussion
### Isothermal Calorimetry  
Experiment results for normalized heat flow are presented in Figure 5. Data on PC00 and PC01 were plotted in Figure 5a,b to draw a comparison with PC02, PC03, PC04, and PC05 in terms of cement hydration reaction periods. Early age heat was produced by cement dissolution to form an amorphous layer of hydration products on the cement particles. [PERSON] et al. [23] used isothermal calorimetry and established that the heat peak appearing during the first couple of hours was due to the formation of dehydrated gypsum from calcium sulfate hemi-hydrate. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Curing Time**} & \multicolumn{6}{c}{**Flexural Strength (MPa)**} \\ \cline{2-7}  & **MC00** & **MC01** & **MC02** & **MC03** & **MC04** & **MC05** \\ \hline
3 d & 9.7 & 9.0 & 8.8 & 8.5 & 9.3 & 9.3 \\
7 d & 9.4 & 9.1 & 9.1 & 8.9 & 9.5 & 9.2 \\
28 d & 9.4 & 9.5 & 9.2 & 9.2 & 9.4 & 9.5 \\
56 d & 11.3 & 10.9 & 10.9 & 10.5 & 10.7 & 10.4 \\
91 d & 11.4 & 11.4 & 11.0 & 10.6 & 11.2 & 11.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Flexural strengths of mortar mixtures. Figure 4: Compressive strength development as a function of time. The induction and acceleration periods of PC00 began after an hour and twenty hours, respectively, as shown in Figure 5. The induction periods were initiated after the formation of a protective layer on the C\({}_{3}\)S particles [24; 25] and were complete when this layer was broken and C\({}_{3}\)S particles were used to form C-S-H and Ca(OH)\({}_{2}\), causing the hydration reaction rates to eventually slow down [26]. That is, Alite was used to form C-S-H and Ca(OH)\({}_{2}\), and the induction period was complete when the layer was broken with C\({}_{3}\)S particles. Setting takes place during the acceleration period, and the silicate begins to quickly hydrate with a maximum rate of heat release [27]. The maximum heat for the second peak of PC00 between the acceleration period and the retardation period was 4.49 mW/g as shown in Table 5. It was observed that maximum peaks of the second curves between the acceleration and retardation periods from PC01 to PC05 were less than that of PC00. An increase in the dosage of CNTs reduced the peak heat because cement hydration at an early age was impeded by CNT particles. However, setting delays were not observed in all mixtures, which means that CNT addition did not significantly influence the setting times of cement composite hardening. It can be considered that setting delays directly shift the acceleration and retardation curve positions to the right-hand side [28]. Cumulative heat at an early age for PC00 and PC01 was higher than those for other PC mixtures. Even though a couple of mixtures, PC03 and PC04, developed higher cumulative heat in some regions, the total heat released for PC00 was the highest. The amounts of heat released from the experiments for PC01 to PC05 were in the order of large to small as shown in Figure 6 and Table 5. \begin{table}
\begin{tabular}{c c c} \hline
**Specimen** & **Heat Flow for the Second Peak (mW/g)** & **Cumulative Heat (cal/g)** \\ \hline PC00 & 4.49 & 54.4 \\ PC01 & 4.44 & 53.5 \\ PC02 & 4.27 & 52.2 \\ PC03 & 4.42 & 51.4 \\ PC04 & 4.39 & 51.3 \\ PC05 & 4.31 & 50.5 \\ \hline \end{tabular}
\end{table} 
Table 5: Maximum heat flow for the second peak and cumulative heat. Figure 5: Specific heat flow of paste samples compared to PC00 and PC01 as a function of time, (**a**) PC02, PC03, (**b**) PC04, PC05. The cumulative heat difference between PC00 and PC05 at 108 hours was 3.9 cal/g, and it does not seem to be significant. It can be considered that 28% water-to-binder ratio and the addition of fly ash does not considerably influence the hydration heat. Thus, a 7.2% difference in hydration heat is large enough to explain the strength development shown in Figure 4.  
Owing to the Van der Waals forces between CNT particles, early age hydration reaction was slow from the experiment result.