## 3 Results and Discussions
### Discussion  
The increasing compressive results when applying heat curing can be explained in that the heat curing enhances the microstructure and results in increasing strengths and fracture energy in a shorter time than the standard curing procedure [5; 25; 26]. This is because heat curing accelerates the hydration reactions of cement and the pozzolanic reaction of mineral admixtures, i.e., SF, FA, and calcium hydroxide generated from the hydration of cement. This was also confirmed by the research results of [PERSON] and [PERSON] [5] regarding the pozzolanic reaction ratio of UHPFRC heat treated at 70 \({}^{\circ}\)C, 90 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 250 \({}^{\circ}\)C and 400 \({}^{\circ}\)C, compared with UHPC cured at 20 \({}^{\circ}\)C. This research also revealed that  
Figure 5: Typical normalized 28-day compressive strength of UHPC with fly ash. the pozzolanic ratio of UHPC heat treated at 90 \({}^{\circ}\)C was 90% for soft cast samples and 93% for pressed cast specimens, whilst this was only 72% for soft cast specimens and 82% for pressed cast specimens which were cured at 20 \({}^{\circ}\)C. Therefore, heat curing condition is known as one of the important principles to make UHPC. Moreover, it can be seen from the experimental results that the UHPC specimens using a combination of 10% SF and different FA contents attained the different highest 28-day compressive strength with the different _W/B_ ratios and curing conditions, which are given in Table 4.  
For a very interesting point, a hyperbolic relationship was observed between the highest compressive strength \(f_{c}^{\prime}\) of UHPC and FA content, as presented in Figure 6. The ratios of the highest \(f_{c}^{\prime}\) of UHPC with early heat curing relative to that of the counterpart UHPC cured under the standard environment corresponded to approximately 1.22, irrespectively of the variation of FA content. This implies that the increasing ratios in \(f_{c}^{\prime}\) of UHPC due to early heat curing were independent of \(R_{f}\). Furthermore, when compared with the highest \(f_{c}^{\prime}\) of the UHPC with \(R_{f}=0\) under the standard environment, UHPC with early heat curing exhibited a higher value when \(R_{f}\) was 50% and slightly lower value when \(R_{f}\) was 50% for UHPC with early heat curing, while considering a comparable value to the compressive strength of UHPC with no FA and cured under the standard environment. Based on the results from Figure 6, it can be also proposed that the optimum UHPC composition to attain the compressive strength of 120 MPa includes the total mineral admixture content of about 62.5% (10%SF + 52.5%FA) and over 80% (10%SF + 70%FA) under standard curing and heat curing conditions, respectively. It means that UHPC can be produced with only about 220 and 450 kg cement per m\({}^{3}\) under these corresponding curing conditions. Recently, the desired compressive strength of UHPC is up to 150 MPa or higher, and this leads to the total mineral admixture content of about 62.5% (10%SF + 52.5%FA) or about 450 kg cement per m\({}^{3}\) which can be used to produce UHPC under the heat curing condition (Figure 6). This plays an important role in the research and development of sustainable construction. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mix**} & **The Highest 28-Day** & \multirow{2}{*}{**Curing Condition**} & \multirow{2}{*}{**W/B** **(by Mass)**} & **SF** & **FA** \\  & **Compressive Strength (MPa)** & & & **(\% by Mass)** & **(\% by Mass of** **Binder)** & **(\% by Mass of** **Binder)** \\ \hline
1 & 146.0 & Standard curing & 0.16 & & \\
162.0 & Heat curing & 0.14 & & 10 & 0 \\
2 & 149.0 & Standard curing & 0.16 & & \\
164.0 & Heat curing & 0.14 & & 10 & 20 \\
3 & 142.0 & Standard curing & 0.14 & & \\
161.0 & Heat curing & 0.14 & & 10 & 30 \\
4 & 134.0 & Standard curing & 0.14 & & \\
155.0 & Heat curing & 0.14 & & 10 & 50 \\
5 & 92.0 & Standard curing & 0.12 & & \\
124.0 & Heat curing & 0.12 & & 10 & 70 \\ \hline \hline \end{tabular}
\end{table}