## 3 Results and Discussion
### Mechanical Properties of the DP/Gypsum Composite  
Figure 8 shows the compressive strength results of all samples at 7 days. The compressive strength of the control mix (G-C) was 18.80 MPa. When 10% and 20% DP was added, the compressive strengths of the G-10%DP and G-20%DP samples decreased by 3.7% and 10.6%, respectively, when compared to the control (G-C). Although the flexural strength of G-10%DP is higher than the control (see Figure 9), it seems that the strength of gypsum generally decreased with increasing DP content. This may be due to the change in the microstructure of the materials. Figure 10 shows the microstructure of the composite materials. As shown in Figure 10a, DP is a discontinuous phase in the gypsum matrix, while diatomic is a porous material (see Figure 2). Additionally, the interfaces between diatomic and gypsum are weak. Therefore, the mechanical strengths of DP/gypsum decreased with the increase in DP content.  
CF, with high tensile modulus values ranging from 230 to 500 GPa, can improve the mechanical properties of the DP/gypsum composites. When 1 wt. % carbon fiber was added, the compressive strengths of G-10%DP-1%CF and G-20%DP-1%CF increased by 29.8% and 9.0%, respectively, compared to the control (G-C). The effect of CF on flexural strength is much more significant. As seen in Figure 9, the flexural strengths of G-10%DP-1%CF and G-20%DP-1%CF increased by 176.0% and 56.0%, respectively, when compared to the control. This remarkable improvement is mainly due the high strength and toughness of CF, as well as its pull-out effect on the gypsum matrix that restrains the crack development. Specifically, as shown in Figure 10b,c, with an average diameter of 7 \(\upmu\)m and  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Thermal** & **T\({}_{\mathbf{M}}\)** & **\(\Delta\)H\({}_{\mathbf{M}}\)** & **T\({}_{\mathbf{F}}\)** & **\(\Delta\)H\({}_{\mathbf{F}}\)** &
\begin{tabular}{c} **Average Enthalpy** \\ **((\(\Delta\)H\({}_{\mathbf{M}}\) + \(\Delta\)H\({}_{\mathbf{F}}\))/2)** \\ \end{tabular} \\ \hline
**0** & 22.94 \({}^{\circ}\)C & 13.75 J/g & 21.75 \({}^{\circ}\)C & 18.75 J/g & 16.25 J/g \\
**300 times** & 22.77 \({}^{\circ}\)C & 15.05 J/g & 21.45 \({}^{\circ}\)C & 17.36 J/g & 16.21 J/g(\(-\)0.04 J/g) \\
**600 times** & 22.79 \({}^{\circ}\)C & 14.02 J/g & 21.37 \({}^{\circ}\)C & 16.89 J/g & 15.46 J/g(\(-\)0.79 J/g) \\ \hline \hline \end{tabular}
\end{table}
Table 3: The phase change temperature and enthalpy after thermal cycles of 0, 300 and 600 times.  
Figure 7: DSC curves of diatomic/paraffin with thermal cycles of 0, 300 and 600 times.  
length of 3 mm, can cross the pores or defects (<100 um) that exist in the cement matrix, and thus it is able to restrain the crack development and separation under high tension and compression strengths. According to the test results, CF is considered effective in enhancing the mechanical properties of DP/gypsum, and the G-10%DP-1%CF composite displayed the best mechanical properties among the samples tested.  
Figure 8: The compressive strength of the diatomite/paraffin/gypsum composites after 7 days curing.  
Figure 9: The flexural strength of the diatomite/paraffin/gypsum after 7 days curing.