## 3 Results and Discussion
This change is linked to a volume increase. Bulk density values after 750 \({}^{\circ}\)C of heat-treatment tend to split into two subgroups with means of 2559.3 and 2361.3 kg/m\({}^{3}\) (Figure 7). Bulk density values decreased from 22 \({}^{\circ}\)C to 250 \({}^{\circ}\)C, followed by a slight increase to 500 \({}^{\circ}\)C. At 750 \({}^{\circ}\)C, they are indicating a significant drop in bulk density values. In terms of mass and volume change--as the primary control factor of bulk density change--a minor but continuous decrease in mass appear in the groups, but volume change tends to differ. Volume increased from 22 \({}^{\circ}\)C to 250 \({}^{\circ}\)C (+1.2%) and further increases to 500 \({}^{\circ}\)C, which was followed by a significant rise in volume at 750 \({}^{\circ}\)C (+8,3%). As the heat expansion difference of the main rock-forming minerals and phase alterations (such as \(\alpha\)-quartz to \(\beta\)-quartz alteration at 573 \({}^{\circ}\)C) occur, significant volume increases, and thereby, cracks develop in the structure of the monzogranite samples [14; 15; 53; 54]. The heat-induced cracks might \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Thermal** \\ **Treatment** \\ **Group** \\ \end{tabular} } & \begin{tabular}{c} **Bulk** \\ **Density** \\ **(Std)** \\ \end{tabular} & \begin{tabular}{c} **Duroskop** \\ **Value** \\ **(Std)** \\ \end{tabular} & \begin{tabular}{c} **P-Wave Vel.** \\ **[PUNDIT]** \\ **(Std)** \\ \end{tabular} & \begin{tabular}{c} **P-Wave Vel.** \\ **[GEOTRON]** \\ **(Std)** \\ \end{tabular} & \
\begin{tabular}{c} **S-Wave Vel.** \\ **[GEOTRON]** \\ **(Std)** \\ \end{tabular} \\ \cline{2-6}  &
\begin{tabular}{c} **(kg/m\({}^{3}\))** \\ \end{tabular} & **(-)** & **(km/s)** & **(km/s)** & **(km/s)** \\ \hline
22 \({}^{\circ}\)C & 2716.0 (23.6) & 51 (1.5) & 5.29 (0.24) & 5.28 (0.32) & 3.23 (0.22) \\
250 \({}^{\circ}\)C & 2678.1 (34.0) & 49 (1.7) & 4.49 (0.15) & 4.56 (0.18) & 3.00 (0.12) \\
500 \({}^{\circ}\)C & 2694.7 (27.1) & 47 (2.4) & 3.32 (0.19) & 3.39 (0.12) & 2.40 (0.15) \\
750 \({}^{\circ}\)C & 2493.3 (125.0) & 19 (5.7) & 0.76 (0.23) & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Bulk density, Duroskop rebound value, P-wave velocity (PUNDIT and GEOTRON), and S-wave velocity (GEOTRON) results in heat-treated groups. Figure 7: Histogram of bulk density values. develop at lower temperatures [24], but at elevated temperatures (750 \({}^{\circ}\)C) can be seen macroscopically on the surface of the specimens (Figure 8). These factors contribute to the bulk density changes. Comparing the measured heat-treated granite results reported in the literature [14; 55; 56; 57; 58], the bulk density value changes after heat treatment shows similar trends as reported in this paper. Similar bulk density value decreases from room temperature to around 200 \({}^{\circ}\)C and a similar increase occurs from \(\sim\)200 \({}^{\circ}\)C to \(\sim\)500 \({}^{\circ}\)C, which is also documented in the literature [55; 56]. From 500 \({}^{\circ}\)C to 800 \({}^{\circ}\)C, a significant decrease in bulk density values occur similarly in the literature as in this paper; thus, the measured data in this research match the processes known from other similar works (Figure 9). Duroskop rebound values show that surface strength also decreases after heat treatment (Figure 10) besides the granite's mass-, volume (Figure 7), phase-, and color changes (Figure 7). The surface strength of the samples slightly decreases from room temperature to 250 \({}^{\circ}\)C and 500 \({}^{\circ}\)C and significantly drops at 750 \({}^{\circ}\)C. The phase alteration- and thermal expansion-driven crack development [14; 15] significantly weakens the surface strength  
Figure 8: Overview images of a (22 \({}^{\circ}\)C) room temperature (**left**) and a heat-treated (750 \({}^{\circ}\)C) sample (**middle**). Examine the crack development on the magnified surface of the 750 \({}^{\circ}\)C heat-treated specimen (**right**).