## 3 Results and Discussion
Figure 9: Scatter plot of bulk density values of heat-treated granite samples of this paper compared to previous works. of the monzogranite. At room temperature, an average of 51 decreases to 49 at 250 \({}^{\circ}\)C. Rebound values at 500 \({}^{\circ}\)C show another slight decrease in surface hardness with an average of 47. After 750 \({}^{\circ}\)C heat-treatment, a significant 63% decrease in Duroskop mean values occurred compared to room temperature averages with a mean of 19 (Figure 10). This tool has been proved to describe well small surface strength changes in heritage stones associated with weathering [45; 46; 47]. It has only been recently applied to test the surface strength of heat-treated stones such as granitic rocks [59], suggesting strength loss with increasing temperature [60]. Our results are in good agreement with these previous studies [59; 60], namely, values around 50 signify high surface strength, while values less than 20 represent weakened surfaces. P-wave velocity data of PUNDIT instrument in heat-treated groups decrease from 22 \({}^{\circ}\)C to 750 \({}^{\circ}\)C with average values of 5.29 km/s (22 \({}^{\circ}\)C), 4.49 km/s (250 \({}^{\circ}\)C), 3.32 km/s (500 \({}^{\circ}\)C) and 0.76 km/s at 750 \({}^{\circ}\)C. Data shows that at 750 \({}^{\circ}\)C, a significant 87% decrease in mean values occur from room temperature to heat-treated average values. The results of GEOTRON instrument P-wave velocity values show similar results as PUNDIT values. At room temperature, an average of 5.28 km/s decreases to 4.56 km/s at 250 \({}^{\circ}\)C. At 500 \({}^{\circ}\)C, the mean P-wave velocity of the specimens was 3.39 km/s. S-wave velocities show a similar decrease in velocity values. At 22 \({}^{\circ}\)C 3.23 km/s average slows down to 3.00 km/s at 250 \({}^{\circ}\)C. At 500 \({}^{\circ}\)C, a mean of 2.40 km/s occurs at the samples (Figure 11). Figure 11: Relative frequency histograms of measured ultrasound velocity properties among heat-treated groups: Histogram of P-wave velocity (PUNDIT device) values (**left**). Histogram of P-wave velocity (GEOTRON device) values (**middle**). Histogram of S-wave velocity (GEOTRON device) values (**right**). Figure 10: Histogram of Duroskop rebound values, the surface hardness of specimens. GEOTRON instrument P-wave velocity values show a high correlation with PUNDIT instrument values with an R-value of 97%, and therefore, presents the same trend and similar values as PUNDIT P-wave values. This high correlation value shows that the quick and easy-to-use PUNDIT-, and the more accurate but time-consuming GEOTRON instrument results can be used side by side. The sensitivity of the GEOTRON instrument did not allow to record the P-, (nor the S-wave) velocity measurements of 750 \({}^{\circ}\)C heat-treated samples, however, the PUNDIT measurement could measure these values. For further studies, these results could be used as complements to each other. S-wave velocity values of the GEOTRON instrument show a similar tendency as the P-wave velocity value decreases among heat-treated groups. At room temperature of 22 \({}^{\circ}\)C, the mean pulse velocity is 3.23 km/s, dropping to 3.00 km/s at 250 \({}^{\circ}\)C. It shows a further decrease to 2.40 km/s after 750 \({}^{\circ}\)C heat-treatment. S-wave and P-wave velocity values of the GEOTRON instrument show a high linear correlation with an R-value of 94% (Figure 12). Ultrasound velocity values as P- and S-wave velocity values from both PUNDIT and GEOTRON instruments show the best way to quantify physical changes in the samples after heat-treatment from the examined measurements. Phase alteration- and thermal expansion generated crack development processes, which tend to slow down ultrasound velocities in the samples [14; 15]. Different heat-treated subgroups form separate data populations and show a continuous decrease in velocities between heat-treated groups. P-wave velocity values decrease similarly in this research and the examined literature [14; 54; 55; 56; 57].