| Summary: | Cylindrical specimens obtained from the monzogranite host rock of the National Radioactive Waste Repository of Hungary were tested at room temperature and 250 °C, 500 °C, and 750 °C of heat treatment. Reflectance spectra (color), bulk density, Duroskop surface hardness, and ultrasound-wave velocity values were measures before and after thermal stress. According to CIE L*a*b* colorimetric characteristics, the specimens’ color became brighter and yellower after the heat treatment. At 750 °C, a significant volume increase was recorded linked to the formation of macro-cracks, and it also led to the drop in bulk density. Smaller temperature treatment (250 °C) caused a minor decrease in density (−1.3%), which is higher than the reduction of density at 500 °C (−0.8%). Duroskop surface strength showed a slight decrease until 500 °C, and then a drastic decline at 750 °C. P- and S-wave velocity values tend to decrease uniformly and significantly from room temperature to 750 °C. P-wave velocity and Duroskop values have a high exponential correlation at elevated temperatures. Physical alterations originated from the differential thermal-induced expansion of minerals, the formation of micro-cracks. Mineralogical changes at higher temperatures also contribute to the volume change and the loss in strength.
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