Physical Alteration and Color Change of Granite Subjected to High Temperature

Abstract

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.

Mineralogical Transformations in Granitoids during Heating at Fire-Related Temperatures

“…Colour parameters, such as chroma and lightness, change in quartz and feldspar due to microcracking development [14]. Hue change is also noticeable in mica, clays, and iron oxides mainly due to iron thermal oxidation, which causes yellowing and reddening [12,[14][15][16][17][18][19]. Volume increase and roughness changes as a consequence of thermal expansion have been also evaluated [14,20,21].…”

“…One of the main objectives of heating tests of rocks is assessing how their cracking dynamics and strength decrease with temperature [22][23][24]. For this purpose, previous research used non-destructive indirect methods such as p-waves velocity, capillary water uptake, and porosity variations [12,13,19,[25][26][27], as well as direct methods such as compression or flexural strength tests [24,28,29]. Most of these tests were carried out after the rock specimens were heated to a certain target temperature and cooled down to room temperature.…”