Effect of thermal damage on mineralogical and strength properties of basic volcanic rocks exposed to high temperatures

In this study, the engineering behaviors of rocks exposed to temperatures ranging from 200 to 1000 degrees C were investigated and changes in the geomechanical properties, as well as related mineralogical changes, were examined. To reduce the effect of rapid heating and prevent the formation of new thermo-mechanical cracks, a heating rate of 10 degrees C/min was used. In addition, a heating time of 2h was applied for each temperature degree to observe the thermal heating effect. After heating, the samples were left to cool to room temperature, both dry or immersed in water. The strength of the samples gradually decreased from 128 to 25MPa with increasing temperature. The strength value decreases by about 70% for the samples cooled in air and by about 80% when cooled in water. The strength decrease was greater for the water-cooled samples because of new microcracks and glass material formed during sudden cooling. The effect of different temperature degrees on rock samples was evaluated separately and approximately 80% of total strength decrease occurred in the 600-700 degrees C range. Also, new mineral formation and melting of the existent minerals were not observed, although the structural properties were significantly changed at these temperatures. At 800 degrees C and above, some secondary minerals (e.g., melilite and augite) were formed. Partial melting was seen at temperatures higher than 1000 degrees C. In the study, a predictive model as a function of the strength for determination of thermal damage on rocks was also suggested. Some equations characterized by a correlation coefficient of 96% were suggested, based on the curves of thermal damage, to predict the uniaxial compressive strength for the designed temperature.