Engineering properties of four different rocks after heat treatment

Abstract

The effect of high temperature on engineering properties of rocks has recently become an important topic for geotechnical engineering applications due to the development in geothermal energy extraction, underground coal gasification, disposal of nuclear waste and deep mining applications. This study aims to investigate the engineering properties of four rocks heated at high temperatures and to identify the corresponding mineralogical changes. Two types of sandstone (Beaudesert sandstone and Jimboomba sandstone), basalt and argillite were subjected to a range of temperatures from 25 to 800 °C and their properties such as porosity and point load index were studied through a series of laboratory experiments. The physical and mineralogical alteration of the heated rock specimens were also investigated to better understand the thermally induced processes that occurred in each rock type using the X-Ray Diffraction, Thermogravimetry (TG), Differential Scanning Calorimetry (DSC) and Derivative Thermogravimetry (DTG) techniques. The obtained data indicated the important role of mineral composition in the thermal behaviour of the studied rocks. Noticeable changes in the mineral compositions at the threshold temperature were observed for each rock type. This behaviour was confirmed with TG/DSC/DTG analysis. This temperature-related process resulted in the formation of cracks, an increase in porosity and a decrease in strength. However, the effects of high temperatures on engineering properties were found to be different in the two sandstone types. While Beaudesert sandstone underwent significant weakening at high temperatures, only a slight change in the strength of Jimboomba sandstone was recorded. The threshold temperatures were identified as 400 °C, 400 °C, 500 °C and 300 °C, for the tested Beaudesert sandstone, Jimboomba sandstone, basalt and argillite samples, respectively. The critical temperatures (obtained using a damage coefficient) were found as 570 °C for both Beaudesert sandstone and basalt, and 440 °C for argillite. Analysis of the laboratory data revealed the difference between the threshold temperatures and the critical temperatures. It is recommended to use both threshold temperature and critical temperature parameters to provide more accurate estimations of the temperature range at which rock undergoes significant changes.