Quantification of thermally-induced microcracks in granite using X-ray CT imaging and analysis

摘要

The influence of heating followed by cooling on the alteration of the microstructural properties of Harcourt granite was comprehensively investigated utilising X-ray computed tomography (CT) imaging and three-dimensional (3-D) image reconstruction and analysis. Two cooling treatments, rapid and slow cooling, were adopted with preheating temperatures ranging from 25 to 1000 degrees C. The pore spaces of the thermally-treated specimens were analysed to obtain their porosity and pore connectivity and to generate pore network models (PNMs). According to the results, no significant microcracking occurs up to the preheating temperature of 200 degrees C and regardless of the cooling treatment, microcracks start to emerge from the outer boundary of the rock specimen at 300 degrees C during heating followed by cooling treatments. Further, coalescence of microcracks into broader intergranular cracks along feldspar, quartz and biotite grain boundaries was observed beyond 400 degrees C and intragranular cracks formed beyond 800 degrees C. Mineral stability analysis revealed considerable unstable deterioration of quartz and feldspar grains subjected to thermal stresses during heating and cooling processes, while biotite minerals showed more stable behaviour against thermal shock-induced microcracking. In addition, thermal microcracking-induced rock matrix deterioration was more significant in specimens subjected to rapid cooling compared to slow cooling, which is probably due to thermal shock-induced stress accumulation in the rapidly cooled rock matrix. Analysis of the local axis connectivity along the height of the rock specimens showed that most of the induced pores are interconnected in the specimens subjected to rapid cooling compared to the slowly cooled specimens. According to the results, no significant microcracking occurs up to the preheating temperature of 200 degrees C and regardless of the cooling treatment, microcracks start to emerge from the outer boundary of the rock specimen at 300 degrees C during heating followed by cooling treatments. Moreover, the range of the equivalent radii of the pores in the PNM widened from 1-2000 mu m to 1-3300 mu m, and from 1-1000 mu m to 1-2700 mu m with the increase of preheating temperature from 600 degrees C to 800 degrees C for rapidly cooled rock specimens and slowly cooled specimens, respectively. A similar phenomenon was observed for the equivalent radii of the throats in the PNM. At temperatures beyond 800 degrees C, regardless of the cooling treatment, more complex and extended pore networks tend to be formed inside the rock pore matrix due to thermal deterioration.