Microstructural evolution of granitic stones exposed to different thermal regimes analysed by infrared thermography

Detailed knowledge of the behaviour of rocks under thermal stress is essential in a variety of fields such as the exploitation of oil and mineral resources, the geothermal sector, the storage of radioactive liquid waste, or even CO2 capture and storage. Granites are widely studied and exploited in these fields, and they show different reactions to hightemperature and thermal cycles due mainly to their high mineralogical and textural heterogeneity. One of the features that influences the most the thermal response is the porosity. The objective of this study is to evaluate the influence of porosity when these rocks are exposed to different thermal treatments. For that purpose, experiments were carried out on four granitoids selected by their similar crystal size, but with variable mineral proportion and porosity values, ranging from 1 to 6%. Two kinds of tests were performed: i) progressive heating cycles from 90 degrees C to 130 degrees C to determine the critical threshold for thermal damage; ii) thermal fatigue with cycles of heating-cooling up to 200 degrees C. The porosity and the water transport phenomena of the samples were characterised before and after each cycle by the monitoring of capillary water uptake coupled with infrared thermography. This technique allowed to follow the capillary fringe migration during the test and the evolution of the cooling rate index. The direct assessment of the damage was carried out by mercury injection porosimetry, optical polarising microscopy, and scanning electron microscopy. The combination of all the results permitted to establish a link between the evolution of temperature and the modification of porous networks in granitoids. Microcracks appeared distinctly at a temperature between 90 degrees C to 130 degrees C for high porosity granitoids whose Quartz/Feldspar ratio was close to 1. For higher temperatures, the low porosity granitoids develop microcracks from the first heating cycle. The porosity then showed a stronger impact on thermal behaviour than the effect of the mineralogy. The results obtained from infrared thermography allowed to detect the strong variations in the microstructure.

Automated summary

The study aimed to assess the behavior of granitoids with different porosities under various thermal treatments. Results showed that porosity has a greater impact on thermal behavior than mineralogy, with high-porosity granitoids developing microcracks between 90°C and 130°C.