## 4 Results  
### Scanning Electron Microscopy Observations  
The porous network of the rocks before and after the main heating cycles viewed under SEM is shown in Figure 4.  
The Chalons chalk showed a homogeneous microporous network. The main components of the rock are well-preserved coccolith fragments of different shapes, although the  
Figure 3: IRT setup. (**a**) Schema of the sample and the measurements carried out. (**b**) IRT image in false colour. (**c**) Image after the application of a subtraction and an Advanced Plateau Equalisation (APE) scale. (**d**) Cooling curves. (**e**) Interpretation of the IRT image.  
majority are elliptical with no preferred orientation. The cococolith length distribution was unimodal with a median of 4.2 \(\upmu\)m and an interquartile range of about 2 \(\upmu\)m. The contacts between the cococolith fragments were mainly punctic as well as between the micrites. The porosity was mainly composed of an interparticle microporosity inside the nanofossil cells (diameter < 1 \(\upmu\)m). A few pores of around several microns in diameter have been observed. During the TT and TF test, the microtexture shows little major modification. The chalk surface at 200(5) \({}^{\circ}\)C appeared more irregular than the fresh samples, showing more grains clustered together and pore regions with enlarged interparticle spaces.  
The Fontainebleau sandstone presented different compactness, with some areas where the grains were strongly cemented. Clumps of grains were grouped and completely silicified. The crystals had a diameter between 100 and 300 \(\upmu\)m. The shape of the porous spaces consisted of networks of channels with widths that ranged from 5 to 30 \(\upmu\)m, although some could reach 60 \(\upmu\)m. This pore size was sufficient to allow correct hydraulic connectivity. Capillary rectilinear ends (less than 1 \(\upmu\)m in diameter) were found connecting the different channels of larger size. During the TT test, the rock showed some crushed crystals that invaded the intergranular porosity and the appearance of dispersed macropores, more numerous with higher temperatures (Figure 4). During the TF test, the intergranular porosity changed in two ways, with closure and widening of boundaries, revealing a pore  
Figure 4: SEM observations of the Chálons chalk (**a**). CC, the Fontainebleau sandstone (**b**). FS, the Gris Alba granite (**c**). GA, and the Golden Ski granite (**d**). GS, in their initial state and after heating treatment. (TT: Thermal Threshold test and TF: Thermal Fatigue test).  
redistribution. Some crushed crystals were also present and intragranular microcracks became visible.  
The Gris Alba granite showed predominantly intergranular cracks in the initial state. The grain size had a diameter between 4 and 6 mm [28]. The average crack width is about 8 \(\upmu\)m [35]. After heating at 130 \({}^{\circ}\)C, the minerals increased in volume and the boundaries looked fused. Some pores of bigger size appeared, due to materials fracturing and material loss. The microcracks seemed angled, decreasing tortuosity and improving connectivity. At 200 \({}^{\circ}\)C, new microcracking appeared on the rock surface, following grain boundaries with less than 1 \(\upmu\)m width. Mica showed a notable opening of the cleavage planes, and a material detachment was observed. The pre-existing cracks became larger and slightly wider in diameter.  
The Golden Ski granite showed initially marked trans, inter, and intragranular microcracks, ranging in diameter from 5 to 50 \(\upmu\)m. The grain size had a diameter between 2 and 4 mm [28]. After heating, minor variation was observed although new microcracks appeared especially in quartz grains. The cleavage of mica was also altered showing irregularities, even fractures that increased the roughness of the granite surface.