## 5 Discussion
### Thermal Threshold Test (TT)  
The progressive heating of the selected stones up to 130 \({}^{\circ}\)C produced changes more or less remarkable depending on the initial microstructure and composition quantified by MIP, capillary water uptake, and CRI (Figure 8). The Chalons chalk (CC) is a microporous rock with a void volume of around 40%. That implies an easy readjustment of the calcite expansion during heating that avoids the microcrack formation. Thus, very slight changes were observed in the properties related to their microstructure, that is, capillary water absorption and MIP (Figure 8). Despite the calcite solubility, the interaction times between this mineral and water were too short during the tests to produce any chemical alteration on chalks [62; 63] or uniform dissolution between intergranular contacts [64]. Most of the studies on the behaviour of chalk with increasing temperatures (around 100-130 \({}^{\circ}\)C) revealed that no mechanical variations were produced [3; 65; 66; 67; 68; 69], although a few times a slight variation of the elastic modulus was quantified [64; 66; 70]. However, the tensile forces on the outer layers during heating may produce weakness in some boundaries that may lead to a granular disaggregation or crushing on the surface [46]. The SEM observations after treatment showed that the porous structure of the chalk consisted of a more heterogeneous zone with the grouping of numerous particles in the same place. The surface roughness for natural chalk is on the order of only a few micrometers and cycles of wetting and drying can erode its surface [71]. This erosion was mainly due to the detachment of the particles. These observations suggest that the study of the surface roughness of the chalk is an important parameter for evaluating the cooling rate index (CRI) because it depends on the exchanges between the surface and the environment. An augmentation in roughness is a valuable  
Figure 8: Variations of the capillary coefficient (C), the cooling rate index (CRI), and the porosity (MIP) along the Thermal Threshold test (TT). hypothesis to explain the increase in the CRI after some heating cycles and capillary tests (Figure 9a). Figure 9: Diagram of the evolution of the porous 2D network of the Chálons chalk (**a**), Fontainebleau sandstone (**b**), and Gris Alba (**c**) according to the temperature. The Fontainebleau sandstone (FS) is composed mainly of quartz, a mineral with high and anisotropic thermal expansion. Microstructural variations through TT were observed with a slight increase in the pore volume and pore access radii under 140 \(\upmu\)m and a decrease in the capillary water uptake kinetics. Acoustic emission studies revealed microcracking in sandstones at temperatures as low as 65 \({}^{\circ}\)C [72] and 100 \({}^{\circ}\)C [73], while [74] discovered grain boundary cracking at temperatures under 125 \({}^{\circ}\)C. The trend of this microstructural variation is aleatory, with an increase in crack density measured by [75] and a decrease in porosity by [76]. Localisation, connectivity to porosity, and the opening of microcracks are important parameters in the analysis of fluid transfer [77]. It has been observed that the water rises more slowly in the wider parts of the porous network of the Fontainebleau sandstone [47]. The SEM images (Figure 4) and the increase in the CRI (Figure 8) helped to schematize the FS behaviour during the TT test (Figure 8(b)). In its initial state, the flow of capillary water uptake was regular thanks to the homogeneous and continuous distribution of the entire pore volume. The size of the flow paths was small (radius centred about 1 \(\upmu\)m) although pores of smaller size were present because of the strongly cemented areas locally. As described by [46], a fine zone of microcracking on the outside of the samples (tensile stresses) and a thick zone of crack closure in the center (compressive stresses) can be observed on rocks following thermal stress.