Résumé
The structural evolution of four granites, heated from 20 to 700 degree C, was examined after cooling. The granites have four typical initial structures, which can be sub-divided into two groups: rocks with high connected porosity; comprising cracks with low aspect ratio and alteration pores (type A), pores and a few cracks (type B), or cracks (type C); and rocks with low connected porosity, with cracks (type D). The connected porosity was measured by mercury injection, and the total porosity by X-ray computerized tomography. At low temperature (T < 300 degree C), the connected porosity decreases with increasing temperature, particularly for the material with high crack density and connected porosity (A and B). The total porosity increases for B, C and D, and decreases for the material with low aspect ratio cracks (A). Above 300 degree C, both porosities increase. The data show that the thermal history of a granite can be decomposed into two phases. In the first stage the connected porosity decreases with increasing temperature, cracks are initiated at low temperature and they form unconnected networks. In the second stage, the connected and total porosities increase, the previously initiated cracks form a connected network by opening, and they interact with newly created cracks.