The influence of thermal damage and cooling method on some physical properties (i.e. open porosity, ultrasonic waves propagation, elasticity and uniaxial compressive strength) of the San Julian's calcarenite stone has been studied. Samples were previously heated at different temperatures (from 105°C to 900°C). For each temperature, the samples were divided into two groups of five units: one group was air-cooled and the other group was water-cooled. The properties obtained with the group that was heated at 105°C were taken as reference. Non-destructive tests (porosity and ultrasonic waves propagation) and destructive tests (uniaxial compressive strength and deformability) were performed over available samples. The results show that uniaxial compressive strength and elastic modulus decrease as the temperature increase for the tested range of temperatures. A reduction of the uniaxial compressive strength up to 35% and 53% is observed in air-cooled and water-cooled samples respectively when the samples are heated to 600°C. In fact, this is the more sensitive parameter to cooling method. Regarding the Young's modulus, there has been a decline of about 80% with both cooling methods at 600°C. Heating temperatures of 700°C and above represents the complete degradation of the material. Other physical properties, closely related with the mechanical properties of the stone, are porosity and propagation velocity of ultrasonic waves in the material. All of them exhibit considerable changes with temperature. Excepting for porosity, the changes produced by the heat treatment are amplified when cooling is realized by immersion in water.
Fire produces physical and chemical changes, from certain temperatures, in the internal structure of the rocks. Rocks are composed of minerals, bounding matrix, cracks and pores. The geometry and density of the cracks and pores are the main controlling parameters for the physical properties of rocks (Yavuz et al., 2010). The temperature induces micro cracks in the material due to the different nature of the constituent minerals (intergranular) (Jansen et al., 1993) or within grains (intragranular). The intragranular microcracks can occur when any of the mineral undergoes a qualitative change or phase transition (such as the case of α/β phase transition in quartz) (Glover et al., 1995). The intergranular cracks are due to the different expansion coefficients of the component minerals, causing a differential expansion thereof with temperature, generating internal stresses resulting in the creation of cracks in the transition phase between components (Jansen et al., 1993). When temperature changes occur in a very short time, intergranular cracks occur by another different process than the previous ones: high temperature gradients in the material.