DIRECT MICRO X-RAY COMPUTED TOMOGRAPHY (μCT) STUDY OF STRUCTURAL MODIFICATIONS OF CERAMICS INDUCED BY FREEZE-THAW CYCLES

Ceramics are multiphase materials usually obtained by the firing of a mixture of clay, water and non-plastic inclusions. According to the raw materials and the production cycle used, ceramics can have different physical properties and, hence, different uses and applications. One of the field of applications in which ceramics are most used is masonry. Like other building materials (concrete, mortars, steel, etc.), ceramics must be subjected to several tests and must follow particular recommendations before the commercialization. One of these tests concerns the assessment of the ability of a ceramic to withstand freeze-thaw stresses (UNI EN 12371). This property is very important in particular for applications where temperature fluctuates around 0 °C. In these conditions, the cycling formation and dissolution of ice may cause the formation of fissures and cracks which can seriously damage the ceramics. In order to understand the causes that may led to structural and microstructural modifications of the ceramics, scanning electron microscopy (SEM) or optical microscopy (OM) observations are usually employed after freeze-thaw tests. However, for this kind of analyses, samples should be usually cut in thin sections or embedded in a resin that can cause further microstructural changes and do not allow to analyse the same sample after a series of freeze-thaw cycles. On the contrary, high-resolution micro x-ray computed tomography (μCT) is a non-destructive powerful method to investigate samples before and after treatments, without any sample preparation. In the present work, μCT equipped with a thermal stage has been used for the characterization of ceramic microstructures after several freeze-thaw cycles. Ceramics were prepared by mixing a kaolinitic clay with 10% (w/w) of water, molding by uniaxial pressing and then fired at 750 and 1000 °C. In order to study the effect of non-plastic inclusions, 5 % (v/v) of quartz or limestone were also added to the mixture. Six samples of 5x5x10 mm (per ceramic) were cut from ceramic briquettes and were subjected to freeze-thaw cycles according to the UNI EN 12371. One of the 6 samples, was also studied via μCT using a cooling stage in order to keep the temperature at -12°C. μCT was equipped with a W source (80 kV and 125 mA) and the following parameters were set up for the analyses: pixel size of 2.4 μm, rotation step of 0.13 deg (0-180 deg) and an exposure of 1980 ms. A 1 mm thick Cu filter was used to improve the signal to noise ratio. Results suggest that the presence of quartz reduces the resistance to freeze-thaw cycles, because the cracks produced by the α-β quartz phase inversion propagates into the ceramics after the crystallization of ice. In conclusion, the possibility to analyze the same sample after several cycles makes μCT a valuable method for the microstructural characterization of ceramic materials subjected to physical stress tests.