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The article evaluates the structure of thermal cracks created on the surface of cement paste. The samples were loaded with an elevated temperature of 250 °C. Due to the volumetric deformations and increasing water vapor pressure, thermal cracks appeared on the material’s surface. The analysis was performed using the computer image analysis; three stereological parameters were proposed for the description of the cracks’ structure: the cluster average area (), the cluster average perimeter (), and the crack average width (). The aim of the research was to determine how the geometry of the thermal cracks changes in a situation in which the shape and size of the sample changes. The tests were carried out in two variants: in the first the sample’s width was variable, and in the second – its height. Using the least squares method (LSM), the dependencies that occur between the sample’s size and the geometry of the cracks were developed. Considering cement paste as a highly concentrated dispersion system, an attempt was made to identify factors shaping the cracks’ structure due to the change in the size of the sample. The considerations were supplemented with microstructural investigations using a scanning electron microscope (SEM) and a X-ray microanalyzer (EDS). The cluster structures were analyzed at a lower level of observation; there were no significant differences in the composition of cement paste, which confirmed its high chemical and thermal stability in the studied temperature range.
