## 1 Introduction
According to the authors of the study, \"over 90% of the test results obtained by participating laboratories can be described as\" satisfactory (\"having (\(|z|\leq 2\)) under EN ISO/IEC 17043\", while the remaining results were questionable or unsatisfactory [15]. In 2014, the interlaboratory tests were extended to the second characteristic--adhesion after immersion in water. In 2018, during the tenth jubilee edition of the study, three characteristics were measured: initial tensile adhesion strength and tensile adhesion strength after immersion in water and open time [17]. In the tenth edition, sixty-seven laboratories from 25 European countries and Asia participated in tests. Overall, 94% of the obtained test results were qualified as satisfactory (\(|z|\leq 2\)) under EN ISO/IEC 17043. One of the project's objectives was to show that the constant participation in laboratories' proficiency testing programs improves the quality of tests. In this respect, the study's organizers achieved the intended goal. In the aspect of meeting the standard requirements by cementitious CTAs, it is worth mentioning the research on the effect of seasoning water on the results of adhesion strength tests [18]. Here, the samples of cementitious CTAs were stored in three types of water: distilled water (pH = 7.09, specific conductance: 0.040 mS/cm), tap water (pH = 8.25, specific conductance: 0.805 mS/cm) and treated tap water (pH = 8.63, specific conductance: 1.228 mS/cm). It was found that the origin and type of water used to season the samples greatly influenced the adhesion strength of adhesives. The samples stored in distilled water were characterized by higher adhesion strength than those stored in tap water or softened tap water. In some cases, the difference between the test results was so significant that it determined the compliance with the standard requirements by cementitious adhesives. The adhesion of the cementitious adhesive to tiles is determined in the following system: concrete substrate-adhesive-ceramic tile. Properties of ceramic tiles approved for use during adhesion tests are specified in the standards referred to the given test method in EN 12004. However, not all parameters characterized by ceramic tiles used for testing the adhesion strength of cementitious CTAs are specified. [PERSON] assessed the influence of the chemical composition and surface structure of ceramic tiles on the tensile adhesion strength of adhesives to tiles [19]. The results obtained in the tests confirm the influence of the quality of auxiliary materials (ceramic tiles) used in the tests on the compliance with the threshold requirements by the tested tile adhesives. Many critical comments are made in connection with the adoption in EN 12004 of the adhesion strength determination determined by determining the tensile strength (vertical force) and not the parallel shear force as crucial for the classification of cementitious CTAs [13; 20]. Shear stresses in the substrate-cemetitious adhesive-ceramic cladding system may arise due to thermal expansion of the ceramic cladding (e.g., on balconies or terraces) or as a result of subsoil contraction (e.g., fresh concrete or unsealed cement floor). Unfortunately, the shear strength measurements are quite complicated as opposed to the relatively simple tensile strength measurements. For this reason, the shear strength of cementitious tile adhesives is not ordinarily determined. Recently, [PERSON] and [PERSON] described a new test method in which a tile adhesive's tensile strength was determined after a sample was subjected to a controlled shear [20]. In the authors' opinion, the research method proposed by them corresponds well to the real effects that the cementitious CTAs are subjected to under operating conditions. Similarly to other experimental tests, the determination of the tensile adhesion strength is characterized by the dispersion of the obtained results [21; 22; 23]. In this study, cementitious CTAs commissioned by market surveillance authorities in 2016-2019 in Poland were analyzed.