## 4 Discussion
The difference, however, was in the grain size of the PET, which corresponded to the grain size of the sand being replaced. The use of 10% by volume of PET with a grain size of 2-4 mm resulted in only an approx. 4%decrease in strength, while PET with a grain size identical to sand resulted in no decrease in compressive strength but as much as a 25% increase in flexural strength [51]. It can therefore be assumed that an appropriate adjustment of the grain size of the artificial aggregate will influence its applicability, without compromising the mechanical characteristics of concrete. In contrast, the inclusion of 10% of PET reduced the flexural strength of the mortars made, while at 5%, the decreases in value compared to the reference sample were not statistically significant. Also, the literature data indicate that PET applied in small quantities does not affect the flexural strength values obtained, and in some cases, this parameter is improved [45; 47]. Glass powder used as part of the binder slightly increased the flow of the mortars. Glass has hydrophobic properties, thus more water is available at the same \(w/c\) ratio compared to the reference mortar, and thus, a better workability is observed [52]. In contrast, this effect is not as strong as mixes with PET. Also, the lower relative density of glass relative to cement can reduce the weight and density of the fresh mortars made, while the decreases in volumetric density of mortars with glass powder were insignificant in the systems analysed. It was indicated that the partial replacement of the cement mass with larger amounts of glass powder, reduces the air content in the mixture. In turn, the data of [PERSON] et al. indicated an increase in the degree of air entrainment of the concrete mixture with an increase in the glass content. They explained this phenomenon as the retention of the air bubbles by the surface energy of the glass being greater the finer the glass grains are [53]. However, [PERSON] et al. found a decrease in air content in concrete mixtures made with waste LCD glass [54]. The formation of this feature most likely depends on the specific surface area of glass powder. Decreases in the strength of mortars in which part of the cement has been replaced by glass powder have been noted--the higher the glass powder content, the lower the compressive strength values. The use of 10% by weight of glass powder in the binder results in an 11% reduction in strength, while 20% of glass powder results in a 25% reduction in strength. The results of the phase composition tests show that the glass powder exhibits very poor pozzolanic properties. The literature indicates that glass powder with a grain size of less than 150 \(\upmu\)m should have good pozzolanic properties and be a partial replacement for cement [55]. Although the glass was ground to dimensions below 63 \(\upmu\)m in the present study, satisfactory results were not obtained. [PERSON], on the other hand, pointed out that the pozzolanic properties of glass powder were only in powder with a grain size below 45 \(\upmu\)m [56]. In addition, the reaction of glass with portlandite, a product of cement hydration, should produce a C-S-H phase that seals the structure and increases the strength of the composite material. It is likely that a greater fineness of the glass powder would improve its reaction with Ca(OH)\({}_{2}\)[57]. However, it can be assumed that glass powder of the grain size used in the study acts as a filler and, when used up to 10%, does not significantly reduce the flexural and compressive strength of the mortars. The lower strengths of mortars with glass as a binder component are also due to the lower cement content, and glass does not adversely affect the hydration processes. No increase in the strength of mortars with the simultaneous use of PET and glass powder was observed, where it was expected that the aggregate-slurry contact zone would be strengthened by the reaction with Ca(OH)\({}_{2}\).