## 4 Discussion  
The presented results show the influence of waste raw materials on the physical and mechanical properties of cement mortars made with them. No research has been carried out on the simultaneous influence of glass powder and PET on the properties of cement composites, both in Poland and worldwide. The obtained research results allowed for the assessment of the possibility of using these materials in cement composites, and in the future, they may help in developing the technical possibilities of introducing these materials in the building industry. The use of packaging waste can contribute to the development of sustainable production of composite materials, effective waste management, and environmental protection. Both glass powder and PET increased the liquidity of the mortar mixes. Different observations of the effect of PET on workability are cited in the literature. As the volumetric proportion of PET increases as a fine aggregate, a deterioration in workability was observed [45], while others have indicated an increase in the flow of mixes with PET [46]. Similar discrepancies have been indicated with the use of PET fibres [12; 47]. PET can lead to the disruption of the structure of mortars and concretes, resulting in a reduction in the flow of mixes. The discrepancies may be due to the fact that the flow of mortar mixes and concretes made with PET depends on, inter alia, the shape, grain size, texture, specific surface area of the artificial aggregate used, and the w/c ratio. [PERSON] et al. indicated that plastic waste can be used for self-compacting mortars and concretes, where the mixtures must have good flow [46]. Plastic particles, due to their smoother surface than sand, do not absorb water, therefore the excess water improves the workability of mixtures. PET reduces the density of mortar mixes with its addition; the reason for this being the lower relative density of artificial aggregate (specific gravity) compared to natural aggregate [15]. The density of the mix with 10% PET decreased by approximately 4%. There were no significant differences between the air content in mixtures with and without waste raw materials; however, PET increases the air content in the mixtures. This is probably due to the voids formed between the PET aggregate and the cement slurry, which results in air being trapped close to the surface of the hydrophobic aggregate [48; 49]. PET decreases the compressive and flexural strengths of the mortars made, with the recorded decreases in strength showing no significant differences from the reference mortars when PET was introduced at 5% by volume. The increased number of weakened zones in the mortar matrix is responsible for the decreases in compressive strength of mortars with PET volumetric addition. The weakening of the contact zone between the PET aggregate and the cement slurry is due to the weak bond between the cement matrix and the hydrophobic surface of the plastic. As [PERSON] et al. indicated, the plastic waste-binder interface is less dense with cracks and poor adhesion between these two materials [46]. When mortars were tested in compression, whole particles of artificial aggregate were pulled out, as also indicated by [PERSON] and [PERSON] in their study [50]. The contact zone between the polymer aggregate and the cement slurry is weaker than the contact zone between the slurry and a mineral aggregate. Interestingly, the decreases in compressive strength of mortars with 5% and 10% by volume of PET are similar and statistically insignificant at around 12-13%. The particle shape of the polymer aggregate is irregular, and the grains are larger than those of silica sand, which can play a significant role in determining the strength parameters. [PERSON] et al. replaced 10% of the sand volume with plastic waste and found no deterioration in concrete strength [51]. The authors similarly substituted 10% by volume of fine aggregate, as in the present paper.