## 3 Results and Discussion
### Physical-Chemical Characterization of the Waste Vitreous Enamels
#### 3.2.2 Effects of Waste Enamels on Mortar Properties  
Test results of the physical and mechanical properties of mortar mixtures with the addition of waste enamels are given in Tables 8-11 and Figures 5-10.  
Figure 5: Flexural strength of mortar samples with WEP.  
Figure 6: Flexural strength of mortar samples with WETM.  
Figure 7: Percentage ratio of flexural of mortar with waste enamels and reference mortar at 90 days.  
Figure 8: Compressive strength of mortar samples with WEP.  
Figure 10: The percentage ratio of compressive strength of mortar with waste enames and reference mortar at 90 days.  
Figure 9: Compressive strength of mortar samples with WETM.  
The physical properties of mortar were not considerably influenced by the use of waste materials as the substitution for cement. As it may be seen from the presented results, the consistency of the mortar, i.e., the spreading, grew slightly, and then decreases with the increase in the replacement of cement with waste enamels. The spreading variation is within \(\pm\)10 mm. Generally, all types of mortars meet the criteria prescribed by the Standard for plastic consistency of mortar. Apart from that, the results of the substitution of cement with WEP and WETM samples show a small decline in the bulk density of the mortar in the fresh and hardened state. This reduction is greater in mortars with the addition of WETM. The absorption of water at atmospheric pressure was the lowest on the reference mortar, and with an increase in the percentage of cement replacements in the mixtures, the absorption of water slightly increase. A possible reason is the higher porosity interfacial transition zone between cement paste and aggregate in a mortar with waste enamels compared to reference mortar.  
Drying cement mortar shrinkage measurement results show that mortar mixtures with waste materials added demonstrated lower shrinkage values in the initial hardening phase (first 7 days) in comparison with the reference mortar. At 14 days, mortar mixtures with WEP featured almost the same shrinkage values, while mortar mixtures with WETM demonstrated somewhat higher shrinkage than the reference mortar. In the later hardening phase (days 21 to 90), the reference mortar showed a lower shrinkage than the mortars in which waste materials partially substituted the cement. Greater shrinkage was observed in mortars where cement was replaced by waste enamel WEP. This does not match with the fact that in the cement hydration process, the pozzolanic reaction occurs later and after the formation of Portland (calcium-hydroxide), which is necessary for its initiation. The hydration products shrank due to the pozzolanic reaction, which caused increased shrinkage of the mortars with waste enamels added [58; 59; 60].  
The quality of cement composites primarily depends on mechanical strengths, especially compressive strength. The flexural and compressive strength tests were performed at the mortar specimen age of 2, 7, 28 and 90 days. A graphical representation of the results is shown in Figures 5-10. Based on the test results, it was observed that for 2 and 7 days, increasing the amounts of cement replacements causes a decrease in the mortar strengths, regardless of the type of waste enamel. On the other hand, for 28 and 90 days, the strength of mortar prisms with waste materials slowly approaches the values of the reference mortar. That may be attributed to the pozzolanic reaction of the waste materials, which is manifested at a later stage of hardening. Due to higher pozzolanic activity, mortar samples with WEP waste enamel have a faster increase in mechanical strengths compared to samples with WETM addition. For that reason, the percentage of possible replacement of cement is estimated on the basis of strengths at 90 days and can be up to 20% for all tested materials. If the drop in strength of mortar samples at 90 days in the amount of 20% is acceptable, then the maximum replacement of cement with WEP is 30%, and with WETM is slightly more than 20%.