## 3 Results and Discussion
### Physical-Chemical Characterization of the Waste Vitreous Enamels
#### 3.2.3 Effects of Waste Enamels on Concrete Properties
On the other hand, an increase in the amount of WETM caused a significant decrease in the slump. That may be due to the porosity of WETM and WETM caused a significant decrease in the slump. Figure 11: Compressive strength of concrete samples with WEP. Figure 12: Compressive strength of concrete samples with WETM. Figure 13: The percentage ratio of compressive strength of concrete with waste enamels and reference concrete at 90 days. its ability to retain water and thereby reducing the amount of free water in the concrete mixture and therefore its consistency. The content of entrained air into fresh concrete decreased with an increasing percentage of cement replacement, which can be attributed to the better placement of concrete prepared with waste enamels. The replacement of cement with waste materials caused an improper change in bulk density. However, those changes vary within the limits up to \(\pm 1\%\), which is acceptable. The most important factors that affect ultrasonic pulse velocity through concrete are the concrete porosity, aggregate type and interfacial transition zone (ITZ). Based on the results presented in Tables 12 and 13, all concrete mixtures had values of ultrasonic velocity higher than 4.5 km/s which is a limit for strong concrete [60]. The differences in the values of the ultrasonic velocities in concretes with the addition of WEP and WETM were insignificant, which indicates that all concrete mixtures had an approximately uniform quality in terms of physical and mechanical characteristics. The test was performed at 28 and 90 days, the samples were tested to flexural strength. Based on results presented in Tables 13 and 14, one can conclude that the flexural strength declined with an increase in the percentage of cement substitution, and the decline compared with reference concrete was the in the range 26-32% for concretes with WEP and 9-16% for concretes with WETM at a test samples age of 90 days. Compressive strength is the most important property of concrete and test samples of 2, 7, 28 and 90 days were used to determine it. Based on the results shown at Figures 11 and 12, it can be observed that with the increase in the content of waste enamels, the compressive strength of concrete samples decreased at all ages of concrete, except for the \"WETM-C-10\" mixture at 2 and 7 days. In the first 7 days, the concrete mixtures \"C-E\" and \"WETM-C-10\" had the highest compressive strength increase, while with increasing age, the compressive strength of concrete treated with waste enamels approached the value for reference due to a delayed pozzolanic reaction. With the increasing concrete age, a small decline in compressive strength was observed with an increase in the percentage of cement replacement by waste materials. The authors [17; 58; 59] in the study of the properties of concrete with the addition of recycled glass and waste vitreous enamel had comparable inferences. Figure 13 shows the percentage ratio of compressive strength of concrete with waste enamels and reference concrete at 90 days. With an increasing percentage of cement replacement, a small decline in compressive strength can be observed, irrespective of the type of waste enamel used. As for the compressive strength, the cement substitution with waste enamel WEP turned out to be more effective. If the level of compressive strength reduction of 15% is acceptable, it can be concluded that up to 30%, i.e., 20% of the cement can be replaced with WEP, i.e., WETM in concrete production. The results from Tables 14 and 15 show that the tensile splitting strength slightly declined with the increase in the percentage of cement substitution, irrespective of the waste enamel type used. The highest strength value (3.9 mPa) was observed for the reference concrete, while the concrete mixtures with 30% cement substitution by WEP and WETP showed 10% lower tensile splitting strength. At a sample age of 28 days, the secant modulus of elasticity of concrete was examined.