## 3 Results and Discussion
### Density of Hardened Mortar  
The density of cement composites based on recycled rubber did not exceed the value of 1300 Kg\(\cdot\)m\({}^{-3}\), more precisely the samples prepared according to mixture Z2 reached the density of 1260 Kg\(\cdot\)m\({}^{-3}\) and the samples of mixture Z3 of 1250 Kg\(\cdot\)m\({}^{-3}\). For the purpose of comparison, a test using cement composites based on natural aggregate (standardized sand) was performed as well. The value of the density of hardened mortar was 2150 Kg\(\cdot\)m\({}^{-3}\). The results of the test determining the density of hardened mortar have confirmed the assumption that the resulting cement composites prepared according to mixtures Z2 and Z3 belong to the category of lightweight concrete LC 1.4 as far as their density is concerned according to CSN EN 206-1 [50]. Their density does not exceed 1400 Kg\(\cdot\)m\({}^{-3}\).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mixture**} & \multicolumn{3}{c}{**The magnitude of the standard deviation of tensile flexural strength [MPa]**} \\ \cline{2-6}  & **3 days** & **7 days** & **14 days** & **28 days** & **90 days** \\ \hline Z2 & 0.08 & 0.04 & 0.04 & 0.09 & 0.05 \\ Z3 & 0.05 & 0.02 & 0.02 & 0.05 & 0.03 \\ Comparative & 0.04 & 0.02 & 0.05 & 1.20 & 0.07 \\ \hline \hline \multicolumn{6}{c}{**The magnitude of the standard deviation of compressive strength [MPa]**} \\ \hline Z2 & 0.06 & 0.03 & 0.03 & 0.42 & 0.07 \\ Z3 & 0.04 & 0.02 & 0.02 & 0.09 & 0.03 \\ Comparative & 0.09 & 0.07 & 0.05 & 0.74 & 1.21 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The standard deviation of tensile flexural strength and compressive strength of experimental mixtures.  
Figure 5: Graphical expression of Compressive strength of mixtures Z2 and Z3.  
Figure 8: Sound absorption of cement composites (thickness 50 mm).  
Figure 9: Sound absorption coefficient dependence on the thickness of the sample of mixture Z2 (the trendline is reported for each set of measurements).  
Figure 11: Sound absorption coefficient dependence on the thickness of the sample of comparative mixture (the trendline is reported for each set of measurements).  
Figure 10: Sound absorption coefficient dependence on the thickness of the sample of mixture Z3 (the trendline is reported for each set of measurements).  
The thermal conductivity measurement results clearly show that a 100% replacement of aggregate with recycled rubber in cement composites significantly affects the thermal conductivity coefficient. Mixture Z2 shows a decrease in the coefficient of thermal conductivity from 2.97 to 0.31, which corresponds to a decrease of 89.5% compared to the comparative mixture. Mixture Z3 shows a decrease in the coefficient of thermal conductivity from 2.97 to 0.35, which is a decrease of 88.1% compared to the comparative mixture. Furthermore, the results of thermal properties of the prepared composites based on recycled rubber show that mixture Z2, with 100% replacement of aggregate with recycled rubber of fractions 0/1 and 1/3 in the ratio of 50:50, have better thermal conductivity result than mixture Z3 with the ratio of recycled rubber fractions of 40:60. This improvement of 11.4% is influenced by a larger share of the finer fraction of recycled rubber, which has positive effect on thermal conductivity [54; 55].