## 4 Discussion  
Table 5 shows the basic parameters of tested concretes performed with three kinds of recycled aggregates: SC--sanitary ceramics, CR--concrete rubble, and BC--building ceramic. The range of tests carried out permits to estimate the suitability of the aggregates as a main ingredient of structural concrete. The aggregates SC and CR are suitable for construction concrete. The water absorption for concrete exposed to the atmospheric humidity should be lower then 5% and for concrete shielded against the direct influence of the rainfall 9% [40].  
Concrete with recycled aggregates is studied in the work [30] and the received values of water absorption are similar to ours (6-10%). Taking into account the absorbability criterion, it could be stated that the BC aggregates are not suitable for structural concrete (water absorption = 22%). The high value of absorbability is affected by the high porosity of the aggregates (volumetric density of the BC aggregate 932.9 kg/m\({}^{3}\)) and high water demand. This leads to high demand of water with a constant amount of plasticizers and decreases the strength parameters. The specimens made of BC indicate the highest shrinkage, about 51% higher than SC and 39% higher than CR. BC aggregates could be used as a main ingredient of light or insulating concretes. The observed failure mode of specimens with BC aggregates is characteristic for plastic materials. It could be of significant benefit for light or insulating concretes and needs further researches to prove it. The SC recycled aggregates have strength parameters similar to natural aggregates and could be used as a substitute for them. This is proved by our own results and by the results of other authors [29; 30; 31]. It should be noted that not all kinds of ceramic could be suitable as an  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & \(G_{1}\)**[g]** & \(G_{2}\)**[g]** & \(\bigtriangleup G\)**[\%]** & \(R_{1}\)**[MPa]** & \(R_{2}\)**[MPa]** & \(\bigtriangleup R\)**[\%]** \\ \hline SC & 1942.9 & 1933.3 & 0.5 & 50.5 & 45.3 & 10.3 \\ CR & 2121.8 & 2076.1 & 2.2 & 55.3 & 53.5 & 3.3 \\ BC & 1674.0 & 1632.7 & 2.5 & 26.7 & 23.5 & 11.9 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Results of frost resistance tests.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **SC** & **CR** & **BC** \\ \hline Water–cement ratio & 0.5 & 0.5 & 0.82 \\ Consistency class & S1 & S1 & S3 \\ Compressive strength [Mpa] & 62.7 & 62.2 & 30.5 \\ Tensile strength in bending [Mpa] & 2.7 & 1.8 & 0.6 \\ Water absorption [\%] & 8 & 7 & 22 \\ Watertightness & W12 & W12 & W12 \\ Frost resistance class & \(\geq\)F25 & \(\geq\)F25 & \(\geq\)F25 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Basic parameters of tested concretes.  
ingredient in concrete. For instance, in paper [28] concrete on a base of aggregates made of ceramic tiles is demonstrated. The concrete had a much lower compressive strength than concrete based on natural aggregates, by about 33%.  
The results of water absorption and frost resistance tests of concrete with SC, CR, and BC allow to rank them as W12 and F25 class, respectively. Table 3 shows the average height of water absorption in the middle of the sample. Analyzing the results, it is concluded that the highest average tightness was observed for samples made on the basis of the SC aggregate, by about 21% higher compared to CR and about 51% higher than BC.  
The forest resistance tests showed the average decrease of compressive strength after cyclic freezing and thawing (25 cycles) to be 3.3% for samples with CR aggregates. The concretes made of SC and BS aggregates showed 10.3% and 11.9% decreases, respectively.