## 5 Conclusions  
This research focused on the alternative use of several materials, natural and recycled, as pervious concrete aggregates as well as on their effect on the final behavior of the concrete. At the same time, this study tried to enrich the research gap regarding the effect of the microstructure of variable recycled materials (wastes) on concrete performance. The main conclusions of the study are summarized below:  
Figure 11: N. 50 cycle of the freeze–thaw test of the produced concrete specimens vs. the permeability coefficient of the produced concrete specimens.  
Figure 10: Specific gravity of the tested aggregates vs. the permeability coefficient (k) of the produced concrete specimens.  
All the tested raw materials showed obvious differences mainly in their petrograph and structural characteristics, as well as in their physical properties, which had classified them into three groups: group 1 (e-wastes and hotel construction wastes), group 2 (slags and limestones), and group 3 (basalt).  
The physical properties of the tested raw materials, dependent on their petrographic characteristics, played a crucial role for the permeability of the produced concrete specimens, their mechanical behavior (concrete strength), as well as for the freeze-thaw test results.  
The geochemical analysis of the used e-wastes showed high concentrations of toxic metals, a fact that determines the suitability of these materials as pervious concrete aggregates.  
The mass loss directly depended on the total aggregate and concrete properties and constituted the critical factor for the evaluation of the suitability of each natural or recycled raw material for pervious concrete.  
Concrete made by variable wastes and by-products can be compatible for concrete production as they showed similar performance both in the mechanical strength test and in the freeze-thaw test with those made by natural aggregates.  
To sum up, sterile materials of igneous and sedimentary natural aggregates such as basalts and limestones can be widely used as raw materials for pervious concretes, while metallurgical wastes (slags) have a generally satisfactory and very promising application. On the contrary, construction wastes, because of their mechanical behavior, and especially e-wastes, because of their potential toxic load contained, are less suitable for pervious concrete aggregates.  
Conceptualization, P.P.; methodology, P.P., P.P.G., A.R., N.L., P.L., P.M. and N.K.; software, A.R. and P.M.; investigation, P.P., P.P.G., A.R. and M.K.; resources, P.P., P.P.G., A.R., M.K., N.L., P.L. and D.P.; data curation, P.P., P.P.G., A.R., N.L., P.L. and D.P.; writing--original draft preparation, P.P., P.P.G. and A.R.; writing--review and editing, P.P., P.G. and N.K.; visualization, P.L. and P.M.; supervision, P.P. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
We kindly thank [PERSON] of the Laboratory of Electron Microscopy and Microanalysis, University of Patras for her assistance for the XRF analyses. We would also like to thank [PERSON] from Wasco Coating Europe BV for providing us slags.  
The authors declare no conflict of interest.