## 4 Discussion  
Previous concrete has been developed for many years. Previous concrete constitutes a benefit to urban developers as being the best and most sustainable way to control urban storm water. Up to now, satisfactory research has been conducted regarding this object; however, a research gap concerning the behavior and the effectiveness of pervious concrete made by exclusively recycled materials as aggregates still remains. It is well known that traditional pervious concrete cannot guarantee both strength and permeability performance [41]. [PERSON] et al. [41] had prepared recycled pervious concrete using waste glass and Recycled Concrete Aggregates (RCAs). In the case that natural aggregates were totally replaced by the recycled ones, their strength was 20 MPa on average and their permeability was 2.3 mm/s. When using a 50% waste glass cullet and 50% RCA as aggregates, their strength reached 22 MPa, but their permeability was reduced (1.2 mm/s). Those studies and applications are considered necessary, increasingly experiencing more of the effects of climate change and following the basic view of the modern era for the use of circular economy in both the construction and mineral resources sector where the \"closure\" of the life cycle of materials and of products from the extraction of natural resources but also their processing lead to the production of secondary raw materials. Important wastes and by-products of modern society that have been used from time to time by various researchers and in various construction applications are food wastes such as beer glasses and animal bones in the manufacture of concrete [42; 43], slags metallurgy [44], construction waste [42], electrical and electronic waste [26], but also plastic waste. However, the novelty of this study is the comparison of e-waste, slags, and hotel construction wastes used as aggregates with the most widely used natural ones (limestones and basalts) in pervious concrete for the first time, encouraging future researchers of the same philosophy in this subject. According to the above, the present work, by studying in detail the structure of raw materials and concretes on a microscopic scale, tries to emphasize the protection of non-newable resources and the utilization of by-products and waste, as well as the recovery and reuse of products in a new application of recycled aggregates for producing pervious concrete. By studying the microstructure of raw materials, many researchers have extracted significant results regarding the final behavior where concretes are likely to have normal requirements, while similar systematic studies in pervious concretes are missing, especially in pervious concrete made by e-wastes as aggregates. In the present study, natural sterile materials of sedimentary rocks such as limestones and igneous rocks of high strength such as basalts are used. These natural rocks are used as they constitute the most widely used aggregates when having as an ultimate goal the comparison with the recycled materials used as aggregates in order to use as a potential financial application. Metallurgical wastes are also used as aggregates (slags), construction wastes, as well as electronic wastes derived  
Figure 8: Diagram of the behavior of different concrete specimens under freeze–thaw conditions. from hotel units mainly consisting of electronic boards of telephone exchanges and old telephones. From their microscopic study, the above materials show obvious differences mainly in their structural characteristics. The most cohesive aggregates in their microscopic study based on the EN 932-3 standard [41] are presented to be sterile natural aggregate rocks (basalts, limestones) followed by metallurgical waste (slags). In contrast, recycled hotel materials such as bathroom wastes and e-wastes present a structure of low cohesion with no particular homogeneity in their mineralogical characteristics, which makes it an important inhibitory factor in finding a representative reference sample.