1. Introduction
The critical parameter to create sustainable buildings is the development of infrastructure systems that are environmentally friendly, climate-resilient, cost-effective, as well as long-lasting [
1]. Concrete constitutes the most used and cost-effective construction product worldwide, having an annual production of 33 billion tons on average [
2,
3]. The unavailability of efficient recovery routes leads to the need of the sustainable disposal and/or recycling of various wastes after their use [
4]. Concrete is made up of cement, aggregates, and water, which may significantly affect its workability, durability, and mechanical performance. Generally, the increase in concrete production has resulted in the exploitation of mineral raw materials and even of natural resources while generating construction wastes in the process [
5,
6]. The prevailing trend in modern research efforts is to focus on protecting the environment and, at the same time, addressing the problems that have arisen from human activity. This is evident from the turn of research, even from the concrete industry, to the development of more rational rainwater management systems. The beginning of these efforts started in the last decade in the USA with the design and development of pervious concrete, a new type of product that has a high porosity and allows rainwater to pass through its mass. From an environmental point of view, the cement production technology has a significant influence on energy consumption and increased CO
2 levels. An important amount of emissions is caused by human activity [
7] and is associated with the high calcination temperature (~1450 °C) of raw materials. Aggregates constitute approximately up to 65% of the final concrete’s volume and increase environmental interests related with their extraction and processing [
8]. The European Union waste management legislations entered a limit on the quantity of FRP wastes relative to landfills and incineration activities [
9]. Therefore, composite industries must identify cost-effective recycling solutions and/or disposal methods for CFRP and GFRP, which would pave the way for composite manufacturers and suppliers to promote sustainability of their products and maintain an upward growth of the composite sector [
9]. Additionally, the quality of hardened concrete depends on the suitable application of various aggregates in various sizes and usually consists of natural sand and gravel or crushed rocks, as well as their cohesion with the cement paste. Thus, it is obvious that the enormous quantities of concrete produced every year require equal significant volumes of natural mineral raw materials both for aggregates and cement, something that causes serious environmental issues. Waste management and natural resource depletion, a worldwide challenge, are becoming even more apparent in driving change. Following this fact, a new legally binding agreement was adopted in December 2015 by the United Nations known as the Paris Climate Change Agreement, aiming to reduce the greenhouse gas emissions on a global basis [
10].
Pervious concrete pavement (PCP) constitutes a special construction application system that is an integral part of sustainability. PCP generally constitutes a structural surface wearing course containing coarse aggregates, cement, water, and admixtures, allowing the infiltration of stormwater into the underlying granular base/sub-base resting over a soil subgrade [
11,
12,
13]. It contains a small amount of fine aggregate or no fine aggregate, leaving gaps to achieve the purpose of water permeability. Due to their porous nature, PCPs suffer from lower strength compared to Portland cement concrete pavements (PCCPs), thereby limiting their applications to parking lots, medians, sidewalks, etc. [
12,
14]. Its design with the use of a minimal amount of cement paste for the coating of coarse aggregates facilitates the formation of this interconnected network of pores in the material, which allows the passage of water at a much higher rate than in conventional concrete.
Pervious concrete belongs to a completely different category from conventional concretes and, therefore, its physical characteristics differ significantly from those of known concretes. It consists of Portland cement, coarse aggregates, little or no aggregates, admixtures, and water, the optimal ratio of which is investigated depending on the nature of the aggregates used and the individual application requirements of the concrete. Finally, the combination of these components leads to the production of a hardened product with interconnected pores ranging in size from 2 to 8 mm, which allows water to easily penetrate the concrete. Its empty spaces range between 18 and 35% and the typical compressive strengths achieved are in the range of 2.8 to 28 MPa [
11,
13]. However, the strength of PC decreases with the increase in PC permeability [
15,
16,
17]. Rizvi et al. [
18] used 15%, 30%, 50%, and 100% recycled concrete aggregate (RCA) instead of coarse aggregate in PC. The strength of samples with an RCA content of 30% or above decreased significantly, while permeability and pore content increased. Zhang et al. [
19] indicated that increasing the ratio of binder to aggregate can improve the mechanical properties of the material but reduce the permeability coefficient of the material. Finally, Ibrahim et al. [
20] added recycled fine aggregate (RFA) into PC, resulting in an increase of 7% in compressive strength and 37% in splitting tensile strength.
Permeable concrete is a type of concrete where the porosity is not in the aggregates but inside the concrete web itself. In recent decades, the steel slags have attracted the attention of the construction industry as they have the potential to be used as a partial replacement of cement due to their pozzolanic characteristics, but also as aggregates replacement in concrete. Moreover, the European Waste Framework Directive [
21,
22] highlights the significance of utilizing industrial by-products such as Fe and steel slag aggregates, which helps to minimize waste and conserve resources. Iron and steel slag is a melt that is silicate and is mainly formed during the manufacture of crude steel and crude iron. This is due to the combination of slag and flow agents used to remove impurities from iron ore and other furnace metal feeds [
23]. The hot slags are collected from the liquid metal and thus transferred to a slag substrate where they are stored after cooling (spraying in open-air or water) and processed into a solid material. In terms of blast furnace slags and steel slags production, it was found to be higher in Europe at around 45 Mt in 2018 [
24], compared to the US where the steel slag production ranged from 8 to 12 Mt [
23]. On the other hand, there are wastes that do not decompose or degenerate, and these are the electronic and electrical waste (e-waste items). The use of these items has created another, significantly dangerous stream of waste called “electronic-waste” (e-waste). Electronics that are already used and classified for reuse, resale, recycling, or disposals are also considered e-waste. The development of these products has been a threat that is considered extremely serious for both public health and the environment, as e-waste has been growing exponentially in recent years, as markets for them are growing rapidly. The e-waste that is generated is usually disposed in the form of land fill, incineration, reuse, or recycling. However, the cost of these disposal measures is high and has a hazardous effect on our environment. It is necessary to arrive at a cost-effective and environmentally friendly recycling process, which may be considered as the real need hour. Manjunath [
25] studied the utilization of e-waste particles as a fine and coarse aggregate in concrete. An experimental study was made on the use of e-waste particles as fine and coarse aggregates in concrete with a percentage replacement ranging from 0% to 30% (in an interval of 10%) for the M20 grade of concrete. The compressive strength, tensile strength, and flexural strength of concrete with e-waste give better strength as compared to concrete without e-waste. The problem of solid waste disposal and management in all countries has become one of the most important environmental, economic, and social issues. Usually, electronic plastics are not recycled, the same type of plastic products made from recycled plastics that are often not recyclable. Several researchers have used e-wastes in concrete mixtures in order to partially replace the natural aggregate rocks such as home appliances, information and communication technology devices, home entertainment devices, electronic utilities, and office and medical equipment [
26,
27,
28,
29], producing concrete specimens of satisfactory compressive strength when sometimes producing specimens characterized by a significant improvement in their compressive strength in contrast to those of conventional concrete [
25,
30]. It is evident that there is potential for further research into the production of environmentally friendly concrete specimens containing different types of wastes as environmental benefits are certainly anticipated. Numerous researchers have studied the use and impact of recycled materials on the production of pervious concrete specimens as required by sustainable development, which are followed by all the developed and modern states. However, most researchers have not focused on the effect of microstructure and on the behavior of these materials on the micro-scale where the initial failures of concrete during loading occur. This is exactly the research gap that the present study fills by studying different wastes on a micro scale. Furthermore, the present study gives the trigger for such a large-scale application where it could provide significant economic benefits as the construction industry is one of the most attractive industries for which there are great benefits, provided that business strategies of the circular economy are followed as the dynamic growth prospects and opportunities arise for the whole construction business, through the circular economy. The construction sector plays a very important role in the European economy as it produces 10% of the EU’s GDP and provides 20 million jobs to respective companies. It is also mentioned that the construction industry is also important as it uses a significant percentage of intermediate products (raw materials, chemicals, electrical, and electrical equipment) and related services. The roadmap for a Europe that uses its resources efficiently (European Commission, Strategy 2020) estimates that the optimal construction and use of buildings could contribute to significant savings of natural resources such as 42% of the total final energy consumption and 35% of total greenhouse gas emissions.
Based on the aforementioned principles, the main goal of this research is to promote the effective use of available by-products and wastes from Greece, thus replacing the natural aggregates in concrete production for a variety of applications, thus improving the sustainability of pervious concrete. This is a case study that, in the near future, can be generalized. Furthermore, the resistance to intense temperature changes—cooling/thawing conditions—as well as their durability in freeze and thaw cycles is studied.
4. Discussion
Pervious concrete has been developed for many years. Pervious 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]. Lu 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 nonrenewable 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 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. All the above regarding the classification of raw materials are verified by all the petrographic and chemical analyses where they are performed on the raw materials. In general, a special feature that should be considered before extensive use of such a pervious concrete is the chemical analysis of the raw materials where they are used as aggregate materials, as aggregates rich in toxic load that may be in extensive wetting is possible to create through leaching, another serious environmental runoff problem, which is also unacceptable. This study when trying to enrich the research gap focuses on the exclusive use of recycled materials and e-wastes in pervious concrete by having a holistic approach. On the other hand, the majority of the researchers such as Lu et al. [
41] partially replaced the natural aggregates with recycled ones, presenting significantly higher strength and permeability values than expected. Therefore, from all the raw materials examined, it appears that the electronic waste as shown in
Table 3 and in the XRD pattern has a particularly high concentration in Ni, Zn, and Cu, creating a strong concern for its use in the field of inhomogeneity of the material. On the contrary, all the other raw materials do not seem to be able to create a significant environmental problem during their continuous leaching in an application. Regarding the physical characteristics of the raw materials such as the specific gravity and the volume porosity, there is a clear division into three groups directly dependent on what is mentioned above for their microstructure. More specifically, group 1, consisting of e-waste and construction waste, presents with increased values of the specific gravity and the porosity and, therefore, with increased water retention capacity in their structure. It is followed by group 2 (slags and limestones) and finally followed by group 3, in which the basalt stands out due to its very low alteration degree, as presented by the LOI values where it has an indirect alteration index.
The Influence of Recycled Aggregates on Pervious Concrete Behaviour
The influence, the behavior, and the suitability of aggregates within the structure of a concrete have been studied by numerous researchers who have shown that the particular microscopic characteristics that determine the structure and composition of aggregates are the most important factors influencing the final behavior of various technical projects. The above view of the relationship between aggregates and concrete seems to have been followed in the present study of the use of recycled materials for the production of pervious concretes. More specifically, it seems that in the general behavior of pervious concrete, the above 3 groups are followed and verified as they are classified during the study of raw materials. The concretes produced from aggregates of group 3 (basalts) are the most durable aggregates and the most cohesive, yielding the concretes with the highest mechanical strength (UCS) as shown in
Table 5.
Samples of group 2 are followed (limestones and slags), while concretes of lower strength are produced from the samples of group 1 (hotel construction wastes and e-wastes). The direct dependence of the effect of aggregate type, natural and recycled, on the final concrete strength is shown in the diagram of
Figure 9, as well as the strong interdependence between the specific surface of the aggregates and the final mechanical strength of the concrete. It is obvious that aggregates with the highest specific surface gravity and porosity (group 1) produce concretes of lower strength, which is probably due to the ability of these aggregates to retain water in their structure by absorbing the water where it is needed for coagulation and curing of the cement, thus preventing it from successfully completing its hydration reactions.
Similar interdependencies are conducted for the relationship between aggregates and normal concretes, which generalizes the present assumption to other types of concrete such as pervious concretes. The initial classification of the three groups is presented to be verified in the results of the water permeability of the concrete, and this property seems to be directly related to the type of aggregate and, in particular, to its physical properties. More specifically, we observe that the samples of group 1 are mainly due to their heterogeneous composition, but also their nature tends to adsorb water in their structure; they work in this way and in a pervious concrete. On the contrary, the samples of group 3 with the particularly low porosity of basalts allow the water to penetrate them without entering either the primary or the secondary porosity of their structure as aggregates presenting the highest water permeability values of the concretes in which they participate. The main matrix of the eco-friendly pervious concrete has a very low porosity; its perviousness mostly hinges on the reserved top-bottom interconnected porosity. All the above that are amenable to the above physical interpretation are obvious and are proven through the strong correlation where it is located between the permeability of the concrete and the specific gravity of the tested aggregates (
Figure 10).
Samples of group 2 and 3 with the recycled aggregates show behavior where the combined use of innovative pervious concrete as the matrix and RCA as the aggregate is attractive and promising to produce clear concretes of satisfactory strength and permeability. Finally, regarding the freeze–thaw test, the mass-loss rates of all three groups are revealed in
Figure 8. It can be clearly observed that the mass of all specimens first increases and then gradually remains stable for the first 15 cycles for all concrete specimens, something that reveals that none of the aggregates present any failure, microcracks, and mass loss for the first 15 cycles of the test. Then, the microcracks and angle loss occurring in concretes with aggregates from group 1 make the mass decrease [
45]. The fluctuation in mass-loss changes becomes larger with the increase in wetting–drying cycles. The reason for this phenomenon is that with the growth of the replacement rate, there is more space for corrosion products to accumulate in group 1, and the compactness of concretes from group 1 decreases, which accelerates the generation and accumulation of sulfate products. Contrary to the above, concrete made by aggregates of group 3 shows the lowest mass loss as using the basalts (low altered), and they produce concretes of higher mechanical strength and with better consistency, as shown in
Figure 6, something that does not appear in the concrete made by the recycled aggregates of group 1. Limestones and slags of group 2 show a satisfactory and similar behavior based on their similar physical characteristics, where they lead to a similar satisfactory consistency. The microstructure of concrete and the physical properties of the aggregates seem to determine to a large extent both the water permeability test of the concrete and freeze–thaw test. These tests seem to be directly interdependent with each other, as shown in the following diagram (
Figure 11), which shows the interdependence of the freeze–thaw test with the water permeability test of concrete.
Summarizing, it seems that sterile materials of igneous and sedimentary natural aggregates such as basalts and limestones can be widely used as raw materials for pervious concretes, respectively, while metallurgical wastes (slags) have a generally satisfactory and very promising application. The results of this study are quite encouraging but they are also characterized as preliminary regarding the construction sector. However, in the near future, these types of wastes can potentially be used in a wider range of applications. On the contrary, construction wastes and e-wastes that are less suitable for such an application should be used under certain conditions, and, in particular, e-wastes that are found to have a toxic load should be examined by the leaching test of pervious concretes so that there are no additional environmental problems.