Sustainable Use of By-Products and Wastes from Greece to Produce Innovative Eco-Friendly Pervious Concrete

Abstract

This study was based on the reduction of the extraction of natural resources and, at the same time, was focused on the use of by-products and various wastes in construction applications by following the principles of circular economy. Sterile natural rocks (limestones, basalts), industrial by-products (slags), hotel construction wastes (bathroom wastes) and electronic wastes (e-wastes) were tested for pervious concrete aggregates. For this reason, ten concrete specimens were prepared and tested petrographically, structurally, and physically. The physical properties of the tested raw materials directly depended on their petrographic characteristics and played crucial role for the permeability of the produced concrete specimens, for their mechanical behavior, and for the freeze–thaw test results. Generally, from this study, strong encouraging results were achieved as concrete made by variable wastes and by-products can be compatible for concrete production as they show similar performance both in the mechanical strength test and in the freeze–thaw test with those made by natural aggregates. Another goal of this study was to recommend to other researchers the extended use of by-products, construction wastes, and e-wastes as concrete aggregates for producing eco-friendly constructions.

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 CO₂ 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.

2. Raw Materials and Methodology

2.1. Raw Materials

In this research, variable waste materials were used as aggregates in concrete. More specifically, sterile natural aggregates from engineering tests such as limestones (derived from Achaia region) and basalts (derived from Veria-Naousa ophiolite complex) were collected, and hotel construction wastes such as bathroom wastes containing tiles, industrial by-products such as slags (derived from Electric Arc Furnace-Wasco Coating Europe BV), and e-wastes (electronic boards of telephone exchanges and old telephones) were also collected, tested, and used as concrete aggregates (Figure 1). Furthermore, cement of Type CEMII 42.5 was used as well as potable tap water.

2.2. Methodology

2.2.1. Methods for Wastes Raw Materials

To study the microstructural features of the natural and recycled aggregates that are used for preparing concrete, polished thin sections were made, and a polarizing microscope was used according to the EN-932-3 [31] standard for their petrographic observation (Figure 2). These thin sections contain important information about the mineralogical composition as well as for the textural characteristics of these materials. Using the petrographic microscope (Leitz Ortholux II POL-BK Ltd., Midland, ON, Canada), thin sections were examined for mean grain size and grain shape. The determination of the bulk mineral composition of the studied samples was made by X-ray Diffraction (XRD) analyses. For the preparation of random powder mounts, the powder was pressed gently into the cavity holder. The area that was scanned for the bulk mineralogy covered the 20 interval 2–70°, and using the DIFFRACplus EVA 12^® software (Bruker-AXS, Gmbtl, Karlsruhe, Germany), the mineral phases were determined, based on the ICDD Powder Diffraction File of PDF-2 2006. The determination of loss on ignition (LOI) of the studied natural samples used as aggregates was made according to the ASTM D7348-13 standard [32]. Moreover, an XRF (X-ray fluorescence) spectrometer and a sequential spectrometer cited at the Laboratory of Electron Microscopy and Microanalysis were used for the determination of the major and trace elements of studied wastes and by-products used as aggregates. To determine the surface areas of the used raw materials, the BET method was used, where argon adsorption isotherms at 87 K were computed by grand canonical Monte Carlo (GCMC) simulations [33,34].

2.2.2. Methods for Concrete Specimens

According to ACI-211.1-91 [35], ten alternative normal concrete cube specimens (150 × 150 mm) were prepared by different raw materials used as aggregates (Table 1). The used raw materials were crushed through specific sieves and separated into the size classes of 2.00–4.75, 4.45–9.5, and 9.5–19.1 mm. Next, 24 h after the production, the samples were removed from the mold, cured in water, and remained for 28 days. The temperature was 20 ± 3 °C for 28 days. After 28 days, these specimens were tested in a compression testing machine at an increasing rate of load of 140 kg/cm² per minute and the compression test was elaborated according to BS EN 12390-3 [36]. After the compressive strength test, the textural characteristics of concretes were examined with specific polished thin sections in a polarizing microscope according to the ASTM C856-17 [37]. Water permeability, as the only property of water to penetrate the pervious concrete, was expressed in cm/s. In general, pervious concrete shows intense high water permeability values compared to conventional-type impermeable concrete. For this reason, the method of water permeability testing for conventional-type concretes according to EN 12390-8 [38] is considered as unsuitable for the control of pervious concrete. As there is no Greek, European, or American standard for this test, the method for the determination of water permeability with a variable-height water perimeter that meets the specifications of soil engineering standard E 105-86 [39] was performed.

$$k=\frac{a\ast L}{A\ast t}\ast \ln (\frac{h1}{h2}),$$

(1)

k: the permeability coefficient (cm/s),

a: the cross-section of the cylindrical tube (cm²),

A: the cross-section of the sample (cm²),

L: the length of the sample (cm),

t: the time required for water to fall from height h1 to height h2.

Table 1. Aggregate components of the produced concrete specimens.

Components/Samples	S1A	S1B	S2A	S2B	S3A	S3B	S4A	S4B	S5A	S5B
Limestone	√	√	-	-	-	-	-	-	-	-
Basalt	-	-	√	√	-	-	-	-	-	-
Slag	-	-	-	-	√	√	-	-	-	-
Construction wastes	-	-	-	-	-	-	√	√	-	-
E-wastes	-	-	-	-	-	-	-	-	√	√

Table 1. Aggregate components of the produced concrete specimens.

Components/Samples	S1A	S1B	S2A	S2B	S3A	S3B	S4A	S4B	S5A	S5B
Limestone	√	√	-	-	-	-	-	-	-	-
Basalt	-	-	√	√	-	-	-	-	-	-
Slag	-	-	-	-	√	√	-	-	-	-
Construction wastes	-	-	-	-	-	-	√	√	-	-
E-wastes	-	-	-	-	-	-	-	-	√	√

Cooling–thawing tests were performed by modifying the ASTM C-666 [40] standard, which stipulates that if there is no automated cooling–thawing system and the operations are performed manually, the specimens should be stored until the start of the next refrigeration–thawing cycle. The cooling temperature was close to −18 °C and the thawing temperature was +4 °C. Two specimens from each different concrete were examined. This process took place in a dry environment while thawing in a wet environment. Defrosting lasted 6 h, which was considered a sufficient time for the complete thawing of the specimens regarding their dimensions. Each time the cycles were interrupted due to time constraints, the specimens remained in a refrigerated state according to ASTM standard C-666/C [40]. The processing followed is described as follows:

• Measurement of initial masses and initial heights of the specimens.
• Place them in deionized water for 2 days until the specimens are saturated in water.
• New weighing of the specimens and measurement of their heights.
• Start of the cycles.

The measurements of the mass and the heights of the specimens were always made at the stage of thawing; specifically, the specimens were weighed after the end of each cycle while the measurements of their dimensions were carried out every 5 cycles. The whole freeze–thawing process was performed for a total of 50 cycles.

3. Results

3.1. Characterization of Raw Materials

The limestones used as components of the produced concrete specimens as can be seen from the petrographic study do not show important cracks or a high percentage of impurities in their mineralogical composition. The used limestones are characterized by a micritic texture with many fossils and microcrystalline calcite veinlets, as well as stylolitic porosity that are filled with clay minerals and Fe-oxides (Figure 3a). Concerning the slags, they are rich in opaque minerals and mainly wüstite and elongated spinels. The examined slags present microcrystalline to cryptocrystalline and mainly anisotropic calcium-alumino silicate phases (Figure 3b). Alternations in isotropics are also observed with anisotropic minerals, which form banded structures. Some of the grains present different compositions around their margins; specifically, they display microcrystalline material that is possibly alumina-calcium-silicate (anisotropic) and it coexists with an amorphous mass (isotropic). The shape of the opaque minerals as well as of those of the spinel group appears polygonal to spherical with some peculiar-sub-dominated plains. The minerals of the spinel group present more dark brown to red colors and less amber, which means that they have more ferritchromit characteristics and they have less aluminum. Most anisotropic minerals have a prismatic shape, except from the silicon dioxide ores that are usually unhedral. The colors of anisotropic minerals are usually gray, white, and yellow, whilst crystals of olivine are presented less frequently and the basalt shows a porphyritic texture, as shown in Figure 3c. Their main composition includes plagioclase, magnetite, clinopyroxene, zircon, as well as ilmenite. Epidote, calcite, quartz, actinolite, pumpellyite, chlorite, titanite, and hematite are observed in small amounts, and they constitute their secondary mineralogy. The matrix is glassy with fine plagioclase crystals. Generally, the slags display a heterogeneous texture owing to an unequal aggregate of minerals or ordinarily multi-organic fragments (Figure 3d). Construction wastes (containing tiles) are brownish-black and have an optically inactive groundmass surrounded by fine-grained quartz inclusions. Quartz inclusions are sub-angular and their sizes range from 0.01 mm to 0.10 mm.

3.1.1. XRD

The X-ray diffraction analysis results of the tested raw materials used as concrete aggregates are in accordance with the results of microscopic features of the studied wastes used as raw materials under the microscope (Figure 4). More specifically, limestone contains mainly calcite and, in lower amounts, dolomite (Figure 4a). Wüstite, spinel group minerals, and lime-alumino silicate phases constitute the modal composition of slag (Figure 4b). Construction wastes (containing tiles) contain mainly quartz in their modal composition (Figure 4c). The modal composition of e-wastes comprises Cu, carbides, and baryte (Figure 4d).

3.1.2. Geochemistry

Table 2. Geochemical analysis results of the tested aggregates.

	S1A	S1B	S2A	S2B	S3A	S3B	S4A	S4B
CaO	54.89	55.00	3.78	6.56	28.75	28.74	44.70	44.90
SiO2	1.12	1.10	59.34	48.40	11.81	11.72	20.10	20.00
Al2O3	0.01	0.01	14.36	13.63	37.29	37.31	4.72	4.70
Fe2O3	0.05	0.04	10.02	14.28	20.41	20.20	1.79	1.68
SO3	-	-	-	-	0.10	0.10	0.04	0.03
MgO	0.42	0.30	5.20	6.74	1.64	1.59	0.69	0.71
MnO	-	-	0.15	0.19	-	-	-	-
K2O	0.07	0.04	0.47	0.05	0.03	0.03	0.50	0.60
Na2O	0.12	0.09	3.89	4.90	0.10	0.11	0.40	0.30
Cl−	-	-	-		0.01	0.01	0.18	0.17
TiO2	0.01	0.01	0.56	2.39	0.57	0.55	0.26	0.24
P2O5	-	-	0.03	0.23	-	-	-	-
LOI	42.30	41.90	2.00	2.20	-	-	26.23	26.30

shows the values of the major elements of the aggregates that are tested. As can be seen from Table 2, there is a significant variety in the concentration of those different raw materials, with basalts having the higher percentage of SiO₂ and limestones containing the higher percentage of CaO. As for the tested slags, they contain a high percentage of Fe. Regarding the tested e-wastes (S5A and S5B), they comprise a heterogeneous mix of metals and metalloids, and this makes it difficult to compare with the other raw materials. As it can be seen from

Table 3. Chemical analysis results of the tested e-wastes.

Chemistry/Samples	S5A	S5B
pH	6.17	6.20
Cd (mg/L)	0.56	0.70
Cr (mg/L)	0.31	0.35
Cu (mg/L)	5.27	5.21
Fe (mg/L)	1.53	1.48
Ni (mg/L)	10.22	10.30
Zn (mg/L)	6.12	5.98
Pb (mg/L)	1.35	1.32

, they contain noticeable concentrations of Ni, Zn, and Cu.

3.1.3. Physical Properties

It should be noticed that natural aggregates display similar values of specific gravity and porosity. Concrete specimens made by e-wastes as aggregates display the highest specific gravity values, while those made by construction wastes have the highest porosity values (

Table 4. Specific gravity and porosity results of the tested aggregates.

Test Results/Samples	S1A	S1B	S2A	S2B	S3A	S3B	S4A	S4B	S5A	S5B
Specific Gravity (m2/g)	0.512	0.510	0.412	0.415	0.729	0.725	1.009	1.020	1.010	1.000
Volume Porosity (cm3/g)	0.002	0.002	0.001	0.001	0.003	0.004	0.013	0.013	0.005	0.004

). The lowest porosity can be observed in specimens produced by basaltic aggregates rocks.

3.2. Characterization of Concrete

3.2.1. Concrete Strength

In

Table 5. Uniaxial compressive strength test results of the produced concrete specimens.

Test Results/Samples	S1A	S1B	S2A	S2B	S3A	S3B	S4A	S4B	S5A	S5B
UCScon (MPa)	12.5	12.0	15.0	14.0	12.0	13.5	9.7	9.0	6.5	6.9

, the results of the UCS test of the produced concretes are shown. The less durable specimens are those made by e-wastes as aggregates (Figure 5). The relative low strength is also presented in concrete specimens made by construction wastes used as aggregates, while all the other produced specimens present a higher durability with those containing basalts as aggregates to present the highest strength values (14 to 15 MPa).

3.2.2. Concrete Petrography

Representative figures of the produced concrete specimens are shown in Figure 6. As can be seen from the figure below, all the used types of aggregates, even if they are natural rocks (limestones, basalts), industrial by-products (slags), or other construction wastes (i.e., bathroom wastes), display satisfactory cohesion between them and the cement paste, a cohesion that makes them suitable for such applications they are made for. More specifically, better bonding between the aggregate and the cement paste is observed in those concrete specimens that are made by natural aggregates (Figure 6a,c), in those made by slags (Figure 6b), and less in those made by construction wastes (Figure 6d). Concrete specimens made by basaltic aggregates show fewer failures and pores among all the other specimens. On the other hand, concrete specimens made by e-wastes present numerous pores and failures and are hence the worst bonding between the aggregate and the cement paste, a fact that is in accordance with the results of the concrete strength as they are characterized as the less durable specimens.

3.2.3. Permeability

The permeability of the investigated pervious concrete specimens is presented in

Table 6. Permeability test results of the produced concrete specimens.

Test Results/Samples	S1A	S1B	S2A	S2B	S3A	S3B	S4A	S4B	S5A	S5B
k	0.15	0.16	0.17	0.17	0.16	0.16	0.14	0.13	0.11	0.10

through the k coefficient. As it can be seen from Table 6, concrete specimens produced by natural aggregate rocks such as sedimentary rocks (limestones), as well as those made by slags and construction wastes, present similar values that range from 0.13 to 0.17 cm/s. Concrete specimens produced by e-wastes as aggregates present lower permeability (k values), while the highest permeability is observed in concrete produced by basaltic aggregates (Figure 7).

3.2.4. Freeze–Thaw

The mass loss measurements of all the concretes are presented in

Table 7. Number of cycles of freeze and thaw tests of the tested concrete specimens.

	Number of Cycles of Freeze and Thaw Test
Samples	0	5	10	15	20	25	30	35	40	45	50
S1A	1300	1300	1300	1300	1300	1250	1250	1250	1250	1200	1160
S1B	1300	1300	1300	1300	1300	1230	1210	1200	1200	1190	1140
S2A	1600	1600	1600	1600	1600	1600	1600	1600	1580	1560	1520
S2B	1600	1600	1600	1600	1600	1600	1600	1600	1560	1520	1500
S3A	1500	1500	1500	1500	1500	1500	1500	1470	1400	1100	900
S3B	1500	1500	1500	1500	1500	1500	1500	1470	1400	990	890
S4A	1220	1220	1220	1220	1100	1080	1020	800	600	580	380
S4B	1220	1220	1220	1220	1100	1080	1020	800	600	580	380
S5A	1180	1180	1180	1180	1180	1140	1165	1150	790	590	580
S5B	1180	1180	1180	1180	1180	1140	1150	1140	800	600	580

and the course of the % change in mass of all the samples is shown in the diagram of Figure 8. As observed from both Table 7 and Figure 8, until the 15th cycle, the samples do not appear to show mass loss. The samples produced with limestone, basalt, slag, and e-wastes almost up to the 35th cycle show no change in mass or show little of it.

Regarding the concrete specimens made by construction wastes, they show a mass loss from the 20th cycle. From the 20th cycle onward, they show a significant reduction in their mass. Sudden mass loss is also observed in concrete with slag from the 40th cycle onward. Concrete with natural aggregates (limestones, basalts), as a whole, shows the least mass loss of all the investigated concretes (Table 7, Figure 8).

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.

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:

• All the tested raw materials showed obvious differences mainly in their petrographic 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.