1. Introduction
Lightweight aggregate (LWA) is significantly different from conventional aggregate. LWA is a solid material whose apparent density does not exceed 2.0 g/cm
3 and whose bulk density does not exceed 1.2 g/cm
3 [
1,
2]. Due to the material from which the aggregate is obtained, in simple terms, it can be divided into three main groups:
- −
Naturally occurring materials that do not require processing, such as pumice, foamed lava, volcanic tuff and porous limestone.
- −
Materials that occur naturally and require further processing, such as expanded clay, slate and vermiculite;
- −
Industrial by-products and wastes such as sintered fly ash, expanded or foamed blast furnace slag or expanded blast furnace slag and hematite [
3].
Unfortunately, the lightweight aggregates industry causes a global environmental problem due to the use of large amounts of natural resources. Natural raw materials suitable for the production of such aggregate are rather unique. There is a continuous increase in the production of aggregates, so non-renewable natural resources are decreasing at an increasingly faster rate due to the high demand in various industrial branches. The development of the production of artificial light materials will help minimize the consumption of natural resources. The obtained modifications may bring benefits and new challenges for designers for many reasons, for example, weight reduction, improved acoustic or thermal properties, drainage or filtration capabilities [
2,
3,
4,
5]. Raw materials and waste used in the production of artificial lightweight aggregate may come from many different sources. These may be processed waste of municipal origin [
6,
7] or different types of ash, for example, bottom ash from incinerators [
8,
9], fly ash in pellet form [
10] or paper ash from recycled newspapers [
10,
11]. They may also be crushed construction rubble, such as foam and ordinary concrete [
8,
11,
12,
13], brick [
8,
10,
11,
14,
15] or other ceramics and foamed and crushed glass [
16]. Sometimes materials of other origin are used as raw materials, such as river sediments [
17], beer production waste [
18], crushed coconut shells [
19], crumb rubber or shredded tires [
20,
21]. Thus, LWA is very desirable and widely used in various industries. Considering that aggregate constitutes approximately 70% of the concrete mix, replacing natural aggregate with lightweight aggregate produced from waste materials will be an effective method of minimizing the use of non-renewable resources. LWA is important in creating lightweight concrete by reducing greenhouse gas emissions in buildings and reducing the self-weight of structures [
22,
23]. Moreover, lightweight aggregate is a key element in the construction of earthquake-resistant buildings [
24]. Regardless of the uses of artificial aggregates in concrete production, LWA is worth investigating specifically to minimize environmental problems, along with maintaining long-term sustainability through improving water quality (filtration) [
25].
Modern rainwater management, aimed at rational protection of water resources in terms of their quantity and quality, must take into account the implementation of sustainable urban drainage systems in urban areas (sustainable urban drainage systems—SUDS), ensuring relief and improved operation of rainwater drainage systems, improving the microclimate and water balance of urban areas and the quality of ecosystems while increasing the aesthetic values of public space. The objects included in this type of system include, among others, roofs covered with vegetation, called green roofs. An interesting solution to the use of lightweight aggregates is to use them as a substrate for green roofs to mitigate the urban heat island effect [
26,
27]. As a sustainable ecosystem system, the green roof is known for its ability to provide thermal resilience and buffer surface runoff of stormwater in urban areas. The shape and type of materials used in the drainage of a green roof and the substrate layer significantly affect energy efficiency and water drainage [
18,
28,
29,
30,
31]. Due to the larger number of internal pores in lightweight aggregate, moisture absorption is faster than in the case of ordinary aggregate. Therefore, one of the most important indicators for green roof materials is the water retention capacity of the substrate materials. Extensive green roofs are less frequently maintained and have shallower aggregate layers, so the cultivation of various types of plants and species is largely dependent on the soil and drainage layers [
30,
32]. In order to impose less load on buildings, the substrate layer should not be deeper than 20 cm; therefore, the composition and physical properties of the aggregate have the greatest impact on the water-retention capacity. The materials used in the drainage layer can contribute to increasing the water-holding capacity of green roof systems. For example, crushed brick, as a lightweight porous material, reduces pressure on the subgrade and drainage layers while increasing the water-holding capacity of the green roof system [
33]. Porous materials listed as raw materials used for the production of lightweight aggregates have similar properties. Ecological issues, such as the limitation of natural resources and huge amounts of waste, are increasingly leading the developing civilization towards sustainable construction. Two basic environmental problems are the depletion of natural resources and the disposal of waste generated during various processes. Substrate mixtures for extensive green roofs containing organic matter should contain 70–90% of the volume of minerals. Some authors suggest that organic matter should be up to 10% and that this is adequate for sustainable plant growth [
34]. One of the key topics in green roof research is replacing these materials with recycled or locally available components to reduce the environmental impact of green roof construction.
The authors attempted to produce lightweight aggregate using various consolidation and grinding methods. The material used to produce the aggregate was moraine clay, which is a by-product in the process of washing gravel aggregates. Warm periods (interglacial) between glaciations (glacials) that occurred repeatedly during the Quaternary period left crumb material on over 70% of Europe’s territory, which can be used in construction. Water flowing from melting ice sheets carried sand and gravel [
33,
35,
36]. Various glaciation ranges mean that they are available in almost every region and easy to extract using open-pit methods. Due to transport costs, they are mainly used in local construction. Glaciations and interglacial periods as well as the river network developed during the Holocene provided billions of tons of raw material. Currently, mainly deposits of glacial and glacial sands and gravels are used, which occur mainly in the central and northern parts of Europe. Glacial clays are treated as by-product material. A characteristic feature of glacial clays, apart from the co-occurrence of rock fragments with extremely different diameters, is also the huge variability of their composition in space. Fragments made of almost pure clay, after a few or a dozen or so meters, can be replaced by a completely sandy sediment, and a little further along, they become a mixture of all fractions—from clay to large boulders. So far, fine-grained materials are consolidated using granulation processes, obtaining a spherical shape of lightweight aggregate. The authors wanted to draw attention to the fact that flat grains are more beneficial when used for garden architecture purposes (green roof) due to their unique properties. The flat grains are arranged in a specific tile-like pattern, which creates additional spaces between the grains called cavities. This hollow space can also be periodically filled with water, which contributes to more efficient drainage and retention, while on the other hand providing space for plant roots to develop. To produce super flat grains, the authors used patent solution no. PL 231748 B1 called multi-deck vibrating screener [
37]. The performance properties of flat aggregates and how they are produced are discussed in detail later in this article.
2. Materials and Methods
2.1. Material Glacial Clay
The occurrence of natural sands and gravels in Europe is common but uneven. The reason for the diversified distribution of natural resources of gravel and sand aggregates are age and genetic factors that affect the conditions of occurrence of deposit series—the diversity of deposits and resources found in Europe. The most sought-after coarse aggregate (gravels—fraction content 2 mm ≤ 30% and sand with gravel—fraction content 30% < 2 mm ≤ 70%) occurs in the southern part of Europe, in the Carpathian-Sudetic zone and in the northern part of the continent [
38]. Deposits of natural sands and gravels also differ significantly in their petrographic composition. The material is located in the Pomellen-Nord natural aggregate mine (Germany) and belongs to Calculus company. Samples were taken from four sedimentation tanks, as shown in
Figure 1. The material was collected at points from different places in the tanks and then averaged. The tests covered a wide analytical spectrum, such as granulometric composition, mineral and chemical composition, calcium carbonate content, organic parts content, natural humidity, hygroscopic water content and pH reaction. The results of these studies provide a rational answer as to whether the accompanying clays will have potential applications.
2.2. Analytical Methods Used in the Research
In order to assess the mineral content of individual samples, X-ray analysis was performed. The tests were performed using a PANanalytical X-ray diffractometer model (Empyrean, Malvern Eindhoven, The Netherlands). The share of individual phases was determined using the Rietveld method. The measurements were made using monochromatic radiation with a wavelength corresponding to the Kα1 emission line of copper in the angle range 5–90° on the 2ϴ scale. The qualitative analysis of the phase composition was carried out using the X‘Pert HighScore plus 3.0 Plus computer program developed by PANanalytical. The reference databases used were PDF-2 (2004) and ICSD Database FIZ Karlsruhe (2012). In order to determine the elemental composition of the samples, the following was used: X-ray fluorescence method XRF (Rigaku—Primini WDXRF spectrometer, Tokyo, Japan).
Investigations carried out in the high-temperature microscope belong to the standard investigations of thermal properties of materials. Not only do they allow determination of characteristic temperatures, but they also allow determination of decomposition temperatures, sublimation temperatures, phase transition temperatures, etc. Measurements in the high-temperature microscope (HSM, Misura® Expert System Solution, Modena, Italy) were performed on particular sets of specimens with the temperature increment 10 °C/min. This allows the influence of temperature on the behavior of the material under investigation to be evaluated. The greatest advantage of this method is the ability to make in situ observations of changes in the dimensions and shape of the sample as it is heated.
In order to determine the basic mechanical properties, an abrasion testing device was used. The tests were carried out on an apparatus from Erweka TAR II, Frankfurt, Germany. The result of the test is the percentage weight loss of the sample with a grain size of less than 2.0 mm. The test time was 10 min, with a tank speed of 20 rpm.
Aggregate imaging studies were performed by digital microscope Keyencee, VHX-7000 N, Osaka, Japan. The microstructure of the tested materials was examined using a scanning microscope (FEI, Nova NanoSEM 200 model, Hillsboro, OR, USA) This microscope allows operation under high vacuum and operation with steam as the working gas in low vacuum mode (10–200 Pa). Achievable magnifications range from 100× to 1,000,000×. The microscope makes it possible to assess the surface of materials, internal structure, changes and deformations.
2.3. Characteristics of the Grinding and Screening Technology Station
The test stand consists of a jaw crusher from Eko-Lab (Brzesko, Poland) (
Figure 2a) and vibrating screens: a three-deck, four-product HTS-(Gliwice, Poland) (
Figure 2b) and a three-deck, six-product HTS-(Gliwice, Poland) (
Figure 2c) developed according to the patented invention No. PL 231748 B1. These machines were used to simulate the production process of irregular (flat) aggregates according to the technological layout shown in
Figure 3. A more extensive description of the method of producing both shaped and unshaped aggregates in the patented screen can be found in the authors’ previous works [
39].
The crushing and two-stage screening system with return as shown is designed to crush the material in a selective way so as to obtain oversize products, i.e., larger than the required final products. The oversize product is screened on the sieve and returned to the circuit. This process involves the addition of one screen deck (16 mm) and the provision of an increased technical capacity of the screen and crusher by several or tens of percents due to the circulating material. In addition, in such an installation, it will be possible to control the crusher outlet gap, which will have the effect of increasing and varying the grain size distribution of the individual product fractions and at the same time minimizing the proportion of undesirable products, e.g., 0–2 mm, or increasing the proportion of other fractions. The second screening stage based on two three-deck six-product vibrating screens is a unique solution. These equipment screens place aggregates into narrow fractions in the first part of the screen decks. In the second part of the screen decks, the fractions are separated by shape into regular (cubic) and irregular (flat, elongated) grains. The shape of the regular and irregular grains in the products obtained is defined in accordance with the adopted standard used in the production of crushed aggregates PN-EN 933-3:2012 [
40]. Such a solution makes it possible to obtain aggregates with different and narrow grain size ranges and different shapes. The red dashed lines and arrows drawn on the diagram illustrate the alternative possibility of producing regular aggregates or turning them back for grinding. This screening process results in three fractions with regular grains and three fractions with irregular grains in a single screen, which can be combined with each other, with the regular grains, IR, being returned for crushing and the irregular grains (IP) being directed to the firing process for the production of lightweight aggregates.