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Article

Improved Artificial Aggregates for Use in Green Roof Design

by
Agata Stempkowska
* and
Tomasz Gawenda
Faculty of Civil Engineering and Resource Management, AGH University of Krakow, Mickiewicza 30 Av., 30-059 Krakow, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(13), 5512; https://doi.org/10.3390/su16135512
Submission received: 27 May 2024 / Revised: 19 June 2024 / Accepted: 26 June 2024 / Published: 28 June 2024
Browse Figures
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Abstract

:
The development of surfaces in cities, as a result of progressive urbanization, not only reduces the natural retention capacity of the environment but also causes changes in the water balance. In urbanized areas, the amount and intensity of rainwater discharged to receivers increase, and the time of water outflow from the catchment area shortens. Low retention does not provide effective responses to the local water deficit and does not limit the effects of excess water during flood periods. Furthermore, aging drainage systems do not always have the required hydraulic efficiency in absorbing runoff after intense and heavy rainfall or snowmelt. The aim of the work was to determine the possibility of obtaining flat aggregates with a grain size of 2–16 mm from clay-silt fractions from sedimentation tanks using selected mechanical processing methods (crushing and screening in a crusher-screener system with recycling). An important issue was the examination of the physical and mechanical properties of the produced aggregates after firing, where the work required a detailed material analysis using various research techniques, such as XRD, XRF, SEM and digital microscopy. The obtained results will allow for further research on developing the concept of technology for the production of lightweight aggregates used, for example, on building roofs. Particular attention was paid to the flat shape of the aggregate, which affects a number of its properties. To obtain a flat-shaped aggregate, the authors used a patented sieving method. The obtained materials had high cavernosity of 69% on average, water absorption of 40.7% and low bulk density of 0.82 g/cm3.

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/cm3 and whose bulk density does not exceed 1.2 g/cm3 [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.

3. Results and Discussion

3.1. Characteristics of Glacial Clay

3.1.1. Grain Distribution Analysis

Samples were taken as suspensions from the sedimentation basins shown in Figure 1. They were preserved for testing and transported in pans. After drying, they can be characterized as varying degrees of sandy clay. The material was separated into individual fractions, and wet sieve analysis was performed. Table 1 and Figure 4 present the research results. Individual samples were clearly different from each other. Sample No. 1 is the thickest material, and the material is divided between individual grain fractions. Sample 1 can be classified as silty clay, with the others as clays [41]. The remaining samples are definitely more homogeneous, and sample no. 4 contained small amounts of sand. For further material processing, grain sizes below 0.063 mm are preferred.

3.1.2. X-ray Analysis

Table 2 shows the quantitative determination of the minerals found, and Figure 5 shows an example of an X-ray.
The mineralogical composition of the samples is similar. It is mainly quartz and carbonates, approximately 30%. Another group with a content of several percent includes clay minerals from the kaolinite-illite and vermiculite groups. These minerals are the basic plastic ingredient for various types of ceramics. Their presence will also be beneficial for material granulation for agricultural applications. Clay minerals also have the ability to store moisture. Next, feldspars were observed, i.e., orthoclase and its polymorphic form, microcline and albite. Large amounts of calcite and dolomite are observed [42,43]. During firing, such raw materials will create porosity and are not suitable as a raw material for highly sintered materials (clinker) [44]. The appropriate amount of calcium carbonate in the mass causes the formation of CaO (decarbonation of CaCO3) and consequently reduces the viscosity of the liquid phase formed at high temperatures and facilitates the sintering process. The second effect is the formation of pores as a result of decarbonation, which create a network of interconnected channels (open porosity increases). The thermal decomposition of calcium carbonate proceeds according to the following reaction. In a stoichiometric process, the mass loss is 44%.
CaCO3 → CaO + CO2↑
In clay, raw materials with excess CaCO3, anorthite, calcium aluminates and silicates are formed at temperatures above 960 °C. The decarbonation reaction of calcite and dolomite is intense under normal pressure. The presence of dehydrated clay and inorganic admixtures of Fe2O3, TiO2, SiO2 and others contributes to the acceleration of the decarbonation reaction, which can be attributed to an increase in phase formation involving CaO and MgO. In the presence of dolomite, the diopside CaO-MgO-2SiO2 can form in the presence of CaO and Fe2O3—aluminosilicates and ferrates [45].

3.1.3. XRF Analysis

The averaged results are shown in Table 3.
A high iron content (of the order of a few percent) is observed in the chemical composition. No harmful elements such as Hg, Cr, Cd or Pb were found. A high amount of calcium was observed, which is confirmed in the XRD analysis by the presence of calcite and dolomite.

3.2. Production of Aggregates

The test methodology adopted was to prepare the averaged samples by drying them (moisture content to about 4%) and naturally cracking to a size < 63 mm. The material was then subjected to a crushing process in a crusher at outlet gaps of 8, 10 and 12 mm, with a crushing process at a slot of 6 mm included to compare the effects of the tests. (This created a no-turn-open system, as the product obtained then did not exceed 16 mm). The graphs (Figure 6) show the distribution of the fractional fractions obtained, depending on the set crusher outlet gap. In the jaw crusher operating at a discharge gap of 6 mm, more than 32% of the <2 mm fine fraction was obtained. In contrast, the highest yield in the range of approximately 43% to 47% was obtained for the 2–8 mm fraction. For the 8–16 mm fraction, the output ranges from about 21% to over 25%. Analyzing the grinding tests, it can be seen that as the outlet gap increases, the outturn of the 0–2 mm fraction decreases from 32% to more than 11%, the proportion of the intermediate 2–8 mm fraction decreases from about 47% to 26% and the proportion of the coarsest fraction increases from about 21% to 62%. At an outlet gap of 12 mm, the material returned would be about 12%. The research shows a possible wide range of product yield control but especially the possibility of reducing the 0–2 mm fraction to around 12%. From the point of view of the use of aggregates for green roofs, dusty fractions (below 2 mm) should be eliminated as far as possible. Aggregates were produced in narrow fractions according to the scheme (Figure 3) with flat grains. Aggregates in the 4–6.3 mm fraction were obtained for further research purposes and for comparison with other available aggregates on the market.

3.3. Sinterability Analysis

Sintering is the basic technological process in the manufacture of materials with a settled shape. When heated to an appropriate temperature, below the melting point, a collection of contacting grains bind to each other to form a polycrystal. At the heart of this process are mass transfer mechanisms that lead to macroscopic changes in the material. These include a reduction in porosity and accompanying compaction and shrinkage of the sintered system and an increase in its mechanical strength. This test provides valuable information about the behavior of the material during heating and often approximates the properties of the material in various industrial processes, where phenomena occurring at the interface between two phases—liquid and solid—play a predominant role. This is particularly true for such materials that undergo phase transformations during heating [46]. Table 4 shows the results of the analyses. Figure 7 shows an example of HSM image analysis—for sample 1. The orange dashed line shows the original shape of the sample The sample for microscopic examination is taken from the raw material and formed into a cylinder 5 mm high and 3 mm in diameter.
All samples show similar sintering parameters. The maximum temperature is 1150 °C. Above 1200 °C, the samples melt, so this firing temperature should not be exceeded (Figure 8). None of the samples showed high-temperature swelling properties.
In general, the finer the mineral fraction, the faster the sintering process. The materials studied are fine-grained so transport of matter will occur quickly and homogeneously. The sintering of clays is a process consisting of three stages:
−
Stage I—the initial sintering phase—observed when the temperature of the material is about 0.25 of the melting point. In this stage, no shrinkage is observed, and the original layering in the aluminosilicates remains intact. This process is characterized by the removal of the rest of the unbound water, the content of which is still a few %.
−
Stage II—the intermediate phase—occurs when the temperature of the material is 0.25–0.75 of the melting point, at which point the onset of shrinkage, grain growth and material thickening can be observed. In the second firing period, also known as the dehydration period, which includes a further increase in temperature to about 600 °C, chemically bound water is released. At the same time, the decomposition of organic matter takes place during the dehydration process, and the decomposition of chemical compounds and minerals also begins.
−
Stage III—the final phase—the end of the compaction phase, the transformation of open pores into closed pores and their partial disappearance, with the grains continuing to expand. This stage, known as the vitrification period, is characterized by major changes in the mineralogical composition of the mass.
The main reactions occurring during firing in clays are as follows:
−
release of hygroscopic water from clay minerals and water from silica-clay gels,
−
oxidation of organic admixtures,
−
release of constitutional water, i.e., dehydration of clay minerals,
−
reactions in solid phases,
−
liquid phase reactions and formation of glassy alloy,
−
formation of new crystalline phases,
−
decarbonation and sulfur-removal reactions.
The first group of reactions is characterized by the formation of water vapor, the pressure of which can burst the material or furnace lining at very high temperature increases. In the second group of reactions—oxidation of organic admixtures—some of these admixtures may remain unfired when the temperature rises rapidly, revealing themselves as a dark core on the break of the product. Virtually no organic admixtures were found in the filter cake tests. The dehydration of clay minerals is primarily influenced by the firing environment. In addition to this, the presence of Fe2+ in the clays promotes the formation of new phases [47,48,49,50]. In further studies, the firing process of the raw material was carried out in an electric chamber kiln, and care was taken to select the firing temperature so as to obtain durable sinter and high porosity at the same time, i.e., to omit stage III. A sample of the fired aggregate is shown in Figure 9. The materials studied, clays, have a complex mineralogical composition, hence the sintering system is complicated. Analysis of the feed showed that the material contains a high proportion of calcium carbonate, above 30% (Table 2), which was present in a fine-grained form dispersed throughout the volume. The authors’ research [51] shows that the standard firing temperatures characteristic of, for example, red ceramics are too high and close the porosity of the material. On the other hand, the full thermal dissociation of carbonates occurs at a temperature of about 1000 °C. It was therefore decided that the sintering process should be carried out at 1050 °C. This is the lowest possible sintering temperature, which at the same time ensures full decarboxylation of the carbonates.

3.4. Analysis of Selected Properties of Burnt Aggregates

3.4.1. Evaluation of Soluble Compounds Based on Conductivity and pH

Evaluation of the leachability of soluble compounds based on conductivity and pH was carried out on an aggregate fired at 1050 °C. Table 5 presents the results of the weekly tests. No significant changes in pH were recorded, but it is quite high due to the presence of calcium. The leachability remains at a very low level throughout the assumed experimental time. The pH of the distilled water was 6.48 and the conductivity 32 µS. The tested aggregate can therefore be regarded as intertie and safe in terms of leaching of potentially hazardous and corrosive compounds.

3.4.2. Bulk Density, Apparent Density and Cavernosity Tests on Samples

To analyze the apparent density of aggregates, a basket stand submerged in liquid and suspended on a balance was used. The method involves weighing the solid under testing in air and then in a liquid of known density (e.g., water). The apparent density of the aggregates (Table 6) varies little between samples, and values range from 1.85 to 2.04 g/cm3. The apparent densities of the aggregates obtained are at the limit of EN 13055:2016, lightweight aggregates [1]. In the following analyses, tests were carried out on the bulk density and cavernosity of the narrow 4–6.3 mm fraction; each measurement was repeated five times, and the average values are given in Table 6. These fractions were obtained by sifting using control sieves from the 2–8 mm fraction in order to narrow down the wide range of grain size distribution and to compare the aggregates obtained with other lightweight aggregates available on the market. The bulk density of all the materials tested is similar, meeting the aforementioned standard and ranging from 0.72–0.88 g/cm3. Due to the fact that the authors obtained flat grains, the bulk density of the materials is low, as the flat grains are arranged in a tile-like manner, causing voids to form between them. The sum of these voids can be referred to as cavernosity.
The method of testing for cavernosity consists of weighing the dry weight of the aggregate (net weight—without container) contained in a container of known capacity, then pouring distilled water over the aggregate to the level of known capacity (the entire aggregate is covered with water) and weighing the aggregate with water (net weight—without container). Aggregate cavernosity is the percentage of inter-grain spaces corresponding to the mass of water that occupies the space between the grains, expressed as a percentage. This parameter is extremely important. Material with high cavernousness maintains the correct levels of moisture, nutrients and air and the correct particle structure required for long-term sustainable plant growth. LECA. Kiryu and Akadama have a spherical shape, and their packing is greater. The flat aggregate produced has a cavernousness of 68–71%, and this is the highest value compared to aggregates available on the market.

3.4.3. Abrasion Resistance of the Aggregate

The lightweight aggregates provided for green roofs do not carry heavy loads, so there is no need for them to have high crushing properties. However, they should be characterized by their abrasion resistance. Firstly, it is a question of transport and incorporation into the green roof—eliminating process dust. The second reason is the formation of a dusty fraction during the operation of the green roof, which settles at the bottom of the layer and can interfere with the drainage of excess water and the aeration of the substrate and prevent their proper growth. Figure 10 shows the results of the measurements, which were carried out five times for each sample. The narrow material fraction tested was 4–6.3 mm. Despite a fairly uniform mineral and oxide composition and identical sintering conditions, a large scatter of test values is observed. On average, the weight loss in all determinations was 3.01%. This is due to the very high porosity of the material and the possibility of an increased number of fissures and critical pores.

3.4.4. Water Absorption, Total Retention, Water Release Kinetics

Successful green roof design depends heavily on moisture management. The need for vegetation to retain moisture and the effective drainage of excess water during large storms are two factors that must be considered by green roof designers. Irrigated designs also need to consider the most efficient means of replenishing water to the plants. The basic principle in any green roof design is to eliminate the need for irrigation. The key to eliminating irrigation (or reducing the need for water) is to choose the right substrate for the green roof. In terms of water buffering capacity, it has been shown that water permeability depends on the porosity and shape of the materials used for the green roof substrate layer [52]. Another study conducted showed that the use of highly porous materials, such as mineral wool, in the substrate layer increased the porosity and permeability of green roof systems [29,53,54]. In the fractions obtained, the absorbability of the produced aggregates does not vary strongly. The values range from 39.5 to 42.2%. This is related to the firing temperature and the grain shape obtained. The unshaped grains have a higher cavity and a higher specific surface area. In addition, the material is more durable and resistant to mechanical factors and temperature changes after firing at approximately 1050 °C. The aggregates compared have different absorption capacities. Japanese Kiryu and Akadama aggregates absorb water at a rate of approximately 30%. Clay shale absorbs water at a significantly lower rate, averaging around 20%. Very divergent results characterize the LECA aggregate. If the grains are cracked or deliberately crushed, the water absorption is very high, up to 45%. Unbroken and uncracked grains have a low absorbability of the order of 10% due to the vitrified outer layer of the aggregate. That is, LECA would meet the conditions for use as a substrate for a retention roof but only after crushing, which generates additional costs and energy consumption. The accumulation of rainwater on the roof is only possible with the prior application of good quality vegetation and drainage layers. Micro-retention in the pores of the aggregate is crucial. In many cities around the world, it has been recognized that the most important ecological advantage of rooftop planting is stormwater management. It is difficult to give average performance values, but around 50% of rainfall can be expected to be retained by an extensive green roof. Water retention during rapid rainfall was simulated. To do this, 100 g of each dried aggregate was taken, and 300 mL of distilled water was poured through it. The water was poured in such a way that the flow was dynamic (simulating a surge rainfall), and the surface of the aggregate was completely wetted. The overflow time for each sample was 10 s. The experiment for each sample was repeated five times, after which the average value was calculated. The results are shown in Table 7. The total water retention is the amount of water retained in the aggregate in percentage terms. A green retention roof is able to store large amounts of rainwater, essentially improving the water balance and counteracting the effects of drought.
Despite its very low bulk density, the typical LECA lightweight aggregate has a relatively low water absorption rate because its structure is dominated by closed porosity. This porosity is irrelevant for absorbing water during heavy rainfall but can be used as a drainage layer. The sintered flat aggregates produced have the highest retention capacity, with an average of 34.5% compared to the other aggregates tested. If the aggregate absorbs water very well, the retention capacity can be equal to the total absorption capacity. This is the case with Japanese Kiryu, whose absorbability and retention capacity are both 30%.
The kinetics of water release is a very important parameter in terms of plant development on a green roof. It appears that the type of aggregate influences the rate of water release, as shown in Figure 11. This is mainly influenced by the type and size of porosity. This is because materials with open porosity will behave differently from materials with partially closed porosity. The more open pores with a corresponding diameter, the longer the water release time. The aggregates produced are excellent at absorbing and slowly giving up plant-available water over a longer period of time. The water release curves are similar for the manufactured aggregates and Japanese Akadama and Kiryu. The water storage capacity for Shale and LECA is limited.

3.4.5. Optical and Scanning Microscopic Analysis

Porosity is therefore of key importance when it comes to water absorption. What is also important, however, is the type of this porosity, as it has been shown that increasing the number of tortuous pathways through the substrate layer increases the retention time and reduces the water permeability of green roof systems [55,56]. That is, the way the transport channels are arranged in the aggregate structure and their size are important. Large pores will cause rapid water drainage, while a developed capillary porosity of approximately 0.5–10 µm will allow water to be retained. Macro and mesopores and transport channels were observed in the resulting aggregate. Example microphotographs are presented in Figure 12.

4. Conclusions

  • Glacial clay, which has not been used to date, is well suited for the production of lightweight aggregates that are designed for “green roofs”.
  • The laboratory machines used in the presented technological system with the use of a unique patented screening unit made it possible to produce aggregates with flat grains in narrow fractions.
  • The control of the crusher outlet gap enables the production of 2–8 and 8–16 mm aggregates in varying proportions and the minimization of <2 mm fractions. The highest yields, ranging from approximately 43% to 47%, were obtained for the 2–8 mm fraction. For the 8–16 mm fraction, the outcrop ranged from about 21% to over 25%, and for the 0–2 mm fraction, it was up to 12%. From the point of view of the use of aggregate for green roofs, dusty fractions (below 2 mm) should be eliminated as far as possible.
  • The aggregate has an apparent density of approx. 2.00 g/cm3, which is close to the standard limit, but due to the special shape of the aggregate has a very low bulk density of 0.82 g/cm3 on average. The aggregate made from clayey-clay raw material has a high water absorption rate of approximately 40% and the highest cavity density of ~68% compared to other aggregates.
  • There are three conditions that a substrate must fulfil to ensure adequate moisture management: effective water absorption and retention; easy drainage; and high void coefficient (air volume)—cavernosity. The produced aggregate meets all these conditions.
  • A high and sufficiently developed porosity, and thus water absorption, was achieved by suitable sintering, while in further studies, the authors would like to focus on the use of additives that improve the formation of winding channels important for water retention.

Author Contributions

Conceptualization, A.S. and T.G.; methodology, T.G.; formal analysis, A.S.; T.G., investigation, A.S; writing—original draft preparation, A.S.; writing—review and editing, T.G; All authors have read and agreed to the published version of the manuscript.

Funding

The research project is partly supported by the program “Excellence initiative—research university”.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

The authors would like to thank Calculus Sp. z o.o. for providing the raw materials for the study and the necessary materials for writing the publication.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Natural aggregate deposit with sedimentation tanks, Pomellen-Nord, Germany.
Figure 1. Natural aggregate deposit with sedimentation tanks, Pomellen-Nord, Germany.
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Figure 2. Machines used to produce the aggregates: laboratory jaw crusher (a), three-deck four-product vibrating screen (b), three-deck six-product vibrating screen (c).
Figure 2. Machines used to produce the aggregates: laboratory jaw crusher (a), three-deck four-product vibrating screen (b), three-deck six-product vibrating screen (c).
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Figure 3. Diagram of the technological system for the production of irregular aggregates with a return. RP—regular particles, IP—irregular particles.
Figure 3. Diagram of the technological system for the production of irregular aggregates with a return. RP—regular particles, IP—irregular particles.
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Figure 4. Grain curve of the analyzed material.
Figure 4. Grain curve of the analyzed material.
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Figure 5. Diffractograms of the test samples.
Figure 5. Diffractograms of the test samples.
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Figure 6. Distribution of the yields of the individual fractions depending on the outlet gap “e” of the crusher.
Figure 6. Distribution of the yields of the individual fractions depending on the outlet gap “e” of the crusher.
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Figure 7. Geometrical changes of sample 1 (beginning of analysis, sintering, spreading).
Figure 7. Geometrical changes of sample 1 (beginning of analysis, sintering, spreading).
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Figure 8. Sintering curves of the test samples.
Figure 8. Sintering curves of the test samples.
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Figure 9. Aggregates obtained after drying, crushing in a jaw crusher and firing at 1050 °C.
Figure 9. Aggregates obtained after drying, crushing in a jaw crusher and firing at 1050 °C.
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Figure 10. Abrasion test on lightweight aggregate.
Figure 10. Abrasion test on lightweight aggregate.
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Figure 11. Water saturation level curves for the tested aggregates.
Figure 11. Water saturation level curves for the tested aggregates.
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Figure 12. Example of microscopic images of aggregate.
Figure 12. Example of microscopic images of aggregate.
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Table 1. Results of sieve analysis of the tested materials.
Table 1. Results of sieve analysis of the tested materials.
Sieve [mm]Yield [%]TotalYield [%]Total Yield [%]Total Yield [%]Total
Sample 1Sample 2Sample 3Sample 4
0.0228.2428.2493.9893.9872.0372.0389.8889.88
0.049.0537.295.3299.3023.0495.077.9897.86
0.06317.7355.020.5699.864.6699.730.7098.56
0.07118.2673.280.0599.910.1399.860.0198.57
0.124.9498.220.0899.990.1199.970.0798.64
0.251.7799.990.011000.031000.3398.97
0.3150.01100-100-1001.03100
Table 2. Averaged phase shares in individual material samples.
Table 2. Averaged phase shares in individual material samples.
MineralChemical FormulaSample 1Sample 2Sample 3Sample 4
Composition [% wt.]
QuartzSiO231.329.931.929.0
CalciteCaCO330.333.833.332.0
DolomiteCaMg(CO3)28.77.38.410.0
AlbiteNa[AlSi3O8]6.65.56.47.0
Orthoclase/MicroclineK[AlSi3O8]7.26.67.68.0
Vermicullite(Mg, Fe, Al)3(Al, Si)4O10 (OH)2·4H2O1.81.30.4-
Illite/KaoliniteAl4[Si4O10](OH)814.115.612.214.0
Table 3. Averaged oxide chemical compositions of the material.
Table 3. Averaged oxide chemical compositions of the material.
OxideSample 1Sample 2Sample 3Sample 4
Content of Oxide [% wt.]
MgO3.663.463.633.49
Al2O310.7211.5510.9811.29
SiO253.4153.5055.2754.45
K2O2.832.912.752.90
CaO19.6519.3021.0219.78
TiO21.161.120.991.07
MnO0.440.500.390.46
Fe2O38.037.604.886.49
ZrO20.110.070.080.08
Table 4. Characteristic temperatures of the materials tested.
Table 4. Characteristic temperatures of the materials tested.
NumberSinteringSofteningSpreading
Specific Temperatures [°C]
Sample 1116011801208
Sample 2116011851208
Sample 3116911901202
Sample 4115811901208
Table 5. Conductivity and pH test results.
Table 5. Conductivity and pH test results.
SampleConductivity [μS]
Day 1Day 3Day 7
1165179284
2145176159
3167189183
4156176210
SamplepH [-]
Day 1Day 3Day 7
18.018.348.45
28.248.218.32
38.148.238.27
48.298.098.14
Table 6. Summary of physical properties of aggregates with grain size 4–6.3.
Table 6. Summary of physical properties of aggregates with grain size 4–6.3.
Type of AggregateCavernosity
[%]		Apparent Density [g/cm3]Bulk Density [g/cm3]
Burnt aggregate, flat grainsSample 171.41.990.72
Sample 268.31.910.81
Sample 367.91.850.85
Sample 469.02.040.88
Kiryu (Japan), regular grains52.21.610.84
Akadama (Japan), regular grains60.01.930.78
Shale (Poland), flat grains55.31.810.89
LECA (Poland), regular grains54.41.220.43
Table 7. Summary of hydrometric parameters of aggregates.
Table 7. Summary of hydrometric parameters of aggregates.
Type of AggregateTotal Absorbability [%]Retention Capacity [g/cm3]
Burnt aggregate, flat grainsSample 142.235.0
Sample 240.032.6
Sample 339.534.2
Sample 441.236.5
Kiryu (Japan), regular grains30.130.0
Akadama (Japan), regular grains30.628.2
Shale (Poland), flat grains21.417.6
LECA (Poland), regular grains10–4519.0
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Stempkowska, A.; Gawenda, T. Improved Artificial Aggregates for Use in Green Roof Design. Sustainability 2024, 16, 5512. https://doi.org/10.3390/su16135512

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Stempkowska A, Gawenda T. Improved Artificial Aggregates for Use in Green Roof Design. Sustainability. 2024; 16(13):5512. https://doi.org/10.3390/su16135512

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Stempkowska, Agata, and Tomasz Gawenda. 2024. "Improved Artificial Aggregates for Use in Green Roof Design" Sustainability 16, no. 13: 5512. https://doi.org/10.3390/su16135512

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