Lightweight aggregate (LWA) is defined as solid substance with an apparent density of less than 2.0 g/cm3 and a bulk density of less than 1.2 g/cm3 (EN 13055:2016). LWAs are porous and often granular materials that are widely used in architecture, landscaping and geotechnics. The porosity of these materials can be either open or closed, which affects their applicability. For example, aggregates with open porosity can provide effective drainage but also micro water retention. Furthermore, they can provide better sound absorption and thermal insulation. Lightweight aggregates can be divided into the following categories (Kumar 2010, Franus 2015, Alqahtani 2019):
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naturally occurring raw materials that require further processing, such as clays and intrusive shales and vermiculite;
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naturally occurring raw materials that do not require processing, such as pumice stone, foamed lava, volcanic tuff and porous limestone.
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industrial wastes such as sintered fly ash or foamed blast furnace slag.
The most commonly used artificial aggregate in many industries is sintered fly ash. The use of lightweight aggregate in concrete has gained popularity due to its low density, good thermal conductivity and strength, environmental friendliness and many other advantages. (George 2020, Sun 2021).
A global environmental problem due to the use of large amounts of natural resources is generated by the construction industry. Most of the aggregates used in this industry are obtained from natural resources. There is a continuous increase in the production of construction materials and therefore non-renewable natural resources are diminishing at an accelerating rate due to the high demand for their use in concrete production. Of course, for heavy-duty, high-strength and cover concretes, the use of quality natural aggregates appears to be essential. However, modern construction is moving away from oversized heavy structures. The development of the manufacture of artificial of lightweight materials such as lightweight aggregate will help minimize the use of natural resources. Lightweight aggregate is significantly different from conventional aggregate. 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. (Kumar 2010, Alqahtani 2019, Hao 2022).
Undoubtedly, the consumption of construction aggregates depletes natural resources and poses a direct threat to the environment. Due to the increasing expansion of construction, resources of natural aggregates are rapidly decreasing. There is a local shortage of resources that requires proper use for sustainable development (Kozioł 2016. Khan 2023). Bibliographic research conducted by the authors shows that the dominant issue in the field of waste and recycled materials is the technology of building materials, and in particular the technology of concrete. It can be said that these issues are present in virtually every scientific and economic field. Starting from mineral and biotic resources, through broadly understood chemical and physical sciences, to geotechnics (Stempkowska 2023). An interesting direction of use of waste materials are fired materials such as bricks, tiles, lightweight aggregates (Danish 2022, Izak 2023,, Simao 2023, Singh 2023).
However, the authors of the above publications point out the difficulties in using waste materials. Despite homogenization processes, mineral waste is characterized by low composition stability and the presence of soluble compounds. Such compounds may cause efflorescence on the surface or increased leachability of harmful substances, which causes functional defects. Natural sand can be replaced using by-products from coarse aggregate production, which are widely available, cheaper and reduce the extraction of natural sand (Altuki 2022). Additionally, manufactured sand has become a popular choice to replace natural sands because it is often extracted from streambeds and sand extraction is considered environmentally harmful (Franus 2015). In addition, the produced sand, which is made from hard igneous, sedimentary or metamorphic rocks, can be produced locally, reducing transportation costs.
The demand for aggregates with a coarser fraction is different. Such materials, obtained from waste raw materials, must be integrated. They can be obtained in several ways, such as:
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cold consolidation ~ 25°C (cold setting, cementation, geopolymerization) (Rehman 2019, Zafar 2021)
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autoclaving to about 200°C
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sintering, at temperatures above 900°C (Hao 2022).
Generally, lightweight aggregate produced at, for example, ~ 1200°C provides the best mechanical properties, but deteriorates its porosity. To obtain the most favorable properties of artificial aggregate, the sintering temperature should be determined individually depending on the raw materials used, using available techniques, e.g. high-temperature microscopy (HSM). The sintering method also enables the production of aggregate in a short time. However, sintering requires a large amount of energy during production, which affects its price. Generally, the sintering method is widely used around the world in some popular commercial products such as LECA and Lytag. These are some of the most popular artificial lightweight aggregates that have been commercialized on the market to replace natural aggregate. Due to costs, energy consumption and CO2 emissions, new methods such as cold bonding and autoclaving are being investigated.
In the autoclaving method, the hardening and strength of the granules are achieved by pressure and temperature. There are research papers describing the autoclaving process for the production of aggregates (Wang 2022, Wang 2020). In their research, the authors dry the aggregate at room temperature for 24 hours and then harden it at a temperature of up to 200° C for several hours. Through this procedure, they obtained aggregate very quickly. Tests have shown that a small amount of binding material is required to consolidate the aggregate. Unfortunately, there is not enough research on the effectiveness of the autoclaving method. This is because this method requires a specialized machine with precise temperature and pressure control to harden the aggregate. Furthermore, the autoclave is very expensive and requires high energy consumption and large production facilities to complete the process.
The third popular bonding method is cold bonding. It is a process of strengthening larger particles obtained using pressure or pressureless agglomeration methods. In the cold setting process, cement or an alkaline activator is usually used as a binder. (Vali 2020, Zafar 2021). The authors indicate that the cold bonding method was considered profitable because consolidation takes place at room temperature. Compared to other production processes, the cold bonding method minimizes energy consumption. In the case of cold gluing, the granules are dried at room temperature for at least 24 hours. Then, such granules require hardening, preferably in a closed chamber with steam, until the required strength is achieved (Abbas 2018, Rehman 2019, Wang 2022). The main challenge with cold set aggregates is the requirement for longer production times, as curing is typically required for 28 days. It is not always reasonable or possible to use cements or other alkaline activators.
In the presented research works, the authors used a combined (hybrid) method - sintering and hydration hardening.
Regardless of the uses of artificial aggregates in concrete production, LWA is worth investigating in particular to minimize environmental problems, along with maintaining long-term sustainability through improving water quality (filtration) (White 2009) or as a substrate for green roofs to mitigate the urban heat island effect ( Liu 2016 Kazemi M 2023). 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 the green roof and the substrate layer significantly affect the energy efficiency and water drainage (Ouldboukhitine 2015, Vijayaraghavan 2016, Farias 2017, Szota 2017, Stovin 2013). Due to the larger number of internal contact pores in the lightweight aggregate, moisture absorption is faster than with regular aggregate.
Ecological issues, such as the limitation of natural resources and huge amounts of waste, are increasingly leading the developing civilization towards sustainable construction. The two main environmental problems are the depletion of natural resources and the disposal of waste generated during various processes. Therefore, the authors attempted to produce a new lightweight aggregate from by-products and waste materials.