# Recycling Red Ceramic Waste as a Raw Material for Lightweight Aggregates
## 1 Introduction  
Lightweight aggregates (LWAs) are of significant importance within the construction industry. Their utilisation in engineering works and services confers numerous advantages, such as a reduction in the structure's own weight, thermal and acoustic insulation, high fire resistance and durability [1]. As demonstrated in the preceding studies, the global consumption of aggregates exceeds 50 billion tonnes per year, and there is a significant prospect of growth in the coming years [2]. This demand reinforces the need to manufacture new LWAs. Typically, commercial LWAs have granulometric fractions suitable for use as fine and coarse aggregates [3, 4, 5, 6, 7, 8, 9]. [PERSON] et al. [10] posited that in the manufacture of LWAs, it is essential to produce materials with good mechanical properties, as the mechanical strength of the lightweight aggregate has a strong influence on the final strength of the lightweight concrete. As posited by [PERSON] et al. [11], a lightweight aggregate with optimal properties for use in mortars and concretes should have the following characteristics: a strongly sintered porous core, a rough and impermeable surface and an approximately spherical shape. Generally speaking, when designing the production of expandable LWAs it is possible to obtain these properties and at the same time modify their density, water absorption and mechanical strength, adapting the material to the most diverse applications [12]. The production of expandable LWAs is typically initiated by subjecting the granules to heating to their respective melting points. At this point, gas formation and pyroplastic deformation must occur simultaneously [13]. In this context, [PERSON] et al. [12] state that for a given sintering temperature, there are three conditions necessary to achieve swelling in LWAs: (a) the production of a quantity of liquid phase capable of transforming the granule into a viscous mass, (b) the development of gas bubbles occurs and (c) obtaining a liquid phase with adequate viscosity for the retention and growth of gas bubbles. In the production of LWAs, clay is the raw material that is most frequently employed. According to [PERSON] et al. [11], clay is commonly used in the manufacture of LWAs mainly due to its availability close to urban areas, its ease of forming into granules and the fact that it results in light and resistant samples when subjected to sintering. However, overconsumption of clay can have significant environmental impacts, such as the depletion of natural resources and the production of waste [14; 15]. Consequently, the use of waste as a raw material for manufacturing lightweight aggregates has been frequently analysed in scientific research. According to [PERSON] et al. [12], interest in this application is growing and is strongly influenced by the search for more sustainable technologies. Successful studies have shown that the production of LWAs has a high potential for incorporating waste [11; 16]. As demonstrated in the extant literature, a wide variety of waste materials is already employed in LWAs, including but not limited to fly ash, sewage sludge, mining and quarrying tailings, sawdust, glass waste, coffee grounds and reservoir sediments [11; 12; 17]. However, there are still no reports on the use of red ceramic waste (RCW) in expandable LWAs. RCW is often generated during the manufacture of ceramic materials. According to [PERSON] et al. [18], construction and demolition waste (CDW) and materials that do not meet technical specifications are the main sources of RCW. It is estimated that approximately 33 per cent of ceramic production can be wasted every day as a result of the generation of defective bricks, blocks, tiles and other articles [19]. This tends to contribute to environmental degradation, causing, for example, water and soil pollution and the modification of the natural landscape and local biodiversity [20]. The chemical and mineralogical compositions of RCW indicate that this waste has significant potential for making lightweight aggregates.