Manufacturing and application of artificial lightweight aggregate from water treatment sludge
Introduction
The global demand for drinking water has increased dramatically due to the rapid population growth and lifestyle changes (Ahmad and Alam, 2016). For this reason, the generation of WTS has also increased rapidly. According to Babatunde et al. (Babatunde and Zhao, 2007), the daily production of WTS worldwide is estimated to exceed 10,000 tons. WTS refers to the inorganic sludge generated during the process of purifying drinking water (Martínez-García et al., 1016; Dassanayake et al., 2015). Thus far, WTS has been sent to landfills and has been used as a filling material (Sanchez-Monedero et al., 2004; Kim et al., 2005a) However, the demand for the recycling of WTS is increasing due to the increased amounts of sludge generated and due to higher landfill costs.
Recently, various methods have been assessed for the recycling and reuse of WTS. The methods include cement composites (Gomes et al., 2020; Godoy et al., 2019), concrete blocks (Liu et al., 2020a, 2020b), geopolymers (Geraldo et al., 2017; Nimwinya et al., 2016; Luukkonen et al., 2019), sintering bricks (Heniegal et al., 2020; Benlalla et al., 2015; Tay and Show, 1992), and lightweight aggregates (Huang and Wang, 2013; Huang et al., 2005). Among them, recycling this material into ceramic products such as sintering bricks and lightweight aggregates requires more energy than other recycling methods, but this issue is drawing attention due to the high stability and durability of the obtained products. According to previous studies (Kim et al., 2006, 2010; Cheeseman et al., 2003), even waste containing a large number of heavy metals can dramatically reduce the leaching of heavy metals through a ceramic process. Therefore, recycling into ceramic products can be said to be an environmentally very stable treatment strategy.
Artificial lightweight aggregates weigh less than natural stone aggregates. According to ASTM330C, lightweight aggregates for structural purposes are defined as those being 880 kg/㎥ or less based on the loose bulk density (C330: “Standard Spe, 2005). Artificial lightweight aggregates are mainly manufactured by the bloating and sintering of expanded clay, slate, and shale in a rotary kiln (Ayati et al., 2018; Bernhardt et al., 2014) and are widely used in various fields, such as construction and landscaping (https://www.leca.com (acc, 2020; http://www.liapor.com (ac, 2019). The manufacture of lightweight aggregates using expanded clay and shale is actively carried out in Europe, the United States, and Japan. In recent years, research on the manufacturing of lightweight aggregates using waste has been active. Examples include waste glass (Chang et al., 2016), sewage sludge (Guo et al., 2017), coal fly ash (Wei et al., 2017), and polymeric wastes (Moreno-Maroto et al., 2018). Most studies have shown that it is possible to manufacture artificial lightweight aggregates using various types of waste. Artificial lightweight aggregate manufacturing technology using waste provides valuable economic and environmental benefits. The Republic of Korea uses 200 million tons of aggregates annually, making this country a very large market. The manufacture of lightweight aggregates using waste can enter very large amounts of other types of waste into the construction market and can greatly reduce the environmental load caused by reclamation and excavation.
A study of the manufacture of lightweight aggregates using WTS was carried out by Huang et al. (2005). In their study, lightweight aggregates were successfully manufactured using WTS. Most of the sludge used in the study met the chemical composition requirements established in Riley (RILEY, 1951), and the content of Fe2O3 was around 10 wt%. According to Wie et al. (Wie et al., 2020a; Wie and Lee, 2020), the content of Fe2O3 is an important factor in the bloating of lightweight aggregates, and the adhesion phenomenon during the sintering process of aggregates is determined by the content of Fe2O3. The adhesion phenomenon during the sintering of aggregate refers to how each molded body melts and sticks to other molded bodies as part of the viscous behavior of the material. It is very important to suppress this phenomenon during the aggregate production process, as large lumps generated by adhesion phenomenon can block the outlet of the rotary kiln and deteriorate the quality of the product. However, the chemical compositions of many types of WTS cannot be identical, and these materials are highly likely to be affected by geological conditions and by the water treatment plant processing conditions. Given that alum, the most representative flocculant, contains a large amount of Al, it is believed to have potentially a strong influence on the chemical composition of these types of sludge depending on the treatment process used. Therefore, the recycling of WTS as lightweight aggregate requires research on various cases, and evaluation of each manufactured aggregate should also be made newly.
Therefore, in this study, the characteristics of WTS generated in Korea were investigated and the manufacturing conditions of lightweight aggregates using these materials were established. The physical properties of the manufactured aggregates were also evaluated, and the performance outcomes were compared with those with existing products by applying them to actual concrete mixes and filter media.
Section snippets
Material and methods
The experimental method is detailed below, and a schematic diagram of the method is shown in Fig. 1.
Raw material analysis
The chemical composition of the raw materials is shown in Table 2. The chemical composition for the manufacture of artificial lightweight aggregates was studied by Riley (RILEY, 1951). Riley defined the overall chemical composition as SiO2, Al2O3, and flux and investigated the bloating area with a three-component system of clay-based raw materials. This is still recognized as an important part of the chemical design of lightweight aggregates. Later, Cougny (1990) discussed the importance of the
Conclusion
To recycle WTS, the manufacturing characteristics of artificial lightweight aggregates were confirmed, and the following conclusions were obtained by reviewing several potential areas of application.
- 1)
WTS has a high Al2O3 content due to the influence of a flocculant, and for this reason, the refractoriness increases when it is added in large amounts.
- 2)
It was possible to manufacture ultra-lightweight aggregates with a density of 0.8g/㎤ or less depending on the firing temperature from aggregates with
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This subject is supported by the Korea Ministry of Environment as part of the “Prospective Green Technology Innovation Project” (2020003160016).
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