# Hydrothermal Manufacture of Zeolitic Lightweight Aggregates from Clay and Marine Plastic Litter
###### Abstract  
Mixed plastic fraction (MPF) from marine litter was investigated as a pore-forming agent in formulations with kaolin and a rejected Mg-clay (rich in smectite and sepiolite) to obtain innovative zeolitic lightweight aggregates. Round granules of \(\sim\)10 mm in diameter were shaped and fired at 600 \({}^{\circ}\)C (mixtures of kaolin with 0, 5, 10, and 20 wt.% MPF) and 900 \({}^{\circ}\)C (mixtures of rejected Mg-clay:kaolin (1.0; 2.1; 1.2; 0.1) with 10 wt.% MPF). The fired specimens were hydrothermally treated in a 3M NaOH solution at 150 \({}^{\circ}\)C for 24 h. Mixtures containing 20 wt.% MPF led to specimen crumbling, while those with 5 and, especially, 10 wt.% MPF favored a significant crystallization of zeolites and feldspathoids (50-80%), highlighting carnitine, nephine, zeolite A, and analcime. The resulting materials were lightweight (1.5-1.8 g/cm\({}^{3}\)) and their crushing strength increased substantially with the hydrothermal treatment, from 0.04-0.5 MPa to 2.3-5.5 MPa after zeoliticization. High content of rejected Mg-clay in the mixture (>67%) negatively affected the zeoliticization and the properties of the final aggregate, while 33 wt.% was adequate, increasing slightly the crushing strength (3.4 vs. 3.1 MPa). These findings contribute to plastic waste circularity and sustainability/technological progress in materials production. hydrethermal treatment; zeolitic construction materials; lightweight aggregates; marine plastic litter; clay +
Footnote †: journal: [PERSON]. It is worth noting that the manufacture of lightweight aggregates can be carried out through four routes: (i) cold bonding: combining raw materials with pozzolanic capacity, such as fly ash, calcium hydroxide, and Portland cement, to obtain hardened aggregates after mixing with water [12; 13]; (ii) autoclaving: starting from raw materials that are also binders, the shaped granules undergo an autoclave curing process at temperatures > 100 \({}^{\circ}\)C to promote hardening [12; 14]; (iii) geopolymerization: mineral precursors rich in aluminosilicates are mixed with an alkaline activator solution until a paste is obtained, which hardens under suitable curing conditions [15]. Foaming agents, such as silica fume, can be added to develop a lighter structure [16]; and (iv) firing or sintering: this is the most common method to obtain lightweight aggregates, as with suitable raw materials and manufacturing conditions (generally using rotary kilns), it can lead to very light and porous materials. From an environmental standpoint, the above processes present aspects for improvement: in the case of cold bonding and autoclaving, the starting raw materials are generally obtained at high temperatures to be activated (for example, Portland cement is manufactured at about 1500 \({}^{\circ}\)C). Likewise, the rotary kiln firing method involves temperatures that in most cases are between 1050 and 1250 \({}^{\circ}\)C [17]. Regarding geopolymerization, although it is not very popular in the manufacture of lightweight aggregates, it is attracting a lot of attention in recent years as a more sustainable method of obtaining alternative binders to Portland cement. Although geopolymerization is usually considered environmentally friendly, the strong impact in the manufacture of its most common activators is usually ignored, such as sodium silicate, which is usually synthesized at 1200-1400 \({}^{\circ}\)C [18]. Against this backdrop, it is essential to develop more sustainable manufacturing methods, especially from an energy efficiency perspective. To this must be added the new objectives of the European Union, which aim to promote the development of advanced properties in materials [19; 20]. [PERSON] and [PERSON] [21] have recently developed a new manufacturing method based on alkaline hydrothermal treatment of macroscopic specimens fired at relatively low temperatures (600 and 900 \({}^{\circ}\)C). This new approach has been proved satisfactory in kaolin mixtures with 10 wt.% of rubber and polyethylene microplastics.