Hydrothermal Manufacture of Zeolitic Lightweight Aggregates from Clay and Marine Plastic Litter

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 ~10 mm in diameter were shaped and fired at 600 °C (mixtures of kaolin with 0, 5, 10, and 20 wt.% MPF) and 900 °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 °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 cancrinite, nepheline, zeolite A, and analcime. The resulting materials were lightweight (1.5–1.8 g/cm3) and their crushing strength increased substantially with the hydrothermal treatment, from 0.04–0.5 MPa to 2.3–5.5 MPa after zeolitization. High content of rejected Mg-clay in the mixture (>67%) negatively affected the zeolitization 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.