Impact of temperature and NaOH concentration on the hydrothermal synthesis of zeopolymers (zeolite-geopolymer composites) from kaolin-marine plastic aggregates

The hydrothermal synthesis of zeopolymers (zeolite-geopolymer composites) using kaolin and marine plastic as a porogenic additive was studied, focusing on the influence of temperature and NaOH concentration. Spherical specimens (Ø ∼ 10 mm) were shaped from a kaolin-based mixture containing 5 wt% marine plastic and fired at 600 °C. These were subsequently subjected to hydrothermal treatment at 90, 120, and 150 °C under three different NaOH concentrations: 2, 3, and 4 mol/L. Results show that zeolitization is enhanced with increasing temperature and NaOH molarity, with zeolite A being predominant at 90 and 120 °C (33–44%), while cancrinite becomes dominant (47%) at 150 °C with 4 M NaOH. The reduction in total porosity after treatment, particularly at 90 °C, led to decreased water absorption and increased density in the resulting lightweight aggregates (1.66–1.82 g/cm 3 ). Crushing strength improved markedly, rising from 0.1 MPa in the solely fired aggregates to 2.1–3.4 MPa in the zeopolymerized samples, a 19- to 30-fold increase. Notably, the samples treated at 150 °C with 2 M and 3 M NaOH combined high strength (3.4 MPa) with low density (1.66 and 1.70 g/cm 3 ). A transport test on a zeolite A-rich sample showed strong ammonium adsorption capacity ( K d = 3782 L/kg), indicating its potential for water decontamination beyond structural uses. These findings highlight the key role of synthesis parameters in tailoring material properties, while emphasizing environmental benefits associated with marine plastic reuse (a major pollutant in marine ecosystems) and lower processing temperatures than in the ceramics industry and lightweight aggregates (600 °C vs. 900–1300 °C). • Hydrothermal synthesis of zeopolymers from kaolin-plastic aggregates was studied. • % zeolite increases with temperature (150 > 120 > 90 °C) and [NaOH] (4 > 3 > 2 M). • Zeolite A generally dominates, but cancrinite prevails at 150 °C–4 M NaOH. • Lightweight zeolite-A rich sample shows high NH 4 + uptake capacity ( K d = 3782 L/kg). • Crushing strength rises up to 30 times by zeopolymerization, reaching 2.1–3.4 MPa.