Utilization of natural opal-CT-bearing mudstone as a precursor of zeolite-geopolymer composites: Synthesis and characterization

This study investigates the potential of natural opal-CT-bearing mudstone from the Duho Formation (Pohang Basin, Republic of Korea) as a precursor for synthesizing geopolymer-based materials without high-temperature pre-treatment. The natural mudstone used in this study, composed predominantly of opal-CT (> 80 wt%) with minor crystalline impurities, was activated using sodium aluminate and NaOH solutions (8–14 M) under varying initial heat-treatment temperatures (room temperature, 40 °C, 60 °C, and 80 °C). Mineralogical, microstructural, and thermal characterizations were conducted using XRD, SEM, and DSC/TGA, and compressive strengths were measured after 7 and 28 days of curing. The XRD and SEM results show that low-temperature curing (≤ 40 °C) promotes in-situ formation of zeolite-geopolymer composites, whereas higher-temperature treatments (≥ 60 °C) favor the crystallization of zeolitic phases with a negligible amount of amorphous gel. Compressive strengths were also strongly dependent on curing temperature and NaOH molarity: low-temperature samples exhibited gradual strength development (up to 15 MPa at 28 days with 10 M NaOH), while high-temperature systems showed rapid early-age strength development (up to 17 MPa with 8 M NaOH). DSC/TGA results showed the phase-specific dehydration and dehydroxylation, revealing the differences in phase assemblages depending on the initial heat treatment temperatures. Our results demonstrate that, although the compressive strengths did not reach the requirements for high-strength structural applications, zeolite-geopolymer composites synthesized from opal-CT-bearing mudstone can be fabricated as bulk-type blocks with compressive strengths above ∼10 MPa. The zeta potential results showed that the zeolite-geopolymer composites exhibited significant negative zeta potential (up to ∼-51 mV), suggesting their promise as a self-supporting cation adsorbent. Therefore, these findings suggest that natural opal-CT-bearing mudstone can serve as a geologically abundant and cost-effective precursor for zeolite-geopolymer composites.