From geopolymerization to ceramicization: How reaction extent and gel chemistry control crystallization in metakaolin-based Na-geopolymers

Metakaolin-based Na-geopolymers can ceramicize during heating, but the extent to which ambient-temperature reaction progress predetermines crystallization remains unclear. In this study, geopolymers were prepared using a 5–10 mol/L activator-composition series. Calorimetry, proton relaxometry, selective dissolution, infrared spectroscopy, X-ray diffraction, thermal analysis, phase quantification, and electron microscopy showed that increasing NaOH molar concentration was associated with greater reaction extent and a shift in the high-temperature response from broad amorphous relaxation to a distinct crystallization exotherm.; the QXRD estimate of nepheline increased from 6.2% to 79.9% after heating at 1000 °C. Additionally, increasing silicate modulus from 1.2 to 1.4 lowered the apparent Kissinger activation energy from 350.9 to 178.5 kJ/mol and reduced the measured image-analysis mean nepheline grain diameter from 1.18 to 0.80 μm. This study establishes an association-based framework linking ambient-age reaction development and gel chemistry with the subsequent fired phase assemblage, apparent crystallization kinetics, and grain size.