In situ transformation of geopolymer gels to self-supporting NaX zeolite monoliths with excellent compressive strength

Geopolymers are a new class of green high-strength aluminosilicate materials, which generally maintain an amorphous structure due to the limitations related to their chemical compositions and reaction conditions but possess the ability to form molecular sieves. In this work, the in-situ transformation of geopolymers into self-supporting NaX zeolite monoliths of relatively large sizes was examined under hydrothermal conditions and various parameters of the synthesis process were optimized. The obtained results indicated that the optimal transformation conditions for geopolymers with a Si/Al ratio of 4.0 are as follows: precursors composed of metakaolin and modified industrial sodium silicate with a Na₂O/SiO₂ molar ratio of 1.0, H₂O/Na₂O molar ratio of 70, hydrothermal synthesis time of 18 h and a temperature of 90 °C. According to the obtained X-ray diffraction (XRD) data, the synthesized NaX Monoliths exhibited characteristic reflections consistent with those of standard NaX zeolites and possessed high degrees of crystallinity, while scanning electron microscopy revealed that the produced molecular sieves exhibited well-defined morphology. In addition, the zeolitic monoliths exhibited excellent compressive strength for potential practical applications.