Valorization of Ornamental Stone Processing Waste for the Synthesis of LTA- and SOD-Type Zeolites

The conversion of ornamental stone waste, rich in quartz and potassium feldspar, into precursors for LTA and SOD zeolite synthesis was investigated via alkaline activation at 850 °C, followed by hydrothermal crystallization without autoclaves. This approach promoted complete amorphization of the material, facilitating the breakdown of the original crystalline framework and the incorporation of sodium ions, which are essential for structural rearrangement during zeolite formation. LTA zeolite was obtained after 6 h of crystallization, reaching 99.94% crystallinity, and progressively transformed into the SOD phase over 48 h, achieving 87.56% crystallinity. This phase transition was closely related to variations in the Na₂O/Al₂O₃ molar ratio and the conditions governing nucleation and crystal growth. Solid-state ²⁹Si NMR analyses revealed local rearrangements in the aluminosilicate framework during the LTA → SOD transformation, highlighting the key role of Na⁺ redistribution in structural evolution. Complementary ²⁷Al NMR confirmed the exclusive presence of tetrahedrally coordinated aluminum and the absence of Brønsted acid sites, indicating a basic character of the synthesized zeolites. These results demonstrate the technical and environmental feasibility of using ornamental stone waste as a raw material, providing a sustainable and economically competitive route to produce functional zeolites with potential applications in catalysis, adsorption, sensing, and gas separation technologies.