Synthesis and Properties of Zeolites from Autoclaved Aerated Concrete (AAC) Waste

Syntheses routes for the conversion of autoclaved aerated concrete (AAC) waste into aluminosilicate zeolites like LTA and related phases were developed. The procedures always started with leaching steps of the pure AAC waste by combinations of strong alkaline (NaOH) and mild acid (citric acid) treatments, before the real crystallization process was performed separately under addition of sodium aluminate. All products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX-analysis) and Fourier-transform infrared spectroscopy (FTIR). Zeolites LTA and related phases basic sodalite (SOD), hydrosodalite (SOD), cancrinite (CAN) and an intermediate phase between SOD and CAN were observed. Depending on the preparation route tailor made synthesis of pure phase zeolite LTA with crystal sizes up to 5 mu m was worked out. In addition to syntheses procedures important properties of the zeolites were discussed with respect to the treatment procedure of AAC. It is shown, that the special synthesis pathway is not only responsible for the product composition and formation of a certain structure type but also exhibits a strong influence on the crystallinity, crystal size, and morphology. The water sorption capacity and the hydrothermal stability of the products were selected for those further investigations. Whereas adequate water sorption capacity up to 272 mg.g1 were measured for zeolites LTA obtained from two different reaction routes, limited hydrothermal stabilities were revealed for other products. Under the conditions of strong hydrothermal treatment at a temperature of 473 K for 72 h, a more or less extended phase transformation into ANA-Type zeolites occurred. This process was least extensive for pure phase zeolite LTA obtained from the alkaline solution of AAC leaching.