“…The white gel was then used for hydrothermal crystallization at 90 C in a sealed Teflon-lined stainless steel autoclave for 1, 6, 12, 24 or 48 h. After hydrothermal crystallization, the resulting solid products, zeolite Y, were filtered out, washed with deionized water, and dried at 90 C overnight. The above new route has been evaluated by comparison with the conventional technique using high-temperature calcination [30]. In the parallel runs, the same amount of calcined kaolin (1.5 g) (i.e., metakaolin) and Na 2 SiO 3 $9H 2 O were used instead.…”