Na-A type zeolites were prepared from two industrial wastes: the solid by-product of cupola slag and aluminum sludge from an aluminum plating plant. Two preparation methods using the same starting material compositions were carried out. In the first method, alkaline fusion was introduced, followed by the hydrothermal treatment to obtain sodium aluminosilicate which was then crystallized in NaOH solution under the condition of 90 +/- 3 degrees C for 1-9 h with different H2O/SiO2 ratios. The result shows that higher H2O/SiO2 ratio increases the rate of crystallization. The largest amount of crystallinity for Na-A was found at 3 h. In the second method, alkaline hydrothermal treatment without fusion was carried out in the same condition as the first method. No Na-A zeolite was obtained by this method. The changes of the dissolved amounts of Si4+ and Al3+ in 3 M NaOH were investigated during the hydrothermal reaction. (C) 2008 Elsevier B.V. All rights reserved.