In this study the effective utilization of two types of municipal solid waste incinerator (MSWI) ashes, namely air-cooled ash (ACS) and water-cooled ash (WCS) samples obtained from a municipal solid waste incineration plant, was examined by applying zeolitic material synthesis and silica extraction. The influence of the experimental conditions including the ratio of sample : NaOH solution, the reaction temperature and time, and the concentration of NaOH solution were investigated. The results for the 25 experimental trials can be summarized as: (1) the formation of tobermorite and/or pectolite-1A as a major component in most conditions; (2) the synthesis of hydroxycancrinite as a major phase at 200 degrees C; (3) a dramatic increase in the extracted SiO(2) yield at 1 : 30 value of sample : NaOH ratio and 200 degrees C, even at short reaction times; and (4) relatively high SiO(2) yields for WCS ashes rather than ACS ashes. An increase in the reaction time improved the quantity of synthesized zeolitic materials. The reaction temperature determined the type of zeolite. An increase in the NaOH concentration can be an essential factor to improve zeolitic material synthesis, but it significantly reduced the yield of SiO(2) extraction. In conclusion, suitable conditions for obtaining both SiO(2) extraction and synthesized zeolites from the ashes of the incinerated solid waste materials should be: 200 degrees C reaction temperature; a 1 : 30 (g : mL) value for the sample : NaOH ratio; 2 mol L(-1) NaOH concentration; and a reaction time of more than 24 h.