When fly ash came into contact with NaOH, leaching of Si, Al and other minor ions started. The amount of leaching was dependent on NaOH concentration and leaching time. The results indicated that leaching time of 5–10 min was sufficient as an increase in leaching time to 20 and 30 min did not significantly increase the concentration of Si4+ and Al3+ ions. The mixing of fly ash with 10 M NaOH for 10 min was, therefore, appropriate for synthesis of geopolymer when economy and practicality were taken into consideration.
Mixing sequence had some effects on geopolymerization of paste and compressive strength of mortar. Separate mixing procedure allowed time and condition for leaching of silica and alumina from fly ash particles into NaOH solution. IR study and compressive strength confirmed that geopolymerization was elevated with S series compared to N series. At high water glass content, its effect became dominant and the effect of leaching of fly ash was less. Geopolymers with relatively high strengths of 60–70 MPa were obtained when 10 M and 15 M NaOH; G/N of 1.0 and separate mixing procedure were used. For normal mixing procedure, a high strength geopolymer of 65 MPa was also obtained but this required high G/N of 1.5 and 2.0.
Optical microscope and SEM studies suggested that geopolymerization started with gel formation in colloidal form varying in sizes from less than 1 μm to about 20 μm. As more gel was formed, it overwhelmed remaining fly ash particles and formed a continuous mass of gel resulting in relatively dense geopolymer matrix.