Optimization of compressive strength of fly ash-based geopolymers using central composite design

In this study, fly ash-based geopolymers (FAGs) were prepared by using fly ash (FA) as a source of aluminosilicates and a mixture of sodium hydroxide and sodium silicate as an alkaline activator. This study aims at optimization of compressive strength of FAG tested after 7 and 28 curing days, using the central composite design. Two factors that mainly affected the compressive strengths of the samples were selected: sodium silicate to sodium hydroxide ratio (three levels 1.5, 2.5 and 3.5) and FA/alkaline solution ratio (three levels 1.5, 2.5 and 3.5). The prepared geopolymers were characterized using several analytical methods, such as X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy with energy dispersive X-ray analyses. The optimum compressive strength obtained was 32 MPa with Na2SiO3/NaOH ratio = 2.5 and solid/liquid ratio = 2.5.