Setting Time and Compressive Strength of Geopolymers Made of Three Indonesian Low Calcium Fly Ash with Variation of Sodium Silicate Addition

In this research, the effect of sodium silicate (Na2SiO3) on the geopolymerization of fly ash type F (low calcium) has been studied. The variations of Na₂SiO₃ used in the synthesized geopolymers were 19, 32, and 41wt%. The fly ash from three different power plant sources was characterized using X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Particle Size Analyzer (PSA), and Scanning Electron Microscopy (SEM). Fly ash-based geopolymers were tested for mechanical strength and setting time. The best geopolymer was obtained by adding 32% Na₂SiO₃, produced a compressive strength of 21.62 MPa with a setting time of 30 hours. Additions of 19wt% % Na₂SiO₃ failed to form geopolymer paste while the addition of 41wt% Na₂SiO₃ decreased the mechanical strength of the geopolymer. Higher calcium content in low calcium fly ash produces stronger geopolymer and faster setting time.