Optimization of SiO2/Al2O3 ratio in the preparation of geopolymer from high calcium fly ash

High-calcium fly ash-based geopolymers have been successfully synthesized. The source of fly ash used in this study is fly ash from PT. IPMOMI, PT. Holcim and Paiton PLTU 9. The variation in the mole ratio of SiO2/Al2O3 carried out in this study was 3,0; 3,3; 3,6; 3,9 and 4,2. Compressive strength tests were carried out on geopolymers aged 7, 14, and 28 days. A geopolymer with a ratio of 3,6 has the best compressive strength. The compressive strength test results for IPMOMI, Holcim, and Paiton 9 geopolymers were 75,37; 50,95; and 39,25 MPa. The highest geopolymers with compressive strength were characterized using XRD and SEM. The results of the XRD showed that geopolymers had an amorphous phase and gibbsite mineral in the three geopolymers which indicated that there was still Al(OH)3 which had not reacted. Microstructure analysis using SEM shows the all of three solid structure of geopolymers. There is efflorescence on Holcim and Paiton 9 geopolymers which causes lower compressive strength than IPMOMI geopolymers.