Effect of SiO2, Al2O3 and Na2O content and fly ash fineness on the structure and mechanical properties of fly ash based geopolymer

Nowadays, the geopolymer technologies using secondary raw materials are more and more widespread; however, some of them (for example fly ash) have originally low reactivity which can be tailored by a mechanical or chemical activation, or by the addition of various high reactive materials. This study investigates the effect of silica, alumina, and sodium oxide contents on the structure and mechanical properties of high calcium fly ash based geopolymers. Metakaolin (MK) was used as an additional alumina and silica source. It was added at various dosages (0; 5; 10; 15; 25; 50 and 75% by weight) as a replacement of the fly ash (FA). The experimental results confirm that the compressive strength of the geopolymer is greatly affected by the SiO2/Al2O3, Al2O3/Na2O, and SiO2/Na2O ratio. The addition of MK improved the compressive strength of geopolymer by 92%. In addition, the effect of mechanical activation of FA on the structure and strength of the geopolymer was investigated in case of a given MK content. Based on the results it can be stated that the mechanically activated FA resulted in higher compressive strength. The addition of MK and the fineness of FA changed the structure of geopolymers, which was detected using FT-IR spectroscopy method.