The Influence of the SiO2/Na2O Ratio on the Low Calcium Alkali Activated Binder Based on Fly Ash

Alkali activated binders (AAB) are potential alternative to ordinary Portland cement. The AAB is produced by activating the source of amorphous SiO2 and Al2O3 with alkali solutions (NaOH, Na2SiO3.nH(2)O, KOH or etc.). As a result of alkali activation the N-A-S-H gel is forming as binding phase in the structure. Influence of different SiO2/Na2O ratio to the alkali activated fly ash compressive strength, chemical composition (XRD, FTIR), crystallinity of hydroxy-sodalite (by-product of the alkali activation) and structure (SEM-EDX) was studied. The highest compressive strength for AAB was achieved with SiO2/Na2O ratio in the range of 1.5-2.3 (max. 56.16 MPa). XRD and FTIR suggest if ratio was too low, rapid sodium carbonation would take place and significantly reducing sodium content for alkali activation. On the other hand, if ratio was higher than 2.3 there would not be enough sodium, that results in low compressive strength. Also, higher SiO2/Na2O ratio reduces hydroxy-sodalite crystallinity.