The role of synthesis parameters on the workability, setting and strength properties of binary binder based geopolymer paste

In this paper the effect of different binder to solution ratio (B/S) and different Si/Al molar ratio in the synthesis of binary binder based geopolymer paste on their workability, setting and compressive strength is investigated. Geopolymer paste was prepared by combination of metakaolin (MK) and silica fume (SF) as aluminosilicate source with a mixture of sodium hydroxide with concentration of 14M and sodium silicate as alkaline activator solution having constant H2O/Na2O and Na2O/SiO2 molar ratio. For having different Si/Al ratio, metakaolin has been replaced partially by silica fume in precursor. Workability of fresh paste was measured by mini-slump test. Compressive test at the ages of 1, 3 and 7 days, setting time, FTIR and XRD analysis were used to characterize the geopolymer paste. Results show that the combination of MK and SF can be used as the aluminosilicate source materials for making geopolymer. Also, results concluded both B/S and Si/Al ratios had a significant effect on workability, setting time and strength. By increasing B/S and decreasing Na2O/binder ratio in the system the compressive strength increased, also, in constant B/S = 1.2, 1.4 ratio, by increasing Si/Al ratio in binder growth compressive strength was obtained, but in B/S = 1.6, 1.8 with high Si/Al ratio increases, the strength will be reduced. XRD analysis showed that geopolymer has an amorphous to semi-crystalline phase which, by increasing Si/Al ratio, will decrease the intensity of crystalline phases. FTIR spectra results illustrate that when both B/S and Si/Al ratio increase in the system, SiOT bonds will shift to higher wave number.