Use of sodium oxalate and calcium formate for chemical activation of high volume fly ash (HVFA) binders

Abstract

This study investigates hydration mechanism of a high volume fly ash (HVFA) binder, comprising 70 wt.% of fly ash (FA), chemically activated by using sodium oxalate (Na2C2O4) and calcium formate (Ca(HCOO)2). Prior to the binder synthesis, mechanical activation of FA was employed. Effects of the activators on the properties of the HVFA binders were determined by measuring setting times and compressive strength of the binders. Heat of hydration, pore solution composition, bound water and portlandite content, mineral composition, and microstructure of HVFA binder pastes were analyzed in order to compare the hydration mechanisms of the HVFA binders activated with the selected activators. The obtained results indicated that using of the two different activators led to the enhancement of the properties of the HVFA binders, but it was achieved in two distinct ways. It was found that the use of Ca(HCOO)2 mitigated adverse effects of the high alkalinity of the pore solution of chemically activated HVFA binder on hydration process and strength development in the later period. Therefore, it was concluded that Ca(HCOO)2 was more suitable for chemical activation of HVFA binders, compared to Na2C2O4.