The cation-dependent effects of formate salt additives on the strength and microstructure of CaO-activated fly ash binders

Abstract

This study investigated whether three formate salt additives (Ca(HCOO)2, NaHCOO, and KHCOO) could improve the strength of a CaO-activated fly ash (FA) system. Various tests were conducted to verify the effects of the formate salts including compressive strength, X-ray diffraction (XRD), thermogravimetric (TG) analysis, solid-state 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy, and mercury intrusion porosimetry (MIP). The results showed that the addition of Ca(HCOO)2 significantly strengthened the FA binder system by (1) substantially increasing C-S-H formation and (2) measurably reducing overall pore size. However, adding NaHCOO or KHCOO was much less advantageous to strength improvement because although helpful in dissolving FA, the high pH environment caused by their reaction with CaO inhibited further dissolution of CaO, which was the main activator used in this study.