Effects of calcium formate on early-age strength and microstructure of high-volume fly ash cement systems

Abstract

Fly ash (FA) is used as supplementary cementitious material to Ordinary Portland cement (PC) to reduce environmental impact and improve long-term properties of concrete. However, the replacement level of PC by FA is currently limited to 15%-30% in the industry, mainly due to problems with early age strength development. This research used calcium formate (CF) as an admixture in high volume fly ash (HVFA) composites to find the optimal dosages of CF that improve the early age strength most. From very low dosage of 0.5%, up to high dosage of 9% of CF were tried on PC and HVFA pastes with 60% and 70% FA. Compressive strength testing was performed as a preliminary identification of optimal mix designs to give the highest strength. Isothermal calorimetry, thermogravimetric analysis (TGA), x-ray diffraction (XRD) and scanning electron microscopy (SEM) were conducted to further investigate the effects of CF regarding hydration and microstructural aspects. The results show that PC pastes reached the highest strength with 2% CF. Both HVFA pastes with 60% and 70% FA achieved the highest strength at the CF dosage of 3%. At the age of 28 days, adding of 3% CF in HVFA mixes lead to more consumption of FA as well as more formation of CH, C-S-H, CaCO₃ and ettringite which contribute to the increase of strength. Adding of very high CF dosage at 9% increased the hydration of C₃A but could hinder the hydration of C₃S in both PC and HVFA pastes with 60% and 70% FA.