Reactions between BOF slag and acidic phosphate: shifting acid-base reaction towards hydration

The growing demand for sustainable cement alternatives, coupled with the underutilization of Basic Oxygen Furnace (BOF) slag in high-value applications, underscores the need for innovative binder solutions to reduce CO₂ emissions and waste. This study addresses the challenge of limited hydration in BOF slag, which restricts its use as a cement-free binder in conventional alkaline activation. A novel approach using monopotassium phosphate (MKP) at dosages below 10 wt% is proposed to activate the latent hydraulic phases of BOF slag, specifically dicalcium silicate (C₂S) and brownmillerite (C₂(A,F)), at ambient temperature. The microstructure and strength development of BOF slag pastes were examined using a multi-technique approach, including quantitative XRD, SEM/EDX with phase mapping, TGA, calorimetric measurements, MIP, etc. The dosage of MKP is pivotal in modulating the transition from acid-base reactions to sustained hydration in BOF slag-based binders. Findings demonstrate that MKP enhances C₂S and C₂(A,F) hydration, producing hydrotalcite, pyroaurite, C-S-H gel, hydrogarnet, and hydroxyapatite-like phases. An optimal MKP dosage of 5 wt% achieves maximum strength at both 7 and 28 days (19.9 and 44.5 MPa), while 2.5 wt% MKP affects slightly 7-day hydration but promotes 28-day hydration. Conversely, excessive MKP (10 wt%) triggers rapid early reactions, forming large pores that impair strength. These results underscore the critical need for balanced phosphate dosages to optimize hydration and mechanical performance, offering a viable strategy for valorizing BOF slag in sustainable construction.