The mineral phase evolution characteristics and hydration activity enhancement mechanism of steel slag under NaOH alkaline excitation

During the slow cooling process of steel slag, crystals of the inert ore phase (C2F) and active ore phase (C2S, C3S) grow dependently, and the irregular interlacement and coating between ore phases strongly affect the hydration activity of the active ore phase in steel slag. Therefore, in this article, a typical alkaline activator, NaOH, was selected, the solvent type and concentration were optimized, and the etching behavior and mechanism of action of the NaOH alkaline activator on the surface of steel slag particles were clarified by analyzing the mineral phase composition, microstructure, particle size distribution, and pore structure evolution characteristics under alkaline excitation. In addition, the water hydration products, microstructure, exothermic hydration and thermogravimetric data before and after the excitation of steel slag were characterized, revealing the mechanism of enhancing the hydration activity of steel slag under alkaline NaOH stimulation. The results showed that both NaOH aqueous solution and ethanol solution could effectively increase the mass fraction of active mineral phases in steel slag; among them, the 0.08 mol/L NaOH ethanol solution had the best chemical activation effect on the steel slag. Compared with that of the raw steel slag, the total mass fraction of the active mineral phases (C2S and C3S) increased from 37.3 wt% to 45.6 wt%, and the mass fraction of the inert mineral phase C2F decreased from 38.3 wt% to 19.1 wt%. The consumption of active mineral phases in steel slag is reduced by the water in the activator. The alkaline activator erodes the dense inert mineral phase C2F on the surface of the steel slag particles, resulting in etching pits that expose the active silicate mineral phase and provide additional hydration sites. The cumulative heat release from the steel slag after alkaline excitation was 116.27 J/g at 72 h, which was 66.88 J/g higher than that of the raw steel slag. These conditions significantly improved the early hydration performance of the steel slag and promoted the formation of amorphous cementitious products (CH, C-S-H and AFt) in the steel slag system, which subsequently filled the capillary pores of the steel slag and formed a denser microstructure.