The effect of gypsum on reaction kinetics and microstructure of alkali-activated CaO‐FeOx‐SiO2 slag

Abstract

Gypsum is commonly used in conventional cement systems to regulate setting time and enhance early strength. However, its role in alkali-activated materials (AAMs) is less well understood due to the distinct chemistry of precursors and reaction products. This study investigates the impact of synthetic and industrial gypsum on the reaction kinetics and microstructure of CaO-FeO_X-SiO₂ slag activated with sodium silicate and sodium hydroxide, supported by dissolution-precipitation tests. Results demonstrate that gypsum addition to sodium silicate solution promotes the precipitation of C-S-H gel, which evolves into two distinct compositional varieties in the paste environment with slag, influencing the reaction kinetics. The early formation of Ca-rich gel accelerates the setting time but initially reduces the strength. The delayed formation of main Si-rich gel matrix leads to strength gain over time, with the 1 % industrial gypsum sample achieving 90 MPa at 28 days. In NaOH solutions, gypsum induces portlandite precipitation but the formation of a rod-like thaumasite phase (CaSiO₃·CaCO₃·CaSO₄·15H₂O) in the slag paste environment. The early formation of sulphate phases improves early mechanical performance but compromises durability due to the expansive nature of thaumasite growth. These findings underscore the dual role of gypsum in controlling setting time and strength in AAMs and highlight the need to optimize gypsum type and content to address challenges posed by precursor chemistry.