Activating secondary hydration in sulfoaluminate cement via reactive carbonate-aluminate phases derived from steel slag

Sulfoaluminate cement (SAC) faces significant challenges in relation to the development of late-stage strength and the reduction of CO₂ emissions due to the absence of secondary hydration. This research develops a steel slag-based negative-carbon supplementary cementitious materials (SCMs) that fundamentally overcomes this limitation by introducing a carbonate-aluminate driven secondary hydration pathway in SAC. Through coupled CO₂ and mechanochemical activation (CAMA), steel slag is converted into highly reactive SCMs containing polymorphic nano-CaCO₃ (vaterite, defect-rich calcite) and Al-Si gels with high surface area and reactivity. These phases act simultaneously as nucleation seeds and chemically active reactants, accelerating AFt and AH₃ precipitation while enabling CaCO₃ and Al-Si gel to react with C₄A₃Š to form monocarboaluminate (Mc) and hemicarboaluminate (Hc). This reaction stabilizes AFt, suppresses AFt to AFm conversion, and enables sustained microstructural densification. Consequently, the 28-day compressive strength of SAC composites increases by up to 20.3%, accompanied by pronounced pore refinement and micromechanical enhancement. Meanwhile, a significant amount of CO₂ is permanently sequestered in carbonate phases, resulting in a carbon-negative SCM. This study establishes a new paradigm for producing high-performance, low-carbon SAC through engineered secondary hydration.

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