Ionic supply and hydration mechanisms in clinker-free cement: New insights into steel slag as an alkaline activator

Clinker-free cement has attracted considerable attention owing to its low carbon footprint. However, the relationship between its hydration process and mechanical properties remains insufficiently understood; hence, clarifying the hydration mechanism is essential for the regulation of strength development. In this study, steel slag was used as the sole alkaline activator and combined with fluorogypsum to synergistically activate the ground granulated blast-furnace slag, forming a steel slag-fluorogypsum-ground granulated blast-furnace slag clinker-free cementitious system (SFSC). The effects of steel slag on the hydration induction period and the microstructural development were elucidated. The results showed that the duration of the induction period of SFSC is associated with Ca 2 + concentration as well as with the availability of [SiO 4 ] 4- , [AlO 4 ] 5- and SO 4 2- relative to Ca 2+ supply. At 6 h, I mis increased from 0.65 for SS15–0.90 for SS25 and 1.90 for SS35, while the corresponding induction-period durations increased from approximately 7–10 and 56 h. Their common monotonic ordering reveals a composition-dependent association between the early dissolved-element imbalance and hydration delay. In addition, combined QXRD‑TG analysis yielded semi‑quantitative estimates showing that the increase in C‑(A)‑S‑H‑equivalent fraction was substantially greater for SS15 than for SS35 from 3 d to 28 d. At 28d, SS15 retained a higher apparent FTIR Q 2 /Q 1 ratio than SS35. Correspondingly, the porosity of SS15 decreased by 26.68 %age points during this stage compared with only 14.23 %age points for SS35, and the compressive strength of SS15 ultimately attained 69.5 MPa at 28d. These findings provide new insights into the hydration mechanism of SFSC and its regulation of mechanical properties.