Thermochemical coupling of CO2 release and transient cooling for regulating early-age behavior of alkali-activated slag

초록

Alkali-activated slag (AAS) binders have attracted increasing attention as sustainable alternatives to ordinary Portland cement (OPC) due to their high mechanical performance and potential for reducing CO2 emissions. However, rapid setting and poor workability remain critical barriers for cast-in-place construction. In this study, solid CO2 (dry ice) is introduced as a dual-function thermochemical additive to regulate early-age behavior without altering activator chemistry. Dry ice was incorporated at 0, 3, 6, and 9% by slag mass into NaOH-sodium silicate-activated AAS pastes. Setting time, flow retention, isothermal calorimetry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), and compressive strength were systematically evaluated. Results show that sublimation-induced cooling delayed precursor dissolution and early hydration kinetics, while released CO2 dissolved in the alkaline pore solution likely reduced alkalinity and induced early carbonate and hydrotalcite precipitation. These coupled effects significantly extended the placement window-up to four times that of the control-and markedly improved flow retention. Although early-age strength decreased due to retarded reaction, transient carbonate and layered double hydroxides (LDHs) phases may later promote matrix densification through nucleation and filler effects, facilitating continued gel formation. By 7 and 28 days, all dry ice mixes exceeded the control in strength, with the optimal dosage (approximate to 6%) achieving an 11% increase. These findings demonstrate that dry ice modifies early AAS reaction pathways through coupled thermal and chemical effects, providing an effective strategy to improve workability while maintaining or enhancing later-age performance. This approach offers a practical pathway for balancing fresh-state control and long-term performance in low-carbon AAS systems.

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