Heat Evolution Analysis for Determination of Nucleation Kinetics of Alkali Activated Slag

Abstract

This study investigates the nucleation mechanism of slag alkali activation at different solid-to-liquid ratios, focusing on kinetics, including growth rates. Heat evolution during activation was monitored, and calorimetric data were analyzed using the Johnson–Mehl–Avrami–Kolmogorov model. Compressive strength and phase evolution (via wide-angle X-ray scattering) were correlated with heat evolution to enhance understanding of reaction mechanisms in alkali-activated material formation. This is essential for producing alkali-activated slag that meets standard requirements for construction applications. Results showed that the highest heat evolved (–360.60 J/g) did not correlate with the best strength performance (22.69 MPa at 1 day and 25.83 MPa at 3 days), since the lowest cumulative heat (–226.15 J/g) at an S/L ratio of 1.4 yielded the best strength. This was supported by the highest growth rate (0.1172 min–1) at this ratio. JMAK analysis indicated instantaneous nucleation with one-dimensional rod-like growth, driven by increased nucleation site availability. From the results obtained, it can be concluded that an increment in S/L ratio significantly increased nucleation and polymerization of alkali-activated slag, thereby hindering heat flow, as evidenced by the lowest total cumulative heat evolved. In addition, the highest growth rate observed corresponded linearly with the compressive strength, further confirming densification by polymeric gels formed during alkali activation.