In Situ Spectroscopic Insights into the Setting Performance of Alkali-Activated Slag

This work investigated the setting performance of alkali-activated slag (AAS) pastes prepared with various activators including sodium hydroxide (NH), sodium carbonate/hydroxide (Nc/NH), and sodium silicate (NS) solutions using in situ attenuated total reflectance Fourier transform infrared spectroscopy (FTIR). Also, the AAS slurry samples at the initial and final setting moments were characterized by multiple microscopic techniques [i.e., X-ray diffraction (XRD), thermogravimetric analysis (TGA), FTIR, and field emission scanning electron microscopy (FESEM)]. The results showed that a higher concentration of NH activator leads to more rapid setting reaction and more calcium-aluminosilicate-hydrate (C A S H) formation in AAS at the setting moments. In AAS prepared with carbonate-rich activators, early-age precipitation of carbonate phases (e.g., gaylussite and calcite) contributed substantially to the initial setting completion; afterward, the C A S H precipitation played a decisive role in final setting realization. In NS-activated slag systems, a relatively high silica modulus in activators facilitated the plasticity loss of fluid paste due to the early precipitation of C A S H products formed by the rapid reaction of aqueous silicates from activators with the calcium ions released from dissolving slag particles. This study contributes to identification of characteristic molecular and microstructural features of AAS that are correlated well with the setting moments.