Characterization of Microstructural Variations in Alkali-Activated Coal Fly Ashes Depending on Their Intrinsic Properties

AbstractThere is significant microstructural variability in alkali-activated coal fly ashes (CFA) depending on the activation conditions and the intrinsic properties of the utilized CFAs. This study systematically presents the impact of intrinsic CFA properties on these variations while keeping the activation conditions constant. Four high-Ca and four low-Ca CFAs were activated with 10 M NaOH under hydrothermal conditions (80°C, 1 day). The mineralogical and morphological changes investigated through X-ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM) point out some poorly zeolitic formations with N─ A─ S─ (H) phase for low-Ca samples and with (N, C)─ A─ S─ H phase for high-Ca samples. Due to their self-cementitious properties, the mechanical performance—determined by compressive strength tests—of the high-Ca CFAs was higher than that of the low-Ca ones; albeit, too much free CaO had a detrimental effect. In all samples, the performance was enhanced with increased fineness. The mechanical performance of alkali-activated CFAs is mainly attributed to the amorphous or poorly crystalline phases. XRD and FTIR were able to confirm the amorphous gel formation. However, a direct correlation of their results with strength development could not be established. On the other hand, the SEM micrographs of the higher-strength samples showed a denser structure. Obtained results map out microstructural variations of alkali-activated CFAs, guiding the way for their effective reutilization.