Comparison Between Fly Ash and Slag Slurry in Various Alkaline Environments: Dissolution, Migration, and Coordination State of Aluminum

Scanning electron microscopy, X-ray diffraction, nuclear magnetic resonance, and inductively coupled plasma spectroscopies were used to investigate the dissolution, migration, and coordination state of Al of fly ash (FA) and ground granulated blast-furnace slag (GGBFS) slurry in three different alkaline environments. The results show that in FA-CH pastes, Al dissolved from FA is inclined to enter the structure of C-A-S-H, C(4)AH(13), and monosulfoaluminate (a small amount); sulfates promote the formation of ettringite and hinder the incorporation of Al in C-S-H. In GGBFS-CH pastes, Al dissolved from GGBFS enters the structure of hydrotalcite (HT), third aluminate hydrate (TAH), and C-A-S-H (a small amount); sulfates promote the migration of Al from TAH (mainly) and C-A-S-H to monosulfoaluminate and ettringite but have little effect on Al in HT. Although the Al2O3 content in FA is significantly higher than that in GGBFS, the content of Al-containing products formed in FA pastes is lower than that in GGBFS pastes. In the formed Al-containing products, the content of Al(VI) is significantly higher than that of Al(IV). The ratio of Al(VI)/Al(IV) for GGBFS pastes is significantly higher than that of FA pastes and increases with the increase of sulfate concentration.