Reaction mechanism of sulfate attack on alkali-activated slag/fly ash cements

In this work, the effects of slag/fly ash ratios, Na2O concentration, and silicate modulus (Ms) of activators on the reaction mechanism of sulfate attack on alkali-activated slag/fly ash cements are elucidated by XRD, FTIR, and TG/DTG analysis. To accelerate the sulfate attack process, the alkali-activated cements (AACs) were crushed into particle samples before immersion in the Na2SO4 and MgSO4 solutions. The results show that upon the Na2SO4 attack, the C-A-S-H gel structures of AACs do not change basically. However, the MgSO4 attack on AACs is severe due to the dissolution of alkali and the intrusion of Mg2+. Gypsum is the main product in AACs upon the MgSO4 attack, and the amount of brucite decreases with the incorporation of fly ash. The NaOH-activated slag (Ms = 0) has a relatively higher MgSO4 resistance compared with the Na2SiO3-activated slag, while it is the opposite when the fly ash replacement level increases to 80%.

Automated summary

This study elucidated the effects of slag/fly ash ratios, Na2O concentration, and silicate modulus on the reaction mechanism of sulfate attack on alkali-activated slag/fly ash cements using XRD, FTIR, and TG/DTG analysis. The results showed that MgSO4 attack on AACs was severe due to alkali dissolution and Mg2+ intrusion, whereas Na2SO4 attack did not significantly alter the C-A-S-H gel structures of AACs. Additionally, the resistance of NaOH-activated slag to MgSO4 was higher compared to Na2SiO3-activated slag, but this trend reversed with an 80% fly ash replacement level.