Performance of slag blended alkaline activated palm oil fuel ash mortar in sulfate environments

• Combination of slag and POFA in AAGU mortar provide good resistance to sulfate attack.
• Sulfate attack of AAGU mortar depends on the cations present.
• Slag content provides better resistance to sulfate attack than activators ratio.
• MgSO4aq attack causes decalcification of C–S–H in AAGU mortar to form M–S–H.
• AAGU mortar has a higher resistance to MgSO4aq than Na2SO4aq attack.

This study investigated the performance of alkaline activated ground steel slag-ultrafine palm oil fuel ash (AAGU) mortar exposed to 5% Na2SO4 and 5% MgSO4aq for 6 months. The samples cured at 60 °C for 24 h were prepared with 10 M-NaOHaq and Na2SiO3aq (Ms-SiO2/Na2O = 3.3) activators. Microstructural and characterization tools were employed to examine the type and nature of the resulting products. The findings showed that there was a better strength retention in the MgSO4aq-exposed samples compared to the Na2SO4aq exposed due to the formation of surfacial deposits and intra-microstructurally crystallized anhydrite (CaSO4). The loss of weight/strength and skeletal disintegrations in Na2SO4aq exposure resulted from the leaching of active elements (Ca, Al and Mg). Slag significantly contributed to the sulfate resistance of AAGU mortars, while there was an insignificant difference in strength retention when the activator ratios (Na2SiO3aq:NaOHaq) varied within 1.0–2.5 for the samples prepared with a workable mixture.