Combined effect of alkali-aggregate reaction (AAR) and internal sulfate attack (ISA): Microstructural and porous structure modifications of portland cement mortars

•Mortar deteriorates faster when ISA and AAR occur concomitantly.•Mortars present initial inertia of reactions when ISA and AAR occur in isolation.•ISA and AAR, when isolated, initially lead to increases in mechanical strength.•ISA and AAR products promote pore refinement and subsequent cracking. This study aims to analyze the isolated and combined action of alkali-aggregate reaction (AAR) and internal sulfate attack (ISA) in Portland cement mortars cured partially immersed in water for 360 days. The investigation focused on microstructural changes, mainly changes in the porous structure of mortars. When isolated, AAR and ISA increase the compressive strengths, with subsequent reductions in these values due to crack formation. When the reactions acted combined, it was possible to identify both ISA and AAR reactions coexisting in an associated and even isolated way. At certain times, intervals of pore refinement and subsequent cracking by the formed products were observed, both by AAR and ISA. These cycles were irreversible and deleterious to the cement matrix.