This study investigates the durability of alkali-activated slag (AAS) mortars in sulfate-rich environments, with a focus on long-term expansion behavior under sodium sulfate (Na₂SO₄) and magnesium sulfate (MgSO₄) exposure. Two AAS mixes were prepared using ground granulated blast furnace slag (GGBFS) activated with alkaline solutions of identical Na₂O content (n = 4.0 g/100 g GGBFS), but differing silicate moduli (Ms): 0.5 (AAS0.5) and 1.9 (AAS1.9). The performance of these mixes was compared to that of an ordinary Portland cement (OPC) mortar. Prismatic specimens (40 × 20 × 160 mm3) were submerged in sulfate solutions for over four months, and the expansion was periodically measured to assess sulfate-induced deterioration. During the initial stages of exposure, both AAS mixes exhibited slightly higher expansion than OPC. However, after approximately 100 days, OPC mortars experienced a marked and progressive increase in expansion, particularly in Na₂SO₄ solution. In contrast, AAS specimens remained dimensionally stable throughout the testing period. The long-term expansion behavior of AAS0.5 and AAS1.9 was comparable, indicating that variation in silicate modulus had minimal impact on the sulfate resistance under the conditions studied. Additionally, AAS mortars displayed similar performance in both Na₂SO₄ and MgSO₄ environments, whereas OPC showed significantly greater expansion in Na₂SO₄, demonstrating higher sensitivity to the sulfate type. This investigation complements previous studies on the same AAS mixes focused on microstructure development, alkali-silica reaction (ASR), and creep, contributing to a comprehensive understanding of their long-term performance.