Cementitious materials used in coastal and offshore infrastructures are frequently subjected to aggressive acidic environments resulting from industrial discharge, marine pollution, and sulfur-based biochemical processes. This study examines the mechanical behavior and acid resistance of mortar incorporating lightweight expanded clay aggregate (LECA) as an environmentally sustainable partial replacement for natural sand, with the added benefit of internal curing. Four mortar mixtures containing 0%, 5%, 10%, and 15% LECA were prepared, water-cured for 28 days, and subsequently exposed to a sulfuric acid solution (pH 1.5) for up to 90 days to simulate severe acid attack. Compressive strength, flexural strength, mass loss, and strength–degradation correlations were evaluated. Results indicate that LECA replacement does not compromise initial mechanical performance, while the mixture containing 10% LECA exhibited the highest long-term durability under acid exposure, demonstrating reduced mass loss and significantly lower strength degradation compared to the control mix. The enhanced performance is attributed to the internal curing effect of LECA and its ability to mitigate microcracking in chemically aggressive environments. These findings highlight the potential of LECA-modified mortar as a sustainable and durable alternative for coastal and offshore structures subjected to acidic conditions.