The durability of reinforced cement-based structures is a critical aspect in construction and civil engineering. Among the most aggressive deterioration factors are chloride ions and CO₂. Chloride ions penetrate the porous matrix of concrete, reaching the steel reinforcement and initiating localized corrosion, which compromises both the concrete-steel bond and the overall mechanical performance of the structure. On the other hand, CO₂ reacts with calcium-rich hydration products such as portlandite (Ca(OH)₂), forming CaCO₃ and lowering the pH of the cementitious matrix, which can depassivate the steel and accelerate corrosion. This study investigates the durability of mortar specimens incorporating foundry slags, a by-product of the steel and cast iron industry, as a fine aggregate replacement. To evaluate their performance under aggressive environments, two accelerated durability tests were conducted: chloride penetration and carbonation. Reinforced mortar samples were subjected to cyclic wet-dry exposure in a NaCl solution for three months, while carbonation tests were carried out in a controlled CO₂-rich atmosphere. Electrochemical measurements, including Open Circuit Potential (OCP) and Electrochemical Impedance Spectroscopy (EIS), were performed to monitor the corrosion behavior of the embedded steel reinforcement. The results indicate that replacing natural sand with foundry slags does not significantly affect chloride penetration or carbonation resistance. Corrosion indicators remained within safe limits, and the carbonation depth showed no significant differences across the different slag replacement levels, with full substitution (100% slags) even demonstrating a slight improvement. These findings suggest that foundry slags can be a viable and sustainable alternative in cement-based materials without compromising durability.