Concrete structures can have their durability affected due to internal sulfate attack (ISA). This attack can occur by the use of aggregates contaminated with sulfides, such as pyrite. The oxidation of pyrite releases sulfate ions into the cement matrix and favors the formation of expansive phases, such as gypsum and secondary ettringite. These expansions cause cracks and progressive degradation of concrete structures. Therefore, this study aimed to evaluate the effect of sulfate-resistant Portland cement in the likely mitigation of ISA. Mortar specimens with two types of cement (CEM I and CEM I SR) were cast with and without pyrite. These mortars naturally aged for 406 days and later were subjected to wetting and drying cycles at 40 °C to stimulate the ISA. The following properties of mortars were evaluated over time: length change and mass variation, tensile and compressive strengths, capillary and immersion water absorption, and electrical resistivity. The results showed that sulfate-resistant Portland cement only managed to reduce the length change in the first ages. The results of water absorption tests showed that the attack by sulfates due to pyrite oxidation is in the early stages. However, water absorption was reduced by 60% during the 56 wetting and drying cycles with water only. The electrical resistivity of the pyrite mortars was greatly affected (about 550%) over the 18 weeks of exposure.