This study investigated the coupled effects of petroleum coke residue (PCR) and alkali equivalent on the strength, shrinkage, and durability of alkali–activated mortars based on a ground–granulated blast–furnace slag–fly ash (GGBS–FA) system. A suitable PCR content improved compressive and flexural strength, whereas higher contents reduced strength. These trends may reflect changes in calcium– and sulfate–bearing reaction products and the concurrent reduction in FA–derived precursors. Increasing the alkali equivalent generally increased strength and shrinkage, while PCR replacement generally reduced post–demolding sealed contraction. Potentially expansive products may contribute to this reduction. At 3% PCR, relatively low carbonation depths and non–steady–state chloride migration coefficients were obtained, whereas higher PCR contents could yield higher sulfate corrosion–resistance coefficients under certain conditions. An appropriate PCR dosage can effectively improve strength, shrinkage resistance, and durability, indicating potential for PCR valorization in alkali–activated mortars.