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Limestone-slag-Portland cement mixtures have a low carbon footprint but have low early compressive strength. This study proposes a method to improve the early strength of ternary mixture mortar using aluminum sulfate, evaluates its effectiveness, and tests the optimal dosage. The fresh paste hydration heat and mortar macroscopic properties, such as mechanical performance, resistivity, and ultrasonic pulse velocity, were measured at different dosages (0, 1, 2, and 3%) of aluminum sulfate. Microscopic testing methods, such as X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, were employed to analyze the interaction mechanism between aluminum sulfate and the ternary mixture. The research findings demonstrated that aluminum sulfate enhanced the early mechanical properties and significantly improved the 28-day resistivity. Dosages of 1% and 2% had no adverse effects on the 28 d strength, whereas a 3% dosage reduced the 28 d strength. In the presence of aluminum sulfate, the early hydration heat release of the mixture increased significantly, ettringite became the main early hydration product, and portlandite decreased significantly. Considering the changes in the early strength, 28 d strength, and surface electrical resistivity, a 1% aluminum sulfate dosage was determined as the optimal addition level.