Portland cement (PC) is the world's most widely used cementitious materials. However, its production process emits substantial amounts of CO2. Reducing carbon emissions from cement is crucial for the sustainable development of the cement industry. Rapid industrial growth has generated vast quantities of industrial solid wastes (ISWs), such as lithium slag and red mud. Utilizing these ISWs to partially replace PC in composite cement represents an effective pathway for the low-carbon transition of the cement industry. Nevertheless, substituting PC with low-activity materials like lithium slag and red mud significantly compromises cement performance. Therefore, this study employed calcium hydroxide to optimize the properties of a composite cement based on lithium slag, slag, and red mud, investigating its influence. The results indicate that calcium hydroxide reduced the fluidity of the composite cement and prolonged its setting time. At early hydration ages (3 d), increasing calcium hydroxide content decreased the compressive strength of the composite cement mortar. After 7 d of hydration, however, the compressive strength gradually increased with higher calcium hydroxide content. By 56 d of hydration, the compressive strength of composite cement mortars with 8% to 12% calcium hydroxide exceeded that of OPC paste. Compared to PC, the composite cement paste exhibited a significant increase in shear stress and correspondingly higher viscosity. The effect of calcium hydroxide on rheology was not distinctly clear, though it generally reduced the paste's shear stress. Calcium hydroxide provided additional alkalinity during the hydration of the composite cement, promoting the pozzolanic reactions of lithium slag and red mud. Furthermore, the alkaline components present in the red mud also contributed to the alkaline environment for composite cement hydration. This research not only provides a feasible technology for the low-carbon and sustainable transformation of traditional cement but also offers effective pathways for the resource utilization of ISWs.