To achieve the resource utilization of construction solid waste, a study on the optimization of weakly alkaline activation components was conducted. The influence of activator proportion on the activation characteristics and microstructure of construction waste powder-slag composite cementitious materials was systematically investigated. Through experimental design, the regulatory effects of different activation systems on mechanical properties, setting behavior, and rheological performance were revealed. Combined with microstructural characterization, the mechanism of activator type and dosage on the crystalline evolution of reaction products and the reconstruction of microscopic morphology was elucidated. The results indicate that Ca(OH)₂ effectively activates the composite material by continuously supplying Ca²⁺ and OH^- to maintain a weakly alkaline environment, thereby promoting the formation of a silicate-aluminate network structure. With increasing Ca(OH)₂ content, the compressive strength was enhanced by up to 20.8%, with an optimal dosage of 40 g demonstrating comprehensive performance: a 28 d compressive strength of 20.1 MPa, an initial setting time of 455 min, and a fluidity of 132 mm. Microscopic analysis revealed that an appropriate amount of Ca(OH)₂ accelerates the synergistic growth of C-A-S-H gel, forming a dense and interlocked structure that significantly improves mechanical properties.