Abstract: To address the issues of low utilization rate, dense particle structure, and insufficient release of potential reactivity of Water-quenched slag (WQS) during the resource recovery of electroplating sludge, this study regulated the particle size distribution and specific surface area of WQS via mechanical activation, and investigated the influence mechanism of WQS at different replacement levels on the mechanical properties, hydration products, and microstructural evolution of cement-based composite systems. Through mechanical testing, three-dimensional response surface methodology, range analysis, XRD, TG-DTG, and SEM, this study revealed the evolutionary behavior of WQS characteristics under mechanical activation and clarified the microstructural strengthening mechanisms. The results demonstrate that the mechanical properties of the composite system exhibit distinct age-dependent response surface evolution characteristics. At 3 days of curing, the range contribution of the WQS replacement level to strength is relatively high, the system is mainly affected by the clinker dilution effect, and the overall response surface shows a unidirectional downward trend. Conversely, at 28 days of curing, the relative influence of mechanical activation time is enhanced, and the response surface presents an asymmetric peak distribution, indicating that moderate mechanical activation is conducive to improving the later-age strength development of the WQS composite system. Considering the later-age strength, WQS replacement level, reactivity enhancement efficiency, as well as particle agglomeration and energy consumption issues caused by excessive grinding, 30 min of ball milling with a 30% WQS replacement level was identified as the optimal balanced parameter within the test range of this study. The WQS30-30 group exhibited excellent strength retention capacity and microscopic compactness at 28 days. Microscopic analyses indicate that 30 min of ball milling promotes the refinement of WQS particles, perturbs the glassy phase structure, and exposes potential reactive sites. During the later hydration stage, the Ca(OH)₂ content in the WQS30-30 group decreased significantly. Combined with the XRD, TG-DTG, and SEM results, it can be inferred that moderate mechanical activation promotes the later-age pozzolanic reaction contribution of WQS, facilitating the formation of gel products and the densification of the matrix structure. Ultimately, this study determined the optimal balanced parameters (30 min ball milling with a 30% WQS replacement level) within the test scope, providing a reference for the resource utilization of electroplating sludge-derived WQS in low-carbon cementitious materials.