Internal Curing Effect of Pre-Wetted Coal Gangue Powder on Hydration and Shrinkage Behavior of Cement–Fly Ash Composite Materials

Coal gangue, one of the largest industrial solid wastes generated during coal mining, has accumulated to more than 7 billion tons in China, creating persistent environmental pollution and occupying substantial land resources. Although various activation approaches, including thermal activation, chemical stimulation, and mechanical grinding, have been developed to improve the reactivity of coal gangue, their practical application is still constrained by considerable energy demand and high processing costs. In this work, a simple pre-wetting strategy was introduced to enhance the utilization efficiency of coal gangue powder (CGP) incorporated into cement–fly ash blended binders. Particular attention was given to its influence on shrinkage characteristics and mechanical performance. Pre-wetted CGP replaced the cementitious materials at replacement levels of 10%, 20%, and 30% by mass. Compared with mixtures containing dry CGP, the specimen with 30% pre-wetted CGP exhibited the most pronounced improvement, with chemical shrinkage, autogenous shrinkage, and drying shrinkage decreasing by 36.5%, 47.4%, and 36.4%, respectively, while the 28-day compressive strength increased by 6.2%. These improvements are mainly associated with the internal curing effect provided by the water stored in the pre-wetted CGP, which facilitates cement hydration and results in an 18.6% increase in the intensity of the main hydration peak. In addition, microstructural characterization confirmed that pre-wetting effectively refined the pore system and reduced capillary pore pressure by 33.7%, thereby producing a denser matrix with improved mechanical performance. The proposed pre-wetting treatment offers an effective, economical, and environmentally friendly strategy for the high-value utilization of coal gangue.