Mechanical properties and microstructural evolution of geopolymers derived from blended raw and calcined coal gangue

To achieve low-cost and environmentally friendly utilization of coal gangue, this study systematically investigates the mechanical properties and microstructural evolution of geopolymers prepared by co-utilizing raw coal gangue (RCG) and calcined coal gangue (CCG). The results show that with increasing RCG incorporation, the compressive strength and workability of the geopolymers gradually decrease, whereas the setting time is significantly prolonged. Thermodynamic and microstructural analyses reveal that kaolinite is nearly inert in the geopolymerization process and mainly acts as a filler, whereas metakaolinite serves as the primary reactive phase. When the RCG replacement ratio is 40 %, the 28-day compressive strength of the geopolymer reaches 21.81 MPa, exceeding the requirements for road base materials. Meanwhile, the initial setting time is extended from 150 min to 400 min, indicating a remarkable regulating effect of RCG on the setting behavior. Furthermore, compared with the geopolymer prepared solely from CCG, incorporating 40 % RCG reduces carbon emissions by 103.7 kg CO 2 -equivalent per ton of geopolymer, corresponding to a 29.39 % decrease, while the production cost is reduced by approximately 9.62 %. This study demonstrates the feasibility of co-utilizing raw and calcined coal gangue to produce high-performance geopolymers and provides a sustainable approach for large-scale coal gangue resource utilization. • Raw and calcined coal gangue can be co-utilized to prepare high-performance geopolymers. • Higher raw coal gangue content reduces strength, fluidity, and density while extending setting time. • Microstructure evolution, carbon emissions, and production costs were systematically evaluated. • Kaolinite remains chemically stable in alkaline environments, acting as a filler without participating in geopolymerization.