Strength Evolution of Concrete with Different Coal Gangue and Fly Ash Contents

To explore the application feasibility of coal gangue and fly ash in underground floor concrete, this study adopted fly ash to partially replace cement and coal gangue to partially replace natural aggregates and prepared concrete specimens with various curing ages to investigate their strength evolution rules and long-term performance of floor concrete. The mix proportion was determined via physical and chemical tests on raw materials, as well as slump and compressive strength tests. The strength, stress–strain, and failure characteristics of concrete at different ages were analyzed through uniaxial compression and Brazilian splitting methods. The research results show that the optimal mix ratio is a water–binder ratio of 0.46, fly ash substitution rate of 30%, coal gangue substitution rate of 40%, and slump ≥160 mm. It meets the needs of underground pumping construction, and its 28-day compressive strength meets the requirements for C30 concrete. The concrete strength increases rapidly at early ages and grows slowly in the later stage. The compressive strength of the optimal group rises to 43.03 MPa at 70 days, an increase of 29.2% compared with its 28-day strength. The compressive strength of groups with high fly ash content is generally low. The failure mode of the specimen is mainly multi-crack breakage, with interlaced cracks and strong penetration. The research findings facilitate the large-scale disposal and utilization of coal gangue and fly ash solid wastes, cut down the consumption of natural aggregates and cement, and satisfy the development demands of green mining.