• Increasing curing age enhances the strength of all backfills, with 28-day strengths exceeding 1.342 MPa.
• The combination of 50% blast furnace slag with 50% cement yields the best mechanical performance (maximum strength 6.129 MPa).
• Blast furnace slag significantly improves the energy index at peak stress, while magnesium slag shows a lesser effect.
• Sharp changes in the energy consumption ratio serve as a reliable criterion for predicting backfill destabilization and failure, providing guidance for mine filling applications.