Abstract: Using cemented rockfill to replace coal pillars offers an effective solution for reducing solid waste while ensuring the safety of gob-side entries. However, achieving the balance among low cost, high waste recycling rates, and adequate strength remains a significant challenge for cemented rockfill. This study used a composite alkali activator to activate gangue cemented rockfill. The compressive strength, scanning electron microscopy, energy dispersive spectrometer, mercury intrusion porosimetry, X-ray diffraction, and thermogravimetric tests were carried out to investigate the effect of the composite alkali activator proportion on the compressive strength, microstructure, and composition of the cemented rockfill. The calcium silicate hydrate (C–S–H) molecular model of cemented rockfill was constructed to explore the fracture evolution of the nucleated molecular structure under tension. The results show that compressive strength initially increased and then decreased with the activator proportion, the optimal activator proportion of 1:2 resulted in a 31.25% increase in strength at 3 d. This reasonable activator proportion strengthens the pozzolanic effect of gangue, and consumes more calcium hydroxide to inhibit its agglomeration, ultimately achieving the densification of microstructure. The activator proportion inevitably substitutes calcium ions with sodium ions in the C–S–H molecular model. The 12% substitution of calcium ions increases the adhesion between silicon chain layers, which is beneficial to the interlayer stress transfer. This work proposes a method for preparing low-cost cemented rockfill from alkali-activated gangue, which can be used for solid waste recycling and reducing cement consumption to achieve low-carbon goals.