Abstract: The high carbon emissions associated with cement production and the persistently high cost of mine backfilling have become major constraints on the green and sustainable development of the mining industry. To address these challenges, biomass power plant ash (BPPA) was used as the primary raw material to prepare a novel cementitious material (NCM) dominated by belite (β–C₂S) and mayenite (C₁₂A₇) through an alkali–thermal synergistic activation process, with the aim of replacing ordinary Portland cement in mine backfilling. Response surface methodology (RSM) was employed to optimize the combined effects of Na₂O mass fraction, CaO mass fraction, and calcination temperature. Predictive models for compressive strength and flowability were established to evaluate factor interactions and determine the optimal preparation parameters. The evolution mechanism of hydration products was further investigated using multiscale characterization techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). The results showed that the interaction between Na₂O and CaO exerted the most significant effect on the properties of the NCM. Under the optimal conditions of 2.1% Na₂O, 41.2% CaO, and a calcination temperature of 976 ℃, the 7 d and 28 d compressive strengths reached 13.85 and 26.98 MPa, respectively, while the flowability was 164.5 mm. Na₂O promoted the depolymerization of the aluminosilicate structure in BPPA and facilitated the release of reactive species, whereas CaO supplied Ca²⁺ and participated in the formation of active mineral phases, including β–C₂S and C₁₂A₇. The two phases exhibited distinct stage-dependent hydration behaviors: C₁₂A₇ hydrated rapidly at early ages and promoted the formation of ettringite (AFt), whereas β–C₂S underwent continuous hydration to generate calcium silicate hydrate (C−S−H) gel. With increasing hydration age, AFt formation gradually stabilized, while C−S−H continued to accumulate. AFt contributed to the establishment of the early-age skeleton, whereas the continuously generated C−S−H gel progressively filled interparticle pores and strengthened matrix connectivity, thereby promoting microstructural densification and strength development of the NCM. These findings provide a feasible technical route for the high-value utilization of BPPA and the development of low-carbon cementitious materials for mine backfilling.