Influence of Gypsum on Fly Ash Activation and Ettringite Stability in Low-Alkalinity Flowable Stabilized Soil

Incorporating massive excavated waste soil typically creates an inherently low-alkalinity environment that severely suppresses pozzolanic reactivity. To overcome this bottleneck, this study develops a high-performance controlled low-strength material by incorporating gypsum as a critical activator into the cement-fly ash system. The investigation integrates macro-scale engineering evaluations with microstructural characterization to elucidate the synergistic hydration mechanisms under reduced pH conditions. The results demonstrate that the mixture achieves an optimal balance, maintaining a high flowability retention rate of 74.5% at 60 min. It exhibits superior mechanical and durability performance: its 56 day compressive strength was 31.7% higher than the cement-only mix, alongside an exceptional 56 day softening coefficient of 92.5%. Microstructural investigations suggest that the proposed gypsum-induced “salt activation” circumvents the alkaline deficiency by accelerating fly ash dissolution, generating massive Calcium−(Alumino)−Silicate−Hydrate gels. Moreover, competitive hydration from the ongoing silicate release effectively prevents the transformation of ettringite to monosulfoaluminate, supporting a proposed “ettringite protection” effect that preserves a robust interlocking skeleton. This microstructural synergy provides a scientific and practical basis for designing resource-efficient, highly durable geomaterials from challenging low-alkalinity solid wastes.