Mix-Proportion Evaluation and Microstructural Characteristics of NaOH-Na2SO4 Composite-Activated Fly Ash-Based Grouting Materials

To improve fly ash utilization in mine grouting materials and address the slow reaction and limited early-age strength of fly ash-based binders, a fly ash-based grout was prepared using fly ash and S95-grade ground granulated blast-furnace slag at a mass ratio of 70:30 and activated with NaOH-Na2SO4. A full-factorial experiment evaluated the effects of water-to-solid ratio, total activator dosage, and NaOH:Na2SO4 on fluidity, bleeding rate, stone formation rate, and compressive strength at 7 and 28 d. Representative 7 d specimens were characterized by SEM, EDS, and XRD. Increasing the water-to-solid ratio improved fluidity but increased the bleeding rate and reduced the stone formation rate and compressive strength. The preferred mixture had a water-to-solid ratio of 0.55, a total activator dosage of 5%, and NaOH:Na2SO4 = 2:1. It exhibited a fluidity of 195.0 mm, a bleeding rate of 1.70%, a stone formation rate of 99.50%, and compressive strengths of 8.5 MPa at 7 d and 11.3 MPa at 28 d. At NaOH:Na2SO4 = 2:1, the specimen showed a denser microstructure with fewer pores and cracks, a local Ca/Si ratio of approximately 0.94, and more evident reaction-product features. The results provide an experimental basis for mix-proportion selection of fly ash-based grouting materials.