A hydraulic cementitious material was prepared by using rice husk ash, CFB desulfurization ash and steel slag, which can replace cement for soil solidification. In order to improve the activity and other properties of this cementitious material, the performance change and mechanism of the clinkerless cementitious material after mechanical grinding were studied. The results show that with the increase of grinding time, the particle size distribution curve of the cementitious materials changes from multi-peak distribution to single-peak distribution, the water consumption of standard consistency decreases and the setting time shortens. The end time of the hydration induction period and the appearance time of the second exothermic peak were significantly advanced, and the cumulative heat release increased. The early compressive strength and autogenous shrinkage of mortar specimens increased significantly. The initial flow expansion degree of the fluidized solidified soil prepared by the cementitious materials increases with the increase of grinding time, and the time-dependent loss accelerates. Mechanical grinding had no significant effect on the early strength of the solidified soil, while the later strength increases first and then decreases. Using this cementitious material without clinker to prepare fluidized solidified soil can meet the strength requirements of general backfill projects.