Abstract: This is an article in the field of ceramics and composites. In order to improve the impermeability and microstructure of the impermeable wall materials, four kinds of calcium-based bentonite modified and filling material with different dosages were prepared under the action of alkali excitation reaction. The modified and filling material samples in the curing process were tested by flexible wall penetration (FWPT), nuclear magnetic resonance (NMR) scanning and electron microscope scanning (SEM). The results show that the permeability coefficient of filling material samples decreases first and then increases with the increase of bentonite content, and reaches a minimum value when the content of bentonite is 4%. The relaxation time (T2) distribution curves of the filled material specimens after curing from 1 to 28 d were all bimodal, with more than 80% of the peaks representing small pores, and the percentage of the peaks representing large pores decreasing with the increase of curing time. According to SEM images, it can be seen that a certain amount of calcium bentonite can promote the occurrence of hydration reaction, and the porosity of the material is significantly reduced. At 4% phosphate slag, the cementitious filler has the best compactness and better impermeability and strength. Accordingly, it can be determined that the better dosage of phosphate slag modified cementitious filler is 4%.