For the repair of narrow cracks in concrete, the potassium magnesium phosphate cement (MKPC)-based material paste should have high fluidity and self-compacting ability, making it convenient for pouring and compacting. A self-compacting MKPC paste that meets the index requirements recommended by the European Federation of National Associations Representing for Concrete (EFNAFC) was prepared by increasing the water–cement ratio and adding water glass and fly ash (FA). Specimens of self-compacting MKPC paste were subjected to long-term water corrosion tests, which found that those high-fluidity MKPC paste specimens (reference sample M0) that were produced with only an increased water–cement ratio lost 15–30% of their strength. The residual ratio of folding to compression was 84.6%, and the volume expansion rate was 7.78 × 10⁻⁴ after immersion in water for 560 days. The strength residual rate of MKPC slurry (M1) modified by sodium silicate and fly ash is over 90% after 560 days of immersion in water, and the residual rate of flexural-compressive ratio is 101.3%, which meets the requirements of hydraulic cement-based materials. The volume expansion rate of M1 is 5.19 × 10⁻⁴, which is 67% of the reference sample M0 with the same water immersion age.

To study the sulfate corrosion behavior of potassium magnesium phosphate cement (PMPC) paste, the sulfate content, strength, and length of PMPC specimens were measured at different corrosion ages under 5% Na₂SO₄ solution soaking conditions, and the phase composition and microstructure were analyzed. The conclusion is as follows: In PMPC specimens subjected to one-dimensional SO₄²⁻ corrosion, the relation between the diffusion depth of SO₄²⁻ (h) and the SO₄²⁻ concentration (c (h, t)) can be referred by a polynomial very well. The sulfate diffusion coefficient (D) of PMPC specimens was one order of magnitude lower than Portland cement concrete (on the order of 10⁻⁷ mm²/s). The surface SO₄²⁻ concentration c (0, t), the SO₄²⁻ computed corrosion depth h₀₀, and D of FM2 specimen containing 20% fly ash (FA) were all less than those of the FM0 specimen (reference). At 360-day immersion ages, the c (0, 360 d) and h₀₀ in FM2 were obviously smaller than those in FM0, and the D of FM2 was 64.2% of FM0. The strengths of FM2 specimens soaked for 2 days (the benchmark strength) were greater than those of FM0 specimens. At 360-day immersion ages, the residual flexural/compressive strength ratios (360-day strength/benchmark strength) of FM0 and FM2 specimens were all larger than 95%. The volume linear expansion rates (S_n) of PMPC specimens continued to increase with the immersion age, and S_n of FM2 specimen was only 49.5% of that of the FM0 specimen at 360-day immersion ages. The results provide an experimental basis for the application of PMPC-based materials.