Abstract: To investigate the regulatory mechanisms of low-alkalinity additives on the performance of clay-cement slurry in mine water hazard prevention and curtain grouting engineering, this study employed an orthogonal experimental design. The rheological properties of the slurry, including flowability, pumpability, setting time, and pH evolution, were systematically analyzed under varying mix conditions. By introducing phosphogypsum, fly ash, and the SJP additive, the effectiveness of these materials in reducing the alkalinity of the slurry was evaluated. The results reveal that phosphogypsum effectively reduced the pH to 11 and prolonged extending the setting time. Although fly ash showed limited efficacy in alkalinity reduction, it significantly enhanced the pumpability of the slurry and shortened the pumping period. The SJP additive demonstrated outstanding performance in both pH reduction and pumpability extension, with the pH decreasing notably to 8 when the dosage was increased from 2.5% to 3.5%. Based on experimental data, an optimized formulation of the SJP additive, clay, phosphogypsum, and cement (CCAS) was developed. Orthogonal tests identified the optimal proportions as follows: composite acid salt 0.5%, calcium sulfoaluminate 1.5%, phosphogypsum 10%, and clay 50%. The optimized slurry achieved a significant reduction in pH to 10, maintained a flowability of 18–23 cm, and extended the pumpability window to 100–220 minutes, fully meeting the engineering application requirements. This study provides a scientific basis for controlling slurry alkalinity in curtain grouting projects, thereby mitigating environmental pollution.