Optimizing High-Water-Resistance Cementitious Materials Using Modified Phosphogypsum Synergy Multiple Solid Wastes with Response Surface Methodology

This study synthesized highly water-resistant composite cementitious materials (HPGCMs) by combining hydrophobically modified phosphogypsum (HM-PG)—produced by milling with sodium silicate and sodium stearate at a specific mass ratio—with cement, slag, and aluminum ash. Orthogonal experiments quantified the significant effects of cement, slag, and aluminum ash within defined compositional ranges, while meticulously optimizing phosphogypsum proportions and water-reducing agent dosage. Material characterization revealed near-identical XRD patterns between HM-PG and regular PG (phosphogypsum), but SEM imaging showed HM-PG possessed a distinctly smoother and finer surface texture. Crucially, water contact angle measurements confirmed effective hydrophobic modification, reaching 113.9°. Multi-objective optimization via Response Surface Methodology (RSM) yielded HPGCMs with exceptional performance: 28-day compressive strength achieved 9.197 MPa, the softening coefficient reached 0.95 (indicating outstanding water resistance retention), and the 1-day water absorption rate was only 9.304% (demonstrating effective early-stage waterproofing). The developed RSM models accurately simulated compressive strength, softening coefficient, and water absorption, with experimental results deviating less than 5% from predicted values, confirming model reliability. Analysis of hydration products and microstructure elucidated the performance enhancement mechanisms. These quantitatively superior properties—high strength, near-perfect softening coefficient, and low water absorption—combined with validated hydrophobic modification and precise optimization, demonstrate HPGCMs' significant potential for waterproof construction material applications.