• • Cement addition significantly boosts early and mid-term strength of solidified sludge; loess dominates later strength gain, with optimal mix ratio loess:fly ash:desulfurized gypsum:cement:sludge = 0.1:0.12:0.12:0.08:1 (by mass).
• • Optimal fly ash and desulfurized gypsum content is 12% each; higher contents cause particle agglomeration, reducing compressive strength.
• • XRD and SEM confirm formation of C-S-H gel and ettringite (Aft), which densify the matrix and effectively immobilize heavy metals, reducing leachability.
• • The optimal solidifying agent composition (by mass) is 23.81% loess, 28.57% fly ash, 28.57% desulfurized gypsum, and 19.05% cement, with total solidifying agent dosage around 40% of sludge mass, balancing cost and performance.