Timely application of slurry seal is one of the effective methods for preventive maintenance and preservation of asphalt pavement on roads and airport runways. With growing environmental concerns and the declining availability of natural resources, the utilization of industrial waste materials in road construction projects has become an essential necessity. In this context, the present study evaluates the feasibility of using ferro-phosphorus powder (FPP) as a filler in slurry seal mixtures, comparing its performance with fillers derived from natural aggregates. Initially, the physical and chemical characteristics of the aggregates and FPP were analyzed. Subsequently, five slurry seal mixtures containing 0%, 15%, 30%, 45%, and 60% FPP as a replacement for natural filler were prepared and tested. Wet cohesion test, wet track abrasion test (WTAT), loaded wheel test (LWT)–sand adhesion, and LWT–displacement tests were conducted in accordance with ASTM D3910 to evaluate the technical characteristics of the mixtures. Furthermore, statistical analysis was performed to determine the effect of FPP on slurry seal performance, and the Toxicity Characteristic Leaching Procedure (TCLP) test was applied to assess environmental impacts. The results indicated that the mixture containing 45% FPP exhibited the optimal performance: increasing cohesion by 30%, reducing moisture susceptibility by 36%, reducing bleeding by 50.4%, and decreasing vertical displacement under traffic loading by 29.7% compared to the control sample. The statistical analysis confirmed that FPP had a significant effect on performance indicators, with P-values revealing its strong influence on improving cohesion and resistance to deformation. Moreover, the TCLP test confirmed the environmental safety of all FPP-containing mixtures, as heavy metal leaching levels in all samples were below permissible limits.