Gel-Dominated Microstructural Evolution and Strength Development in Phosphogypsum-Based PRBA Systems for Road Base Applications

A significant pile-up of phosphogypsum, with utilization under 30% in some areas, has caused serious environmental problems. This study presents a powdered recycled binder-aggregate (PRBA), prepared by activating a phosphogypsum-fly ash-steel slag ternary system via a sodium silicate-calcium hydroxide activator. The optimal L3 at 55% PRBA with PG:FA:SS of 4:3:4 achieved 28-day compressive strength of 9.33 MPa, 22.0% higher than control L0 at 7.65 MPa, reaching 12.47 MPa at 56 days. The C-S-H gel network evolved from 100–200 nm tubular to under 10 nm lamellar structures, reducing average pore diameter from 82 nm to 52.5 nm. Non-isothermal kinetic modeling revealed that C-S-H/AFt dehydration follows Jander three-dimensional diffusion with R2 = 0.998 and activation energy decreasing from 161.3 to 99.3 kJ/mol at 14 days. A relay-race mechanism was identified: steel slag provides early Ca2+, phosphogypsum supplies SO42− forming AFt skeletons, and fly ash densifies the matrix via pozzolanic reactions. Environmental assessment showed P and F solidification rates of 66.7% and 88.3%, plus 10.77 g CO2/kg carbonation. Converting inert industrial wastes into a reactive cementitious system enables 55% natural aggregate replacement and pollutant immobilization, offering scalable pathways for phosphogypsum valorization in road base applications.