Performance and environmental assessment of Portland cement incorporating waste-derived ferrous sulfate as a gypsum substitute: A case study

Natural gypsum in ordinary Portland cement was partially substituted with ferrous sulfate hydrates synthesized from Linz–Donawitz steelmaking sludge and waste sulfuric acid through a cyclic reaction filtration process. Replacement levels of 25%, 50%, 75%, and 100% were evaluated, and the 50% replacement mixture was identified as the optimal level based on mechanical and microstructural performance. The optimal mixture achieved compressive strengths of 37.1, 40.9, 44.2, and 47.5 MPa at 3, 7, 14, and 28 days, representing a 12.3% improvement relative to the control. Setting-time measurements indicated a progressive but manageable delay, increasing from 111/152 min (initial/final) in control to 120/161 min at 25% replacement and 154/191 min at 50% replacement. X-ray diffraction and thermogravimetric analyses confirmed accelerated ettringite formation, moderated AFm development, and sustained portlandite content, resulting in a refined pore structure. The ferrous sulfate also reduced hexavalent chromium to trivalent chromium, providing additional occupational and environmental benefits. A cradle-to-gate life-cycle assessment revealed that substituting natural gypsum with waste-derived ferrous sulfate hydrates reduced the global warming potential by approximately 52%, primarily due to avoided gypsum extraction and lower upstream neutralization burdens. Overall, up to 50% of natural gypsum in Portland cement can be replaced with waste-derived ferrous sulfate hydrates while maintaining acceptable setting characteristics, improving mechanical performance, and significantly reducing environmental impacts.