Microstructural enhancement of concrete paver blocks using steel sludge as a sustainable fine aggregate replacement

The growing demand for sustainable construction materials has accelerated the need for alternatives to natural aggregates in concrete. Although steel slag has been extensively studied, the use of steel sludge as a fine aggregate substitute in concrete paver blocks has not been sufficiently examined, especially regarding durability and microstructural characteristics. This study investigates the performance of steel sludge as a micro-filler and partial replacement of fine aggregate at 10%, 20%, and 30% by mass, maintaining a constant water-cement ratio of 0.38. Mechanical and durability properties were evaluated using compressive strength, water absorption, sorptivity, rapid chloride penetration, and weight loss tests, complemented by SEM, EDX, and XRD analyses. The results indicate a consistent improvement in performance with increasing steel sludge content. The 28-day compressive strength increased from 42.77 MPa for the control mix to 54.13 MPa at 30% replacement, representing an approximate 26.6% increase. Water absorption decreased from 6.12% to 4.20%, the initial absorption rate decreased from 0.0332 to 0.0250 mm/min⁰·⁵. and RCPT values declined from 640 to 552 Coulombs, reflecting reduced permeability. Microstructural analysis demonstrated pore refinement, a lower Ca/Si ratio, and enhanced formation of polymerized C-(A)-S-H gel. XRD analysis confirmed the absence of new crystalline phases. Within the investigated range, steel sludge enhances strength and durability through matrix densification and microstructural refinement. These findings demonstrate its potential as a sustainable industrial waste management to natural sand in paver block applications.