Upcycled engineered earth from recycled sand via hybrid carbonate–hydroxide activation of slag with waste brick powder and recycled rubber fibers

This study investigates the mechanical performance, microstructural evolution, and sustainability of sandy soil stabilized with alkali-activated slag and reinforced with recycled rubber fibers. A hybrid Na2CO3–NaOH activator was used to enhance slag dissolution while reducing reliance on highly caustic activators. The effects of slag dosage, waste brick powder (WBP), and fiber content on unconfined compressive strength (UCS), ductility, and pore structure were evaluated. Increasing slag content promoted C–(A)–S–H gel formation, densified the matrix, and increased particle bonding, achieving a maximum UCS of 4.513 MPa. WBP primarily acted as a micro-filler and nucleation aid, improving packing and matrix densification. X-ray diffraction and field-emission scanning electron microscopy confirmed the development of poorly crystalline binding phases and a more compact microstructure. Image analysis showed pore refinement, with mean pore area decreasing from 0.399 μm 2 to 0.037 μm 2 and pore circularity increasing from 0.450 to 0.706. Fibers improved post-peak behavior via crack bridging, increasing ductility and energy absorption. Embodied carbon of alkali-activated mixtures (62.95–75.42 kg CO 2 -eq/m 3 ) was lower than that of OPC-stabilized systems (80.75–153 kg CO 2 -eq/m 3 ), and Pareto analysis identified S8RF mixtures as balanced options for strength, cost, and carbon efficiency.